Section VI. The Blood & Lymphoid System

Changes in different types of blood cells occur in many diseases (Chapters 24: Blood: I. Structure & Function; Anemias Due to Decreased Erythropoiesis, 26: Blood: III. The White Blood Cells). For instance, changes in leukocyte count provide useful information in infectious diseases (Chapter 13: Infectious Diseases: I. Mechanisms of Tissue Changes in Infection). Anemia is an extremely common clinical problem that has many causes (Chapters 24: Blood: I. Structure & Function; Anemias Due to Decreased Erythropoiesis, 25: Blood: II. Hemolytic Anemias; Polycythemia). Neoplasms of the hematopoietic system include myeloproliferative disorders (Chapters 25: Blood: II. Hemolytic Anemias; Polycythemia, 26: Blood: III. The White Blood Cells), leukemias (Chapter 26: Blood: III. The White Blood Cells), malignant lymphomas (Chapter 29: The Lymphoid System: II. Malignant Lymphomas), and plasma cell myeloma (Chapter 30: The Lymphoid System: III. Plasma Cell Neoplasms; Spleen & Thymus). Leukemias and lymphomas represent the most common malignant neoplasms in persons under age 30 years. Successful treatment of these hematopoietic neoplasms has made their early and accurate diagnosis very important. Students may find it worthwhile to review the discussion of normal hemostasis in Chapter 9: Abnormalities of Blood Supply before undertaking a study of bleeding disorders (Chapter 27: Blood: IV. Bleeding Disorders).

Chapters 28: The Lymphoid System: I. Structure & Function; Infections & Reactive Proliferations, 29: The Lymphoid System: II. Malignant Lymphomas, and 30: The Lymphoid System: III. Plasma Cell Neoplasms; Spleen & Thymus deal with diseases of the lymph nodes, spleen, and thymus. These include infections, reactive proliferations, and malignant lymphomas. The student may find it helpful to review earlier chapters relating to the immune system (Chapter 4: The Immune Response) and immunodeficiency states (Chapter 7: Deficiencies of the Host Response) before embarking on these chapters.

Peripheral Blood

The peripheral blood is composed of cells suspended in plasma (Table 24-1).

Erythrocytes (Red Blood Cells)

Normal erythrocytes are nonnucleated biconcave disks with a uniform diameter of 7.5–8 μm. Erythrocytes are extremely pliable cells, able to change shape as they squeeze through the microcirculation. The erythrocyte cytoplasm contains hemoglobin, a protein complexed to an iron containing porphyrin that gives the cell its characteristic red color. Hemoglobin is vital to oxygen transport in the blood—the main function of the erythrocyte.

Erythrocytes have a life span in the peripheral blood of approximately 120 days. They are derived from normoblasts in the bone marrow, which lose their nucleus in the final stage of development prior to release into the peripheral blood (Figure 24-1; see also Figure 24-3).

Newly released erythrocytes are slightly larger than mature cells and slightly basophilic because of the presence of residual ribosomes and mitochondria. (They demonstrate a finely reticular pattern with supravital stains such as cresyl violet—hence the term reticulocytes.) At any given time, approximately 1% of erythrocytes in the peripheral blood are newly released (eg, reticulocytes). Nucleated normoblasts are not normally found in the peripheral blood.

Granulocytes

There are approximately 1000 red cells for every white cell in the blood.

Granulocytes are nucleated white blood cells that have been classified according to the staining behavior of their cytoplasmic granules as neutrophils, eosinophils, or basophils. They are derived from the myeloid precursor series in the bone marrow (Figure 24-1) and have a life span of only a few hours in peripheral blood. Myelopoiesis is described more fully in Chapter 26: Blood: III. The White Blood Cells.

Neutrophils have a multilobate nucleus (for this reason they are also called polymorphonuclear leukocytes) and fine cytoplasmic granules. Nuclear lobation increases with maturity; when cells are released from the marrow, they have a band-like nucleus (band forms). Fully mature neutrophils have four or five nuclear lobes connected by fine lines of chromatin (Figure 24-2).

Neutrophils participate in the nonspecific acute inflammatory response to injury. They are particularly active against extracellularly multiplying infectious agents, notably bacteria, but are also involved in repair and immune responses.

Eosinophils have large eosinophilic cytoplasmic granules. Nuclear lobation rarely progresses beyond two lobes. Eosinophils are prominent in parasitic infections and allergic reactions.

Basophils have large basophilic granules that obscure the nucleus. Like their counterparts the tissue mast cells, they are involved in type I hypersensitivity reactions.

Monocytes & Lymphocytes

Monocytes are part of the macrophage or mononuclear phagocyte system. Lymphocytes belong to the immune system. These two cell types are more fully discussed with the lymphoid system in Chapters 4: The Immune Response and 28: The Lymphoid System: I. Structure & Function; Infections & Reactive Proliferations.

Platelets

Platelets are very small, nonnucleated cytoplasmic fragments of megakaryocytes. Platelets have a vital function in control of hemorrhage (Chapter 27: Blood: IV. Bleeding Disorders).

Plasma

Plasma is the fluid in which the formed elements of the blood are suspended. The normal ratio of erythrocytes to plasma is approximately 40:60 (also expressed as the hematocrit: normal is 42–50%). The hematocrit is an important determinant of blood viscosity. Disorders in which the proportion of erythrocytes is greatly increased (polycythemia) are associated with increased blood viscosity.

Plasma is composed of water, electrolytes, proteins, and a large number of other constituents such as glucose, products of protein and nucleic acid metabolism, and enzymes. Plasma proteins include albumin and a variety of globulins. Albumin, which is synthesized by the liver, is the major determinant of plasma oncotic pressure, which governs fluid exchange in systemic capillaries. Globulins include immunoglobulins, complement, enzymes, factors involved in blood coagulation, fibrinolysis, and several transport proteins for hormones, minerals, lipids, and nutrients. Measurements of various plasma constituents may provide evidence of disease.

Serum

Serum is the fluid that remains after blood has been allowed to clot in a tube. It resembles plasma except that fibrinogen and other coagulation factors will have been depleted by the process of clot formation.

Blood tests may be performed on whole blood (eg, hemoglobin, blood cell counts), plasma (eg, plasma protein), or serum (eg, serum amylase, iron). Tests performed on whole blood and plasma require collection in tubes containing anticoagulant to prevent clotting. Tests performed on serum require clotted blood.

Bone Marrow

Normal Bone Marrow

After birth, the bone marrow is the sole site of production of erythrocytes, granulocytes, and platelets. It also produces blood monocytes, which are part of the macrophage system. At birth, hematopoietic marrow is present in the medullary cavity of all the bones of the body. With increasing age, the hematopoietic marrow is replaced by adipose tissue in the bones of the extremities, and hematopoietic marrow is found only in the axial skeleton.

Bone Marrow Examination

Specimens of bone marrow in adults must be obtained from an axial bone, most commonly the iliac crest or sternum. In children, bone marrow for biopsy may also be recovered at the tibial tuberosity. Aspiration biopsy utilizes a thin needle and provides material for smears and for histologic sections of centrifuged particles. Trephine biopsy utilizes a larger needle that provides a core of tissue (including bone spicules) from the marrow for histologic sections. Open marrow biopsy is sometimes performed in difficult cases.

The following criteria are evaluated in a bone marrow examination:

1. Cellularity: The proportion of hematopoietic cells to fat cells; this varies with site and age, from near 100% in a child, to 50% in adult axial marrow, to 0% in peripheral marrow from the elderly (ie, marrow is all fat).

2. Amount and maturation of erythropoiesis.

3. Amount and maturation of myelopoiesis.

4. Myeloid:erythroid ratio (normally 3:1 or 4:1).

5. Numbers and morphology of megakaryocytes.

6. Numbers and morphology of plasma cells.

7. Tumor cells: Leukemia, lymphoma, carcinoma.

8. Microbial parasites (bacteria, fungi, protozoa).

9. Iron content (by iron stain).

10.  Presence of fibrosis.

All stages of maturation of erythroid, myeloid, and megakaryocytic series should be present, with more mature cells outnumbering the blasts. The earliest precursors of the various cell lines (stem cells, or hemocytoblasts) cannot be distinguished by light microscopy.

If systemic infection is suspected (eg, tuberculosis, brucellosis), part of the bone marrow specimen should be cultured using appropriate techniques.

Production of Blood Cells

The pluripotent hematopoietic stem cells occur at a frequency of approximately 1:105 nucleated bone marrow cells. Functionally, stem cells have been identified in mice by their ability to form colonies of bone marrow cells in the spleens of lethally irradiated mice—hence the term colony-forming unit—spleen (CFU-S), signifying production of all hematopoietic cells. Theoretically, such a cell can completely reconstitute the hematopoietic system. Morphologically, dividing stem cells appear as blasts (large cells with finely dispersed chromatin and one or more distinct nucleoli). Phenotypically, they express CD34 surface antigen.

With the beginning of differentiation, these cells become progressively committed progenitor cells, able to give rise only to cells of a single series: colony-forming unit—erythroid (CFU-E), colony-forming unit—megakaryocytic (CFU-Meg), colony-forming unit—monocytic (CFU-M), or colony-forming unit—granulocytic (CFU-G). Morphologically, these are recognized as erythroblasts, megakaryoblasts, monoblasts, and myeloblasts (granulocyte series), respectively. Intermediate forms such as colony-forming unit—granulocytes or monocytes (CFU-GM) (granulocytes or monocytes) also exist (Figure 24-1).

Hematopoietic activity is regulated by various growth factors, including the interleukins (IL-3 stimulates most types of progenitor cells) and colony-stimulating factors (eg, CSF-G and CSF-GM). Recombinant deoxyribonucleic acid (DNA) technology has yielded synthetic versions of a number of these factors, some of which have found application in stimulating bone marrow activity in aplastic states or following chemotherapy.

Erythropoiesis is also under the influence of erythropoietin, which is produced by the kidneys in response to arterial oxygen content (recombinant erythropoietin is available). Erythrocyte maturation (Figure 24-3) involves loss of the nucleus by pyknosis in the late normoblast stage, together with hemoglobinization of the cytoplasm, which imparts an increasing eosinophilia to the cytoplasm (the early normoblast is basophilic).

Myelopoiesis

Other aspects of white blood cell production are described in Chapter 26: Blood: III. The White Blood Cells. Platelet production is considered in Chapter 27: Blood: IV. Bleeding Disorders and lymphopoiesis in Chapters 4: The Immune Response and 29: The Lymphoid System: II. Malignant Lymphomas.

Anemia is by far the most common disorder affecting erythrocytes. It is defined as a reduction in the hemoglobin concentration of the blood (with reference to an established normal range for age, sex, and geography), usually associated with a reduction of total circulating red cell mass. Whatever its cause, anemia decreases the oxygen-carrying capacity of the blood and, when severe enough, causes clinical symptoms and signs.

Clinical Features

(Figure 24-4)

Anemia is characterized by pallor of the skin and mucous membranes and by manifestations of hypoxia (decreased oxygen content of the blood), most commonly weakness, fatigue, lethargy, dizziness, or syncope (fainting). Myocardial hypoxia may result in anginal pain. A hyperdynamic circulation, with an increase in heart rate and stroke volume, occurs in response to hypoxia. Ejection-type flow murmurs may develop, and if the anemia is severe, cardiac failure may ensue (Figure 24-4).

Classification

Anemias are classified in one of two ways.

Etiologic Classification

(Table 24-2.) In this book, anemias are considered from the standpoint of their causes: (1) decreased production of erythrocytes by the marrow, (2) blood loss too extensive for replacement by the marrow, and (3) increased rate of destruction of erythrocytes (hemolytic anemias). Anemias due to decreased erythropoiesis or blood loss will be discussed in this chapter; hemolytic anemias are the subject of the next chapter.

Morphologic Classification

(Table 24-3.) Classification of anemias according to changes in size and hemoglobin content of erythrocytes in the peripheral blood is shown in Table 24-3 and in Figure 24-5.

Incidence

The incidence of anemia varies from 2% to 15% in different studies in Britain and the United States. Anemia is about twice as common in females. Mild anemia without symptoms is quite common, but anemia may be the presenting sign of serious disease. If normal range American values are used, anemia is much more common in underdeveloped countries; this observation raises questions about what the normal range really is and whether it should be defined differently for different populations.

Diagnosis

When a patient presents with symptoms of anemia—eg, pallor of mucous membranes, fatigue, and breathlessness—the initial diagnosis may be confirmed by demonstration of a decreased hemoglobin level. Morphologic classification of the anemia into macrocytic, normocytic-normochromic, microcytic-hypochromic, and dimorphic is done by examination of blood values (mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC)) and a peripheral blood smear. Identification of the cause of anemia may require bone marrow examination, hemoglobin electrophoresis, and determination of serum iron, vitamin B12, and folate levels. The tests required will vary with different patients, eg, bone marrow examination is critical in macrocytic anemia and of limited value in microcytic-hypochromic anemia.

Anemias Due to Decreased Red Cell Production

There are many mechanisms by which production of erythrocytes in the marrow is decreased. Because there is no mechanism to decrease the rate of peripheral destruction of erythrocytes, anemia results. The more important of these conditions will be discussed below:

Aplastic Anemia

Aplastic anemia is the result of failure of production, suppression, or destruction of stem cells in the bone marrow, which leads to decreased generation of erythrocytes, leukocytes, and platelets (pancytopenia). The bone marrow shows marked decrease in cellularity.

Etiology

Drugs are the most common cause of aplastic anemia in clinical practice (Table 24-4).

Pathology & Clinical Features

Aplastic anemia shows a markedly hypocellular bone marrow with a reduction of all cell lines. The peripheral blood smear shows the following characteristic features:

Anemia

Erythrocytes are decreased in number, but those that remain are normocytic or macrocytic and normochromic. There is an absence of reticulocytes in the peripheral blood.

Granulocytopenia

Granulocytopenia (neutropenia) occurs rapidly because these cells have a very short life span; infections and oral ulcers result.

Thrombocytopenia

Thrombocytopenia results in a bleeding tendency.

Treatment & Prognosis

Treatment is symptomatic, plus corticosteroids. The prognosis is poor. In severe, irreversible cases, death results from the effects of infections (eg, pneumonia, septicemia), bleeding (eg, intracranial hemorrhage), or anemia (eg, cardiac failure).

Red Cell Aplasia

Pure red cell aplasia occurs rarely. It may be a congenital disorder (Blackfan-Diamond disease), which is manifested in early life by anemia and erythroid hypoplasia. This disorder frequently remits spontaneously or in response to corticosteroid therapy. Acquired red cell aplasia is usually a transient complication that occurs in congenital hemolytic anemias such as sickle cell anemia. Adult red cell aplasia may also occur as a complication of thymoma by an unknown mechanism.

Marrow Replacement (Leukoerythroblastic) Anemias

Involvement of the bone marrow cavity by metastatic neoplasms, lymphoma, or leukemia, disseminated granulomatous diseases (such as tuberculosis), fibrosis, or multiple abscesses displaces and replaces the normal marrow elements. Replacement of proliferating marrow cells of sufficient degree may result in anemia, leukopenia, or thrombocytopenia.

Compensatory phenomena include (1) the development of active marrow in sites outside the marrow cavity (extramedullary hematopoiesis or myeloid metaplasia, especially in spleen and liver) and (2) release of cells from the marrow before maturation is complete. The latter results in a shift to the left (immature white cells, including myeloblasts and myelocytes) and the presence of nucleated red cells (normoblasts) in the peripheral blood—hence the designation leukoerythroblastic anemia. The blood picture is characteristic (Figure 24-5).

Megaloblastic Anemias

Megaloblastic anemias are a subset of macrocytic anemias in which the maturation phase of erythropoiesis in the bone marrow is abnormal, resulting in erythroid precursors that are enlarged and show failure of nuclear maturation (megaloblasts).

Etiology

Megaloblastic anemias result from conditions in which nucleic acid synthesis is abnormal, as in vitamin B12 and folic acid deficiencies (Table 24-5). Vitamin B12 and folic acid play roles as cofactors in the conversion of deoxyuridine to deoxythymidine, an essential step in the synthesis of DNA (Figure 24-6). Note that vitamin B12 reservoirs in the liver are normally sufficient for several years; following gastrectomy (which removes the source of intrinsic factor, thereby reducing vitamin B12 absorption), a decade may pass before vitamin B12 megaloblastic anemia becomes apparent.

Vitamin B12 is present in high concentration in animal liver and to some degree in most meats but is absent in plants. Folate is widely distributed, especially in leafy green vegetables. Dietary deficiency of vitamin B12 is rare except in strict vegetarians. Dietary deficiency of folic acid is common and occurs in many states of malnutrition.

Pathology

Red Cell Changes

When DNA synthesis is abnormal, erythropoiesis changes from normoblastic to megaloblastic.

Megaloblasts differ from normoblasts in that they are larger and show delayed nuclear maturation but normal cytoplasmic hemoglobinization (nuclear-cytoplasmic asynchrony) (Figure 24-7). The late megaloblast, for example, shows a primitive nucleus and fully hemoglobinized cytoplasm—in contrast with the late normoblast, which has a pyknotic nucleus.

Delayed maturation leads to accumulation of erythrocyte precursor cells. The bone marrow is hypercellular and contains large numbers of early megaloblasts; prior to the discovery of the beneficial effect of liver extract (contains vitamin B12) on pernicious anemia, this picture was mistaken for that of acute leukemia.

As a result of intramedullary hemolysis or ineffective erythropoiesis, many megaloblasts undergo destruction in the bone marrow before maturation, aggravating the anemia and producing mild elevation of serum bilirubin and lactate dehydrogenase (LDH isoenzymes 1 and 2).

The peripheral blood smear shows macrocytosis (large red cells with elevated MCV) and marked variation in size (anisocytosis) and shape (poikilocytosis) (Figure 24-5). Oval forms (macro-ovalocytes) are prominent, and Howell-Jolly bodies, consisting of nuclear debris, are occasionally seen. Megaloblastic anemias are therefore macrocytic anemias if morphologic classification is used. Note that not all macrocytic anemias are megaloblastic.

Neutrophil Changes

The neutrophils are also affected. Neutrophil precursors in the bone marrow show marked enlargement; giant metamyelocytes are characteristic. In the peripheral blood, neutrophils show hypersegmented nuclei, with many cells showing more than 5 nuclear lobes (Figures 24-2 and Figures 24-7).

Changes in Other Cells in the Body

The abnormality in DNA synthesis affects many other cells in the body, notably those that have a high rate of cell turnover. These include the intestinal mucosa and other epithelia, which show cell enlargement and nuclear abnormalities. Recognition of these changes is important contextually if cytologic studies are undertaken—eg, in uterine cervical smears, the nuclear changes of folate deficiency may resemble those of dysplasia.

Clinical Features & Diagnosis

Patients with megaloblastic anemia present with symptoms of severe anemia. Megaloblastic anemia should be suspected upon finding macrocytic anemia with hypersegmented neutrophils in the peripheral blood. Bone marrow examination is necessary for confirmation and shows megaloblastic erythropoiesis.

Establishment of the precise cause of megaloblastic anemia requires further clinical examination and laboratory testing (Table 24-6).

There are two principal forms of megaloblastic anemia: folate deficiency, which has several underlying causes (Table 24-5); and vitamin B12 deficiency, again with several causes, including pernicious anemia, which will be discussed later in this chapter at some length.

Note that the megaloblastic anemia of folate deficiency is identical to that of vitamin B12 deficiency; administration of folic acid may thus mask (partially correct) the anemia of vitamin B12 deficiency, and vice versa; however, folic acid administration does not correct the neurologic effects of B12 deficiency. For this reason, it is important to exclude vitamin B12 deficiency before treating megaloblastic anemia with folate.

Pernicious (Addisonian) Anemia

Pernicious anemia is a form of megaloblastic anemia due to vitamin B12 deficiency. It is common (affecting up to 1% of adults over 60 years of age) in western Europe and among Caucasians (particularly of Northern European descent) in the United States; however, it is rare in Asia and Africa. Pernicious anemia occurs predominantly after the age of 50 years, slightly more often in males than in females. Family members show an increased incidence of achlorhydria, decreased vitamin B12 absorption, or overt pernicious anemia. A rare form of the disease called juvenile pernicious anemia occurs in childhood.

Pathogenesis

Adult pernicious anemia is an autoimmune disease, caused by immunologic destruction of the gastric mucosa. The mucosa of the body and fundus of the stomach is characterized by lymphocytic infiltration and progressive loss of parietal cells (chronic atrophic gastritis). This process is associated with failure of secretion of acid and intrinsic factor. Achlorhydria is invariably present in these patients and can be demonstrated by sampling gastric fluid after appropriate stimulation tests. In the absence of intrinsic factor, vitamin B12 absorption is drastically reduced.

The exact mechanism of immune destruction is not fully known (Figure 24-8). Three types of autoantibodies may be demonstrated in both serum and gastric juice: (1) About 75% of patients have an antibody that blocks vitamin B12 binding to intrinsic factor (blocking antibody); (2) about 50% have an antibody that binds with the intrinsic factor-vitamin B12 complex, interfering with the binding of the complex to ileal mucosal receptors, a prerequisite for vitamin B12 absorption; and (3) about 90% of patients have antibodies against gastric parietal cells. This antibody is also present in the serum of a minority of patients with other autoimmune diseases (Addison's disease, Hashimoto's thyroiditis) and rarely in normal elderly individuals.

The serum antibodies are frequently IgG, and gastric juice antibodies are IgA.

While useful for the diagnosis of pernicious anemia, the role played by these autoantibodies in producing the disease is uncertain. A few patients with pernicious anemia have none of the three antibodies, and the disease does occur in patients with hypogammaglobulinemia. The demonstration of cell-mediated immunity (T cell hypersensitivity) against intrinsic factor is therefore of particular interest.

Pathology & Clinical Features

(Figure 24-8)

Chronic atrophic gastritis with achlorhydria is constantly associated with the failure of intrinsic factor secretion that leads to vitamin B12 deficiency. Megaloblastic anemia and other changes resulting from failure of nucleic acid synthesis have been described above. Neurologic changes are due to demyelination by uncertain mechanisms (Figure 24-6).

The most characteristic neurologic abnormality in pernicious anemia is subacute combined degeneration of the spinal cord, which is characterized by demyelination of the posterior and lateral columns of the cord (Chapter 64: The Central Nervous System: III. Traumatic, Vascular, Degenerative, & Metabolic Diseases). This leads to paresthesia, loss of position and vibration sense, weakness, ataxia, and finally spasticity.

Peripheral neuropathy is due to segmental demyelination of nerves. These neurologic changes do not occur in patients with folic acid deficiency (Table 24-6).

Treatment & Prognosis

Patients with pernicious anemia die without treatment (the adjective pernicious indicates the high mortality rate in the past). With adequate replacement therapy with injected vitamin B12 (cyanocobalamin), these patients live a normal life. The anemia reverses rapidly, the bone marrow reverting to normal. Neurologic changes improve slowly and often incompletely.

Patients with pernicious anemia remain at increased risk for development of gastric carcinoma in the atrophic gastric mucosa and must be followed carefully for this complication. Precancerous epithelial dysplasia usually occurs before invasive carcinoma develops and can be detected in gastric biopsies.

Iron Deficiency Anemia

Iron deficiency anemia is by far the most common cause of anemia worldwide. It is common in underdeveloped as well as developed countries, but the causes differ in the two settings. In countries of the third world, hookworm infections account for most cases. In the United States, pregnancy and chronic blood loss due to gastrointestinal ulcers or neoplasms are the most common causes. It has been estimated that in the United States, 50% of pregnant women, 20% of all women, 20% of preschool children, and 3% of men suffer from iron deficiency.

Normal iron balance is regulated mainly by changes in the intestinal absorption of iron to match the normal loss of iron from the body in secretions, exfoliated cells, and menstrual blood (Figure 24-9).

Plasma iron is complexed with the iron-binding protein transferrin. Normal plasma has enough transferrin (iron binding capacity) to bind 250–400 μg of iron per deciliter of blood. In a normal adult, approximately 30% of the transferrin is saturated, with the normal plasma iron 50–150 μg/dL (Figure 24-10).

Note that the massive iron turnover associated with normal erythropoiesis and red cell breakdown is internalized, with the iron from erythrocyte destruction in the reticuloendothelial cells passing to the ferritin storage pool and thence to the plasma and bone marrow for reutilization in erythropoiesis. The plasma iron turnover for erythropoiesis is about 20 mg/d.

Causes of Iron Deficiency

(Table 24-7)

Dietary Deficiency

The daily dietary iron requirement is 5–10 mg/d (equivalent to 0.5–1 mg of absorbed iron) for men and 7–20 mg/d for women. The normal diet in the United States contains about 15 mg of iron, marginally adequate for women. Iron deficiency due to dietary deficiency occurs commonly in underdeveloped countries.

Increased Demand for Iron

In developed countries, iron deficiency occurs in individuals on marginal diets or when there is an increased demand for iron, ie, in the growth phase of early childhood and during pregnancy and lactation. Premature infants are especially at risk. Dietary iron supplements are essential during these times.

Malabsorption of Iron

Iron deficiency may occur in severe generalized malabsorptive states such as celiac disease and tropical sprue. It also occurs after total gastrectomy because (1) gastric acid is necessary for optimal iron absorption and (2) the transit time of food in the small intestine is decreased after gastrectomy.

Chronic Blood Loss

Loss of blood is a major cause of iron deficiency throughout the world. One hundred milliliters of whole blood with a hemoglobin of 15 g contains about 50 mg of iron (1 g hemoglobin contains 3.5 mg iron). Excessive menstrual blood loss from any cause and occult gastrointestinal blood loss (due to hookworm infection, peptic ulcer disease, chronic aspirin ingestion, esophageal varices, hemorrhoids, and neoplasms) are the most common causes of iron deficiency anemia; chronic loss of as little as one teaspoonful (5 mL) of blood per day will tip most people into negative iron balance. Note that the normal marrow is able to compensate for this degree of blood loss. However, with chronic loss the onset of iron deficiency interferes with erythropoiesis, and this leads to anemia.

Pathology

Iron deficiency is first reflected in a decrease in iron stores, demonstrated by a decreased serum ferritin level and an absence of iron in a specimen of bone marrow. Serum ferritin reflects the amount of storage iron in the body.

When iron stores are exhausted, the serum iron level falls. This is usually associated with an increase in the plasma transferrin level (also called total iron binding capacity [TIBC]). The net result is a decrease in saturation of the TIBC, which is a sensitive indicator of iron deficiency (Figure 24-10).

The main effect of iron deficiency is to reduce erythropoiesis, with several consequences.

Anemia

Anemia is due both to a decreased amount of hemoglobin in individual red cells and to a decrease in the total red cell mass. The poorly hemoglobinized erythrocytes appear as pale hypochromic cells with an expanded central clear zone and a thin ring of pink cytoplasm (Figure 24-5). The MCHC is decreased.

Microcytosis

The erythrocytes are abnormally small (Figure 24-5) and have a decreased MCV. Hypochromic microcytic anemia is the typical result of iron deficiency; however, it is not specific for iron deficiency anemia.

Increased Free Erythrocytic Protoporphyrin (FEP)

Reduced availability of iron for chelation with protoporphyrin results in accumulation of increased amounts of FEP in the erythrocyte. This is a simple and inexpensive test.

Bone Marrow Changes

The bone marrow shows variable normoblastic hyperplasia. The normoblasts are small, have decreased hemoglobin, and show an irregular frayed cell membrane. Storage iron is absent, and the number of sideroblasts (normoblasts containing cytoplasmic iron) is decreased (ie, the Prussian blue stain for iron is negative).

Epithelial Changes

Iron deficiency also results in atrophy of many epithelial surfaces, eg, the mucous membrane of the mouth, tongue, pharynx, esophagus, and stomach. In the esophagus, atrophy may be associated with mucosal webs and dysphagia. Gastric changes may result in hypochlorhydria (decreased acid secretion). Koilonychia (concave fingernails with abnormal ridging and splitting) is a specific result of iron deficiency. The association of iron deficiency anemia, koilonychia, atrophic glossitis, and dysphagia is known as Plummer-Vinson syndrome (Chapter 37: The Esophagus).

Treatment

It is vital to establish the underlying cause prior to treatment (Table 24-7). Iron replacement, either orally or parenterally, is effective in treating iron deficiency anemia and replenishing body stores. It is important in the management of patients with iron deficiency anemia to find and correct the reason for anemia.

Anemia of Chronic Disease

Anemia occurs as a complication of many chronic diseases such as chronic infections, collagen diseases, and malignant neoplasms. The anemia in these cases is caused by failure of transport of storage iron into the plasma and therefore into developing erythrocytes. This leads to failure of hemoglobinization and anemia. In most cases, the red cells are normocytic and normochromic (Figure 24-5), but they may be hypochromic.

The diagnosis is made by finding increased iron stores in the bone marrow and elevated plasma ferritin levels, decreased serum iron and TIBC, decreased numbers of sideroblasts in the bone marrow, and increased free erythrocytic protoporphyrin. The percentage saturation of plasma transferrin (TIBC) is usually within normal limits (Figure 24-10).

Anemia is usually mild to moderate, but it is difficult to treat. Iron therapy does not help and may actually cause harm by increasing the tissue iron stores.

Anemia of Chronic Renal Failure

Patients with chronic renal failure develop a normochromic, normocytic anemia due to failure of normal erythropoietin secretion by the kidney. The bone marrow may show mild hypoplasia of the erythroid series.

Sideroblastic Anemia

Sideroblastic anemia is an uncommon type of anemia characterized by the presence in the bone marrow of increased numbers of sideroblasts (erythroid precursors with demonstrable cytoplasmic iron). It has a variety of causes (Table 24-8).

Sideroblastic anemias are characterized by either a hypochromic, microcytic, or dimorphic peripheral blood erythrocyte appearance. A dimorphic peripheral blood picture is one in which there is an admixture of hypochromic, microcytic erythrocytes and macrocytic erythrocytes.

Serum iron, TIBC, and saturation are increased. The marrow shows erythroid hyperplasia, markedly increased amounts of storage iron, increased number of sideroblasts, and defective hemoglobinization. A characteristic feature is the presence of ring sideroblasts—normoblasts in which the amount of iron is so greatly increased that it appears as cytoplasmic granules arranged in a complete ring around the nucleus. It appears that there is a defect in incorporating iron into the hemoglobin molecule within the erythrocyte. In general, the number of sideroblasts is greater in primary acquired sideroblastic anemia than in the secondary type of disease. Secondary sideroblastic anemia frequently responds to treatment with pyridoxine.

In some patients with refractory sideroblastic anemia, increased numbers of immature white cells (mainly myeloblasts) are present. This is believed to be a premalignant change (dysmyelopoiesis), with many (10%) of these patients going on to develop acute leukemia.

Anemia Due to Blood Loss

Acute Blood Loss

Acute hemorrhage results in loss of whole blood from the vascular compartment, leading to hypovolemia and compensatory mechanisms to maintain perfusion to vital organs (see Shock, Chapter 9: Abnormalities of Blood Supply). In the acute bleeding phase, blood values—including red cell count, hemoglobin, and hematocrit—are normal because equivalent amounts of red cells, hemoglobin, and plasma are lost. An important compensation for hypovolemia is retention of water and electrolytes by the kidneys to restore blood volume. This begins immediately and causes dilution of the blood. Within hours, the effect of this dilution is seen as a progressive decrease in red cell count, hemoglobin, and hematocrit in the peripheral blood. The amount of decrease of these values depends on the amount and rate of blood loss and the effectiveness of renal compensation. Regeneration of erythrocytes lost during the hemorrhage occurs over the next several weeks and slowly restores the red cell count, hemoglobin, and hematocrit levels. During this regenerative phase, the bone marrow shows erythroid hyperplasia and depletion of iron stores, and the peripheral blood shows a reticulocytosis proportionate to the increased rate of erythrocyte production. The anemia is therefore temporary. The body's iron stores are replenished over the next few months. In patients who have depleted iron stores and have a marginal dietary intake of iron, an episode of acute hemorrhage may precipitate iron deficiency anemia.

Chronic Blood Loss

Chronic bleeding is compensated for initially by erythroid hyperplasia of the bone marrow and increased production of erythrocytes. This persists until iron stores have been depleted, at which time iron deficiency prevents adequate compensation. Anemia due to chronic blood loss is therefore a form of iron deficiency anemia and is discussed under that heading.

(Table 25-1)

Hemolytic anemias are a group of diseases characterized by shortened survival of red blood cells in the circulation. Red cell destruction may occur chiefly in the reticuloendothelial system (extravascular hemolysis—the usual situation in most types of hemolytic anemia) or in the blood (intravascular hemolysis—uncommon, largely restricted to physical lysis of red cells, as in microangiopathy, or to complement-mediated lysis), or in some combination of the two (Figure 25-1).

Extravascular Hemolysis

Extravascular hemolysis is characterized by the following features.

Hemolytic Jaundice

Increased production of unconjugated bilurubin occurs because of increased breakdown of hemoglobin. Unconjugated bilirubin is complexed with plasma albumin and transported to the liver, where it is taken up by the liver cells for conjugation (Figure 25-1; see also Chapter 1: Cell Degeneration & Necrosis). Jaundice appears only when the amount of unconjugated bilirubin delivered to the liver exceeds the capacity of that organ to conjugate and excrete it. Even when present, the degree of jaundice in hemolytic processes is usually mild.

Unconjugated bilirubin is not excreted in the urine (acholuric jaundice).

Increased Bilirubin Excretion in Bile

The liver excretes increased amounts of conjugated bilirubin in the bile, which is why patients with chronic hemolytic processes tend to develop bilirubin pigment stones in the gallbladder. The bile enters the duodenum, where it is converted to urobilinogen by bacterial action. Increased excretion of fecal urobilinogen is present in hemolytic anemias. The urobilinogen is also absorbed in the portal vein and excreted in increased amounts in the urine (Figure 25-1; see also Chapter 1: Cell Degeneration & Necrosis).

Erythroid Hyperplasia

The anemia stimulates erythroid hyperplasia in the bone marrow, sometimes to the remarkable degree that expansion of the marrow cavity is recognized on x-rays. Anemia occurs only when the erythroid hyperplasia cannot compensate for the hemolysis. The increased rate of erythropoiesis also results in an increase in the number of reticulocytes in the peripheral blood. In severe cases, normoblasts may enter the peripheral blood. The maximum marrow response is an 8- to 10-fold increase in red cell production.

Hemosiderosis

Iron produced by degradation of hemoglobin is retained in the reticuloendothelial cells to cause hemosiderosis. The tendency to hemosiderosis in chronic hemolytic anemias is aggravated by increased intestinal absorption of iron and use of blood transfusions.

Intravascular Hemolysis

Intravascular hemolysis is characterized by the following features.

Hemolytic Jaundice

The hemoglobin released into the circulation is taken up and degraded by the reticuloendothelial system, causing unconjugated hyperbilirubinemia.

Erythroid Hyperplasia

Marked only if hemolysis becomes chronic.

Decreased Plasma Haptoglobin

Haptoglobin is an α2 globulin that rapidly binds free hemoglobin in the plasma. Normal blood has sufficient haptoglobin to bind about 100–200 mg of hemoglobin.

Hemoglobinemia

Free hemoglobin in the plasma appears when hemolysis has exhausted the capacity of plasma haptoglobin to bind hemoglobin. Hemoglobin imparts a pink color to the plasma and can be identified definitively by its typical spectrophotometric bands.

Hemoglobinuria

The hemoglobin molecule is filtered by the glomerulus and appears unchanged in urine, imparting a pink color. The hemoglobin may be taken up by renal tubular cells and degraded therein with deposition of hemosiderin in the cytoplasm. Desquamated tubular epithelial cells containing hemosiderin may appear in the urine (hemosiderinuria).

Methemalbuminemia

Part of the released hemoglobin is oxidized to methemoglobin. This substance may be bound to albumin to form methemalbumin or complexed with hemopexin, a beta globulin, and cleared by the liver. The presence of methemalbumin in the plasma (detected by Schumm's test) is good evidence for intravascular hemolysis.

Lactate Dehydrogenase (LDH)

Increased serum levels of lactate dehydrogenase are commonly present in intravascular hemolysis due to release of this enzyme from red cells.

Erythrocyte Membrane Defects

The Erythrocyte Membrane

The cell membrane of the erythrocyte is a trilaminar bipolar phospholipid structure in which are situated many kinds of integral structural proteins such as glycophorin. The cell membrane is flexible and is often referred to as a fluid mosaic structure.

Rhesus (Rh) blood group antigens represent integral structural molecules of the membrane. Very rare patients who lack Rh antigens (Rh null) have severe membrane abnormalities that result in deformed erythrocytes known as stomatocytes, which have greatly shortened survival times. ABO glycoprotein blood group antigens, on the other hand, are not integral structural proteins of the membrane. They are located on the outer part of the membrane.

The inner surface of the cell membrane is supported by a lattice-like fibrillar network of proteins that include actin, spectrin, ankyrin, and protein 4.1. Spectrin is believed to control red cell shape and deformability. The membrane also has many enzyme systems (cyclic adenosine monophosphate (cAMP), adenosine triphosphate (ATP), and guanosine triphosphate (GTP)-dependent protein kinases) that appear to regulate membrane functions.

Hereditary Spherocytosis

Etiology & Pathogenesis

Hereditary spherocytosis is a congenital autosomal dominant disease with variable penetrance. Patients may present with severe hemolysis in childhood or with mild hemolysis manifested first during adult life. There is deficiency or abnormal polymerization of spectrin; the severity of the defect correlates with the severity of the disease.

The obvious abnormality in hereditary spherocytosis is a change in the shape of the red cell from its normal biconcave shape to a spherical shape. These spherocytes are less pliable than normal erythrocytes and tend to lose membrane substance as they traverse the splenic sinusoids, becoming progressively smaller (microspherocytes); spherocyte life span is shortened, with destruction occurring in the spleen.

Clinical Features

Patients present with hemolytic anemia and jaundice. Splenic enlargement is usually present as a result of reticuloendothelial hyperplasia. Life-threatening aplastic crises may occur in association with infection. The diagnosis of hereditary spherocytosis is established by demonstration of hemolysis, positive family history, peripheral blood examination showing microspherocytes plus reticulocytosis, and increased fragility in the osmotic fragility test (spherocytes are more susceptible to lysis by saline solution). Spherocytes also show autohemolysis when incubated at 37 °C for 24–48 hours; autohemolysis is reduced by the addition of glucose.

Treatment

The erythrocyte membrane defect cannot be reversed by any known therapy. The only treatment available is splenectomy, which acts by removing the site of maximal erythrocyte destruction. The peripheral blood still shows the typical changes of spherocytosis after splenectomy; however, the degree of hemolysis is much reduced.

Hereditary Elliptocytosis

Hereditary elliptocytosis (ovalocytosis) resembles spherocytosis except that the red cells are oval and the disease is usually less severe, associated with mild splenomegaly. Inheritance is by autosomal dominant transmission, resulting in a defect of formation of protein 4.1.

Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria is a rare acquired disease of red cells characterized by increased sensitivity of the membrane to complement, thought to be due to decreased integration of decay accelerating factor (DAF), which normally accelerates inactivation of complement, into the red cell membrane. Erythrocyte lysis in the circulation results in hemoglobinemia, hemoglobinuria, and hemosiderinuria.

Complement activation occurs mainly through the alternative pathway and is precipitated (1) by decreased pH (acidification) in vivo during sleep, as a result of slower respiration and accumulation of carbon dioxide—hence paroxysmal nocturnal; (2) by acidification of serum in vitro (Ham's test); and (3) by addition of sucrose to serum in vitro (sucrose lysis test), which is a useful screening test for this disorder.

Patients with paroxysmal nocturnal hemoglobinuria are usually young adults, and the anemia may be severe. The disease has a close association with aplastic anemia. It is believed that paroxysmal nocturnal hemoglobinuria represents a clone of abnormal erythrocytes developing in a hypoplastic bone marrow.

Erythrocyte Enzyme Deficiency

Normal Erythrocyte Metabolism

The erythrocyte has no mitochondria and lacks enzymes of the citric acid cycle and oxidative phosphorylation. ATP synthesis therefore occurs via glycolysis (Figure 25-2). Pyruvate kinase is essential for this reaction. An important constituent in the glycolytic pathway is 2,3-diphosphoglycerate (2,3-DPG), increased levels of which favor oxygen release from hemoglobin to the tissues; 2,3-DPG production is stimulated by hypoxia, and high erythrocyte 2,3-DPG levels are present in chronic anemia.

The pentose phosphate pathway is important in preventing hemoglobin oxidation (Figure 25-2). Failure of this mechanism results in oxidation, denaturation, and precipitation of hemoglobin as Heinz bodies. Heinz bodies adhere to the erythrocyte membrane, interfering with membrane deformability and making them more susceptible to phagocytosis in the splenic sinusoids. In addition, splenic macrophages remove those parts of the membrane with adherent Heinz bodies (pitting), causing membrane loss and formation of microspherocytes. Glucose-6-phosphate dehydrogenase (G6PD) is an important enzyme in this pathway.

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

G6PD deficiency is the most common erythrocyte enzyme abnormality. Several variants are recognized; most do not cause disease. The G6PD-A (–) variant, inherited as an X-linked trait, is present in about 10% of blacks in the United States and has a worldwide distribution (so-called African type). A second type (Mediterranean) is somewhat more severe. It is the result of two point mutations and is probably the most common mutation-induced disease of humans. Full expression of the enzyme deficiency occurs in males. Heterozygous female carriers may have mild enzyme deficiency (resulting from random expression of the normal or affected X chromosome). Homozygous females also have severe deficiency, but this defect is uncommon.

Deficiency of G6PD increases as the cell ages, and hemolytic episodes therefore tend to affect the older cells. G6PD-deficient red cells are more vulnerable to oxidants, which cause oxidative denaturation of hemoglobin and lead to formation of Heinz bodies and hemolysis. The main oxidant agents concerned are drugs such as primaquine, sulfonamides, and nitrofurantoin. Favism is a disorder occurring in some patients who suffer hemolysis after ingesting certain broad beans (Vicia faba), which contain an oxidant alkaloid.

Most patients with G6PD deficiency are asymptomatic, although the diagnosis may be confirmed by quantitative spectrophotometric assays. Acute intravascular hemolysis may occur after exposure to an oxidant drug; however, because all of the older G6PD-deficient cells have lysed, G6PD levels may appear normal, and the diagnosis is difficult to make in the acute stage. Rarely, patients develop a mild chronic hemolytic anemia.

The Mediterranean variant of G6PD deficiency occurs in Middle Eastern populations and produces G6PD deficiency in red cells of all ages; the disease is therefore more severe.

Other Erythrocyte Enzyme Deficiencies

Pyruvate kinase deficiency is less common than G6PD deficiency but more often produces clinical effects. It is inherited as an autosomal recessive trait and occurs with equal frequency in both sexes. Patients present with acute or chronic hemolytic anemia. The diagnosis is made by enzyme assay.

Deficiencies of other enzymes such as glutathione reductase and triosephosphate isomerase are rare.

Hemoglobinopathies

Several different types of hemoglobin occur in humans. All have four polypeptide chains (two pairs) per molecule of hemoglobin, each chain linked with one heme group composed of an iron-chelated protoporphyrin molecule (Figures 25-3 and 25-4). Diseases manifesting abnormalities of hemoglobin synthesis are termed hemoglobinopathies. Three broad types are recognized: (1) qualitative or structural hemoglobinopathies characterized by synthesis of an abnormal hemoglobin molecule (eg, sickle cell disease, hemoglobin C disease); (2) quantitative hemoglobinopathies (thalassemias), characterized by failure of secretion of one chain type, leading to lack of certain types of hemoglobin and a compensatory increase of other hemoglobins; and (3) combined qualitative and quantitative hemoglobinopathies.

The most common structural hemoglobinopathies are inherited as single gene abnormalities (single base substitution) that dictate substitution of a single abnormal amino acid in the beta polypeptide chain. Numerous different abnormal genes are recognized, producing many (over 400) different abnormal hemoglobins. The most common abnormal hemoglobin is HbS (which results in sickle cell disease); less common abnormal hemoglobins are HbC, HbD, and HbE. Individuals who are homozygous for the abnormal gene have high levels of the abnormal hemoglobin, whereas heterozygotes have lower levels (Figure 25-5). HbS-C disease is double-heterozygous for the S and C genes. Note that abnormal hemoglobins were at first designated by a letter (eg, HbS); later, they became so numerous that place names were used (see Table 25-2). Current molecular techniques permit direct detection of the specific point mutations responsible for most of the common hemoglobinopathies (eg, HbS, β thalassemia).

Sickle Cell Disease

The abnormal HbS gene is common in Africa, India, and among blacks in the United States. It is rare in Caucasian and Asian races. In sickle cell trait, which occurs in 9% of American blacks, one abnormal gene is present. These heterozygous (A/S) individuals usually have no symptoms. Sickle cell trait confers some protection on the erythrocyte against infection with Plasmodium falciparum; this selective advantage is believed to have favored the persistence of the abnormal HbS gene in malaria-endemic areas.

Sickle cell disease represents the homozygous (S/S) state and occurs in 0.1–0.2% of blacks born in the United States; about 50,000 black Americans suffer from sickle cell disease.

Pathology

A single point mutation (codon beta-6 GAG → GTG) dictates replacement of the normal glutamic acid at position 6 in the beta chain with valine. The result is HbS. This amino acid substitution is on the surface of the molecule and results in a tendency to polymerization, yielding semisolid crystalline structures called tactoids, under conditions of decreased oxygen tension. The degree of anoxia required to induce tactoids is small in homozygous (S/S) patients (sickle cell disease), in whom the red cells contain up to 80% HbS, and greater in heterozygous (A/S) patients (sickle cell trait), in whom the red cells contain about 30% HbS and 70% HbA.

Tactoid formation causes (1) decreased solubility of hemoglobin, (2) change in shape of the erythrocyte to a sickle cell, and (3) decreased deformability of the erythrocyte. Change in shape is due to interaction between the tactoids and the spectrin-actin cytoskeleton. Initially, sickling reverses when oxygenation improves, but with repeated anoxia, the erythrocyte assumes a permanently sickled shape. Decreased deformability leads to phagocytosis in the splenic and liver sinusoids.

Clinical Features

The onset of sickle cell disease is in early infancy, as HbS replaces HbF, and death often occurred during early adult life. With improvements in management, many patients now survive longer.

Patients present with evidence of chronic extravascular hemolysis and severe anemia. Growth retardation is common, as is heart failure (high-output type). Mild hemolytic jaundice with absent urinary bilirubin and increased fecal and urinary urobilinogen are usual. The bone marrow shows marked compensatory normoblastic hyperplasia, often leading to expansion of the marrow cavity in bones and causing bony deformities (tower skull and hair-on-end appearance on skull x-rays). Chronic leg ulcers that fail to heal are a typical feature of sickle cell disease but are of unknown pathogenesis.

Diagnosis

Sickle cells in peripheral blood smears are diagnostic (Figure 25-6) but not always present. Addition of metabisulfite to the blood smears induces sickling (metabisulfite sickle preparation). Hemoglobin electrophoresis permits identification and quantitation of HbS in the blood (Figure 25-5). Patients with sickle cell disease have over 80% HbS in the blood with absent HbA; HbF and HbA2 may be variably increased. The traditional diagnostic tests are increasingly being supplemented by direct detection of the responsible point mutation through molecular techniques. These newer procedures permit a genetic diagnosis in children and adults, or prior to birth when a fetal sample is obtained by amniocentesis.

Complications

Aplastic crisis is sudden failure of hematopoiesis in the bone marrow, which may be precipitated by infections, drugs, or other causes. It is usually transient. Hemolytic crisis of unknown cause is characterized by a sudden increase in the level of hemolysis, sometimes due to phagocytosis in the spleen (so-called splenic crisis).

Hemosiderosis and secondary hemochromatosis are common in long-term survivors. A positive iron balance results from stimulation of iron absorption in the intestine due to chronic erythroid hyperplasia and to blood transfusions given for aplastic crises.

Vaso-occlusive crisis is probably due to plugging of the microcirculation by aggregates of sickle cells. Multiple microinfarcts are characterized by fever and ischemic pain, which may affect the heart, muscles, bone (aseptic necrosis of the femoral head is common), kidneys (papillary necrosis and renal failure may occur), lungs, central nervous system, and intestine.

Splenic changes are characteristic. The spleen may be slightly enlarged in early childhood as a result of reticuloendothelial hyperplasia. Subsequently, repeated ischemic episodes cause infarction, fibrosis, and progressive decrease in splenic substance (autosplenectomy). In adults with sickle cell disease, the spleen is usually quite shrunken and composed of multiple brown scars containing hemosiderin (Gamna-Gandy bodies).

The functionally asplenic state predisposes patients with sickle cell disease to systemic infections with encapsulated bacteria. Pneumococcal bacteremia and Salmonella osteomyelitis are uncommon infections that occur with greater frequency in these patients. The presence of hyposplenism may be deduced by the presence of nuclear debris (Howell-Jolly bodies) in red cells. Howell-Jolly bodies are normally removed by the spleen.

Sickle cell trait, by contrast, is usually without symptoms. HbA levels are around 60% and HbS about 40%. Note that heterozygosity with HbC (see below) usually results in HbS levels of 50%, which do produce symptoms.

Other Abnormal Hemoglobins

Many other amino acid-substitution hemoglobinopathies occur and may alter hemoglobin function (Table 25-2). Hemoglobin electrophoresis is of value in recognizing the different variants (Figure 25-5). The double-heterozygous states, HbS-C and HbS-D, produce lower levels of HbS than homozygous S/S and have a disease of lesser clinical severity than sickle cell disease.

Abnormal hemoglobins may have the following consequences:

1.  Hemolytic anemia. In most cases, the severity of hemolysis is less than that seen in sickle cell disease.

2.  Instability of the hemoglobin molecule, causing precipitation of Heinz bodies and decreased red cell survival.

3.  Altered affinity of the hemoglobin molecule for oxygen. When the affinity for oxygen is increased, oxygen release to the tissues is decreased, leading to hypoxia and compensatory polycythemia. When the affinity for oxygen is decreased, oxygen release to the tissues is increased, leading to decreased erythropoietin secretion and anemia and increased amounts of reduced hemoglobin in the blood (cyanosis). In these patients, tissue oxygenation is normal despite the anemia.

Thalassemias

(Table 25-3)

The thalassemias are characterized by a decreased rate of synthesis of hemoglobin chains that are structurally normal. Beta thalassemia is due to a decreased rate of synthesis of beta chains and is the most common form. Alpha thalassemia, in which there is a decreased synthesis of alpha chains, and delta-beta thalassemia, in which both delta and beta chain synthesis are affected, are rare.

Thalassemia syndromes are common in persons of Mediterranean, African, and Asian ancestry. Approximately 3% of the world's population have abnormal β-globin genes that lead to production of deficient β chain (and hemoglobin A [HbA]) (beta thalassemia). Genetic defects include point mutations, with frame shifts, nonsense sequences, and RNA processing defects as well as gene deletions. This complexity makes genetic counseling difficult. In most cases of alpha thalassemia, one or more of the alpha genes are deleted (Table 25-3).

Homozygous Beta Thalassemia (Cooley's Anemia)

Homozygous beta thalassemia is characterized by total or near total absence of synthesis of beta chains, with a marked decrease in the amount of HbA. Gamma chain production then persists into adult life, resulting in persistently elevated HbF levels (to about 40–60%, but sometimes as high as 90%). Delta chain synthesis is also increased to compensate for the absent beta chains, causing an increase in HbA2 levels. In addition, excess free alpha chains precipitate in the cytoplasm of affected erythrocytes and are visible as inclusions. Alpha chain precipitates damage the red cells, resulting in their destruction.

Clinically, homozygous beta thalassemia begins in early childhood, with severe anemia, hemolytic jaundice, and splenomegaly. Growth retardation and delayed puberty are common features. Extreme erythroid hyperplasia causes expansion of the bone marrow and thinning of cortical bone. Involvement of the facial bones produces a characteristic facial appearance.

The peripheral blood picture is commonly that of hypochromic microcytic anemia with marked anisocytosis and numerous target forms (Figure 25-6). Reticulocytosis and the occasional presence of normoblasts attest to the increased rate of erythropoiesis. Hemoglobin electrophoresis shows elevation of HbF and HbA2 with greatly decreased or absent HbA (Figure 25-5).

The main complication of thalassemia is the occurrence of a positive iron balance due to increased iron absorption in the gut and multiple blood transfusions. This secondary hemochromatosis affects many organs and is the most common cause of death, usually from myocardial or liver failure.

Heterozygous Beta Thalassemia (Cooley's Trait)

The heterozygous state of beta thalassemia may be asymptomatic or may present clinically with a mild hemolytic process characterized by mild anemia, jaundice, splenomegaly, and a hypochromic microcytic blood picture. Hemoglobin electrophoresis shows slight elevation of HbA2 (4–7%) and HbF (2–6%). Most of the hemoglobin is HbA.

Sickle Cell-Beta Thalassemia

Sickle cell-beta thalassemia is caused by heterozygosity for both the abnormal sickle cell gene and the abnormal thalassemia gene. Clinically, it produces a hemolytic process that is intermediate in severity between sickle cell disease and sickle cell trait. The diagnosis is made by positive dithionite solubility or metabisulfite sickle tests, which demonstrate the ability to produce tactoids; and hemoglobin electrophoresis, which shows the presence of both HbS and HbA, with the former present in greater concentration. Levels of HbF and HbA2 are variably increased.

Alpha Thalassemia

Whereas there is only one beta gene per haplotype, there are two alpha genes, for a total of four in the normal situation. Deletion of all four leads to complete absence of alpha chain, severe fetal anemia with edema, erythroblastosis, and stillbirth. Absence of three or two alpha genes leads to progressively less severe disease, with varying levels of tetramer hemoglobins (β4 and γ4) compensating for reduced levels of HbA (Figure 25-4). Deletion of a single alpha gene has no clinical effect.

Immune-Mediated Hemolytic Anemias

Autoimmune Hemolytic Anemias

Autoimmune hemolytic anemias are a group of diseases (Table 25-4) in which hemolysis occurs as a result of the presence of autoantibodies with specificity against blood group antigens, including anti-I (in mycoplasmal pneumonia), anti-i (in infectious mononucleosis), and anti-P (in paroxysmal cold hemoglobinuria). Binding of the autoantibody to the erythrocyte membrane may occur maximally at body temperature (37 °C, warm antibodies) or at 4 °C (cold antibodies). The antibodies may be IgG (usually warm), IgM (usually cold), or rarely IgA. Antibody acts as a lysin, opsonin, or agglutinin (Figure 25-7; see also Chapter 4: The Immune Response). Paroxysmal cold hemoglobinuria is an exception; it is caused by an IgG antibody that binds best to the erythrocyte membrane in the cold.

Idiopathic Warm Autoimmune Hemolytic Anemia

Idiopathic warm autoimmune hemolytic anemia (AIHA) occurs mainly in patients over 40 years of age and in women more frequently than in men. It is caused by an IgG autoantibody (Figure 25-7), occurring in isolation (idiopathic) or as a complication of systemic disease (Table 25-4).

AIHA has an insidious onset. Patients may present with symptoms of anemia or jaundice. Destruction of red cells by splenic macrophages leads to mild splenomegaly and microcytosis.

The peripheral blood shows a normochromic normocytic anemia with microspherocytes, fragmented forms, and poikilocytes. The reticulocyte count is increased. Indirect hyperbilirubinemia, often mild, is common. Fecal and urinary urobilinogen levels are increased. The bone marrow shows normoblastic hyperplasia.

Osmotic fragility is usually increased. The diagnosis of AIHA is established by demonstrating the autoantibody in serum by the antiglobulin (Coombs) test (Figure 25-8). It is necessary to exclude diseases that can cause warm antibody hemolytic anemia (Table 25-4) before making the diagnosis of idiopathic AIHA.

Corticosteroids represent the mainstay of treatment and are very effective. Splenectomy and immunosuppressive agents such as azathioprine may be used if steroids are not effective. Most patients have a chronic course, with relapses and remissions occurring at variable intervals.

Idiopathic Cold Hemagglutinin Disease

Idiopathic cold hemagglutinin disease is a rare cause of hemolysis that occurs mainly in older patients, more commonly in women. It is caused by an IgM autoantibody that binds to erythrocytes at low temperatures, fixes complement, and results in hemolysis (Figure 25-7).

Patients present with cold-induced hemolysis or Raynaud's phenomenon, in which ischemia causes blanching and numbness of the hands on exposure to cold, followed successively by cyanosis, redness (reactive hyperemia), and throbbing pain and tingling. Ischemia is caused by sludging of red cells in capillaries, which is due either to agglutination or to hyperviscosity produced by high levels of IgM antibody.

The diagnosis is made by a positive Coombs test, with anti-IgM antibodies showing maximum reactivity at 4 °C. The Coombs test with anti-IgG at 37 °C is negative.

Cold hemagglutinin disease also occurs in association with malignant lymphoma and as a complication of certain infections (eg, with Mycoplasma pneumoniae).

Paroxysmal Cold Hemoglobinuria

This rare disorder is due to the presence of a cold antibody, but in this instance the antibody is of the IgG class (directed against the P antigen on the erythrocyte membrane). Complement fixation is initiated by cold but does not proceed to lysis until the blood temperature rises to 37 °C. Patients suffer chills, fever, muscle pain, and hemoglobinuria following exposure to cold.

An acute variant of the disease follows certain viral infections. A more chronic form occurs in syphilis.

The Donath Landsteiner test, demonstrating hemolysis of a blood sample following cooling to 4 °C and subsequent warming to 37 °C, is a useful pointer to the diagnosis.

Isoimmune Hemolytic Anemia

Isoimmune hemolytic anemias are those in which the red cells of one individual are lysed as a result of the action of antibodies of another individual—either in blood transfusion, where incompatible donor red cells are lysed by antibody in recipient plasma; or in hemolytic disease of the newborn, where fetal red blood cells are lysed by maternal antibodies that have traversed the placenta.

Hemolytic Blood Transfusion Reactions

Hemolytic transfusion reactions follow transfusion of incompatible blood, usually due to ABO incompatibility and rarely due to other blood groups (see Table 25-7). The more severe forms produce intravascular hemolysis and occur within minutes to hours. The transfused (donor) red cells are destroyed by antibody present in the recipient's plasma (Table 25-5). Note that the presence of antibodies in the donor blood usually does not damage recipient red cells because of the dilution effect of donor plasma in the recipient blood pool; this risk may be further reduced by administration of packed red cells from which almost all of the donor plasma (and hence donor antibody) has been removed.

ABO hemolytic transfusion reactions are theoretically avoided by ABO grouping (Table 25-6). In practice, those reactions that do occur are almost all due to human (clerical) error. More rarely, serious transfusion reactions result from incompatibility in another blood group system (Table 25-7). The possibility that these rarer reactions may occur is usually revealed by the cross-match (which reacts the donor's red cells with the recipient's serum in vitro prior to transfusion) (Table 25-6).

Hemolytic transfusion reactions are typically acute. If severe (as in most cases of ABO incompatibility), they cause severe intravascular hemolysis with hemoglobinemia (Figure 25-7). Such patients may rapidly proceed to shock and are at considerable risk of death. Less severe reactions due to non-complement-fixing antibodies produce predominantly extravascular hemolysis and may be manifested several days after the transfusion (delayed hemolytic reactions).

Hemolytic Disease of the Newborn

Clinically significant hemolytic disease of the newborn is usually caused by Rh incompatibility. More rarely, ABO or other group incompatibility is responsible (Table 25-7).

The Rh system is complex, consisting of three pairs of alleles (D,d, C,c, E,e), which produce a variety of phenotypes. D is the strongest antigen and the one routinely tested; thus, usage of the terms Rh-positive and Rh-negative is intended to denote the presence or absence of D antigen. Note that the allele d is hypothetical and that the corresponding antigen and antibody (d and anti-d) have not been identified, while additional variants of the other five basic antigens have been recognized, adding to the complexity of this system.

Anti-Rh Antibodies

Contrary to the situation in the ABO system, an Rh-negative individual does not have natural anti-Rh antibodies. However, an Rh-negative individual may develop immune anti-Rh antibodies (IgG) if Rh-positive erythrocytes enter the circulation, either (1) when Rh-positive blood is transfused into an Rh-negative individual; or (2) during pregnancy, when fetal Rh-positive erythrocytes may enter the maternal circulation from an Rh-positive fetus. Fetomaternal passage of cells occurs across the placenta late in pregnancy, particularly during delivery. Note that the first Rh-positive pregnancy usually serves only to sensitize the Rh-negative mother. The mother does not suffer disease, and the first fetus is unlikely to be affected. Subsequent Rh-positive fetuses are at increasing risk.

Effect on Fetus

If a sensitized Rh-negative woman becomes pregnant, the anti-Rh IgG crosses the placenta into the fetus, producing hemolysis of the fetal erythrocytes in utero if the fetus is Rh-positive. This may cause (1) intrauterine death of the fetus or (2) hemolytic disease of the newborn, characterized by anemia, severe jaundice, edema, and the presence of numerous normoblasts in the peripheral blood (erythroblastosis fetalis). Kernicterus may occur (Chapter 1: Cell Degeneration & Necrosis).

Prevention

Hemolytic disease of the newborn is prevented by avoiding sensitization of Rh-negative women. This can be achieved (1) by accurate Rh typing, thereby avoiding the transfusion of Rh-positive blood to Rh-negative women, and (2) by administration of high doses of Rh antibody (Rhogam) to an Rh-negative woman during childbirth or abortion. The passively administered antibody destroys any fetal Rh-positive cells that enter the mother's blood, thereby preventing sensitization.

Hemolytic disease of the newborn occurs less often with ABO incompatibility because anti-A and anti-B natural antibodies in the mother's plasma are usually IgM and therefore do not cross the placenta. However, ABO incompatibility causes hemolytic disease of the newborn in those rare mothers who have immune IgG antibodies against A and B antigens in the plasma.

Drug- & Chemical-Induced Hemolysis

A large number of drugs are known to cause immune hemolysis with a positive antiglobulin (Coombs) test. Hemolysis in these cases comes about by several different immune mechanisms, described below. Nonimmune types of drug-induced hemolysis exist also (see below).

Immune Drug-Induced Hemolysis

Induction of autoantibody occurs with the antihypertensive agent methyldopa, which leads to a clinical syndrome resembling idiopathic autoimmune hemolytic anemia. The exact mechanism by which autoantibody is produced is not known; it has been postulated that a reduction in suppressor T cells may be involved.

A hapten effect occurs in which the drug combines with an erythrocyte membrane protein to form an antigenic complex that stimulates production of antibody. This occurs with penicillin and cephalosporins.

Hemolysis due to immune complex formation results when a drug induces antibody formation and then combines with the antibody to form a circulating soluble immune complex that adsorbs to the erythrocyte membrane, activating complement. Quinidine, phenacetin, and the antituberculous drug aminosalicylic acid are examples.

Direct alterations in the erythrocyte membrane (cephalosporins) lead to the adsorption of immunoglobulins and macrophage-mediated hemolysis.

Nonimmune Drug-Induced Hemolysis

Certain chemicals and toxins directly affect red cell membranes (amphotericin B, mushroom toxin, snake venoms, lipid solvents) or red cell enzymes (lead, saponin), leading to hemolysis.

Drug-induced hemolysis in G6PD deficiency was discussed earlier in this chapter.

Hemolysis Caused by Infectious Agents

Several possible mechanisms are involved, eg, (1) autoimmune hemolysis (infectious mononucleosis, mycoplasmal pneumonia); (2) production of lytic toxins, especially in clostridial infections or in severe streptococcal septicemia (streptolysin release); and (3) direct infection of red cells, as in bartonellosis or malaria.

In malaria, red cell lysis occurs episodically upon release of proliferating merozoites from infected red cells (Figure 25-9). Intermittent fever coincides with hemolysis every 48 hours (tertian fever—Plasmodium vivax, Plasmodium falciparum) or every 72 hours (quartan fever—Plasmodium malariae). Red cell debris is cleared by the reticuloendothelial system, and splenomegaly is common. Complications include blackwater fever, glomerulonephritis, and cerebral malaria (Figure 25-9).

In most cases of malaria, the malarial parasite can be identified within red cells in peripheral blood smears, and the different species of plasmodia can be identified by their morphologic features.

Antimalarial drugs such as chloroquine are extremely effective in killing the malarial parasite in erythrocytes. A different drug (primaquine) is required to kill the liver stage in P vivax infections. Chloroquine-resistant species of P falciparum have appeared and complicate treatment.

Microangiopathic Hemolytic Anemia

Microangiopathic hemolytic anemia is caused by fragmentation of erythrocytes as they traverse an abnormal microcirculation.

Etiology

Disseminated Intravascular Coagulation (DIC)

The fibrin strands in the microcirculation cause fragmentation of erythrocytes, slicing through the red cell membrane, which reseals over the defect (leaving a smaller red cell [microspherocyte] or deformed red cell [schistocyte]). Any of the many diseases that cause DIC (Chapter 9: Abnormalities of Blood Supply) may be associated with hemolytic anemia.

DIC and fragmentation hemolysis are especially important in (1) hemolytic uremic syndrome, a disorder of unknown cause affecting many young children characterized by renal failure and microangio-pathic hemolytic anemia; and (2) thrombotic thrombocytopenic purpura, a serious disease of unknown cause in young adults characterized by fever, microangiopathic hemolytic anemia, marked central nervous system changes, and renal failure.

Abnormal Blood Vessels

Hemolysis due to blood vessel abnormalities occurs in (1) vasculitides of all types, (2) malignant hypertension, (3) vascular anomalies such as giant capillary hemangioma (Kassabach-Merritt syndrome) and arteriovenous malformations, and (4) malignant neoplasms when abnormal new vessels form in and around the neoplasm.

Prosthetic Cardiac Valves or Aortic Prostheses

Red cells are traumatized during passage through the prosthesis.

Pathology & Diagnosis

Microangiopathic hemolytic anemia can be diagnosed by the finding of abnormal fragmented erythrocytes (called schistocytes) in the peripheral blood smear (Figure 25-10). These cells have variable shapes with pointed ends (helmet cells). Microspherocytes are also present, caused by loss of red cell membrane during fragmentation. Reticulocytosis and evidence of intravascular hemolysis, including hemoglobinemia, are commonly present.

Table 25-8 presents an approach to the diagnosis of anemias based on examination of a peripheral blood smear coupled with commonly available laboratory tests.

Polycythemia is defined as an increased number of red cells in the peripheral blood. It is uncommon and may result from (1) an increase in the total red cell mass (absolute polycythemia) or (2) decreased plasma volume without an increase in total red cell mass (relative polycythemia), as in dehydration. Relative polycythemia is a temporary condition not discussed further.

Secondary Absolute Polycythemia

Secondary polycythemia is a normal compensatory increase in the red cell volume resulting from chronic hypoxemia, mediated by increased production of erythropoietin. It occurs in patients with chronic lung disease or congenital cyanotic heart disease; in individuals acclimatized to living at high altitudes; in cigarette smokers, who have increased carbon monoxide levels in the blood that bind hemoglobin and lead to hypoxia; and in patients with abnormal hemoglobins (eg, Hb San Diego, Hb Chesapeake) that have an increased affinity for oxygen.

Abnormal and uncontrolled erythropoietin secretion occurs in rare patients with neoplastic diseases, most commonly renal adenocarcinoma, cerebellar hemangioblastoma, and hepatocellular carcinoma; and nonneoplastic renal conditions such as renal cysts or hydronephrosis. A familial form of polycythemia with increased production of erythropoietin has also been reported. It is autosomal dominant and rare.

Polycythemia Rubra Vera

Polycythemia rubra vera is a neoplastic myeloproliferative disorder (Chapter 26: Blood: III. the White Blood Cells) that affects chiefly the erythroid series. Granulocytes and platelet numbers are also commonly increased. The bone marrow is hypercellular with proliferation of all three cell lines. Megakaryocyte clustering is characteristic.

Patients who develop polycythemia rubra vera are usually over 40 years of age. They commonly present with ruddy cyanosis of the face and plethora due to the polycythemia. The increased viscosity of the blood caused by the increased hematocrit often results in vascular thrombosis. Many patients present with venous thrombosis, often affecting the portal circulation. Splenomegaly occurs in 50%.

The diagnosis is made by demonstrating an increased total red cell mass in the absence of hypoxemia. Neutrophils have markedly elevated levels of the enzyme alkaline phosphatase, a feature that permits differentiation from granulocytic leukemia, in which neutrophil alkaline phosphatase is greatly reduced.

Treatment is aimed at reducing red cell mass by repeated venesection. Survival for 10–15 years is usual. As with other myeloproliferative diseases, there is an increased incidence of progressive myelofibrosis and acute leukemia. Polycythemia rubra vera may thus be considered to be a low-grade neoplasm of the bone marrow stem cell, characterized by the emergence of a clone of cells that is usually sensitive to erythropoietin. A clonal 20q deletion has been reported.

The peripheral blood contains white blood cells of several types in numbers and proportions that vary between quite narrow limits in health but more widely in disease.

Normal White Blood Count & Differential

As with many biologic parameters, there is no strict definition of normal; however, normal ranges are established by laboratories for their population group. Table 26-1 shows a typical set of normal laboratory values for the United States.

Variations in these parameters, along with changes in leukocyte morphology as seen in blood smears, are important indicators of disease (Table 26-2).

Abnormalities in Lymphocyte Count

Lymphocyte and monocyte origin and function have been considered with the immune system (Chapter 4: The Immune Response).

Lymphocytosis—increased lymphocyte count in peripheral blood—is best considered in relationship to other lymphoproliferative diseases (Chapter 28: The Lymphoid System: I. Structure & Function; Infections & Reactive Proliferations). It may occur as (1) an acute immune response, with many activated or transformed lymphocytes circulating in the blood; (2) a chronic immune response, in which most of the circulating lymphocytes resemble resting small lymphocytes; or (3) neoplastic proliferation (Table 26-3).

Likewise with lymphopenia—decreased peripheral blood lymphocyte count—immunologic analysis by techniques such as flow cytometry is often vital to determine the cause (discussed fully in Chapter 7: Deficiencies of the Host Response).

Abnormalities in Monocyte Count

Monocytosis is less commonly encountered (Table 26-4) than lymphocytosis and is of less importance diagnostically (bearing in mind that the mononuclear cells of infectious mononucleosis are activated lymphocytes and not monocytes). If the distinction of monocytes from partially transformed lymphocytes is in doubt, the following may be of help: (1) monoclonal antibody markers that can be used to identify monocytes, B cells, and T cells, or the presence of surface or cytoplasmic immunoglobulin for B lymphocytes (Chapter 4: The Immune Response); or (2) enzyme reactions such as naphthylacetate (nonspecific) esterase, which is positive in monocytes.

Abnormalities in Granulocyte Count

Neutrophils

Neutrophils develop from the precursor stem cell in the bone marrow (Figure 26-1). Early forms, which include the myeloblast, promyelocyte, and myelocyte, are actively dividing cells normally restricted to the marrow. Maturing cells, which include the metamyelocyte, the band form, and the segmented neutrophil, are nondividing cells. Normally, the segmented neutrophil and band form are released into the peripheral blood. The maturing pool in the marrow also serves as a storage pool, amounting to 20 times the number of neutrophils present in the peripheral blood. This pool provides a mechanism for very rapid (within hours) increase in the peripheral blood neutrophil count.

Neutrophils in the peripheral blood commonly adhere to the vascular endothelium (margination), and many do not figure in the neutrophil count. Neutrophils have a very short half-life in the blood, passing out into the tissues within a few hours after being released from the bone marrow.

During development, primary granules (containing myeloperoxidase, hydroxylases, etc) make their appearance at the promyelocyte stage (Figure 26-1). Specific secondary granules (containing lysozyme, lactoferrin, leukocyte alkaline phosphatase, etc) appear at the myelocyte stage, when the neutrophil, eosinophil, and basophil series become distinguishable. The presence of these granules (and the enzymes therein) is of value in recognizing different types of normal and leukemic cells and evaluating the stage of maturation.

Abnormalities in peripheral blood neutrophil parameters (Table 26-5) may relate to alterations in the neutrophil count (neutrophil leukocytosis and neutropenia) or morphology.

Neutrophil Leukocytosis (Neutrophilia; Granulocytosis)

Neutrophil leukocytosis is present when the absolute neutrophil count exceeds 7500/μL. The term “leukemoid reaction” is used for a very severe reactive neutrophil leukocytosis, sometimes in excess of 50,000/μL, with a leftward shift owing to early release of storage-pool granulocytes. The peripheral blood picture of leukemoid reaction resembles that of chronic myelocytic leukemia but may be distinguished from it by the neutrophil alkaline phosphatase level, which is elevated in the leukemoid reaction and decreased in chronic myelocytic leukemia.

When counts are extremely high (over 100,000/μL—usually only in chronic myelocytic leukemia), the blood viscosity may increase to such an extent as to produce thrombosis and occlusion of vessels.

Neutrophil leukocytosis may result from a variety of mechanisms (Table 26-6). Redistribution of the bone marrow storage pool or tissue neutrophils into the peripheral blood results in rapid increases in the neutrophil count, often within hours after a stimulus. Increased proliferative activity of early neutrophil precursors leads to a slower (several days) increase in peripheral neutrophil count.

Examination of the peripheral blood smear provides valuable clues to etiology (Table 26-5). Reactive neutrophilias are characterized by the presence of predominantly mature forms. In neoplastic proliferations of granulocytic cells, less mature forms are present in the peripheral blood.

Eosinophilia

Eosinophilia is an absolute eosinophil count in the peripheral blood that exceeds 450/μL. It is a common manifestation of parasitic infections, especially with metazoan parasites; type I hypersensitivity, eg, allergic rhinitis (hay fever), bronchial asthma, urticaria, and eczema; immunologic diseases, eg, pemphigus vulgaris, polyarteritis nodosa, and eosinophilic gastroenteritis; and neoplasms, most commonly Hodgkin's disease and mycosis fungoides. Hypereosinophilic syndromes include eosinophilic leukemia, a variant of CML, and Löffler's syndrome (Chapter 34: The Lung: I. Structure & Function; Infections).

Neutropenia (Granulocytopenia)

Neutropenia is a decrease in the absolute neutrophil count below 1500/μL. There are numerous causes (Tables 26-5 and 26-7). The most severe form of neutropenia is agranulocytosis, which is an absence of neutrophils in the peripheral blood.

Neutropenia is clinically significant when the neutrophil count drops below 1000/μL. Infections, usually with pyogenic bacteria, occur frequently in such patients. When the level falls below 500/μL, such infections are inevitable. Oral infections with ulceration of the throat, skin infections, opportunistic infections, and fever are the most common manifestations. This is a common terminal event in acute leukemia and aplastic anemia. Management in cases not associated with malignant neoplasms is aimed at preventing infection (antibiotics, leukocyte infusions, patient isolation) until the marrow recovers. High doses of steroids produce improvement in some cases. Granulocyte-rich transfusions may provide temporary improvement.

Neutrophil Dysfunction Syndromes

This is a rare group of diseases in which patients manifest clinical complications similar to those seen in severe neutropenia but without a decrease in the neutrophil count in the peripheral blood. A variety of different diseases in which different functions of the neutrophil are affected have been described (Table 26-8). The net result is increased susceptibility to infection, and the differential diagnosis is from other immunodeficiency diseases (Chapter 7: Deficiencies of the Host Response). Some of these conditions produce characteristic morphologic changes in the peripheral blood (Table 26-9).

Leukemias

The leukemias are malignant neoplastic proliferations of hematopoietic cells in the bone marrow. In most cases, the neoplastic cells are also present in increased numbers in the peripheral blood. See Acronyms.

Incidence

The number of new cases of leukemia in the United States is about 25,000 per year, with 15,000–20,000 deaths. Death rates have fallen because of increasing effectiveness of treatment.

Acute leukemias account for 50–60% of all leukemias, with acute myeloblastic leukemia (AML) slightly more common than acute lymphoblastic leukemia (ALL).

ALL occurs predominantly in young children (peak age incidence 3–4 years) (Figure 26-2). AML can occur at any age but is most common in young adults (peak 15–20 years).

Chronic leukemias account for 40–50% of leukemias, with chronic lymphocytic leukemia (CLL) slightly more common than chronic myelocytic leukemia (CML).

CLL occurs mainly in patients over the age of 60 years.

CML occurs at all ages, with a peak incidence in the age group from 40 to 50 years (Figure 26-2).

Etiology

The cause of most kinds of leukemia is unknown.

Viruses

Viruses are known to cause animal leukemias and are highly suspect in humans. A retrovirus (human T lymphotropic virus type I; HTLV-I) has been identified as the causative agent in one type of acute T lymphocytic leukemia first described in Japan, while a related virus, human T lymphotropic virus type II (HTLV-II), has been linked to hairy cell leukemia. However, no causal agent has been identified for most cases of human leukemia.

Radiation

Exposure to radiation resulted in an increased incidence of leukemia in the first generation of radiologists and among survivors of the Hiroshima and Nagasaki bombs. Fetuses who have been exposed to radiation in utero and patients who have received radiation in the treatment of ankylosing spondylitis and Hodgkin's disease have an increased incidence of leukemia.

Chemical Agents

The same cytotoxic drugs used in treatment of leukemia and other cancers produce an increased incidence of leukemia. In addition, arsenic, benzene, phenylbutazone, and chloramphenicol have been implicated in some cases.

Marrow Aplasia

Marrow aplasia due to any cause appears to be associated with an increased incidence of subsequent leukemia, as do the refractory anemias.

Immune Deficiency

Immune deficiency states are associated with an increased incidence of leukemia, suggesting that immunologic surveillance is important in preventing the emergence of neoplastic hematopoietic cells.

Genetic Factors

Chromosomal abnormalities are present in a high proportion of patients with leukemia (Table 26-10). The first reported was the association of the Philadelphia chromosome (a small chromosome 22 resulting from the reciprocal translocation of genetic material from chromosome 22 to chromosome 9) with chronic myelocytic leukemia. Also interesting is the increased incidence (20 times normal) of leukemia in patients with Down syndrome (trisomy 21). Chromosome fragility syndromes (Bloom's syndrome, Fanconi's anemia) also carry a high risk of acute leukemia.

Classification

Leukemias are classified in several ways:

According to Onset and Clinical Course

This was the earliest approach because the identity of the cells involved was not known. It still has clinical merit.

1. Acute leukemias have a sudden onset with a rapidly progressive course leading to death within months, if untreated. They are usually characterized by primitive cells (blasts) that are morphologically poorly differentiated.

2. Chronic leukemias have an insidious onset and a slow clinical course, with patients often surviving several years even if untreated. Chronic leukemias are usually characterized by more mature (well-differentiated) type cells.

According to the Peripheral Blood Picture

1. Leukemic, characterized by elevation of the white blood cell count and numerous leukemic cells. This is the common form.

2. Subleukemic, in which the total white count is normal or low but recognizable leukemic cells are present in the peripheral blood.

3. Aleukemic, where the total white count is normal or low and no recognizable leukemic cells are present in the peripheral blood. This is rare, but it may occur early in the disease.

According to Cell Type

Classification by cell type becomes more complex as new criteria evolve for cell recognition (Table 26-11, Figure 26-3).

1. Lymphocytic leukemias are neoplasms of lymphocytes. Their close relationship to certain of the lymphomas is considered in Chapter 29: The Lymphoid System: II. Malignant Lymphomas.

   1. ALL is characterized by the presence in the bone marrow and peripheral blood of uniform large cells that resemble the proliferating lymphoblast of fetal development (Chapter 4: The Immune Response). Acute lymphoblastic leukemia is further classified by its morphologic features (French-American-British (FAB)—system; Table 26-12) or by its immunologic or genetic features (Figure 26-3; see also Tables 26-14, 29-5, and 29-7).

   2. CLL is characterized by the proliferation of small mature lymphocytes that resemble the resting small lymphocytes of the peripheral blood. In 95% of cases, the lymphocytes are B cells; in the rest, they are T cells.

      When lymphocytic leukemia involves lymph nodes, it has the appearance of malignant lymphoma (Chapter 29: The Lymphoid System: II. Malignant Lymphomas). ALL in lymph nodes is identical to lymphoblastic lymphoma (B, T, or nonmarking type—formerly classified within the broader category of poorly differentiated lymphocytic lymphoma). CLL in lymph nodes is identical to small lymphocytic lymphoma (B or T type—formerly termed well-differentiated lymphocytic lymphoma).

      In each case, this phenomenon represents part of the spectrum of a single disease process, lymphoma-leukemia. This concept is discussed further in Chapter 29: The Lymphoid System: II. Malignant Lymphomas.

2. Myeloid (granulocytic) leukemias are characterized by the proliferation of cells of the granulocyte series, usually neutrophils, although concomitant proliferation of eosinophils and basophils is not uncommon.

   1. AML is characterized by proliferation of myeloblasts. Myeloblasts are difficult to differentiate morphologically from lymphoblasts except (1) when they contain Auer rods, which are purple, crystalline cytoplasmic inclusions; (2) when they show some show maturation into promyelocytes, in which coarse granules are seen in the cytoplasm; and (3) when cytochemical or immunologic markers are used (Tables 26-13, 26-14; see also Table 29-5). AML is further classified (FAB system M1, M2, M3, M4) by its morphologic features. (Note that the FAB classification of AML includes monocytic leukemia [M5], erythroleukemia [M6], and megakaryoblastic leukemia [M7], which others consider separately.)

   2. Chronic myelocytic leukemia is characterized by proliferation of cells of the granulocyte series that have matured beyond the myeloblast stage. Less than 5% of cells in the marrow are myeloblasts. When a patient with chronic myelocytic leukemia has a bone marrow containing more than 5% myeloblasts, that patient is defined as being in the accelerated or blast phase of the disease.

3. Monocytic leukemia–Traditionally, two forms were distinguished: acute monocytic (Schilling type) and acute myelomonocytic (Naegeli type). Both are now included under acute myeloblastic leukemia in the FAB classification, in recognition of the known common origin with granulocytes. There is no well-defined chronic form of monocytic or myelomonocytic leukemia, although some myeloproliferative disorders do show monocytic proliferation.

   1. Acute monocytic (monoblastic) leukemia (FAB–M5) is characterized by proliferation of monoblasts. These can be reliably distinguished from other blasts only with the use of cytochemical markers (Table 26-13).

   2. Acute myelomonocytic leukemia (FAB—M4) is characterized by blasts that have the characteristics of myeloblasts and monoblasts, both morphologically and in cytochemical studies.

4. Other types–Erythroleukemia (Di Guglielmo's disease), plasma cell leukemia, eosinophilic leukemia, and megakaryocytic leukemia (Figure 26-3) are all rare.

Clinical Features

(Figure 26-4)

Acute Leukemias

Acute leukemia is characterized by an acute clinical onset and rapid progression of disease. Patients usually present with evidence of a decrease in one or more of the normal hematopoietic elements because the bone marrow is overrun by the leukemic cells. Anemia, often severe and rapidly developing, causes pallor and hypoxic symptoms. Thrombocytopenia may produce abnormal bleeding or purpura. Neutropenia results in infections, fever, and ulceration of mucous membranes. Patients with acute promyelocytic leukemia (M3) frequently present with disseminated intravascular coagulation due to the coagulant properties of the cytoplasmic granules.

Enlargement of lymph nodes is common in ALL and acute monocytic leukemia but usually absent in AML. Involvement of tissue other than lymph nodes occurs rarely in all types of acute leukemia. Rarely, a local tissue mass (chloroma or granulocytic sarcoma) may be the first manifestation of AML. Similarly, tissue masses may occur in acute monocytic leukemia.

Chronic Leukemias

Chronic leukemias usually have an insidious onset and a slow rate of progression. Most patients present with slowly developing anemia and enlargement of organs infiltrated by leukemia cells.

Generalized lymph node enlargement is present in CLL; histologic features of affected nodes are indistinguishable from those of small-cell (well-differentiated) lymphocytic lymphoma (Chapter 29: The Lymphoid System: II. Malignant Lymphomas). Splenomegaly—often massive—and hepatomegaly are usually obvious at presentation in all chronic leukemias. Patients may also manifest nodular masses in the skin, liver, heart, and kidneys.

Massive splenomegaly and hepatomegaly are common presenting complaints in patients with CML. Pain in the left lower chest is evidence of splenic infarction due to vascular occlusion by aggregates of granulocytes.

Diagnosis

Acute Leukemias

Acute leukemias are characterized by the proliferation of primitive cells (blasts) that mature little, if at all. In both AML and ALL, the peripheral blood usually shows an increased total white cell count with increased numbers of blasts (Figure 26-5B). Rarely, the total white count is not increased, although the diagnosis may be made by the finding of blasts in the peripheral blood smear. Extremely rarely, no blasts are seen in the peripheral blood (aleukemic leukemia).

Rarely, marked lymphocytosis as seen in whooping cough or a viral infection may mimic ALL. Infectious mononucleosis, in which there are activated atypical lymphocytes in the blood, may cause considerable diagnostic difficulty (Table 26-3).

The bone marrow is abnormal in all cases of acute leukemia because it is the primary site of the disease. Involvement is diffuse. The bone marrow is hypercellular, with proliferation of the cell type involved at the expense of the normal hematopoietic elements (Figure 26-6). Diagnosis of the type of acute leukemia and subclassification according to the FAB system depends on identification of the cell type.

1. In AML, the primitive myeloblasts may not show any maturation (in the M1 subtype in the FAB classification), or show minimal maturation into promyelocytes and myelocytes (M2 subtype). Acute promyelocytic leukemia (M3 subtype) is characterized by a predominance of promyelocytes. Separation from ALL and distinction of the different AML subtypes (M1–M7) is by subtle cytologic features plus cytochemical (Table 26-13), cytogenetic (Table 26-10), and immunologic markers (Tables 26-14 and 29-5). CD34 (stem cell) is expressed strongly in M1–M3, while CD13 and CD33 are positive in M1–M7.

2. In ALL, there is proliferation of lymphoblasts (Figure 26-5B). While these may be difficult to differentiate from myeloblasts on morphologic grounds, the use of cytochemical (Table 26-13) and immunologic stains (Tables 26-14 and 29-5) permits accurate diagnosis. ALL is considered in more detail in Chapter 29: The Lymphoid System: II. Malignant Lymphomas along with the related lymphomas (Table 29-7).

Chronic Leukemias

Chronic leukemias are characterized by the presence of very high peripheral white blood cell counts. Morphologically, the cells are mature. The peripheral blood picture is diagnostic of chronic leukemia in most cases (Figure 26-5). Rarely, a leukemoid reaction (severe neutrophil leukocytosis in response to an acute inflammatory process) may be difficult to distinguish from CML (Table 26-5). The bone marrow is always abnormal, showing diffuse hypercellularity.

1. In CML, the dominant cells in the peripheral blood and bone marrow are myelocytes, metamyelocytes, and granulocytes. The granulocytes are usually neutrophils, but it is not uncommon to find increased numbers of basophils and eosinophils. Very rarely, the eosinophils are the dominant cells (eosinophilic leukemia). In the bone marrow, general myeloproliferation is present, involving not only the granulocyte series but also erythroid cells and megakaryocytes. Myelofibrosis (Chapter 25: Blood: II. Hemolytic Anemias; Polycythemia) may complicate CML. The Philadelphia chromosome (Chapter 19: Neoplasia: III. Biologic & Clinical Effects of Neoplasms) is present in all of these cell lineages and remains detectable even after remission of the CML. The chronic phase of CML persists for 1–10 years (median, 3 years), following which there is an accelerated phase that eventually (80% of cases) enters blast crisis. In most cases, the process then resembles AML (M2 or M4). Less often (15% of cases), it resembles ALL (usually pre-B ALL, occasionally T ALL) with the corresponding antigenic markers. Acceleration typically is accompanied by further cytogenetic changes, including extra copies of the Ph1 chromosome.

2. In CLL, the neoplastic cells resemble resting small lymphocytes morphologically (Figure 26-5A) but can be shown to be monoclonal. The bone marrow is infiltrated by similar cells, but normal hematopoietic elements remain until an advanced stage of the disease. CLL is considered in Chapter 29: The Lymphoid System: II. Malignant Lymphomas.

Treatment & Prognosis

Combination chemotherapy, using several anticancer agents simultaneously in various combinations, has improved the prognosis of patients with acute leukemias dramatically. Common ALL in children is now considered to be a curable disease in many cases (70% 5-year survival rate). The rate of cure in other types of ALL (T and B cell, 10% 5-year survival rates), AML, and acute monocytic leukemia (both with 5-year survival rates near zero) is much worse. However, chemotherapy does increase the duration of survival in comparison with nontreated cases.

Paradoxically, treatment has little effect on the survival rate of chronic leukemias. Many of these patients survive many years after diagnosis of disease without treatment, and their overall course and outcome does not seem to be altered by therapy.

Recently, acute leukemias have been treated more aggressively with the intention of destroying all the hematopoietic cells in the marrow, including leukemic cells, followed by rescue of the patient by bone marrow transplantation (see Chapter 10: Nutritional Diseases). The transplant consists either of matched heterologous marrow or of autologous marrow taken prior to therapy and purged of leukemic cells by in vitro treatment with monoclonal antibodies.

Hemorrhage and infection are major causes of death of patients with leukemia, occurring as a direct effect of the leukemia or as a complication of cytotoxic therapy or failed transplantation.

Hairy Cell Leukemia (Leukemic Reticuloendotheliosis)

Hairy cell leukemia is a rare neoplasm of the hematopoietic system that chiefly affects individuals over the age of 50 years. The neoplastic cell is medium-sized, with an ovoid nucleus, a fine chromatin pattern, and inconspicuous nucleoli (see Table 29-7). The abundant cytoplasm has a frayed cell membrane (on electron microscopy, the cell membrane shows hairy processes, hence the name). Electron microscopy also reveals the presence in the cytoplasm of a specific spiral organelle (lamellar ribosomal complex). The cytoplasm contains tartrate-resistant acid phosphatase, the demonstration of which is of diagnostic value. The neoplastic cell has been demonstrated to be a B lymphocyte by immunoglobulin gene rearrangement. Hairy cells have a distinctive phenotype, including B cell markers plus CD11c and CD 105.

Patients present commonly with anemia, neutropenia and thrombocytopenia, and splenomegaly, the last often massive. The disease responds poorly to chemotherapy and has a relatively poor prognosis, with median survival approximately 2–3 years after diagnosis. Splenectomy is of value in some cases, but results are unpredictable.

Myeloproliferative Diseases

The term myeloproliferative disease encompasses several closely related entities characterized by neoplastic proliferation of bone marrow stem cells with differentiation along one or more pathways. The class of myeloproliferative diseases includes the following entities:

1.  AML (and variables, M1–M7; see Table 26-12).

2. CML.

3. Polycythemia rubra vera.

4. Myelofibrosis.

5. Primary thrombocythemia.

6. Myelodysplastic disorders.

The first three conditions listed have already been discussed and are quite well defined. While all may show some clinical overlap, the last three are most difficult to distinguish and to diagnose.

Myelofibrosis (Agnogenic Myeloid Metaplasia)

In this condition, a dysfunctional neoplastic clone displays some megakaryocytic features and appears to release platelet-derived growth factor (PDGF) and platelet factor 4. (PDGF stimulates fibroblasts, whereas platelet factor 4 is able to inhibit collagenase.) The net result is progressive fibrosis of the marrow, with extensive compensatory extramedullary hematopoiesis in spleen (splenomegaly), liver, and even lymph nodes. Clonal chromosomal translocations or partial deletions (chromosomes 7 and 13) are found in 50% or more of cases. Philadelphia chromosome may appear.

Aspiration of marrow is often unsuccessful because of extensive fibrosis of the bone marrow (myelofibrosis). Residual marrow is markedly hypercellular with proliferation of all cell lines. Clusters of dysplastic megakaryocytes are characteristic. The peripheral blood typically shows leukoeryothroblastic anemia with neutrophil leukocytosis and a marked shift to the left.

Myelofibrosis tends to have a slow clinical course. As marrow fibrosis progresses, extramedullary hematopoiesis may cause massive enlargement of the spleen and liver. Hemorrhage, thromboembolism, and infection are common complications. Some cases are associated with other myeloproliferative disorders, especially polycythemia rubra vera. In 10% of patients, there is evolution to AML.

Primary (Idiopathic) Thrombocythemia

In this condition, the clonal stem cell proliferation maintains sufficient differentiation to produce excessive numbers of platelets (thrombocytosis), with counts ranging from 600,000 to 2,500,000/μL. Platelet function is often abnormal, however, and patients may present either with excessive bleeding or with occlusion of small vessels (focal ischemia, visual loss, neurologic symptoms).

Peripheral blood shows numerous platelets, many of abnormal shape, plus leukocytosis. The bone marrow is often fibrotic, emphasizing the close relationship of this disorder with myelofibrosis. Patients are typically elderly, and the disease generally progresses to marrow failure. Rarely, AML develops.

Myelodysplastic Disorders (Refractory Anemias)

This category includes several related and ill-understood conditions. Patients are usually elderly and present with refractory anemia and pancytopenia. Some cases follow a history of cytotoxic therapy; others are associated with severe vitamin B12 or folate deficiency. The peripheral blood often shows a dimorphic hypochromic macrocytic picture. Examination of the bone marrow reveals dyshematopoiesis, particularly evident in erythroid cells. Abnormal-appearing blast cells are present, often in increased numbers; some blasts contain a ring of iron deposited around the nucleus (revealed by Prussian blue stain—so called ring sideroblasts [sideroblastic anemias]). Cytogenetic abnormalities (5q–, monosomy of 7) are present in more than 50% of cases. Often termed preleukemias in the past, more than 10% of these cases develop into AML.

The normal vascular system maintains a delicate balance between clotting mechanisms and clot lysis, with the result that internal or external bleeding is controlled and pathologic thrombosis prevented (see Chapter 9: Abnormalities of Blood Supply).

Several factors acting in concert maintain this equilibrium:

1.  Blood vessels maintain anatomic integrity and a smooth endothelium. Following minor injuries, the blood vessels undergo vasoconstriction, thereby preventing bleeding from the injured site and permitting repair. Vasoconstriction is an effective method of hemostasis in small vessel injuries but is not adequate when large vessels are damaged.

2.  Platelets form the initial hemostatic plug that seals a site of vascular injury. Platelets also play a major role in forming the permanent thrombus that seals the injury.

3.  Blood coagulation is the formation of fibrin from plasma precursors. Fibrin and platelets constitute the permanent hemostatic plug.

4.  Fibrinolysis is the production of factors such as plasmin from plasma precursors that lyse and remove fibrin thrombi which have formed in the circulation.

Disturbance of any one of these mechanisms may produce abnormal bleeding on the one hand (Table 27-1) or abnormal thrombosis on the other. Pathologic thrombosis is discussed in Chapter 9: Abnormalities of Blood Supply.

The clinical effects of abnormal bleeding disorders (Table 27-2) are similar regardless of the mechanisms. Laboratory testing is generally required to reach a precise clinical diagnosis, following which appropriate therapy may be selected.

Along with thrombocytopenia, vascular defects represent the most common cause of bleeding diathesis. In certain vascular disorders, the underlying defect relates to production of abnormal collagen or elastin (Table 27-1); in vasculitis, inflammation is the cause.

Henoch-Schönlein purpura (anaphylactoid purpura) deserves special mention as a poststreptococcal disease of childhood. It occurs 1–3 weeks after streptococcal infection and is thought to be mediated by deposition of cross-reactive IgA or immune complexes plus complement on the endothelium. Occasional cases have been reported with apparent hypersensitivity to other bacteria, insect bites, or food (milk, eggs, crab, strawberries). Clinically, there is purpura, abdominal pain (due to mucosal involvement in the gut, often with frank bleeding), arthralgia or arthritis, and glomerulonephritis. Fever is often present. The prognosis is determined by the severity of the renal lesion (focal glomerulonephritis with deposition of IgA and complement; see Chapter 48: The Kidney: II. Glomerular Diseases).

Hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu disease) is inherited as an autosomal dominant trait and manifested by multiple capillary microaneurysms in the skin and mucous membranes. The lesions tend to become more conspicuous with age and are exceedingly fragile, predisposing both to episodes of acute severe bleeding and to chronic blood loss from the intestinal tract with resulting iron deficiency anemia.

Normal Structure & Function

Platelets are anucleate cytoplasmic fragments 2–4 μm in diameter derived from megakaryocytes. The normal platelet count in peripheral blood is 150,000–400,000/μL. The platelets have a clear outer zone and an inner zone containing cytoplasmic organelles (Figure 27-1), which include contractile microfilaments and several types of granules that contain a variety of enzymes, phospholipid, adenosine diphosphate (ADP), adenosine triphosphate (ATP), serotonin, calcium, and thromboplastic substances. Platelets have a life span of 8–10 days in the peripheral blood.

In peripheral blood smears, platelets appear as small granular cytoplasmic fragments about one-fourth the size of an erythrocyte. Platelets that have exited the marrow recently are larger, and a finding of many large forms is evidence that the rate of release of platelets from the marrow is increased.

The main function of platelets is in hemostasis. In the early phase of hemostasis after vascular injury, platelets plug and seal injured areas in the wall of the blood vessel (Figure 27-2). The process of formation of the platelet plug involves two mechanisms: adhesion and aggregation.

1.  Platelet adhesion: Platelets in the blood are attracted to collagen exposed by endothelial damage. Effective adhesion requires the presence of von Willebrand factor (vWF), which is secreted by the endothelial cells into the serum. Adherence is followed by clumping and release of cytoplasmic granules (activation). Degranulation is an active phenomenon that involves contraction of the platelet microskeleton; it is dependent on the presence of prostaglandins and is inhibited by aspirin. Platelet adhesion is a function of the outer zone of the platelet cytoplasm, with the glycoprotein Ib (GPIb) receptors binding to large multimers of vWF at the injury site.

2.  Platelet aggregation: The release of platelet granules containing ADP, various biogenic amines, and thromboxane A2 induces accumulation and aggregation of large numbers of platelets to form the hemostatic plug that prevents bleeding from the injured zone. Aggregation involves another glycoprotein receptor (GPIIb–IIIa) binding with fibrinogen to initiate the fibrin plug (see Table 27-4).

Platelets secrete other factors that also play a role: platelet-derived growth factor (PDGF), which stimulates repair), platelet factor 4 (PF4), chemotactic for neutrophils), and serotonin (a vasoconstrictor).

Abnormalities of Blood Platelets

Abnormalities of blood platelets include (1) decreased numbers (thrombocytopenia), (2) increased numbers (thrombocytosis), and (3) abnormal platelet function. There are numerous causes for such abnormalities (Table 27-3). In general, thrombocytopenia and abnormal platelet function impair the normal hemostatic mechanism and cause increased bleeding. Thrombocytosis is associated with an increased tendency to thrombosis.

Idiopathic Thrombocytopenic Purpura

Incidence & Etiology

Idiopathic thrombocytopenic purpura (ITP) is a common disease in which severe reduction of platelet numbers in the blood is caused by immune destruction of platelets. It occurs in two clinical forms:

1.  Acute ITP: Acute is seen mainly in children. About 50% of cases are associated with a history of viral infection 2–3 weeks before onset. It is believed that immune complexes bind to the surface of platelets, resulting in phagocytosis by splenic mac rophages. Acute ITP is characterized by spontaneous recovery in the majority of patients; 80% are normal after 6 months; many recover within 6 weeks.

2.  Chronic ITP: Chronic ITP occurs mainly in adults, with a predilection for females (3:1) and frequent occurrence of relapse during pregnancy. Thrombocytopenia is the result of peripheral destruction of platelets that have bound an IgG antiplatelet autoantibody on their surface. Chronic ITP is a long-standing disorder characterized by multiple relapses and remissions.

3.  Neonatal thrombocytopenic purpura: Neonatal disease occurs in children born to mothers with chronic ITP. It results from transfer of the IgG antibodies across the placenta.

Pathology

Platelet survival is impaired. The platelet count is markedly decreased, often to the 10,000–50,000/μL range. The bone marrow typically shows increased numbers of megakaryocytes (compensatory hyperplasia).

The spleen is the major site of destruction of antibody-coated platelets. It is usually either normal in size or slightly enlarged and shows sinusoidal congestion and hyperplasia of macrophages.

As with any patient with severe thrombocytopenia, the bleeding time is prolonged and capillary fragility (tourniquet test) is increased. Tests of coagulation (clotting time, partial thromboplastin time, and prothrombin time) are normal. Clot retraction, which depends on platelets, is defective.

Clinical Features & Treatment

Patients present with a bleeding tendency. Clinical bleeding occurs when the platelet count falls to very low levels (< 40,000/μL). The most common clinical feature is purpura. Bleeding may also occur from mucosal surfaces, leading to hematuria, melena, menorrhagia, and hemoptysis. Rarely, intracerebral hemorrhage occurs—a complication associated with a high mortality rate.

The diagnosis of ITP is one of exclusion, reached by ruling out all other causes of thrombocytopenia.

Treatment with high-dose corticosteroids suppresses splenic phagocytic activity. There is evidence that autoantibody synthesis is also decreased. The response to steroids has a lag phase of several days during which platelet transfusions are frequently necessary to provide adequate hemostasis. Splenectomy is effective in treatment by removing the main site of platelet destruction but does nothing to correct the basic abnormality.

Abnormalities of Platelet Function

Platelet function abnormalities are characterized by symptoms and signs of platelet deficiency (ie, abnormal bleeding) but with a normal platelet count. Tests of platelet function such as clot retraction, platelet adhesion, and aggregation in response to different agents are abnormal and are valuable in separating the different disease processes (Table 27-4).

Normal Blood Coagulation

Coagulation represents the method of permanent healing of a vascular injury. It follows vasoconstriction and formation of the platelet plug and maintains hemostasis when normal blood flow is restored. Coagulation is achieved by the interaction of several plasma protein factors (Table 27-5) that ultimately results in formation of fibrin when they are activated sequentially (Figure 27-2).

Disorders of Blood Coagulation

Etiology

There are three principal groups of coagulation disorders.

Deficiency of Coagulation Factors

Deficiencies of individual coagulation factors may occur as inherited diseases (Table 27-6). Of these, factor VIII deficiency (hemophilia A and von Willebrand's disease) and factor IX deficiency (Christmas disease) are the most common. Acquired deficiencies of coagulation factors occur in severe liver disease (affects all factors produced by the liver) and in vitamin K deficiency (prothrombin and factors VII, IX, and X).

Presence of Circulating Anticoagulants

Factor deficiencies may also be induced by anticoagulant therapy with coumarin derivatives (which interfere with vitamin K, thereby inhibiting synthesis of prothrombin and of factors VII, IX, and X). Directly acting anticoagulants that antagonize some of the coagulation cascade include (a) drugs such as heparin (an antithrombin), (b) antibodies (factor VIII inhibitor and lupus anticoagulant, an antibody in systemic lupus erythematosus), and (c) natural anticoagulants (antithrombin and fibrin degradation products).

Fibrinolytic Activity

Increased fibrinolytic activity in the blood results from increased activation of the plasmin system.

Clinical Features

Patients with disorders of coagulation tend to bleed excessively following minor trauma such as dentistry. In severe cases, spontaneous bleeding occurs (ie, bleeding without evident trauma)—commonly into joints (hemarthrosis) and muscles. Bleeding is usually slow but persistent and can be halted by replacement of the deficient factor.

All coagulation disorders have similar clinical manifestations. Determination of the cause of the abnormality requires bleeding and coagulation testing (Figure 27-3 and Tables 27-7 and 27-8).