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Erythrocyte Membrane Defects
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The Erythrocyte Membrane
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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.
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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.
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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.
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Hereditary Spherocytosis
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Etiology & Pathogenesis
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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.
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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.
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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.
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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.
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Hereditary Elliptocytosis
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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.
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Paroxysmal Nocturnal Hemoglobinuria
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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.
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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.
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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.
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Erythrocyte Enzyme Deficiency
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Normal Erythrocyte Metabolism
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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.
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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.
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Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency
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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.
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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.
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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.
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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.
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Other Erythrocyte Enzyme Deficiencies
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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.
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Deficiencies of other enzymes such as glutathione reductase and triosephosphate isomerase are rare.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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).
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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.
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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.
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Other Abnormal Hemoglobins
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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.
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Abnormal hemoglobins may have the following consequences:
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Hemolytic anemia. In most cases, the severity of hemolysis is less than that seen in sickle cell disease.
Instability of the hemoglobin molecule, causing precipitation of Heinz bodies and decreased red cell survival.
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.
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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.
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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).
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Homozygous Beta Thalassemia (Cooley's Anemia)
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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.
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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.
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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).
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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.
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Heterozygous Beta Thalassemia (Cooley's Trait)
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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.
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Sickle Cell-Beta Thalassemia
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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.
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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.
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Immune-Mediated Hemolytic Anemias
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Autoimmune Hemolytic Anemias
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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.
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Idiopathic Warm Autoimmune Hemolytic Anemia
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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).
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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.
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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.
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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.
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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.
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Idiopathic Cold Hemagglutinin Disease
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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).
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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.
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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.
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Cold hemagglutinin disease also occurs in association with malignant lymphoma and as a complication of certain infections (eg, with Mycoplasma pneumoniae).
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Paroxysmal Cold Hemoglobinuria
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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.
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An acute variant of the disease follows certain viral infections. A more chronic form occurs in syphilis.
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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.
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Isoimmune Hemolytic Anemia
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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.
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Hemolytic Blood Transfusion Reactions
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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.
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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).
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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).
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Hemolytic Disease of the Newborn
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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).
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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.
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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.
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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).
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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.
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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.
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Drug- & Chemical-Induced Hemolysis
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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).
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Immune Drug-Induced Hemolysis
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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.
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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.
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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.
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Direct alterations in the erythrocyte membrane (cephalosporins) lead to the adsorption of immunoglobulins and macrophage-mediated hemolysis.
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Nonimmune Drug-Induced Hemolysis
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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.
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Drug-induced hemolysis in G6PD deficiency was discussed earlier in this chapter.
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Hemolysis Caused by Infectious Agents
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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.
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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).
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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.
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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.
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Microangiopathic Hemolytic Anemia
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Microangiopathic hemolytic anemia is caused by fragmentation of erythrocytes as they traverse an abnormal microcirculation.
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Disseminated Intravascular Coagulation (DIC)
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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.
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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.
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Abnormal Blood Vessels
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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.
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Prosthetic Cardiac Valves or Aortic Prostheses
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Red cells are traumatized during passage through the prosthesis.
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Pathology & Diagnosis
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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.
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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.
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