Table 25–1. Classification of Hemolytic Anemias. ||Download (.pdf)
Table 25–1. Classification of Hemolytic Anemias.
|Intrinsic defect of erythrocytes|
|Hemolysis due to extrinsic factors|
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).
Clinicopathologic effects of hemolysis. (LDH, lactate dehydrogenase.)
Extravascular hemolysis is characterized by the following features.
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).
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 ...