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Chapter 454 Hereditary Stomatocytosis
George B. Segel, Lisa R. Hackney
Hereditary stomatocytosis includes a rare group of dominantly inherited hemolytic anemias in which there are characteristic morphologic changes in the RBCs and increased red cell cation permeability. The RBCs are cup shaped, creating a mouth-shaped area (stoma) of central pallor instead of the usual circular area of central pallor. Hereditary stomatocytosis is classified by the RBC hydration status. The two major varieties are either overhydrated (hydrocytosis) or dehydrated (xerocytosis).
Stomatocytes of the hydrocytic variant have excess intracellular sodium and water content and decreased intracellular potassium content. The principal defect in this variant is an increase in Na+ and K+ permeability, caused by mutations in Rhesus-associated glycoprotein (RHAG). However, the amount of Na+ influx exceeds the K+ efflux, and the cells subsequently develop increased cation content and water, and thus swell. These hydrocytic cells have increased osmotic fragility. Additionally, the integral membrane protein, stomatin or band 7.2 b, is decreased or absent from the erythrocyte membrane. The function of stomatin is not fully understood, although it might act as a switch that influences the function of GLUT1, the glucose transporter. It has been found that stomatin in the hydrocytic variant is synthesized early in RBC development but is lost as the cell matures. The mechanism for this loss of stomatin is not yet determined. It also is unclear how the loss of stomatin expression contributes to the cation leak, which is characteristic of this variant.
The hydrocytic variant is the most severe form of hereditary stomatocytosis and is characterized by moderate to severe hemolysis, macrocytosis, and large numbers of stomatocytes on the blood smear. Patients commonly develop jaundice, splenomegaly, and cholelithiasis.
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