Such defects result in RBC shape alterations as in hereditary elliptocytosis or to fragmentation of the RBC as in hereditary pyropoikilocytosis. Hereditary elliptocytosis is usually inherited as an autosomal dominant trait. The defect is a structural abnormality of spectrin or a deficiency of the RBC membrane protein 4.1, without anemia and usually with no splenomegaly and only mild hemolysis. Most patients are asymptomatic. The number of elliptocytes does not correlate with the severity of the hemolysis.
Osmotic fragility is usually normal.
RBC hemolysis occurs in the spleen, thus splenectomy corrects the hemolysis, but not the RBC membrane defect. Hereditary stomatocytosis is an autosomal dominant disorder, in which a membrane abnormality leads to increased permeability to sodium. This causes water to move into the RBC decreasing the mean corpuscular hemoglobin concentration. The PBS shows RBCs with narrow mouth- like areas of hypochromia. These RBCs are known as stomatocytes.