Paroxysmal nocturnal hemoglobinuria
Paroxysmal nocturnal hemoglobinuria is a rare disease characterised by aplastic anemia, thrombosis and red urine in the morning due to breakdown of red blood cells.
PNH is caused by a mutation in the GPI anchor. This molecule links many proteins to the cell membrane, but in blood cells the main GPI-linked molecules are CD16, CD55 and CD59 (CD is an acronym of cluster of differentiation). These molecules protect blood cells from destruction by the complement system
Acquired complete or partial deficiency of glycosyl-phosphatidylinositol linked proteins causes the erythrocyte to be exquisitely sensitive to hemolysis by complement. White cells and platelets share the deficiency.
Signs and symptoms
White cells and platelets share the deficiency. Clinical manifestations include episodic intravascular hemolysis with hemoglobinuria frequently occurring at night, increased susceptibility to infection, and recurrent thrombotic episodes. As the name implies, the hemoglobin appears in urine in attacks (paroxysms). The symptom in itself is harmless, although excessive hemolysis could cause renal failure.
More importantly, PNH can cause aplastic anemia and thrombosis. Aplastic anemia may warrant blood transfusion (transfused blood is not hemolysed), and thrombosis can occur in the vein tree and less commonly in arteries. A very characteristic form of venous thrombosis is Budd-Chiari syndrome, thrombosis of the hepatic vein, and cerebral venous thrombosis.
Many patients with aplastic anemia develop PNH (10-33%), possibly signifying an escape mechanism.
The diagnosis is based on the presence of a positive sucrose and acid hemolysis test. Iron deficiency may be present secondary to the chronic loss of iron in the urine.
Traditionally, this was done with Ham's test. Modern methods include flow cytometry for CD55, CD16 and CD59 on white and red blood cells. Dependent on the presence of these molecules on the cell surface, they are classified as Type I, II or III PNH cells.
Treatment is symptomatic (blood transfusion, anticoagulant medication). Recent research has suggested that prophylactic use of anticoagulants might be warranted. A new agent, eculizumab, might protect blood cells against immune destruction by inhibiting the complement system. In severe aplasia, bone marrow transplants are occasionally undertaken.
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