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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.
Causes
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.
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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.
Diagnosis
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
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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