Paroxysmal nocturnal haemoglobinuria is an acquired clonal, benign, haematopoietic stem cell disorder which results in the formation of defective red cells, white cells and platelets. It is characterised by intravascular haemolysis and frequent venous thrombosis or bleeding.
It is caused by a mutation in an X-linked gene involved in the formation of membrane phosphatidyl inositol anchors. The membranes of these cells are deficient in those proteins normally anchored to the cell through a phosphatidyl inositol linkage, including complement deactivating factors.
PNH is a very rare disorder and should be suspected in confusing cases of haemolytic anaemia. There is a median survival of 10 years.