The natural history of PNH is highly variable, ranging from indolent to life-threatening.

Free plasma hemoglobin scavenges nitric oxide and depletion of nitric oxide at the tissue level contributes to numerous PNH manifestations, including esophageal spasm, male erectile dysfunction, renal insufficiency, and thrombosis. 8 Intravascular hemolysis releases free hemoglobin into the plasma. The loss of complement regulatory proteins renders PNH erythrocytes susceptible to both intravascular and extravascular hemolysis, but it is the intravascular hemolysis that contributes to much of the morbidity and mortality from the disease.

6, 7 CD55 inhibits C3 convertases and CD59 blocks formation of the membrane attack complex (MAC) by inhibiting incorporation of C9 into the MAC. Two of these proteins, CD55 and CD59, are complement regulatory proteins the absence of these proteins is fundamental to the pathophysiology of the disease. Consequently, the PNH stem cell and all of its progeny have a reduction or absence of glycosyl phosphatidylinositol (GPI)–anchored proteins. The PIG-A gene product is required for the biosynthesis of glycophosphatidylinositol anchors, a glycolipid moiety that attaches dozens of proteins to the plasma membrane of cells. 4, 5 Expansion and differentiation of the PIG-A mutant stem cell lead to clinical manifestations of the disease. 3 The disease originates from a multipotent hematopoietic stem cell that acquires a mutation of the PIG-A gene. 1, 2 PNH can arise de novo or in the setting of aplastic anemia (AA). Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematopoietic stem cell disease that can present with bone marrow failure, hemolytic anemia, smooth muscle dystonias, and thrombosis.

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