Erythrocyte fragmentation and hemolysis occur when red cells are forced through partial vascular occlusions or over abnormal vascular surfaces at high shear stress. “Split” red cells, or schistocytes, are prominent on blood films under these conditions, and considerable quantities of lactate dehydrogenase are released into the blood from traumatized red cells. In the high-flow (high-shear) microvascular (arteriolar or capillary) or arterial circulation, partial vascular obstructions are caused by platelet aggregates in the systemic microvasculature during episodes of thrombotic thrombocytopenic purpura by platelet-fibrin thrombi in the renal microvasculature in the hemolytic uremic syndrome and by malfunction of a cardiac prosthetic valve in valve-related hemolysis. Less-extensive red cell fragmentation, hemolysis, and schistocytosis occur under conditions of more moderate vascular occlusion or endothelial surface abnormalities, sometimes under conditions of lower shear stress. These latter entities include excessive platelet aggregation, fibrin polymer formation, and secondary fibrinolysis in the arterial or venous microcirculation (eg, disseminated intravascular coagulation); in the placental vasculature in preeclampsia or eclampsia and the syndrome of hemolysis, elevated liver enzymes and low platelets (HELLP) in march hemoglobinuria; and in giant cavernous hemangiomas (the Kasabach-Merritt phenomenon).
Acronyms and Abbreviations
ADAMTS13, a disintegrin and metalloproteinase with thrombospondin domain 13; ALT, alanine transaminase; aPTT, activated partial thromboplastin time; AST, aspartic acid transaminase; AT, antithrombin; DIC, disseminated intravascular coagulation; HELLP, hemolysis, elevated liver enzymes, and low platelet count; LDH, lactate dehydrogenase; MAHA, microangiopathic hemolytic anemia; NO, nitrous oxide; PGF, placental growth factor; PGI2, prostaglandin I2; PT, prothrombin time; PTT, partial thromboplastin time; sEng, soluble endoglin; sFlt-1, soluble form of fms-like tyrosine kinase 1; sVEGFR-1, soluble vascular endothelial growth factor receptor-1; TGF-β, transforming growth factor-β; TTP, thrombotic thrombocytopenic purpura; VEGF, vascular endothelial growth factor; VWF, von Willebrand factor.
PREECLAMPSIA OR ECLAMPSIA AND HELLP SYNDROME
A life-threatening condition of pregnancy denoted by eclampsia, hemolysis, and thrombocytopenia was first noted in the German literature by Stahnke in 1922.1 Subsequently, Pritchard and coworkers described three cases in English and suggested that an immunologic process might account for both the preeclampsia or eclampsia and the hematologic abnormalities.2 Although initially known as edema-proteinuria-hypertension gestosis type B,3 a catchier phrase, HELLP syndrome (H for hemolysis, EL for elevated liver function tests, and LP for low platelet counts), was later applied by Louis Weinstein in 1982.4
HELLP syndrome occurs in approximately 0.5% of pregnancies,5 in 4% to 12% of those complicated by preeclampsia (hypertension + proteinuria), and in 30% to 50% of those complicated by eclampsia (hypertension + proteinuria + seizures); however, approximately 15% of patients ultimately diagnosed with HELLP syndrome present with neither hypertension nor proteinuria.6 Two-thirds of HELLP patients are diagnosed antepartum, usually between 27 and 37 weeks. The remaining third are diagnosed in the postpartum period, from a few to 48 hours after ...