Heparin-induced thrombocytopenia-associated thrombosis: from arterial to venous to venous limb gangrene.

Heparin-induced thrombocytopenia (HIT) is an acquired immune-mediated hypercoagulability state that is strongly associated with thrombosis. During the 1970s and 1980s, the prevailing concept was that HIT was associated only with arterial thrombosis, through its unique pathogenesis via heparin-dependent, platelet-activating IgG antibodies. However, in 1990, when I began to encounter HIT in my clinical practice, I found that most such patients developed symptomatic venous thrombosis. This historical sketch summarizes some of the research that challenged the dogma of HIT being a mainly arterial prothrombotic disorder. Two studies - one a substudy of a randomized trial of post-orthopedic surgery thromboprophylaxis, and the second a retrospective five-hospital analysis of consecutive patients with positive test results for HIT antibodies - showed a marked predominance of venous over arterial thrombosis complicating HIT (~ 4 : 1). By the end of the 1990s, an even more dramatic manifestation of HIT-associated venous thrombosis was recognized: venous limb gangrene. Here, ischemic limb necrosis occurs despite palpable arterial pulses, as a result of both macrovascular and microvascular venous thrombosis. The surprising explanation was natural anticoagulant impairment (severe depletion of protein C, a vitamin K-dependent anticoagulant) resulting from treatment of HIT-associated deep vein thrombosis with warfarin (vitamin K antagonist). These insights from HIT research helped to elucidate the pathogenesis of ischemic limb losses in other intense non-HIT hypercoagulability states, including warfarin-associated venous limb gangrene complicating cancer-associated hypercoagulability, and symmetrical peripheral gangrene complicating disseminated intravascular coagulation of critical illness, in which proximate 'shock liver' helps to explain the profound failure of natural anticoagulant systems (protein C; antithrombin) in predisposing to peripheral limb microthrombosis in circulatory shock.