microthrombi

Microthrombi are small clots that form within the microvasculature—arterioles, capillaries, and venules. They consist mainly of platelets embedded in a network of fibrin and, in some contexts, leukocytes. By occluding small vessels, microthrombi can restrict blood flow and cause tissue ischemia in the organs supplied by those vessels.

They arise from local endothelial injury, inflammation, and dysregulated coagulation. Their appearance and composition vary: platelet-rich 'white' microthrombi are typical in thrombotic microangiopathies, whereas fibrin-rich thrombi can predominate in disseminated intravascular coagulation.

Microthrombi are central to thrombotic microangiopathies (TMAs), such as thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). In TTP, deficiency of ADAMTS13 prevents cleavage of ultra-large von Willebrand factor multimers, leading to platelet-rich microthrombi that can cause neurologic symptoms and renal impairment. In DIC, systemic activation of coagulation can produce widespread fibrin deposition in small vessels, with potential organ ischemia and simultaneous bleeding risk. Microthrombi also occur in sepsis, malignant hypertension, HELLP syndrome, and after organ transplantation or bone marrow transplantation.

Histology and detection: Microthrombi are identified in tissue samples by microscopy as intravascular occlusions composed of platelets and fibrin; in TMAs they may appear as hyaline thrombi. They are often not visible on standard imaging but may be inferred from organ dysfunction and laboratory markers of microangiopathy: schistocytes on peripheral smear, elevated lactate dehydrogenase, thrombocytopenia, and rising creatinine.

Management: Treatment focuses on addressing the underlying cause and preventing further thrombus formation; plasma exchange is a standard therapy for TTP; treating the trigger (infection, obstetric complication, malignancy, sepsis) is essential in DIC; anticoagulation may be considered in selected conditions but carries bleeding risk.