Pathophysiology
Clinical meaning
Budd-Chiari syndrome (BCS) refers to hepatic venous outflow obstruction occurring anywhere from the small hepatic venules to the junction of the inferior vena cava (IVC) with the right atrium, excluding obstruction caused by cardiac disease or sinusoidal obstruction syndrome (formerly known as veno-occlusive disease). This obstruction impedes normal hepatic venous drainage, causing increased sinusoidal pressure, hepatic congestion, hepatocellular hypoxia and necrosis, and if unrelieved, progression to liver fibrosis, cirrhosis, and liver failure. The pathogenesis of BCS is fundamentally based on Virchow triad: the majority of cases (approximately 75-85%) are associated with an identifiable prothrombotic or hypercoagulable condition that predisposes to venous thrombosis. Myeloproliferative neoplasms (MPNs) are the most common underlying cause, identified in approximately 40-50% of BCS patients. Polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) all carry increased risk of hepatic vein thrombosis through multiple mechanisms: elevated blood cell counts increase blood viscosity, MPN-associated platelet activation promotes thrombus formation, and the JAK2 V617F mutation (present in >95% of PV and approximately 50-60% of ET and PMF) directly promotes thrombus formation by enhancing platelet adhesion and increasing expression...