Pathophysiology of Primary Budd-Chiari Syndrome: A Narrative Review.

Budd-Chiari syndrome (BCS) is a rare hepatic vascular disorder characterized by obstruction of hepatic venous outflow, leading to sinusoidal congestion, portal hypertension, and centrilobular necrosis - a classical model of postsinusoidal portal hypertension. This review provides an integrated analysis of the pathophysiology of BCS according to the site of obstruction (major hepatic veins, small/centrilobular veins, or inferior vena cava), describing how increased sinusoidal pressure, reduced portal flow, and compensatory mechanisms (intrahepatic collaterals, caudate lobe hypertrophy, and arterialization) shape clinical phenotypes that range from asymptomatic to acute, subacute, and chronic forms. The review highlights the etiopathogenesis of prothrombotic states - particularly myeloproliferative neoplasms (predominantly JAK2 V617F), hereditary thrombophilias, and acquired conditions, such as antiphospholipid syndrome and paroxysmal nocturnal hemoglobinuria - as well as the contributory role of pregnancy, the puerperium, and systemic diseases. It also discusses epidemiologic variability between Western and Asian populations and its clinical implications. Finally, it emphasizes a diagnostic approach based on Doppler ultrasonography (supported by computed tomography (CT) or magnetic resonance imaging (MRI)) and a stepwise management strategy prioritizing early anticoagulation, treatment of portal hypertension complications, decompression via transjugular intrahepatic portosystemic shunt (TIPS), and liver transplantation in selected cases. Understanding these mechanisms is key to improving diagnostic accuracy and guiding therapeutic decisions that have substantially enhanced patient survival.