Bilateral Deep Vein Thrombosis in a Young Man With Inferior Vena Cava Agenesis: A Case Report.

Inferior vena cava (IVC) agenesis is a rare congenital vascular anomaly that may predispose young individuals to venous thromboembolism in the absence of classical risk factors. We report the case of a 35-year-old man, an active smoker with no significant medical history, who presented with a 10-day history of bilateral lower limb swelling and severe back pain radiating to the lower limbs. Doppler ultrasonography demonstrated extensive bilateral deep vein thrombosis (DVT), involving the right external iliac, common femoral, and femoral veins, with complete thrombosis of the left lower limb venous system extending to the posterior tibial veins. The patient was initiated on therapeutic anticoagulation with enoxaparin (1 mg/kg twice daily). Contrast-enhanced computed tomography (CT) of the abdomen failed to visualize the subhepatic IVC and revealed thrombosis in the left hemiazygos vein and bilateral external iliac veins, raising suspicion for IVC agenesis. CT pulmonary angiography confirmed bilateral lower segmental pulmonary emboli. Echocardiography showed no evidence of right ventricular strain but revealed an incidental bicuspid aortic valve (type 1 Sievers) with mild aortic stenosis and regurgitation. The patient was subsequently transitioned from enoxaparin to fondaparinux and later to a direct oral anticoagulant for long-term management. This case highlights the importance of considering congenital venous anomalies such as IVC agenesis in young patients presenting with extensive or bilateral DVT, particularly in the absence of traditional risk factors. Early recognition is essential for guiding appropriate long-term anticoagulation and reducing the risk of recurrent thromboembolic events.