Inferior vena cava agenesis presenting as iliofemoral deep vein thrombosis in a 49-year-old male

Abstract Introduction Inferior vena cava (IVC) agenesis is a rare congenital anomaly, often asymptomatic until early adulthood, when it may manifest with extensive deep vein thrombosis (DVT). In patients without apparent thrombophilic risk factors, identification of anatomical abnormalities such as IVC agenesis is crucial. Renal anomalies such as horseshoe kidney are occasionally associated. This case highlights a rare but important consideration in younger patients presenting with unprovoked iliofemoral DVT. Case Presentation We present the case of a 49-year-old man with a past medical history of hypertension managed with losartan and impaired glucose tolerance treated with metformin. The patient presented with 7 days of progressive right lower extremity swelling, pain, and distal discoloration. Imaging revealed a right iliofemoral deep vein thrombosis. A CT scan demonstrated the presence of a horseshoe kidney and agenesis of the infrarenal inferior vena cava, with collateral circulation via the azygos vein. The patient underwent diagnostic angiography confirming extensive thrombosis and collateral pathways. A mechanical thrombectomy using the Penumbra system and balloon angioplasty of the right iliac vein were performed, resulting in partial recanalization. Postoperatively, the patient experienced resolution of pain and swelling. He was started on enoxaparin and later transitioned to rivaroxaban for outpatient anticoagulation. At one-month follow-up, the patient demonstrated symmetric lower limbs with preserved right iliofemoral vein patency and no evidence of venous reflux. Discussion Agenesis of the IVC, particularly of the infrarenal segment, is rare but recognized as a cause of unprovoked DVT in young adults. It often results in the development of extensive collateral venous networks such as the azygos and hemiazygos systems. The presence of renal anomalies such as horseshoe kidney further suggests a congenital developmental disruption. Mechanical thrombectomy using the Penumbra system has shown utility in re-establishing venous patency in extensive thrombosis. This case emphasizes the importance of considering IVC anomalies in young patients with unprovoked DVT, especially in the absence of inherited or acquired thrombophilia. Long-term management should include anticoagulation and compression therapy, along with regular surveillance for venous patency. Conclusion This case underscores the diagnostic and therapeutic challenges in managing iliofemoral DVT due to IVC agenesis. Early identification and intervention can lead to favorable outcomes, particularly with the use of endovascular techniques and comprehensive follow-up.