Systemic Thrombolytic Resistance in Antithrombin III Deficiency Rescued by Catheter‐Directed Thrombolysis: A Case Report

BACKGROUND: Hereditary antithrombin III (AT III) deficiency is a rare thrombophilic disorder that may cause resistance to systemic thrombolytic therapy. We report a case of massive iliofemoral deep-vein thrombosis (DVT) with high-risk pulmonary thromboembolism (PTE) refractory to systemic thrombolysis, successfully treated with catheter-directed thrombolysis (CDT). CASE SUMMARY: A previously healthy 33-year-old man presented with dyspnoea and left-leg swelling. Imaging revealed bilateral pulmonary-artery thrombi with acute right-ventricular failure and extensive iliofemoral DVT. Despite systemic alteplase infusion, thrombus resolution was incomplete, and hereditary AT III deficiency (53% activity) was diagnosed. CDT via the right femoral vein was performed using alteplase (0.02 mg/kg/h for 48 h), achieving near-complete thrombus resolution and restoration of venous flow without bleeding complications. DISCUSSION: This case highlights systemic thrombolysis failure associated with AT III deficiency and demonstrates that CDT can serve as an effective, low-bleeding-risk rescue therapy for extensive DVT when systemic treatment is inadequate. These findings underscore the importance of early recognition of AT III deficiency and consideration of catheter-based intervention in similar high-risk settings.