Influenza A-triggered complement-mediated aHUS and high-grade block in a young adult: A case report

Abstract Background: Influenza infection is an uncommon precipitant of thrombotic microangiopathy (TMA) and an even rarer cause of transient high-grade atrioventricular (AV) block. A combined presentation has not been described. Case: A 25-year-old man presented with fever, cough, nausea, vomiting, and gross hematuria. Studies revealed platelets 23 → 16 ×10³, creatinine 3.5 → 5.2 mg/dL, LDH 2,692 U/L, D-dimer 14,439 ng/mL, fibrinogen 159 mg/dL, positive influenza A antigen, and ECG demonstrating ventricular rate 37 bpm with high-grade AV block. ADAMTS-13 activity was 74%, excluding thrombotic thrombocytopenic purpura (TTP). He received five sessions of plasma exchange (PLEX) and pulse methylprednisolone followed by a slow prednisone taper. Inpatient eculizumab was not available; outpatient infusion was arranged. Platelets rose to 295 ×10³ and creatinine fell to 2.5 mg/dL at discharge (01/03/2025) with restoration of sinus rhythm without pacing. Conclusion: This case underscores the importance of recognizing influenza-triggered complement-mediated atypical hemolytic uremic syndrome (aHUS) and concurrent influenza myocarditis with conduction disease. Early PLEX, immunosuppression, and timely complement inhibition, even if initiated after discharge, are critical.