Severe Rhabdomyolysis in the Setting of Influenza A Infection and Clinically Suspected Anti-signal Recognition Particle-Positive Immune-Mediated Necrotizing Myopathy.

Anti-signal recognition particle (SRP)-positive immune-mediated necrotizing myopathy (IMNM) is a rare autoimmune myopathy characterized by symmetric proximal weakness, markedly elevated creatine kinase levels, and muscle fiber necrosis with minimal inflammation. A previously healthy 23-year-old man presented with influenza-like symptoms and intractable gastrointestinal symptoms. Testing was positive for influenza A, and initial evaluation demonstrated severe rhabdomyolysis, with a creatine kinase level of 21,694 U/L, oliguric acute kidney injury, and major electrolyte abnormalities. The clinical course progressed to renal failure requiring continuous renal replacement therapy followed by hemodialysis, metabolic encephalopathy secondary to uremia, acute pancreatitis, anemia requiring transfusion, left upper-extremity deep venous thrombosis, and persistent symmetric extremity weakness. A myositis-specific antibody panel was positive for anti-SRP antibodies at 20 SI (reference range, <11 SI), and broad infectious and autoimmune testing was otherwise unrevealing. The patient received high-dose intravenous methylprednisolone for three days, with substantial improvement in strength and a decline in creatine kinase from a peak of 121,627 U/L to 227 U/L. Although mild weakness persisted at discharge, urine output recovered, and hemodialysis was discontinued. This presentation emphasizes that IMNM should be considered in young patients presenting with severe nontraumatic rhabdomyolysis and weakness, particularly when the clinical course is disproportionate to an apparent viral trigger. Early recognition may expedite targeted immunotherapy and multidisciplinary follow-up.