When Sepsis Is Not Sepsis: A Case Report of Autoimmune Myopathy Masquerading as Postoperative Infection

A 79-year-old male with multiple comorbidities underwent left hemiarthroplasty following a neck of femur fracture. Necrotizing autoimmune myopathy (NAM), a rare idiopathic inflammatory myopathy that can mimic postoperative sepsis and lead to diagnostic delays, complicated his postoperative course. From the first day after surgery, he experienced persistent pyrexia with markedly elevated inflammatory markers. Despite extensive septic screening and broad-spectrum antimicrobials, all microbiological investigations remained negative, and the patient continued to deteriorate. Nine days postoperatively, he developed severe pain and progressive symmetrical weakness, rapidly evolving into quadriparesis with significantly reduced muscle power in all limbs and elevated creatine kinase levels. A comprehensive myositis-specific antibody panel was entirely negative, despite the strong clinical suspicion for idiopathic inflammatory myopathy. Given the clinical deterioration and negative infectious workup, empirical low-dose corticosteroid therapy was initiated. The patient demonstrated dramatic clinical improvement within days of steroid initiation. Temperature spikes ceased, muscle power rapidly improved in all limbs within 10 days, and inflammatory markers significantly decreased. The patient progressed from bedridden to mobilising with a mobility aid within two weeks. This case highlights the diagnostic challenge of distinguishing NAM from postoperative sepsis. The seronegative presentation emphasises the importance of clinical acumen when conventional serological markers are absent. Early recognition and corticosteroid therapy are crucial for optimal outcomes in statin-associated NAM. Clinicians should maintain high suspicion for autoimmune myopathy in patients with unexplained postoperative weakness and negative infectious investigations.