Early-Onset Statin-Induced Immune-Mediated Necrotising Myositis: A Rare Case Report from a Resource-Constrained Setting

Background: Statin-induced necrotising autoimmune myositis (SINAM) is a rare but serious complication of statin therapy characterized by progressive proximal muscle weakness, markedly elevated creatine kinase (CPK) levels, and anti-HMG-CoA reductase (anti-HMGCR) antibodies.
Case Presentation: We report a case of a 40-year-old Sri Lankan female with type 2 diabetes and non-alcoholic fatty liver disease who developed severe myalgia and proximal muscle weakness three months after starting rosuvastatin. Laboratory evaluation revealed a CPK level of 92,000 U/L, AST/ALT ratio favouring muscle origin, transient creatinine elevation, and nephrotic-range proteinuria (with myoglobinuria). Electromyography showed features of active myopathy, and muscle biopsy confirmed necrotising myositis. Extensive autoimmune and infectious workups were negative, and anti-HMGCR antibody testing was unavailable locally. Despite this limitation, the diagnosis of SINAM was made by exclusion of other etiologies and supported by histological and clinical findings. The patient showed dramatic improvement with high-dose corticosteroids alone, without the need for additional immunosuppressants. Relapse upon steroid tapering further supported an autoimmune aetiology.
Discussion: Unlike most SINAM cases, which present years after statin initiation, this case highlights an unusually early onset. It underscores the diagnostic challenge in resource-limited settings lacking anti-HMGCR testing. Additionally, the case demonstrated an atypical renal manifestation of overflow proteinuria without albuminuria, likely due to massive myoglobinuria. Timely discontinuation of the statin and initiation of steroids resulted in full recovery, avoiding long-term complications.
Conclusion: Clinicians should maintain a high index of suspicion for SINAM in patients on statins who present with progressive myopathy and elevated CPK, even in the absence of confirmatory serology. Early diagnosis and corticosteroid therapy can lead to excellent outcomes.