Polyarteritis nodosa and sarcoidosis presenting with myalgia and myositis-mimicking magnetic resonance imaging findings: A report of two cases.

Diseases presenting with muscle symptoms are diverse. We report two cases initially suspected of idiopathic inflammatory myopathies that were ultimately diagnosed as polyarteritis nodosa and sarcoidosis, respectively. Case 1 was a 30-year-old man with fever, limb muscle pain, and elevated inflammatory markers. Muscle magnetic resonance imaging showed high signal intensity on fat-suppressed T2-weighted images, and biopsy revealed vasculitis with fibrinoid necrosis, leading to a diagnosis of polyarteritis nodosa. Case 2 was a 70-year-old man with proximal muscle pain and weakness and elevated muscle enzymes. Magnetic resonance imaging showed high signal intensity in the thigh muscles, and biopsy revealed noncaseating granulomas consistent with sarcoidosis. These cases highlight the diagnostic pitfalls of interpreting magnetic resonance imaging findings and serum creatine kinase levels in isolation when evaluating suspected idiopathic inflammatory myopathies and underscore the critical role of histopathological evaluation, including muscle biopsy, in establishing an accurate diagnosis.