Isolated muscle vasculitis as a mimic of inflammatory myopathy.

Isolated muscle vasculitis (IMV) is an uncommon vasculitic entity restricted to skeletal muscle and may present without weakness or creatine kinase elevation, making diagnosis difficult. A woman in her 60s presented with a 3-week history of migratory myalgia involving the thighs, shoulders and flanks. Laboratory tests showed leukocytosis, normocytic anaemia, thrombocytosis, markedly elevated inflammatory markers, normal creatine kinase and mildly increased aldolase. MRI of the thighs demonstrated patchy muscle oedema, while extensive infectious and autoimmune evaluation was unrevealing. Muscle biopsy revealed perimysial lymphohistiocytic vasculitis composed of CD4+ T cells and histiocytes without myofibre necrosis, confirming IMV. She improved rapidly with oral prednisone, and methotrexate was initiated as a steroid-sparing agent. This case highlights the importance of considering IMV in patients with unexplained myalgia and systemic inflammation despite normal muscle enzymes and underscores the essential role of MRI and muscle biopsy in diagnosis. Early immunosuppressive therapy leads to favourable outcomes.