Anti-mitochondrial antibody-positive myositis

A 56-year-old Japanese woman presented with a 1-month history of low back and bilateral thigh pain. Symptoms are aggravated by movements like turning or standing. Her medical history was unremarkable. Physical examination revealed a temperature of 37.0°C, pulse rate of 96 beats/minute, blood pressure of 115/63 mmHg, respiratory rate of 18 breaths/minute, and SpO2 of 98% (room air). Grasping pain was present in the lumbar back and both thighs, with lumbar lordosis observed in standing posture. Manual muscle testing indicated proximal muscle strength decreased to approximately 4, and an agitated gait was observed. Laboratory tests revealed elevated C-reactive protein (7.8 mg/dL) but no other abnormal findings, including the hepatobiliary enzyme and creatinine kinase levels. Lower extremity magnetic resonance imaging (MRI) showed high-intensity lesions in bilateral adductor muscles, gluteus maximus, and tensor fascia femoris (Figure 1). Her anti-mitochondrial M2 antibody (AMA-M2) level was 524 U/mL (reference <7 U/mL); she was diagnosed with AMA-positive myositis. Her symptoms improved with steroid therapy.