An autopsy case of anti-MDA5 antibody-positive amyopathic dermatomyositis with an initial manifestation of panniculitis on the left upper arm.

A 53-year-old man presented with refractory panniculitis on the left upper arm that had persisted for 10 months. The patient was diagnosed with lupus profundus, wherein oral glucocorticoid therapy was initiated. Four months prior, ulceration was observed in the same area. Dapson was administered instead, scarring the ulcer but enlarging the panniculitis. Five weeks earlier, he developed a fever, productive cough, and dyspnoea. Three weeks earlier, a skin rash was observed on the forehead, left auricle posterior to the neck, and extensor aspect of the left elbow. Chest computed tomography showed pneumonia in the right lung, after which the patient's dyspnoea worsened. The patient was admitted and diagnosed with anti-MDA5 antibody-positive amyopathic dermatomyositis (ADM) based on skin findings, hyperferritinaemia, and rapidly progressive diffuse lung shadows. Glucocorticoid pulse therapy, intravenous cyclophosphamide, and tacrolimus were initiated, and later, plasma exchange therapy was combined. However, his condition worsened and required management with extracorporeal membrane oxygenation. The patient expired on day 28 after hospitalisation. An autopsy revealed hyalinising to fibrotic stages of diffuse alveolar damage. Strong expression of myxovirus resistance protein A was observed in three skin biopsy specimens from the time of initial onset, consistent with ADM. Anti-MDA5 antibody-positive ADM not only manifests typical cutaneous symptoms, but also rarely occurs with localised panniculitis, such as in the present case. In patients with panniculitis of unknown aetiology, the possibility of initial symptoms of ADM should be included in the differential diagnosis.