A checkpoint inhibitor induced immune mediated dermatomyopathy

<h3></h3> A 58-year-old male was referred to the neuromuscular clinic with 6-month history of diffuse myalgia and weakness in his upper and lower limbs. He had a background of malignant melanoma of his neck diagnosed 1 year earlier. Sentinel node biopsy was positive. He was deemed high-risk and commenced on Nivolumab. Examination revealed mild bilateral weakness of elbow flexion/extension, small muscles of the hand and proximal weakness in lower limbs. Significant trophic changes were noted over the skin of all limbs. His creatine kinase was normal and myositis immunoblot was negative. MRI of his lower limbs demonstrated extensive perimysial oedema involving all muscle groups of both lower legs with extensive subcutaneous oedema of both calves and ankles. Neurophysiological investigations were unremarkable. Muscle biopsy of his vastus lateralis demonstrated inflammatory changes with predominantly perimysial inflammation and MHC1 upregulation. He was diagnosed with an immune checkpoint inhibitor mediated dermatomyopathy. The Nivolumab was stopped. Pulsed intravenous methylprednisolone was administered followed by high-dose steroid treatment with good effect. As steroids were weaned, he had clinical relapse. Following discussion in a multi-disciplinary forum he was commenced on methotrexate noting excellent clinical and radiological improvement. Steroids were concomitantly weaned and he remains on a low maintenance dose.