Unmasking Idiopathic Inflammatory Myopathy: A Case of Proximal Weakness in a Young Male With Co‐Occurring Vitamin D Deficiency

Idiopathic inflammatory myopathy (IIM) encompasses rare autoimmune disorders causing muscle inflammation and weakness, with subtypes including dermatomyositis, necrotizing myopathy, antisynthetase syndrome, and inclusion body myositis. This case report details a 25-year-old South Asian male presenting with a 60-day history of progressive proximal muscle weakness, dysphagia, and dysphonia, alongside coexisting vitamin D deficiency (serum 25-hydroxyvitamin D: 19.3 ng/mL). Initial vitamin D supplementation (50,000 IU weekly) failed to halt symptom progression. Diagnostic workup revealed markedly elevated creatine kinase (4569 U/L), electromyography findings of myogenic motor unit potentials with abnormal spontaneous activity, and a biceps muscle biopsy showing inflammation, necrosis, and fibrosis, confirming IIM, most suggestive of likely necrotizing myopathy. Myositis-specific autoantibodies (MSAs) were not tested, limiting definitive subtype classification. Comprehensive immunological tests were negative, ruling out other autoimmune conditions. No paraneoplastic workup was performed. Treatment with oral prednisolone (1 mg/kg/day) and azathioprine (50 mg twice daily) led to significant improvement in muscle strength, resolution of dysphagia, and normalized CPK levels within weeks, supported by physical therapy and ongoing vitamin D supplementation. This case highlights the diagnostic complexity of IIM in the presence of vitamin D deficiency, which was coincidental in this instance and may potentially exacerbate disease severity by amplifying inflammation and muscle damage, though it is not causative. However, no temporal relationship between vitamin D supplementation and symptom improvement was observed, and follow-up vitamin D levels were not reassessed. Routine vitamin D screening in IIM patients may be considered recommended to identify and address deficiency as an adjunctive measure, potentially mitigating severity and corticosteroid-related complications like osteoporosis. We recommend complete autoimmune myositis panels, including MSAs, and malignancy screening in similar cases. Further research is needed to define vitamin D's therapeutic role in IIM management, particularly through longitudinal studies and randomized trials.