Aseptic pyomyositis in Behçet's disease: a case report and narrative review of muscle involvement phenotypes.

We report the case of a 54-year-old male patient with Behçet Disease (BD) complicated by aseptic pyomyositis with features of medium vessel vasculitis. His clinical course was characterized by severe myalgia, a palpable mass in the left quadriceps, blistering and ulcerative skin lesions, and systemic inflammation. Laboratory tests revealed neutrophilic leukocytosis, markedly elevated C-reactive protein, and approximately 13-fold increased creatine kinase levels. MRI demonstrated diffuse and asymmetric skeletal muscle oedema in the lower limbs, suggestive of multifocal abscess-like lesions. Muscle biopsy revealed sterile pyomyositis with granulocytic-necrotizing infiltrates and fibrinoid vascular necrosis with massive endomysial inflammatory infiltration. Muscle symptoms responded well to high-dose corticosteroid therapy, while cutaneous ulcers were initially refractory and improved only after multiple immunosuppressive treatments. This case highlights the complexity of diagnosing and managing severe muscular involvement and treatment-resistant skin manifestations in the context of BD.