S4978 A Rare Case of Focal Myositis in a Patient With Ulcerative Colitis Presenting as Left Hip and Thigh Pain

Introduction: Ulcerative colitis (UC) is a chronic inflammatory bowel disease with known extraintestinal manifestations, most commonly involving the joints, skin, and eyes. Muscular involvement is rare and typically presents as generalized myopathy or inflammatory myositis. Focal myositis, characterized by localized muscle inflammation, is an unusual and under-recognized complication in UC. Case Description/Methods: A 51-year-old man with a history of UC presented with progressive left hip and thigh pain and weakness. Computed tomography imaging revealed extensive myositis and myonecrosis of the left hip and posterior thigh musculature. Magnetic resonance imaging demonstrated diffuse myositis involving the left lower extremity with a large area of non-enhancement in the left adductor muscles, consistent with myonecrosis, but no abscess or fluid collection. Laboratory tests including creatine phosphokinase, aldolase, and a comprehensive autoimmune panel (rheumatoid factor, antinuclear antibody, anti-neutrophil cytoplasmic antibodies, anti-Jo-1, Mi-2) to evaluate for dermatomyositis and polymyositis or other autoimmune process were all negative. Infectious, rheumatological, and vascular workups were unremarkable. Electromyography and lumbar plexus magnetic resonance imaging showed no abnormalities. Muscle biopsy revealed round atrophic and angulated fibers, rare necrotic and regenerating fibers, but no inflammation, vasculitis, or infection. Special stains (GMS, Gram, Fite) were negative. The patient was diagnosed with focal myositis and treated with high-dose corticosteroids, resulting in clinical improvement in pain and strength. He was discharged on a steroid taper with neurology follow-up. Discussion: This case represents a rare presentation of focal myositis associated with UC, highlighting the need to consider localized muscle inflammation in UC patients presenting with focal pain and weakness. Negative autoimmune and infectious workups and biopsy findings helped exclude inflammatory myopathy and infectious causes. Early recognition and corticosteroid treatment can lead to favorable outcomes. Awareness of this rare entity expands the spectrum of extraintestinal manifestations of UC and underscores the importance of a multidisciplinary approach for diagnosis and management.