Background: Infectious myositis, a rare but significant inflammatory muscle disease, involves infection of skeletal muscle tissue, typically caused by bacteria, viruses, fungi, and parasites. Staphylococcus aureus is the most common bacterial pathogen. Symptoms range from localized pain and swelling to systemic manifestations like fever and malaise. Early recognition and management are crucial to prevent complications like abscess formation, sepsis, and chronic disability 1. Here, we detail the case of a 21-year-old female with right lower extremity (RLE) pain and swelling who developed infectious myositis and an abscess in the anterior leg compartment despite having no skin defects.

Case Presentation: A 21-year-old female with systemic lupus erythematosus (SLE) presented to the ED after a mechanical fall with RLE pain, swelling, and fevers for two days. Physical examination revealed RLE swelling and tenderness without overlying skin findings. Lab results showed creatine phosphokinase (CPK) 125, C-reactive protein (CRP) 12.3, erythrocyte sedimentation rate (ESR) 72, and D-dimer 4.19 FEU. Initial CT of the RLE indicated subcutaneous edema, muscle atrophy, and fascial plane blurring, suggesting myositis. A RLE Doppler ultrasound was negative for thromboembolic processes. Given symptom worsening despite a negative workup, repeat CT of the RLE with contrast was obtained five days later which revealed a complex, multiloculated fluid collection indicative of an abscess within the anterior compartment. She was treated with IV vancomycin and underwent incision and drainage of the RLE. Intraoperative cultures grew methicillin-resistant S. aureus. Despite persistent fevers, extensive workup was unrevealing, and blood cultures were negative. At discharge, she was afebrile with improved RLE symptoms and uneventful wound healing.

Discussion: This case highlights an atypical presentation of infectious myositis in an immunosuppressed patient following a mechanical fall without any obvious historical or exam findings of skin lacerations. Differential diagnosis included myositis, pyomyositis, deep vein thrombosis, hematoma, and compartment syndrome. Despite high suspicion for myositis, normal CPK levels, lack of leukocytosis, and unrevealing CT imaging complicated diagnosis and management. Elevated ESR and CRP levels were ambiguous due to her SLE history. CT imaging, although useful, has limitations in detecting early or subtle inflammatory changes in soft tissue and persistence of symptoms necessitated further investigation. This case underscores the importance of maintaining a high index of clinical suspicion for infectious myositis and the need for repeat imaging or alternative diagnostic modalities 2 when initial tests are inconclusive, given the risk of serious complications like sepsis, osteomyelitis, and necrotizing fasciitis with delayed treatment.