An Atypical Case of Infectious Myositis in a Young Woman on Immunosuppressive Therapy.

Infectious myositis is a rare but serious condition typically caused by bacterial pathogens. In immunocompromised patients, including those on long-term immunosuppressive therapy, clinical signs of myositis can be subtle or delayed. We present the case of a 21-year-old woman with systemic lupus erythematosus (SLE) on immunosuppressive therapy who presented with pain, fever, tachycardia, and swelling of the right lower leg. Initial evaluation revealed no skin defects or rash and normal creatine phosphokinase (CPK) levels. A non-contrast computed tomography (CT) scan of her leg showed some soft tissue changes, but it was only after a week of worsening symptoms that contrast-enhanced CT imaging revealed a multiloculated, large abscess, measuring 23.7 cm in length, in the anterior compartment of the leg. The abscess was drained surgically, and intraoperative cultures grew methicillin-resistant Staphylococcus aureus. The absence of early definitive findings, including a normal CPK level, may have contributed to the delay in diagnosis. This case highlights the diagnostic challenges of infectious myositis in immunosuppressed patients, where early imaging and laboratory findings can be misleading, underscoring the importance of repeated clinical assessment and timely advanced imaging to ensure early detection and appropriate treatment.