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

Abstract

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.

Keywords: acute myositis; bacterial myositis; bacterial pyomyositis; infectious myositis; pyomyositis; soft tissue infection.

Figure 1. Non-contrast CT of the right lower extremity showing nonspecific soft tissue changes.

Axial (A) and coronal (B) non-contrast CT images of the right lower extremity demonstrate subcutaneous edema and heterogenous hypodensity within the anterior muscle compartment, concerning for muscle atrophy or early inflammation. The red arrow in A highlights the anterior compartment with blurring of fascial planes, suggesting early infectious myositis. No discrete fluid collection is seen. Osseous structures are intact without signs of periostitis, endosteal scalloping, or radiopaque foreign bodies. CT: computed tomography

Figure 2. Contrast-enhanced CT of the right lower extremity showing a large multiloculated abscess.

Contrast-enhanced CT images of the right lower extremity demonstrate a complex, peripherally enhancing, multiloculated fluid collection within the anterior compartment musculature, measuring approximately 5.0 × 3.4 × 23.7 cm and tapering near the right ankle. Subcutaneous fat stranding is present, with no evidence of gas, cortical erosion, or joint effusion. These findings are consistent with infectious myositis complicated by abscess formation. A includes intersecting red dotted lines marking the central area of the fluid collection in the axial view. B shows the full longitudinal extent of the abscess in the coronal plane. CT: computed tomography