Necrotizing Soft Tissue Infection at a Self-Administered Subcutaneous Etanercept Injection Site in an Immunosuppressed Patient With Rheumatoid Arthritis: A Case Report

Necrotizing soft tissue infection (NSTI) is rapidly progressive and can be fatal; outcomes depend on early recognition and prompt surgical debridement. In immunocompromised patients, local findings and inflammatory responses may be atypical, increasing the risk of delayed diagnosis. Although injections breach the skin barrier, NSTI originating at injection sites can be overlooked as a benign injection-site reaction; procedure-associated NSTI has been reported to have poor outcomes. A woman in her 60s with rheumatoid arthritis was receiving methotrexate, tacrolimus, and prednisolone and had started self-administered subcutaneous etanercept injections four months earlier. One week prior to presentation, she developed discomfort and mild pain in the medial right thigh at her usual injection site, and, presuming it was a routine injection-site reaction, delayed medical attention. She presented with fever and difficulty walking and was transferred to our hospital in septic shock. Examination revealed ill-defined erythema and swelling without overt skin necrosis, but pain was disproportionately severe. Contrast-enhanced computed tomography (CT) showed fascial thickening and deep soft-tissue edema without gas. Given a strong clinical suspicion for NSTI, emergent surgical exploration (finger test) and debridement were performed, confirming necrosis of the subcutaneous tissue and fascia. Group A β-hemolytic streptococcus (<i>Streptococcus pyogenes</i>) was isolated from wound and blood cultures, and the clinical course was consistent with streptococcal toxic shock syndrome (STSS). Molecular typing (e.g., emm typing) and laboratory assays proving toxin production were not performed; STSS was diagnosed clinically. The patient required additional debridement, intensive care, and targeted antimicrobial therapy and survived without amputation. She was discharged ambulatory on hospital day 105. This case highlights that, in immunosuppressed patients who self-inject biologic agents, injection-site symptoms should not be dismissed as routine reactions when pain is disproportionate or systemic symptoms develop. Early imaging and prompt surgical evaluation are essential when NSTI is suspected, even if skin findings are subtle.