Case Presentation: A 10-year-old girl with recently diagnosed rheumatic fever presented to the emergency department with two days of right thigh pain. The pain began after her first dose of intramuscular (IM) benzathine penicillin G (BPG) in her right lateral thigh. The medication was administered by her mother, a pediatric nurse, with sterile technique. She subsequently developed refusal to bear weight, abdominal pain, emesis, malaise, and fever to 38.9˚C. On presentation, temperature was 38.2˚C. Her right lateral thigh was very tender to palpation, but no swelling, warmth, or skin changes were noted. Laboratory evaluation revealed: 6.6 x109 white blood cells/L, C-reactive protein (CRP) 152.4 mg/L, and erythrocyte sedimentation rate (ESR) 21 mm/h. Ultrasound demonstrated edema of the right rectus femoris but no joint effusion. Contrast-enhanced magnetic resonance imaging (MRI) revealed edema and enhancement of the right vastus lateralis with involvement of adjacent musculature and an ill-defined rim-enhancing fluid collection (8.0 x 2.0 x 1.3 cm) in the right vastus lateralis. Findings were interpreted as myositis and intramuscular abscess of the right vastus lateralis. She was admitted for intravenous (IV) antibiotics. Pediatric orthopedic surgery performed incision and drainage on hospital day 2 and noted edematous tissues with serous interstitial fluid but no purulence or abscess. Operative Gram stain and cultures were negative.She was afebrile for the rest of her admission and improved post-operatively. Labs on hospital day 3 were notable for decreased CRP of 51.1 and ESR of 34. She received 2 days of IV antibiotics while awaiting culture results. She was discharged on hospital day 4 and changed to oral penicillin for rheumatic fever prophylaxis. By 21 days after her injection, she had resumed normal activity.

Discussion: Clinicians should be aware of potential non-infectious side effects of intramuscular penicillin injections. This presentation mimicked pyomyositis, prompting hospital admission and presenting a diagnostic challenge for the hospitalist team. Injection site pain and inflammation are well-described. Other reactions include sterile abscess formation, intravascular injuries, and Nicolau syndrome, a rare complication of ischemic necrosis of skin and deep tissue. Ultimately, we concluded that our case represented a non-infectious myositis. BPG injection-associated myositis has been described in two prior case reports, but is not listed by the manufacturer as an adverse reaction.

Conclusions: IM BPG is safe and effective when indicated, but providers should be aware of the possibility of injection-site myositis and sterile abscess, which can lead to unnecessary surgical procedures and prolonged antibiotic courses.

IMAGE 1: Figure. MRI (transverse view)