Case Presentation: An otherwise healthy 16-year-old male presented with two days of left thigh pain and swelling. He was initially evaluated in a community affiliate emergency department where the differential diagnosis included trauma with secondary compartment syndrome, septic arthritis, deep vein thrombosis and deep space muscle infection. Labs were remarkable for elevated inflammatory markers (ESR of 37, CRP of 131) and radiographs showed a large left knee effusion. The patient was started on antibiotics prior to transfer to a tertiary care center. In the receiving hospital’s emergency department, orthopedics was consulted and performed a joint aspiration with turbid fluid obtained. Further investigation with MRI showed a rim-enhancing multi-loculated left thigh lesion of the vastus intermedius whose largest collection measured 11cm x 2.6cm x 3.8cm; this was reported in close proximity to the joint capsule with communication unclear on imaging. Extensive disease prompted tissue debridement and wound VAC placement by orthopedic surgery. Biopsies were sent to pathology and outside hospital laboratory. Following surgical intervention, the patient was admitted to the hospitalist service, infectious disease was consulted and the patient was continued on IV antibiotics. Gram stain and culture of tissue samples were negative; however, antibiotics were continued and several days into admission, Lyme Western Blot sent by the community emergency department returned positive on 9 bands. Lyme PCR from the synovial fluid subsequently returned positive confirming the diagnosis of Lyme arthritis complicated by pyomyositis. The patient was seen at follow-up several weeks into therapy with resolution of thigh swelling, minimal pain with activity and downtrending inflammatory markers.

Discussion: Lyme disease is the most common tick-borne illness in the United States. While the number of reported cases continues to rise, the true incidence of Lyme disease is difficult to determine due to passive reporting of cases, frequent misdiagnosis, and false positive serologic test results. Clinical manifestations of Lyme disease vary widely and are classified into the early localized, early disseminated and late stages. Late Lyme disease, which occurs weeks to months after the initial infection, includes arthritis, polyneuropathy and encephalitis. Myositis is a very rare manifestation of late borrelial infection. To our knowledge, Lyme myositis has not been reported in the pediatric literature. A literature search revealed six case reports of Lyme myositis occurring in the United States and 28 total reported cases. The most recent case report describes a 43-year-old man who presented similarly to our patient.  In both cases, MRI showed localized myositis associated with a contiguous monoarthritis; Western blot confirmed Lyme disease with multiple positive IgG bands. In addition, Lyme PCR of synovial fluid from involved knee and involved musculature was positive in both cases. Our case is distinguished from the literature given that it is first known reported pediatric case of Lyme myositis. 

Conclusions: Lyme myositis is a rare manifestation of late stage Lyme disease. Accurate diagnosis is paramount for timely treatment with appropriate antibiotic therapy, as reported cases have shown improved outcomes with prompt recognition and treatment of underlying Lyme disease.