Atypical Pediatric Lyme Arthritis Mimicking Juvenile Arthritis: 3 Cases

Background Lyme disease incidence is rising in Canada.[1] While Lyme arthritis typically presents as monoarthritis of a large joint, most often the knee, it can also mimic juvenile idiopathic arthritis (JIA) in pattern and chronicity.[2] We describe 3 children whose initial presentations resembled JIA but were ultimately diagnosed with Lyme arthritis.

Case Report We reviewed the charts of 3 pediatric patients referred for suspected JIA and later diagnosed with Lyme arthritis. We describe the demographics, exposure history, symptoms and physical examination findings, investigations, treatment, and outcomes. Case 1: An 11-year-old boy from rural Norfolk County, Ontario, developed chronic right-knee arthritis with a large Baker’s cyst and significantly limited range of motion. He reported a suspected tick bite 1 year earlier. He was initially thought to have oligoarticular JIA with MRI confirming active synovitis and a larger Baker’s cyst. However, Lyme serology was positive. After 8 weeks of doxycycline and naproxen, he achieved complete clinical resolution of arthritis and Baker’s cyst that has been sustained for the past 4 years. Case 2: A 13-year-old boy from Mount Pleasant, Ontario with subacute left-knee swelling accompanied by fever and highly elevated CRP of 163 was admitted due to suspected septic arthritis. Synovial fluid was markedly neutrophilic but culture-negative, and he later developed contralateral knee and right wrist effusions, prompting consideration of JIA. A detailed history revealed a febrile illness with erythema-migrans-like rash the prior summer. Lyme serology was positive and Lyme PCR in synovial fluid later returned positive. He completed 1 week of IV ceftriaxone, 8 weeks of doxycycline, and a subsequent 4-week course of IV ceftriaxone for persistent mild arthritis with full recovery. There has been no recurrence at 1 year of follow-up. Case 3: A 13-year-old girl from Oakville, Ontario, developed right-knee arthritis and left TMJ pain with visible swelling, tenderness, and limited range of motion. She had been camping extensively in Nova Scotia, PEI, and Maine 8 months earlier. While she was initially suspected to have oligoarticular JIA because of TMJ involvement, Lyme disease was suspected due to the mismatch between arthritis affecting only 2 joints and elevated inflammatory markers (CRP 68, ESR 45). Her Lyme serology was positive. After 8 weeks of doxycycline and naproxen, she was asymptomatic with no recurrence over the past 5 years.

Conclusion These cases highlight the importance of maintaining a high index of suspicion for Lyme arthritis in children and adolescents with inflammatory joint disease, including presentations that suggest JIA. Recognizing the variable presentations of Lyme arthritis and incorporating appropriate testing can ensure timely, targeted therapy and prevent unnecessary immunosuppression.

References [1.] Murison K. PLoS One 2023;18):e0295909. [2.] Huynh N. Curr Opin Rheumatol 2022;34:335-43.