Working Diagnosis:
Ruptured Bakers cyst secondary to underlying Lyme arthritis, complicated by post-infectious Lyme arthritis.

Treatment:
In addition to rest, ice, compression and elevation and physical therapy, treatment included doxycycline 100 mg twice daily for 28 days. The patient continued to have ongoing swelling and was treated with doxycycline 100 mg twice daily for an additional 30 days.

One month later, he re-developed a right knee effusion despite initial improvement. He was treated with 40mg triamcinolone intra-articular injection for suspected post-infectious Lyme arthritis, with subsequent resolution of his swelling.

Outcome:
Patient had modest improvement after two courses of oral antibiotics. However, he had a recurrence of the knee effusion one month later in the setting of post-infectious Lyme arthritis, a process involving an ongoing immune response involving a proliferative synovitis. He was treated with intra-articular steroid injection which resolved his knee effusion.

Author's Comments:
Lyme arthritis is treated with oral antibiotics, with persistent arthritis treated with either a second course of oral antibiotics or a course of intravenous antibiotics.
Persistent arthritis despite multiple courses of antibiotics may be a post-infectious Lyme arthritis, involving an immune-mediated proliferative synovitis. Treatment involves a course of disease-modifying antirheumatic drugs such as methotrexate or hydroxychloroquine, or intra-articular steroids which seems to be more efficacious in pediatric patients that adults.

References:
Arvikar SL, Steere AC. Diagnosis and treatment of Lyme arthritis. Infect Dis Clin North Am. 2015 Jun;29(2):269-80. doi: 10.1016/j.idc.2015.02.004. PMID: 25999223; PMCID: PMC4443866.

Nimmrich S, Becker I, Horneff G. Intraarticular corticosteroids in refractory childhood Lyme arthritis. Rheumatol Int. 2014 Jul;34(7):987-94. doi: 10.1007/s00296-013-2923-9. Epub 2014 Jan 4. PMID: 24390634.

Jutras BL, Lochhead RB, Kloos ZA, Biboy J, Strle K, Booth CJ, Govers SK, Gray J, Schumann P, Vollmer W, Bockenstedt LK, Steere AC, Jacobs-Wagner C. Borrelia burgdorferi peptidoglycan is a persistent antigen in patients with Lyme arthritis. Proc Natl Acad Sci U S A. 2019 Jul 2;116(27):13498-13507. doi: 10.1073/pnas.1904170116. Epub 2019 Jun 17. PMID: 31209025; PMCID: PMC6613144.

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