Mimicry of lyme arthritis by synovial hemangioma.

To report on the differential diagnosis of lyme arthritis and synovial hemangioma due to similar clinical and radiological signs and symptoms. A 15-year-old boy presented at the age of 9 with recurrent rather painless swelling of the right knee. Altogether four episodes lasting for 1-2 weeks each occurred over a period of 18 months before medical advice was sought. Physical examination revealed only a slightly limited range of motion. Living in an endemic area of borreliosis, he reported a tick bite 6 months prior to onset of his symptoms with erythema migrans and was treated for 10 days with amoxicillin. Serology revealed two positive unspecific bands in IgG immunoblot (p41 and 66) with slight positivity for ELISA. Ultrasound revealed synovial thickening and increased fluid. Despite the weak positive serology a diagnosis of lyme arthritis could not be excluded and intravenous antibiotic treatment with ceftriaxone was started. After two further relapses antiinflammatory therapy including intraarticular steroids were introduced with no long lasting effect. A chronical disease developed with alternate periods of swelling and almost complete remission. Ultrasound as well as MRI demonstrated ongoing signs of synovitis, therefore after further progression, a diagnostic arthroscopy was performed showing an inconspicuous knee joint. A second MRI showed focal suprapatellar enhancement and was followed by open arthrotomy revealing a histopathological proven synovial cavernous juxtaarticular hemangioma. To our knowledge, the differential diagnosis of lyme arthritis and synovial hemangioma has not yet been reported despite obvious clinical similarities. In conclusion, in children and adolescents synovial hemangioma has to be considered in differential diagnosis of recurrent knee swelling. Early diagnosis is important to prevent prolonged suffering from chronic joint swelling with probable joint damages, unnecessary treatment procedures and as well school and sports absenteeism.