Lyme arthritis with subarticular cyst formation in metacarpal and metatarsal bones.

A young woman was treated for seronegative rheumatoid arthritis for more than 9 years, without clinical improvement, before the diagnosis of chronic Lyme arthritis of the metacarpal and metatarsal phalangeal joints was made. Roentgenograms showed subarticular cyst formation in the metacarpal and metatarsal bones. The patient's response to treatment with a 2-week course of ceftriaxone was dramatic.