Lyme arthritis.

Infection with B. burgdorferi can cause a large joint inflammatory arthritis in patients who have not been treated for early Lyme disease; the knee is the most common joint affected. The diagnosis depends on a history of known exposure to the spirochete, characteristic clinical features, and serologic studies (ELISA and Western blot) confirming exposure to the spirochete. In most patients, antibiotic therapy is curative, but in a smaller percentage of patients, the presence of the HLA-DR beta 1*0401 haplotype can trigger treatment-resistant arthritis, in which antibiotic therapy is ineffective; in these instances, remittive agents, such as hydroxychloroquine and methotrexate, are indicated. Arthroscopic synovectomy may be considered when antibiotic therapy is not curative. Fibromyalgia can follow infection with B. burgdorferi but is unresponsive to antibiotic therapy; it is treated with tricyclic antidepressants and an exercise program. Lyme arthritis is the only chronic inflammatory arthritis in which the specific cause is known and can be cured. As such, it serves as an excellent model with which to study the pathogenesis of more common inflammatory arthritides, such as rheumatoid arthritis.