Undifferentiated arthritis and reactive arthritis.

The terms undifferentiated arthritis and undifferentiated characterize arthritides that do not fit into well-known clinical disease categories (e.g., seronegative rheumatoid arthritis and reactive arthritis) and that are an early stage or forme fruste of a definite rheumatic disease, an overlap syndrome between such diseases, or an unknown, etiologically undefined disease that remains to be differentiated from other types of arthritis or spondylarthritis. Undifferentiated arthritis and undifferentiated spondylarthritis share some clinical features with reactive arthritides. Recent data suggest that, at least in Chlamydia-induced reactive arthritis, the triggering bacteria persist in affected joints for some time during the course of the disease in a viable but nonreplicative state, providing an antigenic stimulus for a bacteria-specific immune reaction in the joint. The clinical manifestations of reactive arthritis include not only Reiter's syndrome or clinically suspected postinfectious arthritis but also undifferentiated oligoarthritis and spondylarthritis. The optimal treatment remains to be defined, but there is increasing data that antibiotic therapy is not as effective in cases of well-established reactive arthritis as has been suggested.