Seronegative arthropathies usually involve only a few joints, are less severe than rheumatoid arthritis, and tests for rheumatoid factor are negative, (hence the term "seronegative"). Psoriatic athritis, the most important type in elderly people, is a specific type of arthritis seen in association with psoriasis. Other disorders include reactive arthritis, colitic arthritis in inflammatory bowel disease, and the peripheral arthritis of anklyosing spondylitis, a disorder characterized by involvement of the spine.
Classification and causes. Unifying themes are the presence of an infective trigger and the genetic risk associated with HLA-B27, one of the genes found in normal human white blood cells. This is found 95 percent of cases of ankylosing spondylitis and less often in the other disorders.
Ankylosing spondylitis, colitic arthritis, and reactive arthritis usually begin in early adult life; in the elderly the predominant clinical concern is their late consequences, mainly due to joint failure. Psoriatic arthritis may present de novo in the elderly.
Clinical features. There are a number of clinical patterns of psoriatic arthritis, including oligoarthritis (arthritis that involves only a few joints); a symmetrical polyarthritis, often indistinguishable from rheumatoid arthritis; distal arthritis of the distal interphalangeal joints; and arthritis mutilans, a rare cause of severe joint damage. Other forms of seronegative arthritis predominantly involve oligoarthritis.
Investigations. There are no specific laboratory tests. Acute-phase reactants like the ESR and C-reactive protein may be elevated. Rheumatoid factor is usually negative. X-rays may show marginal erosions. Isolated destruction of individual joints with pencil and cup deformities suggests psoriatic arthritis. Axial disease with sacroilitis and spinal fusion characterize ankylosing spondylitis.