The patterns or subsets of joint involvement in psoriatic arthritis include the following:
A symmetric polyarthritis that resembles rheumatoid arthritis. Usually, fewer joints are involved than in rheumatoid arthritis.
An oligoarthritis that may lead to considerable destruction of the affected joints.
A pattern of disease in which the DIP joints are primarily affected. Early, this may be monoarticular, and often the joint involvement is asymmetric. Pitting of the nails and onycholysis frequently accompany DIP involvement.
A severe deforming arthritis (arthritis mutilans) with osteolysis.
A spondylitic form in which sacroiliitis and spinal involvement predominate; 50% of these patients are HLA-B27 positive.
Arthritis is at least five times more common in patients with severe psoriatic skin disease than in those with only mild skin findings. Occasionally, however, patients may have a single patch of psoriasis (typically hidden in the scalp, gluteal cleft, or umbilicus) and are unaware of its presence. Thus, a detailed search for cutaneous lesions is essential in patients with arthritis of new onset. Also, the psoriatic lesions may have cleared when arthritis appears—in such cases, the history is most useful in diagnosing previously unexplained cases of monoarthritis or oligoarthritis. Nail pitting is sometimes a clue. “Sausage” swelling, or dactylitis, of one or more digits is a common manifestation of enthesopathy in psoriatic arthritis.
The ESR is elevated in approximately 50% of patients with psoriatic arthritis; normal values do not rule out the diagnosis. Rheumatoid factor and anti-CCP antibodies are not present. Uric acid levels may be high, reflecting the active turnover of skin affected by psoriasis. There is a correlation between the extent of psoriatic involvement and the level of uric acid, but gout is no more common than in patients without psoriasis. Desquamation of the skin may also reduce iron stores.
Radiographic findings are most helpful in distinguishing the disease from other forms of arthritis. There are marginal erosions of bone and irregular destruction of joint and bone (eFigure 20–26), which, in the phalanx, may give the appearance of a sharpened pencil. Fluffy periosteal new bone may be marked, especially at the insertion of muscles and ligaments into bone. Such changes will also be seen along the shafts of metacarpals, metatarsals, and phalanges. Psoriatic spondylitis causes asymmetric sacroiliitis and syndesmophytes, which are coarser than those seen in ankylosing spondylitis. In psoriatic arthritis as in ankylosing spondylitis, MRI is more sensitive in detecting axial abnormalities than plain radiographs, especially in the first few years of disease onset. Ultrasonography and MRI are more sensitive than conventional radiographs in detecting peripheral arthritis, enthesitis, and dactylitis.
Erosions at the distal (and proximal) interphalangeal joints in a patient with psoriatic arthritis. (Used, with permission, from Nicole Richman, MD.)