Psoriatic arthritis is a type of arthritis linked with psoriasis, a chronic skin and nail disease. Psoriasis causes red, scaly rashes and thick, pitted fingernails. Psoriatic arthritis is similar to rheumatoid arthritis (RA) in symptoms and joint swelling (inflammation). But it tends to affect fewer joints than RA. And it does not make the typical RA antibodies.
Although psoriasis usually precedes the onset of arthritis, arthritis precedes (by up to 2 years) or occurs simultaneously with the skin disease in approximately 20% of cases.
Symptoms and Signs
The patterns or subsets of psoriatic arthritis include the following:
- A symmetric polyarthritis that resembles rheumatoid arthritis. Usually, fewer joints are involved than in rheumatoid arthritis.
- An oligoarticular form 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 monarticular, and often the joint involvement is asymmetric. Pitting of the nails and onycholysis frequently accompany DIP involvement.
- A severe deforming arthritis (arthritis mutilans) in which osteolysis is marked.
- 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 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 mono- or oligoarthritis. Nail pitting is sometimes a clue. “Sausage” swelling of one or more digits is a common manifestation of enthesopathy in psoriatic arthritis.
Laboratory studies show an elevation of the ESR; 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, 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.
NSAIDs are usually sufficient for mild cases. For patients with peripheral arthritis, methotrexate (7.5–20 mg orally once a week) is generally considered the drug of choice for patients who have not responded to NSAIDs; methotrexate is not effective for axial arthritis. Methotrexate can improve both the cutaneous and arthritic manifestations.
For patients with axial disease or peripheral arthritis that is refractory to methotrexate, treatment with a TNF inhibitor (at doses similar to the treatment of ankylosing spondylitis) is usually effective for both arthritis and psoriatic skin disease. Patients who do not respond to TNF inhibitors can be treated with ustekinumab, a monoclonal antibody that inhibits IL-12 and IL-23, or secukinumab, which inhibits IL-17.
Apremilast, an oral phosphodiesterase-4 inhibitor, is an option for patients who cannot use, or choose not to use, biologic agents. Corticosteroids are less effective in psoriatic arthritis than in other forms of inflammatory arthritis and may precipitate pustular psoriasis during tapers. Successful treatment directed at the skin lesions alone (eg, by PUVA therapy) occasionally is accompanied by an improvement in peripheral articular symptoms.