How Common is Psoriatic Arthritis?About a third of patients with psoriasis have some degree of joint stiffness. A true inflammatory arthritis affects 6 to 10% of patients with moderate to severe psoriasis. There is some genetic overlap in the two diseases, but there are also some genetic markers that are more common in PsA than psoriasis.
What Triggers Psoriatic Arthritis?It is not known exactly what causes PsA, but there are several theories: "deep" Koebner phenomenon, emotional stress, and bacterial infections have all been proposed as triggers. Like psoriatic skin, the joint space in psoriatic arthritis shows high levels of TNF-alpha. Biologic drugs that block TNF-alpha are extremely effective in treating PsA.
Risk Factors for Psoriatic ArthritisSeveral factors have been correlated with PsA. It is more common in Caucasian people and those who have a relative with PsA. Although children can be afflicted with PsA, adults between the ages of 30 to 50 are at highest risk. Being HIV positive increases the likelihood of PsA.
The Patterns of Psoriatic ArthritisFive distinct patterns of arthritis are seen in PsA, and once a particular pattern in established in any given patient, it is unlikely to change:
- Symmetric Polyarthritis
This most common form of PsA can involve any joint, but it typically involves the knuckles near the hands, and is very similar to rheumatoid arthritis.
- Asymmetric Oligoarticular Arthritis
Typically, five or fewer joints in fingers or toes are inflamed creating a distinct "sausage digit" type of swelling known as dactylitis.
- Distal Interphalangeal Joint Arthritis
In this form, the joints further away from the knuckles are involved; psoriatic nail changes are seen in almost all cases.
- Arthritis Mutilans
This severe form of arthritis fortunately represents only 1 to 2% of PsA. It is more common in early onset psoriasis and has the worst prognosis, or outcome.
- Sacroiliitis and Spine Pain
X-ray findings of sacroiliitis (inflammation where the tailbone or sacrum meets the hipbone or ilium) help distinguish PSA from rheumatoid arthritis. X-ray findings similar to ankylosis spondylitis can also be seen, but lumbosacral stiffness and range of motion loss in PsA is not as severe.
Other Features of PsASometimes, there are other symptoms with PsA such as fever, fatigue and loss of appetite. PsA symptoms usually appear after skin psoriasis is evident, but less frequently occur at the same time or even before psoriasis of the skin appears. When PsA and psoriasis are occurding together, it is often difficult for a doctor to differentiate PsA from rheumatoid arthritis when blood test results are not conclusive(ie: rheumatoid factor or RF for rheumatoid arthritis is negative). Blood tests which may be altered in PsA include CBC, platelet counts, erythrocyte sedimentation rate, C-reactive Protein, and uric acid. In the near future, a new blood test for antibodies to something called "proteosomes" may be helpful in diagnosing PsA when and if it becomes commercially available.
Since there is no single specific confirmatory test for PsA, the diagnosis of this disease remains a clinical one which usually relies on the the overall findings as assessed by a rheumatologist.
Camisa C. Handbook of Psoriasis, 2nd Ed. Blackwell Publishing, USA 2004.