About 2.5% of the Caucasian population has the chronic skin condition called psoriasis, while fewer people of African descent have psoriasis. 10% to 30% of Canadians with psoriasis will also suffer from inflammation, pain, or disability in the joints. When this is the case, they have a condition called psoriatic arthritis (PsA), a joint disease related to psoriasis.
Psoriatic arthritis typically appears when people are between the ages of 20 and 50, and it is equally common in men and women, and can occur at any age. Most people with psoriatic arthritis develop joint problems some months or years after the first skin symptoms appeared. But in about 15% of the cases, the arthritis appears before the psoriasis. Over 80% of psoriasis sufferers notice changes in the toenails or fingernails.
Psoriatic arthritis is part of a type of arthritis called spondylopathies, a category of arthritis that also includes ankylosing spondylitis and reactive spondylitis. These diseases tend to be asymmetrical (both sides of the body are not affected equally). Spondylopathies also affect the spine in characteristic ways.
Psoriasis and psoriatic arthritis are inflammatory conditions of the seronegative type. That means that blood autoantibodies (a type of antibody made by the immune system, which attacks a person's own cells and tissues) found in other autoimmune diseases are usually not found. Nevertheless, psoriasis belongs to the same basic class of diseases as rheumatoid arthritis, which is usually a seropositive disease.
Researchers don't know exactly what causes psoriatic arthritis, but they believe it is autoimmune in nature. In these conditions the body's immune system mistakenly attacks its own tissue. Most experts believe this is probably triggered by an infection or changes in the environment, though the exact mechanism as to how and why this occurs is still not known.
Genetics and a family history of psoriasis and psoriatic arthritis play a role in the condition. 40% of people with psoriasis have a first-degree relative with the same disease. Despite the strong genetic role, psoriatic arthritis sometimes appears in children with no family history of the disease. Up to 30% of people with psoriasis develop psoriatic arthritis as well.
Symptoms of psoriatic arthritis involve both skin and joint symptoms.
Skin symptoms include:
For more detailed information about skin symptoms, see the condition article on psoriasis.
Joint symptoms of psoriatic arthritis include pain, swelling, redness, and stiffness. Inflammation can occur in almost any joint but typically affects the joints of the fingers and toes, which may cause the fingers to swell and become sausage-shaped. Pain and swelling is also common where tendons and ligaments attach to bone (i.e., at the elbow or back of the heel) called enthesitis. Large joints are less commonly affected.
The patterns of joint involvement in psoriatic arthritis include:
The most common pattern of inflammation is the asymmetrical type. Basically, this term is used to distinguish psoriatic arthritis from rheumatoid arthritis, in which inflammation in one joint is often mirrored on the other side of the body. Asymmetrical arthritis affects joints in an apparently random fashion. Someone might have pain in the left shoulder and right knee, for example. On the other hand, it may be that the left shoulder and left knee are affected. There's no set pattern.
Most people with psoriatic arthritis do not have back pain but if they do have inflammation in the spine, usually find it worse at night and in the morning, and eases with activity during the day.
In very severe psoriatic arthritis, the shape of the joint and the surrounding bone and tissue can change greatly. When severe arthritis causes disfiguration it's called arthritis mutilans. Rheumatoid arthritis is more likely than psoriatic arthritis to cause such severe damage.
Psoriatic arthritis inflammation tends to rise and fall in severity. The remissions (symptom-free periods) tend to be more complete than in rheumatoid arthritis; the arthritis may totally disappear during these periods. In some people, joint pain tends to rise and fall with the degree of skin inflammation; in others, the two symptoms seem to occur entirely independent of one another.