The Facts
About 2.5% of the Caucasian American population has the chronic skin condition called psoriasis, and fewer African Americans have it. 5% to 8% of people with psoriasis (over 1 million) 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, but it 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 (see "Symptoms and Complications"). They also affect the spine in characteristic ways.