Up to 8 percent of people who have psoriasis (a skin condition that is characterized by scaling and swelling) develop psoriatic arthritis. In a person with psoriatic arthritis, the joints (and sometimes other areas of the body) become inflamed. Some other symptoms of psoriatic arthritis include low back pain or neck pain; pitting, thickened, and discolored toenails or fingernails; and eye pain and redness. Although research scientists do not know the exact cause or causes of psoriatic arthritis, they believe that the condition is caused by a combination of environmental, genetic, and immune system factors. Treating psoriatic arthritis can involve lifestyle changes, medications, and routine monitoring and ongoing care.