Psoriatic arthritis is a chronic disease characterized by inflammation of the skin (psoriasis) and joints (arthritis). Psoriasis is a common skin condition affecting 2% of the Caucasian population in the United States. It features patchy, raised, red areas of skin inflammation with scaling. Approximately 10% of patients who have psoriasis also develop an associated inflammation of their joints. Patients who have inflammatory arthritis and psoriasis are diagnosed as having psoriatic arthritis.
The onset of psoriatic arthritis generally occurs in the fourth and fifth decades of life. Males and females are affected equally. The skin disease (psoriasis) and the joint disease (arthritis) often appear separately. In fact, the skin disease precedes the arthritis in nearly 80% of patients. However, the arthritis may precede the psoriasis in up to 15% of patients. In some patients, the diagnosis of psoriatic arthritis can be difficult if the arthritis precedes psoriasis by many years. In fact, some patients have had arthritis for over 20 years before psoriasis eventually appears. Conversely, patients can have psoriasis for over 20 years prior to the development of arthritis, leading to the ultimate diagnosis of psoriatic arthritis.
Psoriatic arthritis is a systemic rheumatic disease that also can cause inflammation in body tissues away from the joints other than the skin, such as in the eyes, heart, lungs, and kidneys. Psoriatic arthritis shares many features with several other arthritic conditions, such as ankylosing spondylitis, reactive arthritis (formerly known as Reiter's syndrome), and arthritis associated with Crohn's disease and ulcerative colitis. All of these conditions can cause inflammation in the spine and other joints, and the eyes, skin, mouth, and various organs. In view of their similarities and tendency to cause inflammation of the spine, these conditions are collectively referred to as "spondyloarthropathies."