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. Psoriasis often affects the tips of the elbows and knees, the scalp, the navel, and around the genital areas or anus. 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. 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 twenty years before psoriasis eventually appears! Conversely, patients can have psoriasis for over 20 years prior to development of arthritis, leading to the ultimate diagnosis of psoriatic arthritis.

Psoriatic arthritis is a systemic rheumatic disease that can also 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 ankylosising spondylitis, reactive arthritis (formerly 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."

The cause of psoriatic arthritis is currently unknown. A combination of genetic and immune as well as environmental factors are likely involved. In patients with psoriatic arthritis who have arthritis of the spine, a gene marker named HLA-B27 is frequently, but not always, found. Blood testing is now available to test for the HLA-B27 gene. Several other genes have also been found to be more common in patients with psoriatic arthritis. Certain changes in the immune system may also be important in the development psoriatic arthritis. For example, the decline in the number of immune cells called helper T cells in AIDS patients may play a role in the development and progression of psoriasis in these patients. The importance of infectious agents and other environmental factors in the cause of psoriatic arthritis is being investigated by researchers.

In most patients, the psoriasis precedes the arthritis by months to years. The arthritis frequently involve the knees, ankles, and joints in the feet. Usually, only a few joints are inflamed at a time. The inflamed joints become painful, swollen, hot, and red. Sometimes, joint inflammation in the fingers or toes can cause swelling of the entire digit, giving them the appearance of a "sausage."

Joint stiffness is common, and is typically worse early in the morning. Less commonly, psoriatic arthritis may involve many joints of the body in a symmetrical fashion, mimicking the pattern seen in rheumatoid arthritis. Psoriatic arthritis can also cause inflammation of the spine (spondylitis) and the sacrum, causing pain and stiffness in the low back, buttocks, neck and upper back. In approximately 50% of those with spondylitis, the genetic marker HLA-B27 can be found. In rare instances, psoriatic arthritis involves the small joints at the ends of the fingers. A very destructive form of arthritis, called "mutilans," can cause rapid damage to the joints. Fortunately, this form of arthritis is rare in patients with psoriatic arthritis.

Patients with psoriatic arthritis can also develop inflammation of the tendons (tendinitis) and around cartilage. Inflammation of the tendon behind the heel causes Achilles tendinitis, leading to pain with walking and climbing stairs. Inflammation the of chest wall and of the cartilage that links the ribs to the breastbone (sternum) can cause chest pain, as seen in costochondritis.

Aside from arthritis and spondylitis, psoriatic arthritis can cause inflammation in other organs, such as the eyes, lungs, and aorta. Inflammation in the colored portion of the eye (iris) causes iritis, a painful condition that can be aggravated by bright light as the iris opens and closes the opening of the pupil. Corticosteroids injected directly into the eyes are sometimes necessary to decrease inflammation and prevent blindness. Inflammation in and around the lungs (pleuritis) causes chest pain, especially with deep breathing, as well as shortness of breath. Inflammation of the aorta (aortitis) can cause leakage of the aortic valve valves, leading to heart failure and shortness of breath.

Acne and nail changes are commonly seen in psoriatic arthritis. Pitting and ridges are seen in finger and toe nails of 80% of patients with psoriatic arthritis. Interestingly, these characteristic nail changes are observed in only a minority of psoriasis patients who do not have arthritis. Acne has been noted to occur in higher frequency in patients with psoriatic arthritis. In fact, a new syndrome has been described, characterized by inflammation of the joint lining (synovitis), acne and pustules on the feet or palms, thickened and inflamed bone (hyperostosis), and bone inflammation (osteitis). This syndrome is therefore given the eponym SAPHO syndrome.

Psoriatic arthritis is a diagnosis made mainly on clinical grounds, based on a finding of psoriasis and the typical inflammatory arthritis of the spine and/or other joints. There is no laboratory test to diagnose psoriatic arthritis. Blood tests such as sedimentation rate may be elevated and merely reflect presence of inflammation in the joints and other organs of the body. Other blood tests such as rheumatoid factor are obtained to exclude rheumatoid arthritis. When one or two large joints (such a knees) are inflamed, arthrocentesis can be performed.

Arthrocentesis is a office procedure whereby a sterile needle is used to withdraw (aspirate) fluid from the inflamed joints. The fluid is then analyzed for infection, gout crystals, and other inflammatory conditions. X-rays may show changes of cartilage or bone injury indicative of arthritis of the spine, sacroiliac joints, and/or joints of the hands. Typical x-ray findings include bony erosions resulting from arthritis. The blood test for the genetic marker HLA-B27, mentioned above, can be found in over 50% of patients with psoriatic arthritis who have spine inflammation.