A disease of the immune system

While the exact cause of psoriatic arthritis has not been determined, it is believed that certain changes in the immune system are involved. The immune system normally protects your body against foreign invaders such as bacteria, fungi and viruses. But with psoriatic arthritis, the immune system loses its ability to tell the difference between these foreign invaders and the body's normal cells and begins to attack the body's cells, too. This causes inflammation in the lining and connective tissues of the joints, as well as scaling, redness, and inflammation on the skin.

The symptoms of PsA

Joint symptoms can include:

• Swelling of the joints of the hands, feet, knees, and ankles
• Inflammation of only a few joints at a time, but these joints can become painful, swollen, hot, and red. Sometimes joint inflammation in the fingers or toes can cause swelling, making them appear like "sausages"
• Joint stiffness, which is usually worst in the morning
• Pain and stiffness in the lower back, buttocks, neck, and upper back

The symptoms of PsA

Skin and nail symptoms can include:

• Dry, scaly, and inflamed skin
• Changes in the nails. Pitting and ridges are seen in the fingernails and toenails of 80% of patients with psoriatic arthritis

When symptoms appear

• The skin lesions and the arthritis symptoms often appear separately
• In approximately 67% of PsA patients, skin lesions appear before arthritis symptoms
• Skin symptoms tend to be present for years before the joint symptoms appear

Who gets PsA?

PsA affects up to 1% of Canadians. PsA occurs in up to 30% of the people who have psoriasis. It is much more common in those whose skin symptoms are severe. Psoriatic arthritis affects men and women equally. Onset generally occurs between the ages of 20 and 50.