Psoriatic Arthritis (PsA) is a disorder of the immune system that affects the skin and joints, causing joint pain, stiffness, swelling and deformity. It typically appears in people who have psoriasis, a chronic disorder characterised by scaly, reddish skin rash that usually appears on the elbows, shins and scalp. The nails may appear thickened, rugged and cloudy.

A healthy immune system releases antibodies that fight off viruses and bugs that invade the body. In autoimmune disorders such as psoriatic arthritis, the body turns against its own tissues, sending white blood cells to attack and inflame the skin, the joint capsule (synovium) and the tendon around the joints. The commonly affected joints are the hand joints, wrists, elbows, knees, ankles and toes. 

Fingers may also inflame and swell up to be shaped like sausages (dactylitis). Over time, the inflammation invades the cartilage and bone, causing joint damage or even shortening of fingers in severe cases. It also affects the spine in some patients, causing stiffness and pain. As joint damage may occur early in the process of the disorder, diagnosing psoriatic arthritis as quickly as possible and treating it properly is important.

PsA is a systemic disorder that can affect other organs in the body, like the eye, the heart valve and the bowel. The inflammation is chronic and can lead to heart disease.

PsA affects men and women equally. The peak age of onset is between 30 and 55 years old. It can be diagnosed during childhood too. Most people develop psoriasis first, then arthritis.