Psoriatic arthritis is a form of arthritis that accompanies the skin rash of psoriasis in approximately 20% of people with that condition. This form of arthritis can vary from relatively mild disease to severe, deforming arthritis. It can cause arthritis of peripheral joints such as fingers and toes but can also cause arthritis in the spine and painful tendonitis. It's severity does not necessarily correlate with the severity of the skin disease. It's treatment is similar to the treatment of rheumatoid arthritis. There are medications, generally prescribed by a rheumatologist, that can help with symptoms and prevent progression of joint deformity.