Psoriatic arthritis is a type of arthritis that affects people with severe psoriatic dermatitis. This causes joint inflammation and rashes. The areas most commonly affected are the fingers, neck and lower back. eye, nail and heart inflammation The rash usually starts before joint pain, but some people don't even realize they have a rash until they start to feel pain. Psoriasis can affect the nails, scalp, navel, and genital area. Psoriatic arthritis occurs most often between the ages of 30 and 50, equally in men and women.