Psoriatic arthritis

What it is

Psoriatic arthritis is an inflammatory arthritis associated with psoriasis, and it can vary in severity from mild stiffening and soreness of a joint to a joint completely destroyed. It occurs in 10-20% of patients with psoriasis and usually within 10 years of the skin changes, so the skin disease is present for months and the arthritis follows. It typically affects people in middle age but can occur at any age. It is part of the seronegative spondyloarthropathy group, alongside ankylosing spondylitis, enteropathic arthritis and reactive arthritis, which are the HLA-B27-associated conditions.

How it presents

There is no single pattern of affected joints. The recognised patterns are:

• Symmetrical polyarthritis, more common in women, presenting similarly to rheumatoid arthritis and affecting the hands, wrists, ankles and DIP joints, with the MCP joints less commonly affected.
• Asymmetrical pauciarthritis, affecting mainly the digits and the feet; pauciarthritis is when only a few joints are affected.
• Spondylitic pattern, more common in men, with back stiffness, sacroiliitis and atlanto-axial joint involvement.

The spine, the Achilles tendon and the plantar fascia can also be affected. As in the rest of the seronegative group, morning stiffness lasts more than 1 hour and the pain improves with exercise, and there is typically an asymmetrical oligoarthritis of large joints with enthesitis. The signs are:

• Plaques of psoriasis on the skin.
• Pitting of the nails and onycholysis, separation of the nail from the nail bed.
• Dactylitis, inflammation of the full finger, which is the sausage digit; the disease may target just one finger.
• Enthesitis, inflammation of the entheses, the points of insertion of tendons into bone.

Psoriatic arthritis targets the DIP joints of the hands, and characteristic nail changes are usually present.

The sequence and the site are given too: the patient has skin disease for months and then develops arthritis; the arthritis is asymmetrical and polyarticular and falls on the upper extremities; and it is characterised by the sausage digit, the pitting nail, and telescoping, which is the mutilans form. Its treatment there is the treatment for psoriasis.