Medicine – Patterns of Disease in Psoriatic Arthropathy

Psoriatic arthropathy, more commonly called psoriatic arthritis (PsA), is a chronic inflammatory arthritis associated with psoriasis. It belongs to the spondyloarthritis group of disorders and can affect peripheral joints, the spine, sacroiliac joints, digits, and entheses. The pattern of joint involvement varies considerably between patients and may also change during the course of the disease. Traditionally, psoriatic arthritis has been divided into several characteristic patterns, including peripheral oligoarthritis or polyarthritis, spondylitis, distal interphalangeal joint disease, rheumatoid-like arthritis, and arthritis mutilans.

1. Peripheral Oligoarthritis or Polyarthritis

Peripheral oligoarthritis or polyarthritis is the most common pattern and has traditionally been reported in approximately 60% of patients. Oligoarthritis involves only a few joints, whereas polyarthritis affects a larger number of joints. The arthritis may involve both small and large peripheral joints and is often asymmetrical, particularly when relatively few joints are affected.

Patients may develop pain, swelling, stiffness, and reduced movement of the affected joints. Involvement of an entire finger or toe can produce dactylitis, giving the characteristic appearance of a “sausage digit.” Enthesitis, caused by inflammation where tendons or ligaments attach to bone, may accompany the peripheral arthritis.

2. Spondylitis

Approximately 15% of patients in traditional classifications have a predominantly spondylitic pattern. Inflammation affects the spine and sacroiliac joints, producing symptoms similar to other forms of axial spondyloarthritis. Patients may experience chronic inflammatory back pain, morning stiffness, and reduced spinal mobility.

Sacroiliitis may be asymmetrical, particularly during the earlier stages of psoriatic arthritis. Spinal involvement can progress and eventually produce significant restriction of movement. Some patients have axial disease together with peripheral arthritis.

3. Distal Interphalangeal Joint Disease

A predominantly distal interphalangeal (DIP) joint pattern has traditionally been described in approximately 10% of patients. The DIP joints are the joints closest to the fingernails and toenails, and their involvement is particularly characteristic of psoriatic arthritis.

DIP arthritis is frequently associated with psoriatic nail disease, including nail pitting, separation of the nail from the nail bed (onycholysis), and other dystrophic nail changes. The close anatomical relationship between the nail apparatus and DIP joint helps explain this association. DIP involvement can be useful in distinguishing psoriatic arthritis from rheumatoid arthritis, in which these joints are usually spared.

4. Rheumatoid-Type Polyarthritis

Approximately 10% of patients in older classifications develop a rheumatoid-like pattern of arthritis. This consists of a more symmetrical polyarthritis involving multiple small peripheral joints and may clinically resemble rheumatoid arthritis.

Despite the similarities, several features can suggest psoriatic arthritis, including coexisting psoriasis, nail changes, dactylitis, DIP joint involvement, and enthesitis. Most patients with psoriatic arthritis are also negative for rheumatoid factor, although rheumatoid factor positivity can occasionally occur and therefore does not completely exclude the diagnosis.

5. Arthritis Mutilans

Arthritis mutilans is the rarest and most severe traditional pattern, historically accounting for approximately 5% of cases. It is characterised by aggressive osteolysis and destruction of the small joints, particularly those of the hands and feet.

Progressive destruction of bone can result in marked shortening, instability, and deformity of the digits. Severe resorption of the phalanges may produce a characteristic “telescoping” appearance, in which the affected digit can be shortened or extended because of extensive loss of supporting bone. Although arthritis mutilans represents a particularly destructive manifestation of psoriatic arthritis, it is uncommon in modern clinical practice.

Key Clinical Pattern

The traditional patterns of psoriatic arthropathy can be remembered as peripheral oligoarthritis/polyarthritis (60%), spondylitis (15%), DIP-predominant disease (10%), rheumatoid-type polyarthritis (10%), and arthritis mutilans (5%). These categories can overlap, and an individual patient’s pattern of joint involvement may change as the disease progresses.