Psoriatic arthritis

Last revised by Rohit Sharma on 22 Mar 2024

Psoriatic arthritis (PsA) is inflammatory arthritis associated with psoriasis. It is usually negative for rheumatoid factor and hence classified as one of the seronegative spondyloarthritides.

Overall prevalence is ~0.5% (range 0.1-1%), however, it affects up to ~25% (range 6-41%) of patients with psoriasis 1,11. In contrast to many other arthropathies, there is no gender predilection in psoriatic arthritis. The median age of diagnosis is 48 years 11.

Psoriatic arthritis is associated with 11:

Dermatological features of psoriasis precede arthritis in ~65% (range 60-70%) whereas arthritic symptoms proceed dermatological features in 15-20% 11. There is a strong association with nail involvement, particularly for distal interphalangeal joint arthritis. It most commonly presents as an asymmetrical oligoarthritis with spondylitis common; oligoarthritis may progress to polyarthritis in the clinical course of the disease 10,11.

Both environmental and genetic factors are thought to play a role. Up to 60% are HLA-B27 positive 2. A proportion of patients have serum rheumatoid factor 6.

Extra-articular manifestations are common 11:

One of the classification systems is the one by Moll and Wright, which classifies psoriatic arthritis into five subtypes 7:

  1. symmetric polyarthritis (similar in appearance to RA)

  2. asymmetric mono- or oligoarthritis

  3. spinal column involvement (spondylitis)

  4. distal interphalangeal arthritis of the hands and feet

  5. arthritis mutilans

The hallmark of psoriatic arthritis is the combination of erosive change with bone proliferation, in a predominantly distal distribution (e.g. interphalangeal more than metacarpophalangeal joints). The disease most commonly involves the hands, followed by feet. It can also affect sacroiliac joints and spine. Knees, elbows, ankles, and shoulders are less frequently involved 2.

In the hands and feet, the pattern of distribution may be that of a symmetric polyarthropathy, or asymmetric oligoarthropathy, with a distal predominance. 

Imaging findings include:

  • enthesitis and marginal bone erosions; "pencil-in-cup" deformities are common, but not pathognomonic for PsA 3

  • bone proliferation results in an irregular, “fuzzy” appearance to the bone around the affected joint 2

  • joint subluxation or interphalangeal ankylosis may be present

  • periostitis: may appear as a periosteal layer of new bone, or as irregular thickening of the cortex itself 2

  • dactylitis: which can present as a “sausage digit” which refers to soft tissue swelling of a whole digit; ultrasound examination of a sausage digit demonstrates underlying synovitis and tenosynovitis 4

  • acro-osteolysis 12

  • arthritis mutilans: osteolysis and articular collapse can cause a phenomenon referred to as "telescoping fingers"

  • ivory phalanx: classically involving the distal phalanx of the great toe

  • sacroiliitis: often asymmetrical

  • spondylitis: asymmetric paravertebral ossifications and relative sparing of the facet joints

General imaging differential considerations include:

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