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Psoriatic arthritis

Dr Yuranga Weerakkody and Dr Alexandra Stanislavsky et al.

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

Epidemiology

Overall prevalence is at around 0.1 - 1%, however it affects up to 30% of patients with psoriasis ¹. In contrast to many other arthropathies, there is no gender predilection in psoriatic arthritis.

Pathogenesis

Both environmental and genetic factors are thought to play a role. Up to 60% are HLA-B27 positive ².

Clinical features

Dermatological features of psoriasis precede arthritis in 90% ³. There is a strong association with nail involvement, particularly for DIP joint arthritis.

Radiographic features

The hallmark of PsA is the combination of erosive change with bone proliferation, in a predominantly distal distribution (i.e. interphalangeal more than MCP 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.

Imaging findings include :

enthesitis and marginal bone erosions. "pencil-in-cup" deformities are common, but not pathognomonic for PsA ³.
joint subluxation or interphalangeal ankylosis may be present.
bone proliferation results in an irregular, “fuzzy” appearance to the bone around the affected joint ²
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
arthritis mutilans : a severe form of either PsA or rheumatoid arthritis caused by marked bony resorption and the consequent collapse of soft tissue; when this affects the hands, it can cause a phenomenon sometimes referred to as "telescoping fingers."
“ivory phalanx” : classically involving the distal phalanx of the great toe
sacroiliitis : which is often asymmetrical
spondylitis : asymmetric paravertebral ossifications and relative sparing of the facet joints.

Differential diagnosis

General imaging differential considerations include

rheumatoid arthritis
- there is a MCP joint predominance in rheumatoid arthritis (RA) vs interphalangeal predominant distribution in PsA
- bone proliferation not a feature in RA
- osteoporosis not a feature in PsA
erosive osteoarthritis
- “gull wing” central erosions are present in erosive OA vs “mouse ears” peripheral bare area erosions in PsA
reactive arthritis (Reiter syndrome)
- tends to involve feet > hands