General Considerations

Psoriatic arthritis is defined as arthritis, spondylitis, or dactylitis occurring in association with the skin disease psoriasis (PsO). The recent CASPAR criteria for PsA (Box 69-1) dramatically explain the spectrum of this disorder by allowing the inclusion of patients who do not currently have psoriasis, but have a past history of psoriasis or a family history of psoriasis.⁷ The criteria also recognize the importance of changes in the nails, as well as changes in the skin.

Box 69-1

From Taylor WJ, Gladman D, Helliwwell P et al: Classification criteria for psoriatic arthritis; development of new criteria from a large international study. Arth Rheum 54(8):2665–2673, 2006.

Caspar Criteria for Psoriatic Arthritis

Established inflammatory articular disease (joint, spine, or entheseal) with three or more of the following:

1 Psoriasis

• Current: Psoriatic skin or scalp disease present today as judged by a physician

• History: History of psoriasis that may be obtained from patient, family physician, dermatologist, or rheumatologist

• Family history: History of psoriasis in a first- or second-degree relative according to patient report

2 Psoriasis: Typical psoriatic nail dystrophy including onycholysis, pitting, and hyperkeratosis observed on current physical examination

3 Negative test for rheumatoid factor (RF): By any method except latex but preferably by enzyme-linked immunosorbent assay (ELISA) or nephelometry, according to the local laboratory reference range

4 Dactylitis

• Current: Swelling of an entire digit

• History: History of dactylitis recorded by a rheumatologist

5 Radiologic evidence of juxta-articular new bone formation: Ill-defined ossification near joint margins (but excluding osteophyte formation) on plain radiographs of hand or foot

Epidemiology

The National Psoriasis Foundation has estimated that PsO affects 1% to 3% of the population.^3,⁴ Estimates of the prevalence of PsA in patients with PsO are 15% to 39%, with the higher estimates being based on patient series in which patients have actually been examined. As a consequence, PsA is likely a more common disorder than rheumatoid arthritis.

Unlike rheumatoid arthritis, which affects women more frequently than men, the incidence of PsA is equal between men and women and the disorder may present at any age. PsO usually precedes the appearance of PsA, but, in up to 15%, the arthritis may precede the psoriasis, making the diagnosis more difficult. In patients in whom the skin disease occurs first, the mean delay until the onset of PsA is 10 years.

Although there is no linear correlation between severity of skin disease and severity of arthritis, patients with more severe psoriasis are more likely to develop arthritis. Patients with psoriatic nail changes often have distal interphalangeal joint (DIP) arthritis.

Patterns of Arthritis

Moll and Wright have described five patterns of PsA, which are listed in Box 69-2.⁵ Any given patient may present with a combination of arthritis, spondylitis, dactylitis, and enthesitis (insertional tendinitis).

The arthritis itself tends to be preferentially involves large joint, is lower extremity predominant, and is often asymmetrical (Fig. 69-1). It frequently involves the foot and ankle and may involve the axial skeleton. The arthritis is often palindromic, unlike the additive pattern seen in rheumatoid arthritis (RA). Spine involvement often begins in the cervical spine rather than the lumbar spine, which serves to distinguish it from ankylosing spondylitis. Insertional tendinitis is a prominent feature that may dominate the clinical presentation. The net effect of the disease is to cause stiffening rather than gnarling and the end result may be ankylosis of the spine or arthrofibrosis of the knee. Each of these criteria serve to distinguish it from rheumatoid arthritis. (Table 69-1).

Figure 69-1 Psoriatic arthritis: joint inflammation. A, Distal interphalangeal synovitis. B, Proximal interphalangeal and distal interphalangeal synovitis. C, Asymmetrical oligoarthritis. D, Dactylitis with nail changes.

Table 69-1 Contrasting Clinical Features of Rheumatoid Arthritis and Psoriatic Arthritis

Rheumatoid Arthritis	Psoriatic Arthritis
Female predominance	No gender predominance
Peak onset at 45-55 yr	Variable age of onset
Sporadic	Familial occurrence
Symmetrical	Asymmetrical
Upper extremity	Lower extremity
Small joint	Large joint
Polyarticular	Oligoarticular
No axial disease	Axial disease
Synovitis	Enthesitis

Radiographic Features

PsA may be distinguished from RA by its radiographic findings. These are summarized in Table 69-2. The earliest lesion of PsA is bone marrow edema, best demonstrated on T2 images. Figure 69-2 contrasts the magnetic resonance imaging (MRI) appearance of PsA with that of RA. As expected, the inflammation in PsA centers on the enthesis rather than the synovium as seen in RA.

Table 69-2 Radiographic Differences Between Rheumatoid Arthritis and Psoriatic Arthritis

Rheumatoid Arthritis	Psoriatic Arthritis
Periarticular osteoporosis	Periarticular osteosclerosis
Small marginal erosions	Atypical,large erosions
No periostitis	Periostitis
MCP, PIP changes; no DIP changes	DIP common
Bone resorption	Proliferative changes
Spine changes limited to C1-2	Sacroiliitis, spondylitis

Figure 69-2 Fat-suppressed MRI scan of psoriatic arthritis, enthesitis. A, Psoriatic arthritis (straight arrow, anterior patella; curved arrow, patellar tendon insertion; S, superior insertion of the posterior cruciate ligament; *, inferior insertion of the posterior cruciate ligament). B, Rheumatoid arthritis (E, knee effusion; arrows, vessels posterior to the distal femoral diaphysis).

(From McGonagle D, Gibbon W, O’Connor P, et al: Characteristic magnetic resonance imaging entheseal changes of knee synovitis in spondylarthropathy. Arthritis Rheum 1998;41:694–700.)

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