Arthritis, Psoriatic

KEY FEATURES

Essentials of Diagnosis

• Psoriasis precedes arthritis in 80% of cases

• Arthritis usually asymmetric, with "sausage" appearance of fingers and toes; polyarthritis that may resemble rheumatoid arthritis (RA) also occurs

• Sacroiliac joint involvement common

• Radiographic findings

  • Osteolysis

  • Pencil-in-cup deformity

  • Relative lack of osteoporosis

  • Bony ankylosis

  • Asymmetric sacroiliitis

  • Atypical syndesmophytes

General Considerations

• Although psoriasis usually precedes the onset of arthritis, arthritis may precede skin disease by up to 2 years or occur simultaneously in ∼20% of cases

• Arthritis is at least five times more common in patients with severe psoriatic skin disease than in those with only mild skin findings

CLINICAL FINDINGS

• The patterns or subsets of psoriatic arthritis include

  • A symmetric polyarthritis that resembles RA but usually, fewer joints are involved

  • An oligoarticular form that may lead to considerable destruction of the affected joints

  • The distal interphalangeal (DIP) joints are primarily affected

    • Early, this may be monarticular

    • Joint involvement is often asymmetric

    • Pitting of the nails and onycholysis frequently accompany DIP involvement

  • A severe deforming arthritis (arthritis mutilans) with osteolysis

  • A spondylitic form in which sacroiliitis and spinal involvement predominate; 50% of these patients are HLA-B27-positive

• Severe psoriatic skin disease is likely

• Occasionally, however, patients may have a single patch of psoriasis (typically hidden in the scalp, gluteal cleft, or umbilicus) and are unaware of its presence a detailed search for cutaneous lesions is essential

• Psoriatic lesions may have cleared when arthritis appears; in such cases, the history is most useful in diagnosing previously unexplained cases of mono- or oligoarthritis

• Nail pitting is sometimes a clue

• "Sausage" swelling, or dactylitis, of one or more digits is a common manifestation of enthesopathy in psoriatic arthritis

• Uveitis or inflammatory bowel disease (IBD) (or both) may be present

DIAGNOSIS

Laboratory Tests

• Elevated erythrocyte sedimentation rate in ∼50% of patients

• Rheumatoid factor and anti-CCP antibodies are not present

• Uric acid levels

  • May be high, reflecting the active turnover of skin affected by psoriasis

  • There is a correlation between the extent of psoriatic involvement and the level of uric acid, but gout is no more common than in patients without psoriasis

• Desquamation of the skin may also reduce iron stores

Imaging Studies

• Radiographic findings are most helpful in distinguishing the disease from other forms of arthritis

• There are marginal erosions of bone and irregular destruction of joint and bone, which, in the phalanx, may give the appearance of a sharpened pencil

• Fluffy periosteal new bone may be marked, especially at the insertion of muscles and ligaments into bone

• Such changes will also be seen along the shafts of metacarpals, metatarsals, and phalanges

• Psoriatic spondylitis causes asymmetric sacroiliitis and syndesmophytes