A 46-Year-Old Man With Spine Pain and a Rash

Herbert S. Diamond, MD, MACP


February 26, 2020

Gastrointestinal symptoms are more common in patients with psoriatic arthritis than in the general population. Eye disease and aortic valve disease are less common than in ankylosing spondylitis. In contrast to rheumatoid arthritis, other systemic features are atypical. Subcutaneous nodules do not occur, and other organ involvement is rare.

Laboratory studies are not helpful in establishing the presence of psoriatic arthritis, with no tests being diagnostically specific for the disease. A mild anemia may be present, but white blood counts are generally normal. RF and ANA test findings are positive only at the same frequency as in the healthy population. Uric acid levels may be elevated in patients with extensive skin disease but is usually normal. Similarly, the erythrocyte sedimentation rate and C-reactive protein levels also are often normal, although they may be elevated in some cases. Synovial fluid shows nonspecific inflammation with an elevated neutrophil count. Skin biopsy with pathology can establish the diagnosis of psoriasis, but biopsy is not required in most cases.

Radiologic findings can help to establish the diagnosis of psoriatic arthritis.[4] Features of the condition that distinguish it from rheumatoid arthritis include asymmetrical joint involvement, DIP joint disease, periosteal new bone formation, pencil-in-cup erosions (Figure 3), joint fusion, sacroiliac and lumbar spine involvement, and absence of periarticular osteoporosis.[1,3,4]

Figure 3.

In rheumatoid arthritis, symmetrical arthritis, sparing of the DIP joints and lumbosacral spine, and juxta-articular osteoporosis on radiography are common. Less tendon sheath and entheseal involvement is associated with rheumatoid arthritis, patients are usually RF-positive, and skin rash is not a feature. Ankylosing spondylitis, an RF-negative disease, can involve the entire spine; peripheral arthritis is generally limited to large central joints, such as the hip and knee. Wrist, ankle, or DIP joint involvement is unlikely in ankylosing spondylitis, as is rash.

The differential diagnosis of psoriatic arthritis also includes gout and osteoarthritis. Gout is characterized by an episodic, self-limited, acute, monoarticular or pauciarticular arthritis; the spine and sacroiliac joints are rarely involved, and no associated rash is present. Uric acid crystals are present in the fluid from affected joints. In osteoarthritis, inflammation is usually absent, wrist and ankle involvement is unusual, inflammation is generally absent, and no rash occurs.


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