Psoriatic Arthritis
The term “psoriatic arthritis” denotes a heterogenous group of arthritides ranging from peripheral monarticular, oligoarticular and polyarticular disease, to axial skeletal involvement. Yet, despite this apparent clinical heterogeneity, these various presentations are unified in their occurrence in individuals with cutaneous manifestations of psoriasis, rheumatoid factor sero-negativity, similar human leukocyte antigen (HLA) associations and radiographic similarities.
*Images within this article are from the American College of Rheumatology Slide Collection.
Approximately 2% of the Caucasian population in North America has psoriasis. Of these, 5-7% are affected by an inflammatory arthritis in some form. Overall, men and women are affected with equal frequency, though the actual male:female ratio may vary depending upon the subset in question. The peak incidence is in the 4th through 6th decades. Psoriatic skin disease pre-dates the onset of arthritis in 70% of cases, presents coincident with arthritis in 15% of cases, and follows the onset of arthritis in 15% of cases.
We do not know the cause of either psoriasis or psoriatic arthritis. However, we do have the following clues to their etiology and pathophysiology:
- Peripheral joint activity in psoriatic arthritis parallels cutaneous activity in 1/3 of cases, whereas activity of the skin and axial skeletal disease are likely to be discordant.
- Genetic factors appear to play an important role. There is a 70% concordance for psoriasis in monozygotic twins. There is a 50-fold increased risk of developing psoriatic arthritis in first-degree relatives of patients with the disease. There is a 2-fold increased risk of disease “transmission” by an affected father as compared to an affected mother.
- There are epidemiologic associations with the expression of both class I and class II HLA alleles.
- Environmental factors have been implicated. Streptococcal infection can precipitate the development of guttate psoriasis. HIV infection can present with both psoriasis and psoriatic arthritis, as well as worsen existing disease. Physical trauma has been reported to precipitate the development of arthritis, suggesting that psoriatic arthritis is the manifestation of a "deep Koebner phenomenon".
- The inflammatory and autoimmune character of the disease is supported not only by the clinical presentation, but also by the role that T-cells and various cytokines have been demonstrated to play in both the initiation and perpetuation of disease activity.
Factors that may portend a worse prognosis include extensive skin involvement; a strong family history of psoriasis; female gender; disease onset at <20 years of age; expression of HLA-B27, -DR3 or -DR4 alleles; and polyarticular or erosive disease.
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*Images within this article are from the American College of Rheumatology Slide Collection.
