The burden of autoimmune disease: a comparison of prevalence ratios in patients with psoriatic arthritis and psoriasis.

BACKGROUND

Current research has confirmed that many inflammatory autoimmune (AI) diseases are due to derangements in multiple cytokine pathways. Some of these cytokines appear to play comparable key roles across diseases, suggesting that a similar underlying systemic inflammatory cascade could be responsible for various disease states. A recent study supports the hypothesis of a common cytokine-based pathology by showing that having one AI disease significantly increased the risk of having another AI disease. Psoriasis is an AI, manifesting as a chronic inflammatory skin condition and is clearly associated with other conditions, most obviously psoriatic arthritis (PsA).

OBJECTIVE

We sought to examine whether patients with PsA enrolled in managed health care plans carry a higher AI disease burden than patients with cutaneous psoriasis only (PsO).

METHODS

Patients 18 years or older enrolled in a health claims database were classified by two clinical subtypes: PsA and PsO. Control subjects were matched 3:1 to patients with psoriasis on age, sex, census region, and length of previous medical insurance coverage. AI disease diagnoses were identified through International Classification of Diseases, Ninth Revision codes. The association of other AI diseases with each psoriasis cohort was assessed using a prevalence ratio.

RESULTS

PsO was associated with a higher prevalence ratio for the 3 gastrointestinal diseases: Crohn disease (1.6 [confidence interval {CI} 1.4-2.0]), ulcerative colitis (1.3 [CI 1.1-1.6]), and inflammatory bowel disease (1.4 [CI 1.2-1.6]). PsA was also associated with a higher prevalence ratio for the gastrointestinal diseases: Crohn disease (2.1 [CI 1.3-3.3]), ulcerative colitis (2.0 [CI 1.3-3.1]), and inflammatory bowel disease (1.8 [CI 1.3-2.5]). Patients with PsA had an increased prevalence ratio associated with giant cell arteritis (4.8 [CI 1.5-15.7]) and pulmonary fibrosis (1.9 [CI 1.2-3.0]).

LIMITATIONS

Detection and misclassification biases may have affected these findings.

CONCLUSIONS

These findings support the hypothesis that PsA and PsO are associated with development of other AI diseases. The data suggest that evaluating patients with psoriasis for other associated disorders in a prospective manner may be important, because they may be more likely to experience the onset of another AI disease. Treatment planning for these patients could, therefore, require the medical management of more than one AI disease. Further, our data suggest that PsA and PsO may be divergent in ways previously not described that could inform future research.