Increased incidence and prevalence of psoriasis in multiple sclerosis

Background: Psoriasis and multiple sclerosis (MS) share some risk factors, and fumarates are effective disease-modifying therapies for both psoriasis and MS, suggesting a common pathogenesis. However, findings regarding the occurrence of psoriasis in the MS population are inconsistent. Objectives: We aimed to estimate the incidence and prevalence of psoriasis in the MS population versus a matched cohort from the general population. Methods: We used population-based administrative data from the Canadian province of Manitoba to identify 4911 persons with MS and 23,274 age-, sex-and geographically-matched controls aged 20 years and older. We developed case definitions for psoriasis using ICD-9/10 codes and prescription claims. These case definitions were compared to self-reported psoriasis diagnoses. The preferred definition was applied to estimate the incidence and prevalence of psoriasis over the period 1998-2008. We used multivariable Cox regression to estimate the risk of psoriasis in the MS population at the individual level, adjusting for sex, age at the index date, socioeconomic status and physician visits. Results: In 2008, the crude incidence of psoriasis per 100,000 person-years was 466.7 (95% CI: 266.8-758.0) in the MS population, and 221.3 in the matched population (95% CI: 158.1-301.4). The crude prevalence of psoriasis per 100,000 persons was 4666.1 (95% CI: 3985.2-5429.9) in the MS population, and 3313.5 (95% CI: 3057.4-3585.3) in the matched population. The incidence and prevalence of psoriasis rose slightly over time. After adjusting for sex, age at the index date, socioeconomic status and physician visits, the risk of incident psoriasis was 54% higher in the MS population (HR 1.54; 95% CI: 1.07-2.24). Conclusion: Psoriasis incidence and prevalence are higher in the MS population than in the matched population.