Psoriasis in Norway as observed in a population-based Norwegian twin panel.

Background Psoriasis is a chronic T-cell-mediated immunological skin disease. The occurrence of the disease appears to differ with geography and ethnicity. There is a need for epidemiological data obtained from defined population-based studies, and the sex-specific differences observed in the natural history of the disease require more attention. Objectives To describe the occurrence and risk of psoriasis in Norway by age and sex. Patients/methods A population-based health survey was conducted in 1998 in Norwegian twins aged 19-31 years. The present study is based upon the self-reported history of psoriasis among the 8045 questionnaire responders. Results Altogether, 334 (4.2%) reported a positive history of psoriasis. There were no sex differences in the overall prevalence rates, but significantly higher point-prevalences emerged in females in the teenage-year intervals. A fairly linear increase in incidence rates by every 4-year age-interval peaked at a lower age in females. The mean age at onset was also significantly lower in females (14.8 years) than in males (17.3 years). The absolute risk of developing psoriasis appeared higher for females across the entire age range. However, by the age of 31 the cumulative risks were similar in females and males (0.056 and 0.053, respectively). Conclusions In this historical cohort of Norwegian twins, we find a high prevalence of psoriasis in congruence with previously reported data among whites in north-western Europe. We have found sex-specific characteristics in point-prevalences and incidence rates which may contribute to the understanding of the earlier age at onset of the disease in females.