Incidence, prevalence and regional distribution of systemic sclerosis and related interstitial lung Disease: A nationwide retrospective cohort study

Objective To investigate incidence and prevalence of Systemic Sclerosis (SSc) and association with interstitial lung disease (SSc-ILD) in a nationwide population-based study. Methods Patients with an incident diagnosis of SSc in 2000-2016 were identified in the Danish National Patient Registry and categorised based on diagnosis of ILD. Incidence- and prevalence proportions were calculated based on the annual population estimates. A cox proportional hazards model was used to evaluate the association between age, sex, region and marital status and presence of ILD. Results In total, 1869 patients with SSc were identified; 275 patients (14.7%) had SSc-ILD. The majority of patients were females (75.5%). The percentage of males was higher in SSc-ILD than in SSc alone (30.9% and 23.4%, p = 0.008). Median time from SSc to ILD diagnosis was 1.4 years (range 0-14.2). ILD was diagnosed from <= 4 years before to >= 7 years after SSc. Development of ILD was associated with male gender (HR 1.75, 95% CI 1.15-2.66), age 41-50 (HR 1.81, 95% CI 1.07-3.05) and residency in the North Denmark Region (HR 1.95, 9 5% CI 1.12-3.40). Mean annual incidence proportion of SSc was 2.9/100,000 and mean annual prevalence proportion was 16.8/100,000. The incidence remained stable, but prevalence proportion increased from 14.1 - 16.5/100,000 in 2000-2008 to 17.9-19.2/100,000 in 2009-2016. Conclusion The prevalence of SSc increased during the study period, while the incidence remained stable. The prevalence of SSc-ILD was 14.7% and thus less frequent than expected. Male sex and age between 41 and 50 years were associated with ILD.