Comorbidity and mortality in systemic sclerosis and matched controls: Impact of interstitial lung disease. A population based cohort study based on health registry data.

Objective: This population-based, matched cohort study evaluates the impact of comorbidities on mortality among systemic sclerosis (SSc) patients with and without interstitial lung disease (ILD).

Method: Patients with a first-time SSc diagnosis between 2002 and 2015 were identified in the Danish National Patient Registry, separated into two cohorts - with ILD (SSc-ILD) and without ILD (non-ILD SSc), and matched 1:4 with controls from the general population on age, sex, residency and marital status. Comorbidity and mortality data were obtained from national registries. The Deyo-Charlson comorbidity score (DCcs) was used for assessment of the burden of comorbidities.

Results: 1732 patients with SSc and 6919 controls were included; 258 (14.9%) patients had SSc-ILD. The hazard ratio (HR) for death was 2.8 (95% CI 2.4-3.3) in SSc, and especially increased in SSc-ILD (HR 4.2 (95% CI 3.2-5.4)), males (HR 3.1 95% CI 2.4-4.1) and younger adults (aged 18-40 (HR 6.9, 95% CI 3.4-14.2) and 41-50 (HR 7.7, 95% CI 3.8-15.6)). In non-ILD SSc, mortality increased with increasing DCcs. Cancer was the most frequent cause of death in SSc (24.9% of deaths) and in controls (33.5%), in SSc followed by musculoskeletal and connective tissue diseases (22.7%); the cause of only 0.8% of deaths among controls.

Conclusion: The high prevalence of comorbidities in SSc had extensive impact on mortality. Mortality was increased in males, in young adults and in SSc-ILD, underlining the excess mortality associated with ILD. These findings emphasise the importance of timely diagnosis and optimal management of organ involvement and comorbidities in SSc.