Substantial Within-Country Variation in the Incidence of Subarachnoid Hemorrhage: A Nationwide Finnish Study.

Abstract

To study whether the incidence of subarachnoid hemorrhage (SAH) varies between geographic regions of Finland. By utilizing the nationwide Causes of Death and Hospital Discharge Registers, we identified all first-ever, hospitalized, and sudden-death (dying before hospitalization) SAH events in Finland between 1998 and 2017. Based on the patients' home residence, we divided SAHs into 5 geographic regions: southern, central, western, eastern, and northern Finland. We calculated crude and European age-standardized (European Standard Population [ESP] 2013) SAH incidence rates for each region and used a Poisson regression model to calculate age-, sex-, and calendar year-adjusted incidence rate ratios (IRRs) and 95% confidence intervals for regional and time-dependent differences. During the total 106,510,337 cumulative person-years, we identified 9,443 first-ever SAH cases, of which 24% resulted in death before hospitalization. As compared to western Finland, where the SAH incidence was the lowest (7.4 per 100,000 persons), the ESP-standardized SAH incidence was 1.4 times higher in eastern (10.2 per 100,000 persons; adjusted IRR, 1.37 [1.27-1.47]) and northern Finland (10.4 per 100,000 persons; adjusted IRR, 1.40 [1.30-1.51]). These differences were similar when men and women were analyzed independently. Although SAH incidence rates decreased in all 5 regions over 2 decades, the rate of decrease varied significantly by region. SAH incidence appears to vary substantially by region in Finland. Our results suggest that regional SAH studies can identify high-risk subpopulations, but can also considerably over- or underestimate incidence on a nationwide level.