Substantial Within-Country Variation in the Incidence of Subarachnoid Hemorrhage

OBJECTIVE: To study whether the incidence of subarachnoid hemorrhage (SAH) varies between geographic regions of Finland. METHODS: 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. RESULTS: 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. CONCLUSION: 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.