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

We thank Frösen et al. for their interest in our study.¹ As the readers speculated, the discrepancy between the findings of their hospital-based study² and our registry-based study¹ is attributable to the inherent selection bias of hospital-based subarachnoid hemorrhage (SAH) studies.³ Based on their response, we reanalyzed our data using only the data within the Tampere University Hospital catchment area and found a substantial decrease from 28% to 18% in the sudden death rate of SAH during our study period. Moreover, our well-validated registry-based data showed that the proportion of patients with SAH who were admitted to other hospitals in the area, and presumably treated conservatively outside of the neurosurgical unit, decreased from 17% to 4%. Consequently, the proportion of the catchment area patients with SAH admitted for neurosurgical care at Tampere University Hospital increased from 55% to 78%. In fact, when we included only patients with SAH who were admitted to the neurosurgical unit, similar to Frösen et al., we observed no age- or sex-adjusted incidence trends (p = 0.192).