Examination of seasonal variations in the incidence of subarachnoid hemorrhage

Purpose: While some reports have indicated that the incidence of subarachnoid hemorrhage (SAH) exhibits seasonal variation, others have yielded conflicting results. In the present study, we investigated whether differences in the number of cases, severity, and outcomes of SAH could be observed to vary with each month of the year. Methods: A total of 814 patients with SAH were admitted to our hospital from 2003 to 2016. The following variables were retrospectively examined each month during the study period: number of patients, age, sex, activities of daily living before onset, number of days from onset to admission, method of admission (presence/ absence of ambulance), grade at the time of admission, and outcome at discharge. Average monthly temperature and average high/low temperatures in this region were obtained from the Japan Meteorological Agency website. Results: SAH incidence was lowest in January and highest in October. There were no significant monthly differences in age, sex, activities of daily living before onset, number of days from onset to admission, method of admission, grade at the time of admission, or outcome at discharge. The difference between the average high and low temperatures was small in January, June, July, August, and September, yet large in March, April, May, October, and November. Conclusion: These findings suggest that SAH is more likely to occur when the temperature change is large, highlighting the need for more rigorous blood pressure management in patients with unruptured cerebral aneurysms at the turn of the season.