Circannual and latitudinal variation in the incidence of subarachnoid hemorrhage

Numerous epidemiological studies have shown a preponderance of cerebrovascular accidents occurring during the winter or colder months in both hemispheres. However, many of these studies used no or inappropriate statistical tests to demonstrate cyclic trends or relied on a small number of poorly documented cerebrovascular accident cases. Data were obtained for this study from the International Cooperative Aneurysm Study that evaluated 3,521 clinical records from 68 neurosurgical centers in 14 countries. We used the Edwards cyclic trends method to examine the monthly occurrence of subarachnoid hemorrhage between March 1981 and February 1983. Significant (p < 0.05) seasonal variation was noted with more subarachnoid hemorrhage cases occurring during the winter relative to the summer in the northern hemisphere, whereas the reverse of this was noted in the southern hemisphere. Significant (p < 0.025) seasonal periodicity was noted above the latitude of 43.2° N, but not below it. This report, and others, suggest that subarachnoid hemorrhage may be a seasonal and geographic risk disease. An overview of the literature examining seasonality and cerebrovascular accidents is also presented.