Seasonal and meteorological determinants of aneurysmal subarachnoid hemorrhage: a systematic review and meta-analysis.

Many studies have assessed the relationships between seasonal or meteorological determinants and the occurrence of aneurysmal subarachnoid hemorrhage (SAH), but the data are conflicting. We systematically searched the literature and meta-analyzed data from all relevant articles when possible. We searched MEDLINE (1966-2011), EMBASE (1980-2011) and the Cochrane Library to identify all observational studies examining the relationship between seasonal and meteorological determinants (temperature, atmospheric pressure and relative humidity) and the occurrence of SAH. Two authors independently extracted data from articles that were included based on predefined criteria. We pooled relative risks (RR's) with corresponding 95 % confidence intervals (CI's) from the individual studies on season and month by means of the random effects method. We included 48 articles, totaling 72,694 patients. SAH occurred less often in summer than in winter (RR 0.89, 95 % CI 0.83-0.96), and was statistically significant more often in January than in the summer months of June-September. For atmospheric pressure seven of 17 studies found a significant association, six of 18 studies were significant for temperature, and three of 15 studies were significant for humidity, but the direction of these associations was conflicting and data on these determinants were too heterogeneous to pool. Seasons influence the occurrence of SAH, with SAH occurring less often in summer than in winter, and most often in January. The explanation for the seasonal differences remains uncertain, due to the lack of sound data on the influence of meteorological factors on SAH occurrence.