Subarachnoid hemorrhage incidence in the United States does not vary with season or temperature

Abstract

Background and purpose: Previous studies have suggested seasonal variations in rates of spontaneous rupture of intracranial aneurysms, leading to potentially devastating SAH. In an effort to identify a seasonal effect, variation in SAH incidence and in-hospital mortality rates were examined as they relate to admission month, temperature, and climate using HCUP's Nationwide Inpatient Sample.

Materials and methods: Cases of nontraumatic SAH and subsequent in-hospital mortality were extracted from the 2001-2008 NIS and associated with month of occurrence, local average monthly temperatures, and USDA climate zone. Multivariate regression analysis was used to study how admission month, temperature, and climate affected SAH admission and mortality rates.

Results: Among 57,663,486 hospital admissions from the 2001-2008 NIS, 52,379 cases of spontaneous SAH (ICD-9-CM 430) and 13,272 cases of subsequent in-hospital mortality were identified. SAH incidence and in-hospital mortality rates were not significantly correlated with a monthly/seasonal effect (incidence, χ(2) = 2.94, P = .99; mortality, χ(2) = 6.91, P = .81). However, SAH incidence significantly varied with climate (P < .0001, zones 11 and 7) but not with temperature (P = .1453), whereas average monthly temperature and climate had no significant correlation with in-hospital mortality (temperature, P = .3005; climate, P = .0863).

Conclusions: We identified no significant monthly or temperature-related effect in the incidence of SAH. Our data suggest that certain climate zones within the United States may be associated with significantly different SAH incidence, but the origins of these differences remain unclear and are probably unrelated to meteorologic variables.

Fig 1.

USDA hardiness zone map. Used with permission from the Arbor Day Foundation (http://www.arborday.org/media/zones.cfm).

Fig 2.

Monthly stratified relative rates of (A) admission frequency and (B) in-hospital mortality after admission for SAH. Relative rates of SAH admission and in-hospital mortality, stratified by month, were determined from regression models. RRs greater than 1.0 suggest that the event (admission or death) is more likely to occur relative to the reference month (Jan), whereas RRs less than 1.0 suggest the event is less likely to occur relative to the reference month. Significant differences relative to the reference month are represented by an asterisk (*).

Fig 3.

Temperature (A, C) and climate (B, D) stratified relative rates for (A, B) admission frequency and (D, E) in-hospital mortality after admission for SAH. Relative rates of SAH admission and in-hospital mortality, stratified by temperature and climate, were determined from regression models. RRs greater than 1.0 suggest that the event (admission or death) is more likely to occur relative to the reference variable (temperature strata [<30°F]; climate strata [USDA zone 3]), whereas RRs less than 1.0 suggest the event is less likely to occur relative to the reference variable. Significant differences relative to the reference temperature stratum or zone are represented by an asterisk (*).