Association of High and Low Temperature with Hospital Admissions for Subarachnoid Hemorrhage in Susceptible Populations in Korea

Background: A number of studies have examined the influence of meteorological factors and seasonal variation on the incidence of cardiovascular disease. An association between temperature and congestive heart failure or ischemic heart disease seems to be fairly robust; however, the relationship between temperature and subarachnoid hemorrhage stroke (SAH) is complex and the findings are contradictory. There has been consistent evidence for the short-term effects of ambient temperature on increased risk of myocardial infarction (MI) and death due to cardiovascular disease. However, the association between environmental temperature and risk of SAH remains controversial. Most research in SAH has concentrated on the relationship between season and incidence of SAH. It is important to include threshold temperatures, temperature lag effects, and nonlinear exposure-response relationships when evaluating the effects of temperature on stroke, MI, and acute coronary syndrome. The aim of this study was to investigate the association between the daily temperature and risk for SAH by analyzing the hospital admission records of 111,316 SAH patients from 2004 to 2012 in Korea.

Methods: We used climate variables from the Korean Meteorological Administration, air pollution data provided by Korea National Institute of Environmental Research, and SAH admission data from the National Health Insurance Service. We estimated the temperature-SAH association by applying generalized additive models (GAMs) with nonparametric smoothing functions (splines) to describe nonlinear relationships. Associations were adjusted according to humidity, barometric pressure at sea level, and air pollutants including PM10 and NO2. We estimated the threshold temperature using the piecewise-defined function. The analysis was performed for the following subgroups: gender (male or female), age (<75 years or ≥75 years), insurance type (National Health Insurance for the general population (NHI) or medical care (Medicaid) for the poor), and area (rural or urban) with climate zones based on cooling degree-days (CDD) for summer and heating degree-days (HDD) for winter. Results: We found a delayed effect, between 22-28 days, on the incidence of SAH due to hot temperature. The maximum threshold temperature during heat exposure was 31.5°C. The maximum temperature increase of 1°C above the threshold temperature was associated with a significant increase in relative risk (RR) of 1.07 for the >75 years age group, and 1.03 for males, respectively. Apart from longer lasting heat effects, short-term cold effects were observed between 4-7 days. The mean threshold temperature during cold exposure was -3.5°C and the minimum threshold temperature was -13.5°C. A mean temperature decrease of 1℃ was associated with a significant increase in RR of 1.03 for the <75 years age group, and 1.02 for females. The increased risk associated with minimum temperature was especially strong for male patients (RR =1.09, CI=1.05-1.14). With regard to heat exposure, the Medicaid group showed RR of 1.11 (lag: 1 day), which was higher than the NHI group who showed RR of 1.02 (lag: 22-28 days). With regard to cold exposure, the RRs in the Medicaid group were 1.05 with the mean temperature lag of 4-7 days, and 1.11 at the minimum temperature with a lag of 15-21 days, respectively. Meanwhile, patients in the NHI group only showed a risk of 1.01 at the mean temperature. The RR due to heat exposure was higher in hot areas with higher cooling degree-days (CDD) than in warm areas with lower cooling degree-days (CDD). Meanwhile, the RR due to cold exposure was higher in cold areas with higher heating degree-days (HDD) than in warm areas with lower heating degree-days (HDD).

Conclusions: This is the first study to find an association between temperature and incidence of SAH related to heat and cold exposure (defined by threshold temperatures). An increase in temperature above the heat temperature threshold or decrease in temperature below the cold temperature threshold correlated with increased risk of SAH in susceptible populations with different lag effects and risks. Our findings provide useful information for identifying the risk of SAH in vulnerable groups that can be used to establish climate change adaptation strategies.