Strokes occur when the blood supply to the brain is interrupted by a blockage (ischemic stroke) or burst vessels (hemorrhagic stroke), causing brain damage or even death. Although we cannot predict when or if an individual will suffer a stroke, the risks can be identified and minimized. A Chinese study recently found that high temperatures are associated with stroke mortality.
Stroke is the second most common cause of global mortality. Strategies for primary prevention focus on managing risk factors such as hypertension, smoking and diet. Environmental risks may also play a role in stroke mortality. High temperatures in particular are associated with increased risk of total stroke mortality. However, information regarding the effects of heat on stroke subtypes (ischemic and hemorrhagic) is inconsistent. A study published in Science of the Total Environment looked at the effects of heat exposure on total, ischemic and hemorrhagic stroke mortality and whether patient characteristics modified these effects.
This retrospective study analyzed daily stroke mortality data for 12 cities in the Jiangsu Province between 2009-2013 during the warm season (May-September). Data were obtained from the Jiangsu Provincial Center for Disease Control. Daily mortality counts were classified into three groups: total stroke deaths (n = 146 427), ischemic stroke deaths (n = 49 119) and hemorrhagic stroke deaths (n = 63 369). Daily mean temperatures and relative humidity observations were obtained from the China Meteorological Data Sharing Service System and averaged for each city. Daily air pollution data for each city in 2013 was collected from the National Air Pollution Monitoring System.
Using a two-stage statistical approach, the authors initially analyzed heat-related total, ischemic and hemorrhagic stroke mortality risks for each city. Long-term trends, seasons, relative humidity and the day of the week were also considered. The second round of analyses pooled the risk estimates for each city. Confounding factors such as air pollution, age (0-64, 65-74, ≥ 75 years), gender, education level (low: illiterate/primary school, high: high school/college) and location of death (in hospital or out of hospital) were also taken into account.
Significant associations were found between heat and total and ischemic stroke mortality in all 12 cities, and between heat and hemorrhagic stroke mortality in eight cities. Heat had a greater effect on ischemic stroke mortality, although the reason for this is unknown. Neither air pollutants nor relative humidity influenced the risk of heat-related stroke mortality. Heat-related mortality risks were higher in women, older people (≥ 75 years), poorly educated people, and for deaths that occurred away from a hospital.
These results are in contrast to studies performed in other countries, which found no association between heat and ischemic stroke mortality or admission risk and suggested that heat could protect against hemorrhagic stroke. The lifestyle and climatic differences between the cities and inter-study variation may account for the conflicting results. Despite this, the present study provides valuable data about risk factors for stroke subtypes. This data can be used to develop effective prevention strategies for individuals at high risk of heat-related stroke mortality.
Written By: Natasha Tetlow, PhD