A decrease in air temperature might be associated with an increasing number of deaths from stroke, especially in people older than 65 years, according to a new study published in the International Journal of Biometeorology. Further, the investigators also found that in older women the incidence of stroke associated with the colder weather was higher compared to older men.
To find the association between temperature variation and stroke mortality, the team used data collected by the city’s Death Records Improvement Program (PRO-AIM).
- Time series analysis showed that there were 55,633 deaths due to stroke in São Paulo between 2002 and 2011.
- A breakdown of the data by subtypes of stroke pointed to 12,183 deaths from ischemic stroke and 17,250 deaths from hemorrhagic stroke, for a subtotal of 29,433. The difference between this and the total sample (55,633) consisted of deaths from unspecified types of stroke or other cerebrovascular diseases.
- Cross-referencing of the stroke mortality data with mean temperatures for São Paulo City in the period showed a higher risk of stroke when mean temperatures were below 15 °C.
- Relative risk was not statistically significant for mean temperatures in the range of 17 °C-24 °C. However, when mean temperatures exceeded 26 °C, the risk of ischemic stroke was significant for men over 65.
- In the case of hemorrhagic stroke (the most severe subtype), lower temperatures were found to be a risk factor for both men and women, especially below 10 °C.
- Among over-65s, lower temperatures represented a higher risk of hemorrhagic stroke for women. This finding was unexpected and surprised the researchers.
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“At the beginning of the study, we thought pronounced temperature variability, whether in a warmer or cooler direction, would correlate similarly with both stroke subtypes. In other words, there would be more deaths from both subtypes on very cold or very hot days. That’s not what we found, however. In the case of hemorrhagic stroke, cold is a far more important factor, especially for women,” said Ikefuti.
The reason for stroke being the most frequent cause of death in people over 65 could be diminished metabolism in them. And also, in response to temperature changes, older people are less capable of maintaining homeostasis – the tendency to resist change in order to maintain constant physiological conditions required for survival.
“We also found that for all cases of stroke, and hemorrhagic stroke in particular, women are more vulnerable than men,” said Ikefuti. “The data also show that stroke mortality is higher among women on average, albeit slightly. The relative risk of having a stroke in correlation with mean temperature variations was also higher for women than men. Similarly, lower mean temperatures affected women more than men in respect of both stroke subtypes.”
Stress due to cold, results in increased blood viscosity and platelet count, raising arterial blood pressure enough to pose the threat of hemorrhagic stroke, explains Ikefuti.
“Women’s health is more influenced than men’s by such risk factors as diabetes and high blood pressure because women differ from men in a multitude of ways, including anatomy, vascular biology, immunity, neuroprotective factors, coagulation, hormonal profiles, vascular risk factors, lifestyle, and societal role,” the researchers note.
According to Alfesio Luis Ferreira Braga, a Professor at Unisantos who co-authors the study, another important explanation for the higher risk of stroke among women is menopause, when the body produces less estrogen, the hormone that stimulates or controls the development and maintenance of female characteristics. Lack of estrogen during menopause subjects women to a higher risk of vascular disease, among several other conditions.
“Our study contributes to the understanding of the impact of temperature on stroke mortality in a tropical country, where the temperature might be assumed not to be a concern regarding the risk of stroke. It proves that isn’t the case, at least not in São Paulo City,” Braga said.
“Our findings showed that mean air temperature is associated with stroke mortality in the city of São Paulo for men and women and IS and HS may have different triggers. Further studies are needed to evaluate physiologic differences between these two subtypes of stroke,” conclude the authors.
For more information log on to 10.1007/s00484-018-1554-y
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