Cold fronts may increase stroke mortality

Study conducted in subtropical region of Brazil detects correlation between drop in temperature and rise in deaths from stroke, especially among women and older people.

By Peter Moon  |  Agência FAPESP – With the advent of colder winter weather in the South and Southeast of Brazil, falling temperatures may be accompanied by rising numbers of deaths from stroke, especially among people over 65. 

The correlation between lower mean air temperature and higher stroke mortality has been demonstrated by a study based on death records and meteorological data for the period 2002-11 in São Paulo City, Southeast Brazil. The authors also found that in the case of older people, the incidence of stroke associated with colder weather was higher among women. 

Results of the study have been published in an article in the International Journal of Biometeorology by researchers at the University of São Paulo (USP) and the Catholic University of Santos (Unisantos). The study was part of a research project supported by FAPESP. 

In Brazil, chronic diseases such as cardiovascular disorders, diabetes and cancer cause most of the deaths in both men and women. Among cardiovascular disorders, stroke is the main cause of death, accounting for 10% of the total. 

“In the United States, which has a temperate climate and freezing winter weather, a correlation has been found between a rise in stroke mortality and temperature highs and lows. No such study had been done before in Brazil, even for the populations in the South and Southeast, where the climate is subtropical,” said physician Alfésio Luís Ferreira Braga, a professor at Unisantos and coauthor of the study.

To find out whether temperature variation correlated with stroke mortality in São Paulo, geographer Priscilla Venâncio Ikefuti used data collected by the city’s Death Records Improvement Program (PRO-AIM). The principal investigator for the study was Ligia Vizeu Barrozo, a professor in the University of São Paulo’s School of Philosophy, Letters & Human Sciences (FFLCH-USP).

Time series analysis showed that there were 55,633 deaths due to stroke in São Paulo between 2002 and 2011. Daily air temperature and humidity readings were based on data collected from USP’s weather station, run by its Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG). 

Mean temperatures were used instead of highs and lows because they corresponded to the average of several observations made on the same day and were considered a good estimate of exposure to heat or cold, according to the researchers. The mean air temperature in São Paulo between 2002 and 2011 was 21 °C, with a range of 15 °C-25 °C depending on the season. 

To adjust the data for the effects of air pollution on mortality, daily 24-hour averages of particulate matter, ozone, sulfur dioxide and nitrogen dioxide were collected from 14 stations operated by CETESB, the São Paulo State Environmental Agency, around the city.

The next step was statistical analysis of the data using quasi-Poisson regression models to estimate the effects of mean temperature on stroke mortality in the total population and among over-65s.

The results showed that mean temperature correlated with stroke mortality and that relative risk varied with age and gender. Cooler temperatures (below 15 °C) were considered more statistically significant for stroke mortality than were warmer temperatures (above 22 °C).

Based on the total number of stroke deaths in the city between 2002 and 2011, Ikefuti calculated a daily average of 15.24 for the total sample. The average was slightly higher for women (7.99) than for men (7.25). In absolute terms, this small difference between averages was significant, corresponding to approximately 2,000 more stroke deaths among women than among men. 

Among over-65s, there were more deaths from hemorrhagic stroke among women (5,236, for a daily average of 5.81) than men (4,071, for a daily average of 4.6).

A breakdown of the data by subtype 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. 

Ischemic and hemorrhagic stroke

A stroke is a serious medical condition that occurs when the blood supply to part of the brain is interrupted or reduced, causing cell injury and impairing neurological functions. 

Ischemic stroke is the most common subtype, accounting for more than 80%. It is caused by obstruction of a blood vessel, thereby interrupting the supply of blood to the brain.

The main risk factors for ischemic stroke are age (it is more frequent in the elderly), smoking, alcohol abuse, high blood pressure, obesity, high cholesterol, and a family history of heart disease.

Hemorrhagic stroke is the most severe subtype, accounting for 10%-15% of all cases. It is an emergency condition in which a ruptured blood vessel causes bleeding, swelling and pressure inside the brain, damaging cells and brain tissue. It can kill, and survivors usually suffer from temporary or permanent disabilities.

Diminished metabolism and menopause

The 26,200 deaths from unspecified types of stroke or other cerebrovascular diseases may have included deaths from hemorrhagic or ischemic stroke but were not reported as such. Deaths from hemorrhagic and ischemic stroke as notified averaged 4.72 and 3.34 per day, respectively, for all ages. Both averages were higher for women. 

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, 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. 

“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, who is currently employed by the São Paulo State Department of Health’s Epidemiological Surveillance Center (CVE) and was previously a professor at the Federal University of the Southern Frontier (UFFS). 

The finding that stroke is a more frequent cause of death among over-65s could be explained as due to diminished metabolism in the elderly. In response to changes in temperature, older people are less capable of maintaining homeostasis, the tendency to resist change in order to maintain constant physiological conditions necessary to survival. 

“We also found that for all cases of stroke, and hemorrhagic stroke in particular, women are more vulnerable than men,” Ikefuti said. “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, she explained, results in increased blood viscosity and platelet count, raising arterial blood pressure enough to pose the threat of hemorrhagic stroke. 

The researchers note that recent studies have pointed to the key reasons for which women are more susceptible than men to stroke. 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. 

According to Braga, 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 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. 

“Although the city is in a subtropical region, so that mean temperatures are higher than in countries with a temperate climate, substantial daily temperature variations and the occurrence of cold fronts or heatwaves are indeed risk factors for stroke, especially among the elderly, and above all among women.”

The article “Mean air temperature as a risk factor for stroke mortality in São Paulo, Brazil” (doi.org/10.1007/s00484-018-1554-y) by Priscilla V. Ikefuti, Ligia V. Barrozo and Alfésio L. F. Braga can be retrieved from: https://link.springer.com/article/10.1007/s00484-018-1554-y.