"How a change in weather could trigger a stroke: Very cold or humid temperatures can cause a fatal clot to develop," reports the Mail Online website. The headline comes from a US study of the records of 134,510 people who had been hospitalised for a stroke.
Researchers cross-matched the records with the average temperature and humidity in each county. They report that each 1°F increase in average temperature was associated with a 0.86% decrease in the chances of being admitted to hospital for a stroke and a 1.1% decrease in the chances of dying in hospital after a stroke.
They also found that an increased change in daily temperature and humidity was associated with slightly increased odds of stroke hospitalisation. Previous research found an association between cold weather and a rise in blood pressure that could contribute towards this slightly increased risk.
But in this study, the temperature and humidity data were only matched with hospital discharge dates, even though many people who have had a stroke require prolonged hospital stays of up to six months. The researchers also did not take factors such as air conditioning and heating into account, instead using outdoor temperatures.
Although the results of this study are limited, vulnerable people should avoid extreme weather conditions if possible. While we cannot control the weather, there are several proven risk factors for a stroke that can be compensated for. These include seeking treatments for chronic conditions such as high blood pressure, diabetes and coronary heart disease, and making lifestyle changes such as quitting smoking, eating a healthy diet and taking regular exercise.
Where did the story come from?
The study was carried out by researchers from Yale School of Public Health, Harvard School of Public Health and the Duke Comprehensive Stroke Centre in the US. Funding was not reported.
It was presented as a short abstract at the American Stroke Association and American Heart Association International Stroke Conference 2014 in San Diego, California.
The Mail Online story includes advice from the UK Stroke Association that "people should take extra precautions to stay warm and reduce their risk of stroke". However, the story did not explain that in this study, the temperature and humidity conditions were not recorded at the time of stroke, but at the time of discharge from hospital.
What kind of research was this?
This was a retrospective study to see if weather temperature was associated with the incidence and outcome of stroke.
As this was a retrospective study, any conclusions that can be drawn are more likely to suffer from errors as a result of bias and confounding (other factors that could account for the results) than prospective studies, for example.
What did the research involve?
The researchers used data from the US Nationwide Inpatient Sample from 2009-10 to identify adults over the age of 18 who had an ischaemic stroke (a stroke caused by a blood clot in the brain). These people were identified by their International Classification of Diseases (ICD)-9 code.
The researchers collected temperature and dew point (humidity) information from the US National Climatic Data Center for each county and matched these with the discharge dates.
They then performed statistical analyses to assess any relationship between the changes in temperature, stroke hospitalisation and deaths that occurred in hospital after a stroke.
The researchers accounted for some factors that may have influenced the results, including patient age, sex, ethnicity and comorbidities (other illnesses). They report that they also took into account the region and season, but it is not clear how this was done.
What were the basic results?
The researchers identified 134,510 people who had been hospitalised for an ischaemic stroke with an average age of 72.
Each 1°F increase in average temperature was associated with:
- a 0.86% decrease in the odds of stroke hospitalisation
- a 1.1% decrease in the odds of dying in hospital after a stroke
Bigger falls in daily temperature were associated with increased odds of stroke hospitalisation (odds ratio [OR] 1.02). Higher average dew point (humidity) was also associated with increased odds of stroke hospitalisation (OR 1.01).
The results were similar after taking age, sex and ethnicity into account. No results were provided for whether other illnesses affected the results.
How did the researchers interpret the results?
The researchers concluded that, "Larger daily temperature changes and higher average dew point were associated with higher stroke hospitalisation, while lower average annual temperatures were associated with both hospitalisation and mortality after stroke.
"Further research is needed to understand these effects to develop prevention strategies for vulnerable populations during periods of extreme weather conditions."
This retrospective study reports that there is an association between lower temperature and increased humidity and a slightly increased risk of ischaemic stroke. It was a large study, but there are several major limitations of this study design:
- The temperature was matched with discharge dates. There is a wide variation in the length of hospitalisation after a stroke, so the temperature at discharge may be very different to the temperature when the stroke occurred.
- The researchers assumed that changes in outdoor air temperature would have been experienced by all of the people who suffered from a stroke. It is possible that many people were indoors at the time, with either air conditioning or heating on to create normal ambient conditions.
- It is not clear how the researchers adjusted for region or season when performing the analyses.
- The data was collected retrospectively, which leaves it open to bias and confounding.
- No results were provided for whether the presence of other illnesses had an effect on the results.
Overall, this study does not change the commonsense advice for vulnerable people to try to avoid extremes of temperatures. It also remains important to reduce proven risk factors for ischaemic stroke, including:
- optimal treatment of conditions such as high blood pressure, diabetes and heart disease
- making changes to smoking status, poor diet, physical inactivity and obesity
Read more advice about reducing your risk of a stroke.