Heat, Variable Temperatures Found to Increase Risk of MS Hospital Visits
Visits to the hospital related to multiple sclerosis (MS) are more common when it’s hot or when temperatures are more variable, but less likely to be needed when it’s humid, a new analysis suggests.
The results were published in the International Journal of Environmental Research and Public Health, in a study titled “Heat Exposure and Multiple Sclerosis — A Regional and Temporal Analysis.”
It is well-established that increasing a person’s core body temperature can aggravate symptoms of MS, and many people with the neurodegenerative disorder experience an abnormal sensitivity to heat, with unusually warm weather linked to more emergency room visits.
“We’ve known for quite some time that neurological symptoms of multiple sclerosis can be exacerbated as a result of exercise that increases core body temperature and metabolism,” Naresh Kumar, a professor at the University of Miami and co-author of the study, said in a press release.
“What we don’t have a lot of data on is the relationship between ambient meteorological conditions and symptoms of the disease,” he said.
To explore the relationship between weather and MS symptoms requiring hospital care, Kumar and his colleagues conducted an analysis of data from Veterans Affairs medical centers across the U.S.
In total, the team assessed data on 27,290 patients, who made 530,075 total hospital visits from 2010 to 2013. The patients had a mean age of 59.5 years, 81.2% were male, 92.0% non-Hispanic, and 75.2% white. A total of 18.3% of the patients were Black.
The team then investigated the frequency of MS-related hospital visits based on weather factors — determined from information culled from the National Climatic Data Center.
In general, the researchers found, hospital visits were 8.9% less likely to occur in the winter, as compared with the spring, summer, and fall. Visits were particularly frequent when weather was changing from hot to cold, or vice versa.
“Overall, there are two seasonal peaks of MS clinic visits: spring (March and April) and early fall (August and September),” the researchers wrote.
Kumar noted that these peak times correspond with variable weather temperatures. Such variability — the rate at which temperature changes day to day or month to month within a single year — “most likely” had an effect on increased rates of clinical MS visits, he said.
“For example, we observed a significant change in temperature in the month of March, a transition month between a long winter and spring season, which corresponded with the highest frequency of MS clinic visits,” Kumar said.
For their analysis, the researchers used statistical models to examine the impact of temperature and humidity on the relative risk of a hospital visit. In nationwide analyses, they found that for every 10-degree Celsius increase in temperature, there was a statistically significant 0.3% increased likelihood of going to the hospital because of MS.
By contrast, every 10% increase in relative humidity, known as RH, was associated with a statistically significant 0.08% lower likelihood of a hospital visit. Notably, analyses that looked at both ambient temperature and RH generally found a weaker effect on risk, compared with those that looked at temperature alone.
“This suggests that while temperature alone is a risk factor of MS clinic visit, its interaction with RH, a proxy of heat index, is a stronger risk factor of MS clinic visit than exposure to ambient temperature,” the researchers wrote.
According to the team, these findings suggest that humidity may play a protective role in MS, which “is consistent with some of the [previously published] studies and inconsistent with others.” Such differences highlight the need for further research on the impact of humidity in MS, the investigators said.
Importantly, in these analyses, the strongest predictor of risk for MS hospitalization was the standard deviation (SD) of temperature, which essentially is a measurement of how variable the temperatures are.
“In our analyses, temperature variation was the strongest predictor of visit risk among all variables, both at the national and regional level,” the team wrote.
Differences were seen across geographic regions in the U.S., with the highest rate of MS clinic visits in the Pacific Northwest (67.6 per 10,000 patient visits) and the Northeast (64 per 10,000), according to Kumar. The regions with the lowest MS clinic visit rates were the Lower Midwest (15.1 MS clinic visits per 10,000 patients), followed by the Upper Midwest (24.6 per 10,000). The subtropical U.S. had 46.8 MS clinic visits per 10,000 patients.
“Miami is subtropical, so we are used to the high humidity and high temperatures,” Kumar said. “In other areas of the country that undergo massive swings in temperatures in a short time, some patients might not be used to it.”
The researchers speculated that increased sensitivity to changing weather — particularly to variable temperatures — may be due to problems with thermoregulation, which are the processes by which the body maintains a stable temperature. Such problems can be caused by neurological damage that occurs in MS.
“These findings may become more relevant given increases in daily temperature variation and heatwave intensity and frequency with global warming,” the scientists concluded. “In the meantime, these epidemiological associations can be incorporated into practice, especially for exposure avoidance and/or mitigation in susceptible patients.”
The results highlight the importance of implementing a “multimodal approach” to managing the care of people with chronic diseases such as MS, said Anat Galor, a professor of ophthalmology at Bascom Palmer Eye Institute of the UHealth – University of Miami Health Systems and a study co-author.
“In addition to medical therapy, it is important to understand how environmental conditions impact specific diseases so that individuals may make appropriate lifestyle modifications,” Galor said.
The researchers noted several limitations of this study. In particular, since the team relied on insurance data, they did not have access to information on patients’ disability status or disease duration. The fact that the patients were mostly white males also was a noted limitation.