It was the fall of 2019 and Holly Elser was in her third year of medical school at Stanford University when a patient came into the primary care clinic where she was working for an annual checkup. The patient had multiple sclerosis, or MS, a chronic disease that occurs when the immune system destroys the protective sheaths that cover our nerves. Elser asked about how the patient’s symptoms had progressed and how she was managing at home, which is when the patient said something that made Elser stop and think. “She essentially said, somewhat off-handedly, ‘You know, my MS is really bad when it’s hot out,’” Elser said.  

Heat intolerance is a well-established phenomenon in MS. As many as 80 percent of patients living with the disease experience a worsening of their neurological symptoms when their core body temperature rises. But when Elser scoured the existing literature for studies on the implications of weather, temperature, and climate for individuals living with MS, she couldn’t find much. “That was really what made me decide that this was something I wanted to pursue in a systematic fashion,” she said. 

Last month, Elser’s study investigating the connection between MS and climate change was published in the medical journal PLOS Medicine. Using a database of more than 75 million privately insured individuals between the ages of 18 and 64 who filed insurance claims between 2003 and 2017, Elser and a team of researchers from Stanford and Columbia University identified roughly 100,000 patients who filed MS-related claims and compared their claims data to local temperature data. The data showed that periods of unusually warm weather — defined as months that were 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer than the long-term average — were associated with an increased risk of inpatient and emergency department visits, and to some extent outpatient visits, in patients with MS. Older individuals were especially likely to end up in the hospital during warm periods of weather. 

The implications are serious: Individuals with MS won’t only be more susceptible to the worsening effects of climate change; they’re already being impacted by rising temperatures now. 

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“It’s really interesting,” Jonathan Howard, a doctor of MS at New York University’s Langone medical center who was not involved in the study, said. “I think this paper is an important warning of the effects of high temperatures on patients with MS.” 

Joan Casey, the lead author of the study and an assistant professor of environmental health sciences at Columbia University, was surprised by some of the findings. She thought patients might present with worsened MS symptoms, which can vary from patient to patient and include vision loss, pain, fatigue, and impaired coordination, during the summer, when heat is at its most extreme. But the data showed above-average temperatures had stronger effects on patients with MS in the shoulder seasons — fall and spring. 

“I’m speculating,” she said, noting that the researchers looked at the data at the county level and did not actually talk to any of the patients themselves about their experiences, “but my guess is that it’s when temperatures don’t match up with expectation of what the temperatures will be outside that we really have this problem.” People with MS know temperature can worsen their symptoms, so in the summer they’re prepared to cope with heat. “But if it’s fall and suddenly there’s an anomalous month where temperature is much different from what people are expecting to experience,” Casey said, “they may be dressing inappropriately or they might not be seeking out air conditioning.” 

The researchers also found that the association between temperature anomalies and acute care visits was strongest in humid climates and very cold climates. “Perhaps there’s some interaction between temperature patterns and humidity,” Elser said. A large number of claims occurred in the South, which Casey said might be due to higher poverty rates in that region of the country. Low-income people can’t always afford air conditioning, and folks unable to afford health insurance might be more likely to wait till the last second to seek out care and end up in emergency rooms as a result. And in cold climates, even a slight increase in temperature feels noticeable and could catch someone living with MS off guard.  

There’s a lot of ground that this study doesn’t cover. The data set the researchers used doesn’t include information about socioeconomic status, race, health behaviors, or other factors, so at this point, they can only speculate as to what is influencing the regional differences in the claims data. 

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Scott Otallah, a doctor at Wake Forest Baptist Health who treats MS patients, said the hypothesis the researchers used was solid but it’s tricky to draw solid conclusions from the study. Whether there’s a connection between climate and MS is “a difficult question to answer,” he said, noting that the researchers were looking for relatively small changes in the number of people showing up to emergency rooms with MS on warm days. Due to the nature of the data, it’s hard to know whether some of these claims are due to issues other than MS, something the researchers themselves note. “You don’t know all of the other factors that could have caused these patients to come to the ER,” Otallah, who was not involved in the study, said. 

Elser and Casey hope that this research is just the beginning of a wider body of work that will investigate the links between climate change and MS. “My north star for ‘do I want to do this research project?’ is: Does this have the potential to improve the quality of life for people?” Elser said. “It would be ideal if this information improved patients’ quality of life.” 

Doctors can keep an eye out for how their patients react to unusually warm days, but governments also need to do their part in reining in rising temperatures. At least, that’s what a man in Austria with MS argued in a lawsuit filed last month before the European Court of Human Rights. A lawyer for the plaintiff, who is identified as Mex M. for privacy reasons, told the Agence France-Presse that the climate crisis is already affecting her client’s quality of life. Legal fees for the lawsuit are being crowd funded by Fridays for Future, the climate group founded by Swedish climate activist Greta Thunberg. The lawsuit is one of three health and climate-related lawsuits recently filed before the European court. 

It’s entirely possible that studies like this one will end up being fodder for the Austrian lawsuit or for similar lawsuits in the future. “My hope in researching the health effects of climate change is to spur us to make the necessary changes sooner rather than later,” Casey said.