Article In Brief
An overview of studies suggests climate change may have an impact on the risk for stroke, sleep problems, epilepsy, multiple sclerosis, Parkinson’s disease, neuroinfectious disease, and even neurodevelopment. But experts say the studies to date are associative and do not reflect a direct cause and effect.
Daniel Kondziella, MD, PhD, a neurologist at Rigshospitale University Hospital in Copenhagen, was giving a talk about neuroinfectious diseases at a meeting of the Danish Neurological Society when he described a recent rare case of West Nile Virus in his institution.
“I had one slide about climate change and how that will become an important issue,” said Dr. Kondziella. “With rising temperatures, it’s probably just a matter of a few years before West Nile will be endemic in Denmark as well.”
Instead of the physicians in attendance looking worried, however, “they seemed amused,” he said. “I suppose it’s an idea they hadn’t been thinking of. Unfortunately, it’s a serious problem. We neurologists need to get accustomed to climate change being an issue for our patients and our practice.”
Just how unprepared are neurologists for the looming changes that climate change will bring to the field? When Dr. Kondziella published a systematic review of the subject in 2021, he and his team found 84 studies investigating the relationship between temperature and neurologic disorders, but none were rigorously designed and focused specifically on the effects of rising global temperatures.
“There are studies comparing stroke admissions during the winter versus during the summer, or during heat waves,” he said. “But there is no study looking at how many more cases of stroke or other neurological diseases we can expect to see if global temperatures rise by, say, 1.5 degrees Celsius. Climate change will affect stroke, it will affect Parkinson’s, it will affect headache—you name it. Neurologists need a new mindset to become aware of what’s coming.”
Awareness does appear to be dawning. Last September, Neurology and more than a hundred other journals simultaneously published an editorial entitled “Call for Emergency Action to Limit Global Temperature Increases, Restore Biodiversity, and Protect Health.”
“I don’t think we can see the effects on neurology in isolation from other health impacts,” said Jose Merino, MD, FAAN, professor of neurology at Georgetown University and editor-in-chief of Neurology. “In addition to rising temperatures, climate change is increasing the number and severity of floods, droughts, hurricanes, forest fires. By 2050, some areas of the world will be uninhabitable. All of that is going to pose a major threat to neurology and medicine more broadly.”
To address the issue, Neurology Today undertook a review—by no means exhaustive—of the many ways in which climate change is expected to affect the risks and morbidity of stroke, sleep, epilepsy, multiple sclerosis (MS), Parkinson’s disease, neuroinfectious disease, and even neurodevelopment. No one can say whether the changes will amount to a tropical storm or a category 5 hurricane. But as Dr. Kondziella put it: “It’s better to be prepared when a disaster strikes.”
Numerous studies from around the world have established that the fine particulate matter ejected into the atmosphere by vehicles and factories significantly raise the risk of stroke and other neurologic disorders. A 2019 paper out of China, for instance, measured the risk of incident stroke in 15 provinces against their average airborne level of fine particulate matter of 2.5 micrometers or less (PM2.5).
Among 117,575 Chinese men and women who had not had a stroke at baseline, those living in one of the regions with the highest quartile for PM2.5 had an odds ratio for stroke of 1.53 (95 percent odds ratio 1.34 to 1.74), ischemic stroke (1.82, 1.55 to 2.14) and hemorrhagic stroke (1.50, 1.16 to 1.93), compared with those living in the lowest quartile.
Heat itself also appears to increase the risk of stroke. A systematic review and meta-analysis published this past June in Lancet Planet Health, for instance, reported that for every 1°C increase in temperature, the risk of cardiovascular disease-related mortality increased by 2.1 percent, “with the highest specific disease risk being for stroke and coronary heart disease.”
In addition, the paper reported: “Heat waves were also significantly associated with a 17 percent increase in risk of mortality and increasing heat wave intensity with an increasing risk (RR 1.067) for low intensity, 1.088 for middle intensity, and 1.189 for high intensity settings.”
“The problem is what’s going to happen when it’s not just a heat wave,” said Dr. Merino, who is an acute stroke specialist. “What will the effect be when average temperatures remain elevated for months? It will have impacts on all areas of health.”
The editorial published last year in Neurology and other journals mentioned an international initiative called Epilepsy Climate Change (EpiCC). The group’s first conference, held virtually on November 25, 2021, examined what health care providers, researchers, and the pharmaceutical industry can do to limit the effects of climate change, including rising temperatures on the risks of acquiring epilepsy and on people with existing epilepsy.
“We know that people with Dravet syndrome, most of whom have mutations in the SCN1A gene that encodes a key brain sodium channel, are very sensitive to ambient temperature,” said Sanjay Sisodiya, MD, professor of neurology at University College London’s Queen Square Institute of Neurology, where he specializes in epilepsy.
Dr. Sisodiya, who helped to organize EpiCC, published a paper last year in the journal Epilepsy & Behavior which reported: “Climate change-induced increase in seizure precipitants such as fevers, stress, and sleep deprivation (e.g. as a result of more frequent extreme weather events) or vector-borne infections may trigger or exacerbate seizures, lead to deterioration of seizure control, and affect neurological, cerebrovascular, or cardiovascular comorbidities and risk of sudden unexpected death in epilepsy.”
In an interview with Neurology Today, he emphasized that much of the data linking temperature to the risk of seizures is, for now, circumstantial.
“We’re gathering data, but there is already enough information to be concerned,” Dr. Sisodiya said. “We’re all going to have to adapt.”
In 2020, Turkish researchers published a paper on the effect of air pollution on the risk of MS. The cross-sectional study compared the prevalence of MS among 32,261 people in Ereğli, a Turkish city with an iron and steel factory and high levels of air pollution, to 21,963 people in Devrek, a rural city nearby.
“Crude prevalence was found to be 96.1/100,000 in Ereğli and 45.5/100,000 in Devrek,” the paper reported. “The results of this study indicate a more than double MS prevalence rate in the area home to an iron and steel factory when compared to the rural city.”
“There’s a very clear correlation between heat and worsening of symptoms of MS,” said Nancy L. Sicotte, MD, FAAN, chair of neurology at Cedars-Sinai, in a recent article on the health system’s website.
One of the best studied areas of research linking climate change to neurologic disease is on the steady movement of insects carrying arboviruses from tropical environments into areas once considered temperate. For instance, the prime carriers of dengue—Aedes aegypti and Aedes albopictus—are now projected soon to reach northward from South and Central America into the United States.
“Based on current predictions of greenhouse gas emissions continuing to rise, the geographic distribution of these mosquitos is expected to increase, in some cases substantially,” said James Sejvar, MD, FAAN, a neuroepidemiologist with the Centers for Disease Control and Prevention (CDC). “This doesn’t necessarily equate directly to more dengue, but it does mean there are going to be lots and lots more people at risk for transmission of the virus.”
Dr. Sejvar is sometimes called upon to travel to the sites of natural disasters where mosquitoes and other disease vectors can proliferate.
“I’ve done a number of hurricane responses, including after Katrina,” he said. “You definitely see areas that are just rife with standing water, where mosquitoes breed, and in those cases, we have seen spikes in mosquito-borne viruses.”
Arun Venkatesan, MD, PhD, professor of neurology and director of the Johns Hopkins Encephalitis Center, published a paper last year on emerging infectious encephalitides. Climate change, he told Neurology Today, is going to make arthropod-borne diseases more common and to occur over a greater portion of the year. Not just dengue but Powassan virus in North America, Chikungunya virus in Central and South America and tick-borne encephalitis virus in Europe are all likely to spread northward.
“Neurologists need to be familiar with the evolving epidemiology of these diseases, techniques for prevention, vaccination guidelines, approaches to diagnostic testing, and clinical manifestations in order to best serve patients,” he said.
Perhaps the most disturbing trend related to climate change is the effects it might have on neurologic development in children. A paper published in September in the journal Environmental Pollution found detrimental changes in white matter microstructures among children aged 9 to 12 years old who had lived in areas with the highest levels of nitrogen dioxide and particulate matter, compared to the lowest, from conception to age 5. The study controlled for potential confounders.
“It’s been shown that these particles cross the blood-brain barrier and the placenta, so there is biological plausibility,” said the senior author of the paper, Monica Guxens, MD, MPH, PhD, an associate research professor at the Barcelona Institute for Global Health and the department of child and adolescent psychiatry/psychology at Erasmus Medical Center.
“These particles come primarily from cars, not just from the exhaust but from the brake lining and tire wear. They carry trace elements such as iron, copper or zinc.”
The best way to lower such pollutants, she said, is through government regulation. “You cannot tell someone to go live somewhere else to protect their child’s brains,” she said. “You can’t tell everyone to live far from a busy road. This is a public health problem. But you can say that using public transportation rather than driving a car will reduce the amount of air pollution you produce. If everybody would do that, it would reduce air pollution dramatically.”
Another study, conducted by Harvard researchers and published in 2015 in the journal Environmental Health Perspectives, examined IQ in 8-year-old children based on how close they lived to a major roadway. Those who lived less than 200 meters from such a roadway, the study found, had a nonverbal IQ 7.5 points lower than those living farther away. On the other hand, the study found, “Prenatal and childhood exposure to traffic density and PM2.5 did not appear to be associated with poorer cognitive performance.”
With such discordant results, the authors concluded that living near a major roadway “may” affect cognitive development.
In a June 2021 paper in Environmental Research, Italian researchers proposed that the rising rates of Alzheimer’s, Parkinson’s disease (PD) and motor neuron diseases may be due, in part, to rising temperatures.
The same group followed up that report with a preprint report posted in August of 2021 looking specifically at PD. They compared the median rates of PD in 25 countries based on their level of warming between 1990 and 2016 and their average temperature. Those countries with higher-than-average climate change and average temperature, the study found, had higher rates of PD.
“We propose that neurodegeneration derives from oxidative stress caused by neuroinflammation,” said a coauthor of both papers, Renata Del Carratore, PhD, a biologist and senior researcher at the Institute of Clinical Physiology, National Research Council, in Pisa. “The impact of Parkinson’s on thermoregulation may make them especially sensitive to the effects of climate warming.”
A systematic review published in 2018 found six studies related to the effects of rising temperature on sleep, seven related to extreme weather events, and three related to floods or wildfires. The most common effect, the review found, was diminished and disrupted sleep.
Considering the recent devastating effect of Hurricane Ian in western Florida, in which entire neighborhoods were razed to the ground, Dr. Sisodiya said the impact on not just sleep but on epilepsy and other medical conditions is obvious.
“If you were about to fill a prescription for an antiseizure medication when this hit, you might find your supply is not replenished in time,” he said. “If you’ve just been through this terrible event and your house is flooded and you’re sleeping in a giant school hall, your sleep will be disrupted, and we know disruption of sleep can be a potent factor in provoking seizures.”
Given the global scale of climate change, neurologists might wonder what they can do to make a difference.
“Every individual can play a part in forestalling global warming,” said Dr. Sejvar of the CDC. “Neurologists need to be cognizant of emerging diseases and on the lookout for mosquito-borne diseases. Including the encephalitic arboviruses on your differential is a good move.”