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Neurologic health has worsened due to climate change and airborne pollution, a scoping literature review suggested.

In a review that spanned 3 decades of research, temperature extremes and variability were associated with stroke, dementia hospitalization, and multiple sclerosis (MS) exacerbation, reported Andrew Dhawan, MD, DPhil, of the Cleveland Clinic, and colleagues.

Exposure to air pollutants -- especially fine particulate matter (PM_2.5) and nitrates -- was tied to stroke incidence and severity, headache, dementia risk, and MS exacerbation, they added.

Moreover, favorable conditions for zoonotic diseases are expanding beyond traditional borders, presenting opportunities for neuroinfectious disease in new populations, the researchers wrote in .

"As the effects of climate change becomes increasingly apparent, we need to understand how it impacts our patients and the conditions we treat as healthcare professionals," Dhawan told MedPage Today. "This is the first wide-scale study to summarize what is already known about climate change as it pertains to neurological conditions."

"We provide three key priorities for further study in our work -- the effects of temperature and variability in temperature on neurological conditions, changes in neuroinfectious diseases, and understanding how airborne pollutants affect the nervous system -- which might provide a roadmap for future studies," he said.

The international community aims to reduce global temperature rise to under 1.5°C before 2100, but irreversible environmental changes already have occurred and will continue, Dhawan noted.

"In addition to understanding the scale of the issue, neurologists need to be aware of how the work they do might change," he added. "This will be different depending on where they practice and what types of patients they see."

The review examined studies about environment and neurologic disease in adults published between 1990 and 2022. The researchers identified 364 relevant papers: 289 about pollution, 38 about extreme weather events and temperature fluctuations, and 37 about neuroinfectious diseases. Most research was from Asia (149), North America (105), and Europe (93).

Temperature changes

A total of 24 studies evaluated climate and ischemic stroke incidence, with evidence suggesting rising stroke risk at temperature extremes. Six studies showed ischemic stroke incidence rose with increasing temperature and relative humidity increases. Two studies showed increased stroke admissions at lower temperatures, "perhaps because cold temperatures induce vasoconstriction and increase blood viscosity," Dhawan and colleagues noted.

Two studies examined dementia-related hospital admissions and weather; one showed that mean temperature increases of 1.5ºC raised the risk of dementia hospitalization by 12%. Three studies related seizure frequency to meteorological changes, four identified MS symptom exacerbation, and two looked at headache. One study showed that a temperature increase of 5ºC raised the relative risk of headache visits to the emergency department.

Neuroinfectious diseases and climate

Neuroinfectious diseases were related to climate change in 11 studies about West Nile virus, 13 about tick-borne encephalitis, six about meningococcal meningitis, five about Japanese encephalitis virus, one about unspecified viral meningitis, and one about coccidioidomycosis. Several reviews examined the effect of extreme weather like floods on mosquito- and rodent-borne diseases.

Predictive modeling of West Nile virus incidence in North America suggested suitable conditions for the virus could increase in the southern U.S. due to higher temperatures, lower rainfall, longer mosquito seasons, and more droughts. In addition, transmission periods for tick-borne encephalitis were expected to lengthen in Europe and a new focus on Scandinavia may emerge, based on analyses of epidemiological data between 1969-2018.

Airborne pollutants

Overall, 166 studies looked at stroke and air pollution. "Long- and short-term exposures to airborne pollutants had substantial support for association with ischemic stroke incidence and mortality," Dhawan and colleagues observed. A global burden of disease analysis concluded that 9% of stroke disability-adjusted life years and 8.5% of stroke deaths could be attributed to PM_2.5 exposure, they noted. Airborne pollution also was linked with intracerebral hemorrhage.

Airborne pollutants were related to dementia in 51 studies. Overall, 6.1% of incident dementia cases were estimated as attributable to PM_2.5 and nitrogen dioxide (NO₂) exposure.

Particulate matter exposure and Parkinson's incidence was examined in 19 papers and showed varying associations for each pollutant. "NO₂ exposure was examined in seven additional studies and its impact remains controversial," Dhawan and colleagues noted.

Three studies related ALS incidence to air pollutant exposure with varying results. Fourteen studies examined headaches and pollution; the largest one showed an association between higher frequency of migraine-specific urgent care visits and elevated average annual PM_2.5 and NO₂ levels in California. Among 19 MS studies, short-term exposure to airborne pollutants were generally associated with exacerbations of disease activity.

"Climate change poses many challenges for humanity, some of which are not well-studied," Dhawan said. "For example, our review did not find any articles related to effects on neurologic health from food and water insecurity, yet these are clearly linked to neurologic health and climate change."

Most studies were conducted in resource-rich regions of the world, "suggesting a discordance between where research occurs and where changes are most acute," the researchers acknowledged.

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