Exposure to heat and cold early in life may affect the development of white matter in the brain

Brain scans of more than 2,000 preadolescent children have shown that early exposure to heat and cold can have long-term effects on the microstructure of the brain's white matter, especially in those living in poor areas. The study, published in Nature Climate Change, highlights the vulnerability of fetuses and children to extreme temperatures. The study was led by the Barcelona Institute for Global Health (ISGlobal).

In the current climate emergency, the impact of extreme temperatures on human health is of great concern to the scientific community and society. Children are particularly vulnerable to temperature changes as their thermoregulatory mechanisms are still immature.

"We know that the developing brain of fetuses and children is particularly sensitive to environmental influences, and there is some preliminary evidence that exposure to cold and heat may affect mental well-being and cognitive abilities in children and adolescents," says Monika Guxens, a researcher at ISGlobal, Erasmus MC and CIBERESP. "However, there are few studies assessing potential changes in brain structure as a result of these exposures," she adds.

In this study, Guxens' team looked at the structure of white matter in the brains of pre-adolescents to identify periods of vulnerability to cold and heat exposure in early life. The analysis included 2,681 children from the Generation R study in Rotterdam who underwent magnetic resonance imaging (MRI) scans between the ages of 9 and 12. The MRI protocol assessed brain activity by measuring the amount and direction of water diffusion in the brain's white matter.

In more mature brains, water flows more in one direction than in all directions, giving lower values for a marker called average diffusivity and higher values for a marker called fractional anisotropy. The research team used an advanced statistical approach to estimate, for each participant, exposure to average monthly temperatures from conception to age 8 and their impact on these MRI parameters (mean diffusivity and fractional anisotropy) measured at ages 9–12.

The period of receptivity between pregnancy and three years

The results showed that cold exposure during pregnancy and the first year of life, as well as heat exposure from birth to 3 years of age, were associated with higher mean diffusivity in preadolescence, indicating slower white matter maturation. In this case, cold and heat are defined as temperatures that are at the lower and upper ends of the temperature distribution in the region under study.

"White matter fibers are responsible for connecting different areas of the brain, allowing them to communicate. As white matter develops, this communication becomes faster and more efficient. Our study is like a photograph at a certain point in time, and what we see in this image is shows that participants more exposed to cold and heat show differences in a parameter - mean diffusivity - that is associated with lower levels of white matter maturation," explains Laura Granes, IDIBELL and ISGlobal researcher and first author of the study.

“In previous studies, changes in this parameter have been associated with worsening cognitive function and some mental health problems,” she adds.

“The greatest changes in communication parameters are observed in the first years of life,” says study co-author Carles Soriano from IDIBELL, UB and CIBERSAM. "Our results suggest that it is during this period of rapid brain development that cold and heat exposure may have long-lasting effects on white matter microstructure."

No association was found between early temperature exposure and fractional anisotropy at ages 9–12 years. The authors suggest that a possible explanation is that the two parameters reflect different microstructural changes, and that mean diffusivity may be a more reliable indicator of white matter maturation compared with fractional anisotropy.

Children from poor families are more at risk

Analysis stratified by socioeconomic conditions found that children living in poor areas were more vulnerable to exposure to cold and heat. In these children, the windows of susceptibility to cold and heat were similar to those found in the general cohort, but began earlier. These differences may be related to living conditions and energy poverty.

One important mechanism that may explain the effect of ambient temperature on neurodevelopment may be deterioration in sleep quality. Other possible mechanisms include placental dysfunction, activation of the hormonal axis leading to increased cortisol production, or inflammatory processes.

"Our results help draw attention to the vulnerability of fetuses and children to changing temperatures," says Gouksens. The results also highlight the need to develop public health strategies to protect the most vulnerable communities in the face of a looming climate emergency.