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Barcelona Exposure to extreme heat or cold early in life can have lasting effects on the microstructure of the white matter of the brain. This is what researchers who evaluated brain scans of more than 2,000 children report in the journal Nature Climate Change (2024; DOI: 10.1038/s41558-024-02027-w).

The developing brains of fetuses and children are particularly vulnerable to environmental factors, and there is already evidence from studies that heat and cold can affect the mental health and cognitive performance of children and adolescents, reports senior author Mnica Guxens from the Barcelona Institute for Global Health.

The research group led by Guxens and lead author Laura Grans from the Department of Psychiatry at the Bellvitge University Hospital in Barcelona analyzed MRI images of 2,681 children from a birth cohort in Rotterdam, the Netherlands. The brain scans were performed when the children were between 9 and 12 years old.

Impaired maturation of the white brain matter

They assessed the microstructure of the white brain matter using two parameters, the mean diffusivity and the fractional anisotropy in diffusion tensor imaging. These two parameters provide information about the microstructure of the tissue by measuring the distribution of water molecules in the white matter.

In regions where axons are densely packed and well myelinated, and thus more efficiently organized, water molecules diffuse predominantly along the axis of the axons, resulting in lower mean diffusivity and higher fractional anisotropy.

The research group reports that heat exposure during infancy and early childhood as well as cold exposure during pregnancy and infancy were associated with increased mean diffusivity at the age of 9-12 years. This indicates reduced myelination and maturation of the microstructure of the white matter. However, no corresponding associations were found for fractional anisotropy.

Cold or hot temperatures were defined as temperatures that were at the lower or upper end of the temperature distribution in the region studied (Rotterdam, Netherlands).

Vulnerable time window between pregnancy and the third year of life

The nerve fibers of the white brain matter enable communication between different brain regions. As the child develops, this communication becomes faster and more efficient. Our study shows us a snapshot [im Alter von 9-12 Jahren]and what we see on the scans is that children who were more exposed to heat or cold show differences in a parameter that is associated with a lower level of maturation of the white matter of the brain, says Grans.

In other studies, changes in this parameter have already been associated with poorer cognitive function and certain psychological problems.

The greatest changes in this connectivity parameter were observed when the children were exposed to heat or cold in the first years of life. From this, the researchers conclude: During this phase of rapid brain development, exposure to heat or cold can have lasting effects on the microstructure of the white matter.

But why was there no association between temperature exposure at a young age and fractional anisotropy at age 9-12 years? Grans and her colleagues speculate that the two parameters may reflect different microstructural changes and that mean diffusivity is a more robust indicator of white matter maturation.

Poorer children have higher risk

An analysis stratified by socioeconomic circumstances also showed that children from poorer residential areas were more vulnerable to heat and cold exposure. In their case, the window of opportunity in which extreme heat or cold led to permanent changes in the white matter of the brain opened earlier in life than in children from wealthier families. The authors suspect that these differences are due to housing conditions and energy poverty.

The mechanisms underlying the association between temperature exposure and the development of white brain matter are unclear. According to the researchers, it is possible that there is a connection with poorer sleep quality due to ambient temperatures that are too high or too low, which could affect neurological development. But a disruption of placental function, an activation of the hormonal axis that leads to higher cortisol production, or inflammatory processes could also play a role.

Our data underscore the vulnerability of fetuses and children to heat and cold, said Guxens. Strategies must be urgently developed to protect the most vulnerable members of our community from the impacts of the looming climate crisis. © nec/aerzteblatt.de