Researchers have long known that autism is a disorder related to the amygdala but have been unable to pinpoint when it begins. A new study by a team from North Carolina has, for the first time, provided insights into when the condition starts to develop.
Amygdala. Credit Amber Rieder
This research solves a longstanding puzzle by showing that the overgrowth of this vital brain structure begins when a baby is between six and 12 months old. This discovery could enable a much earlier diagnosis of the disorder than is currently possible.
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The study, which appeared in the American Journal of Psychiatry, also highlighted differences between autism and Fragile X syndrome, another brain disorder. There was no abnormal amygdala growth in cases of Fragile X syndrome.
The research was conducted by scientists from the Infant Brain Imaging Study (IBIS) Network, which includes 10 universities across the U.S. and Canada.
Amygdala and Autism
The amygdala is a small, almond-shaped cluster of cells located in the lower part of the brain. Humans have two amygdalae, one in each hemisphere of the brain.
This brain structure is essential for decoding social and emotional cues, such as interpreting facial expressions and recognizing potential threats.
For a long time, scientists have believed that the amygdala plays a key role in the social behavior challenges seen in autism. School-age children with autism typically exhibit an abnormally sized amygdala.
However, it was previously unknown when the amygdala begins to increase in size.
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Amygdala Overgrowth in Infants
To investigate when this brain structure starts to grow, researchers recruited 408 infants. This group included 58 infants at higher risk for autism (those with an older autistic sibling) who later developed the disorder. The remaining infants included 212 at higher risk for autism but who did not develop the disorder, 109 typically developing infants, and 29 infants with fragile X syndrome.
The IBIS team conducted magnetic resonance imaging (MRI) scans on these babies during natural sleep at six, 12, and 24 months old.
At six months, researchers found nothing unusual in the amygdala size of infants who later developed autism. However, the story changed after that point, with amygdala growth accelerating faster than in other babies. By 12 months, the amygdala of infants who developed autism was noticeably enlarged.
This enlargement continued through 24 months (age two), the point at which the disorder is typically diagnosed, with autistic behaviors becoming evident.
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“Our research suggests the optimal time to start interventions and support for children at the highest risk of developing autism may be during the first year of life,” said Joseph Piven, MD, the study’s senior author and Professor of Psychiatry and Pediatrics at the University of North Carolina at Chapel Hill. “The focus of a pre-symptomatic intervention might be to improve visual and other sensory processing in babies before social symptoms even appear.”
The research also showed that babies with Fragile X syndrome were already exhibiting cognitive deficits by six months old. In contrast, those who later developed autism did not show such deficits at that time but began experiencing a slow cognitive decline between six months and two years of age.
As for what might cause the amygdala overgrowth, researchers theorize that increased stress on the structure may be a factor.
Research into other mental health conditions, such as anxiety and depression, has shown a link between amygdala overgrowth and chronic stress. Researchers now hope this will help further understanding of autism development in infants.
References
Shen, M. D., Swanson, M. R., Wolff, J. J., Elison, J. T., Girault, J. B., Kim, S. H., Smith, R. G., & for the IBIS Network. (2022). Subcortical brain development in autism and Fragile X syndrome: Evidence for dynamic, age-, and disorder-specific trajectories in infancy. American Journal of Psychiatry, 179(8), 569–581. https://doi.org/10.1176/appi.ajp.21090896
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