Brain Overgrowth Linked with Autism May Begin in Infancy

There is a portion of the brain that tends to grow larger in children with autism. While this link has been known for some time, it wasn’t clear how soon this occurs. Now, a new study has found the overgrowth may begin in infancy, meaning doctors could have a new tool for even earlier diagnosis.

The amygdala is a portion of the brain that helps process emotions, from recognizing the meaning behind others’ facial expressions to recognizing a fearful situation. An international team of researchers, whose work was funded by the National Institutes of Health, investigated its growth in a large group of young children. The findings, published in The American Journal of Psychiatry, show that the children in the study who were later diagnosed with autism had overgrowth of the amygdala at 12- and 24-months-old. This gives doctors an idea of when these changes first occur, opening up the possibility of even earlier intervention. It also provides further insight into autism’s development.


Dr. Stephen Dager, study co-author and radiology professor at the University of Washington School of Medicine, says, “We are getting closer to understanding why autism occurs by learning more about brain growth alterations early during development, in this case how amygdala growth may be influenced by early sensory processing difficulties and, conversely, how amygdala growth alterations may influence a baby’s interaction with their environment.”

To investigate amygdala growth, the team studied the brains of just over 400 infants, with 270 considered at higher risk of ASD due to having an older sibling on the spectrum. They also included 109 infants with typical development and 29 infants with a developmental and intellectual disability known as Fragile X syndrome. The team gave MRI scans to all of the children at 6, 12, and 24 months.

They found that the 58 children who went on to develop autism had a normal-sized amygdala at 6 months, but it had enlarged by the scans performed at 12 and 24 months. The differing growth rate among these children also had implications.


Dr. Mark Shen, first author and assistant professor of psychiatry and neuroscience at the University of North Carolina Chapel Hill, explains, “We also found that the rate of amygdala overgrowth in the first year is linked to the child’s social deficits at age two. The faster the amygdala grew in infancy, the more social difficulties the child showed when diagnosed with autism a year later.”

On the flip side, the children with Fragile X syndrome did not demonstrate a different amygdala growth rate, but other parts of their brains became enlarged.

For the children with autism, the team has a theory as to why the amygdala begins its overgrowth when it does. They say that past studies have shown infants who develop autism already demonstrate a difference in attention to visual stimuli in their first year of life. The growth in the amygdala may be due to increased stress caused by this type of information processing.


The University of Washington, which was involved in this study as part of the Infant Brain Imaging Study Network, is conducting further studies to see if brain imaging can identify infants who will likely develop autism.

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