UC Davis study does not find differences in brain size of girls with autism or boys with early onset autism.

David Amaral and Christine Wu Nordahl, UC Davis

In the largest study of brain development in preschoolers with autism to date, a study by UC Davis MIND Institute researchers has found that 3-year-old boys with regressive autism, but not early onset autism, have larger brains than their healthy counterparts.

The study is published online today (Nov. 28) in the Proceedings of the National Academy of Sciences Early Edition. It was led by Christine Wu Nordahl, a researcher at the UC Davis MIND Institute and an assistant professor in the Department of Psychiatry and Behavioral Sciences and David G. Amaral, Beneto Foundation chair, MIND Institute research director and University of California Distinguished Professor in the Department of Psychiatry and Behavioral Sciences.

“The finding that boys with regressive autism show a different form of neuropathology than boys with early onset autism is novel,” Nordahl said. “Moreover, when we evaluated girls with autism separately from boys, we found that no girls — regardless of whether they had early onset or regressive autism — had abnormal brain growth.”

Brain enlargement has been observed in previous studies of autism. However, prior to this study, little was known about how many and which children with autism have abnormally large brains.

“This adds to the growing evidence that there are multiple biological subtypes of autism, with different neurobiological underpinnings,” Amaral said.

Autism is a neurodevelopmental disorder whose symptoms include deficits in language and social interaction and communication. The condition affects 1 in 110 children born today, according to the U.S. Centers for Disease Control and Prevention. It is diagnosed more frequently in male children than female children — at a ratio of 4 to 1.

The current study is one of the first published from data collected by the UC Davis MIND Institute Autism Phenome Project (APP). The project’s goal is to recruit and enroll as many very young children as possible in order to collect sufficient biological and behavioral information to characterize different autism subgroups and to explore different neural, immunologic, and genetic signatures of autism.

For the study, the authors enrolled a total of 180 children between age 2 and 4. One hundred and fourteen of the participants had autism spectrum disorder; the remaining participants were 66 age-matched typically developing controls. Of the children with autism, 54 percent were diagnosed with the regressive form and 46 with the non-regressive type.

The researchers collected magnetic resonance imaging (MRI) scans on 180 participants at age 3. To evaluate the rate of brain growth prior to age 3, they analyzed head circumference measurements taken from pediatric well-baby visits from birth through 18 months. Roughly half of the children with autism were reported by their parents as having experienced a regression, characterized by the loss of previously acquired language and social skills.

The MRIs were carried out on study participants during natural, nighttime sleep using protocols developed specifically for the Autism Phenome Project by Nordahl.

“Obtaining MRI scans in 3-year-old children without the use of sedation may seem quite challenging. But, by working closely with the parents, we actually were successful more than 85 percent of the time. Patience on the part of everyone and the dedication of the families was critical for our success,” Nordahl said.

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