The differences in brain anatomy between girls with autism and girls without autism are greater than the differences between autistic boys and non-autistic boys.

That’s the conclusion reached by researchers at the University of California, Davis, who have completed a study that included a higher percentage of autistic girls than previous research.

The researchers said the findings eventually could explain why autistic girls have more severe symptoms than boys and possibly lead to new treatments.

They published their work today in Molecular Autism.

Autism Girls

The team analyzed the corpus callosum,a fiber bundle that bridges the left and right cerebral hemispheres and facilitates communication.

They found that those with autism had a different organization of callosalfibers, especially in the fibers projecting to the frontal lobe, where goal-directed behavior and executive function happens. 

Researchers also observed differences between the sexes in the ability of the callosal fibers to diffuse information.

“The study started out as a general evaluation of the corpus callosum in young children with autism. I didn't set out to look at sex differences,” says lead study author Christine Wu Nordahl, Ph.D., an assistant professor in U.C. Davis’ department of psychiatry and behavioral sciences. “But, I was surprised when the sex differences started emerging as the most clear result.”

Previous studies have found alterations in the corpus callosum in children and adults with autism, but most studies were focused on males only or had small sample sizes of females, Wu Nordahl said.

Get the Facts: What Is autism? »

Research Focused on Differences Within the Sexes

This study included 112 boys and 27 girls with autism spectrum disorder (ASD) as well as 53 typically developing boys and 29 typically developing girls. Researchers used an imaging technique called diffusion tensor imaging to evaluate the organization of fibers that project from the corpus callosum to different parts of the brain.

The study was Wu Nordahl’s first look into the neural phenotype of autism in girls. What she and her team found was that the difference between girls with autism and typically developing female peers is far greater than the difference between boys with autism and typically developing male peers. Wu Nordahl will be presenting these behavioral findings at the International Meeting for Autism Research, held this week in Salt Lake City.

“These differences may affect how boys and girls are diagnosed with autism. As well as potentially the types of treatments or interventions that boys and girls with autism receive,” she said.

Ultimately, the goal is to delineate the biological and behavioral differences between boys and girls with autism so they can better understand the underlying cause. Eventually that could lead to better, more focused treatments.

The bottom line is that researchers still don’t know why the deviation of girls with autism from their typically developing female peers is greater than it is in the male population.

“Recent genetic studies suggest that girls have a greater number of mutations than boys with autism. There are some MRI studies suggesting that girls have more neural abnormalities than boys,” Wu Nordahl says.

Which means that when girls have autism, it may have a greater effect.

Learn More: Which States Have the Highest Autism Rates? »

Researchers Recruiting More Girls for Studies

The U.C. Davis MIND Institute Girls with Autism Imaging of Neurodevelopment (GAIN) study is ongoing. Its goal is to expand the research around autism in girls.

Researchers with the GAIN study want to recruit an additional 100 girls with autism to bolster current studies.

“We don't yet know enough about females with autism because most research studies do not have equal numbers of females and males with autism in their samples,” Wu Nordahl says.

“There is a prevailing theory called the ‘female protective effect’ that suggests that females are protected from autism up to a certain point, and this may be why there are fewer girls with autism,” she says.

The lack of research on females with autism isn’t necessarily surprising considering incidence is much higher among males. An estimated 1 in 42 boys has autism, while only 1 in 189 girls have autism, according to the .

“We need to do a better job of trying to recruit females with autism into our studies so that we can fully explore differences between males and females with autism,” Wu Nordahl says.

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