Swollen amgydalae suggest causes, and possibly even cures, for the debilitating disorder.

Researchers have discovered that the part of the brain known as the amygdala, which is involved in emotional and social processing, is abnormally large in young autistic children. What’s more, it reaches adult size far sooner than it would in non-autistic peers, despite containing fewer neurons than normal amygdalae.

“The amygdala has been implicated in a variety of different functions, probably most commonly in fear,” said David Amaral, a neuroscientist at the University of California, Davis and a co-author of the new report, which appeared in the July 12 issue of The Journal of Neuroscience. “If you look at lots of individuals with autism, one of the prevalent features is anxiety. It’s likely that the abnormal amygdala would probably participate in these abnormal fears.”

Amaral and his colleague Cynthia Schumann measured and counted the neurons in the post-mortem amygdalae of nine autistic males and 10 normal males.

They found that while the neurons were the same size across both groups, the males with autism had far fewer neurons in their amygdalae. A normal structure averaged 12 million neurons, Amaral said, while those of the autistic males had just 10.5 million. 

He added that it’s unlikely that all kids with autism have amygdala abnormalities or even that such abnormalities would account for the broad spectrum of autism’s effects.

“Fewer neurons is clearly an abnormality, and it’s probably responsible for some of the problems of autism,” said Elizabeth Aylward, a neuroscientist at the University of Washington.

It’s still hard to determine however whether the low neuron count causes some of autism’s symptoms or is a result of them, but according to University of Pittsburgh neuroscientist Nancy Minshew, the finding—the first actual neuron count in any part of the autistic brain—is significant. 

“This is really the first good quality quantitative data that we have,” she said.

Amaral and Schumann are currently conceptualizing a new developmental model of autism that reconciles their findings on neuron number as well as the earlier research on amydgala size: They postulate that the early overgrowth of the amygdala in autistic children likely produces an excess of fear and anxiety, traits typical of autism. Over time, this heightened stress response could have a damaging effect leading to neuron death and a reduction in the size of the amygdala.

It’s also possible that even though the amygdalae of autistic children start out large, they may always contain fewer neurons than those of their non-autistic peers. 

If further research supports the idea that the amygdala starts out with enough neurons and then loses them later, the implications would be stunning: The cell death may be preventable, possibly by anti-anxiety drugs—already on the market—that dampen activity in the amygdala.

“Given that you could detect who has risk factors for autism and who has the kind that affects the amygdala,” Amaral said, “I don’t think that it’s so far-fetched to think that you could intervene and stop the neuron loss.”

Originally published July 26, 2006

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