Findings from another study indicate that several brain structures, including the cerebrum and cerebellum, are larger in young children with autism than similarly aged normal children or developmentally delayed children.

In the first study, Dr. Elizabeth H. Aylward, from the University of Washington in Seattle, and colleagues used MRI to compare the brain volumes of 67 autistic patients with those of 83 healthy volunteers. The head circumference of all subjects was also measured. The subjects ranged in age from 8 to 46 years.

The researchers found that in subjects 12 years of age and younger, brain volumes were significantly larger in the autistic group than in the control group. In older subjects, however, no significant differences in brain volume were observed. In both age groups head circumference was greater in autistic patients than controls, suggesting that the older patients probably had increased brain volumes during childhood.

In another study, Dr. Stephen R. Dager, also from the University of Washington, and colleagues used volumetric MRI to compare the sizes of several brain structures in 3- to 4-year-old children with and without autism. The study included 45 children with autism, 14 with developmental delay, and 26 normal children.

Cerebral volume was significantly increased in autistic children compared with the other two groups. In addition, volumes of the cerebellum, amygdala, and hippocampus were increased in autistic children, but all of these increases were proportional to the overall increase in total cerebral volume.

Structural abnormalities were observed more often in boys with autism than in girls, but the statistical power of this observation is limited because the study group only included seven girls. The structural findings did not correlate with nonverbal IQ, the authors note.

In a related editorial, Dr. Jonathan W. Mink, from the University of Rochester in New York, and Dr. Robert C. McKinstry, from Washington University in St. Louis, discuss the implications of the current findings and make suggestions for the focus of future research.

"We now have pretty good evidence that the brains of young autistic patients are larger than those of healthy subjects," Dr. Mink told Reuters Health. "However, we still really don't know much about the biology of the disorder."

Dr. Mink believes that future research should move beyond just describing the differences between normal and autistic brains. "We really need to have specific hypotheses," he explained.

"For example, a fundamental problem in autism is language development and we know the areas of the brain that are involved in language," Dr. Mink noted. "Therefore, a study could be done focusing specifically on abnormalities in those areas," he said. However, for studies like this to be successful a homogeneous autistic population is needed, which can be difficult to find because autism is such a heterogeneous disorder, he added.

Neurology 2002;59:158-159,175-192.