A study by researchers at the University of North Carolina at Chapel Hill and Duke University found that, by age 2, children with autism show a generalized enlargement of their brains.
Exactly why this roughly 5 percent greater brain growth occurs and what it means are not yet clear.
Indirect evidence suggested that the increased brain growth probably began during the later months of the children's first year of life.
A report is published in the Archives of General Psychiatry.
" Our team conducted brain magnetic resonance imaging scans on 51 autistic and 25 control children at age 2, making it the largest study of two-year-olds with autism," Cody Hazlett said. "Analysis of brain tissue volumes showed significant enlargement, across all regions in both gray and white tissue, in the cerebral cortex of the autistic children.
" While we saw the greatest volume increases in the temporal lobe, an area of the brain involved in language, we concluded that at this age, tissue enlargement is present throughout the cortex."
In the same paper, the team reported on the largest retrospective study of head circumference in autism reported to date, comparing head circumference measurements on 113 autistic children to 190 other youngsters who served as controls. Measurements took place periodically from birth to age 3.
" Our head circumference data suggest that enlarged head size is not present at birth and that the onset of enlarged head size in autistic children begins, on average, at around 12 months," Cody Hazlett said. " These findings, together with our brain volume data, give us reason to believe that a period of brain overgrowth in autism may occur between 12 months and 2 years of age.
" We do not know whether this brain enlargement plays a primary role in autism, or is a downstream effect of another process," she said. " Further studies of very early brain development may help us better understand the timing and nature of this brain overgrowth."
About four times as many male children suffer from autism as females, Cody Hazlett said. The disorder varies in severity, but affected patients often face mental retardation, poorly developed language skills and difficulty developing social relationships.
Source: University of North Carolina at Chapel Hill, 2005