Total brain volumes were measured by MRI in 67 non-mentally retarded children with autism and 83 healthy controls, aged 8 to 46 years, in a study at University of Washington, Seattle: Johns Hopkins University Hospital, Baltimore: and University of Pittsburgh School of Medicine, PA. Head circumference was also measured. Children with autism 12 years of age and younger had significantly larger brain volumes (average 5% increase) compared to normal children. Autism and control groups older than age 12 years showed no differences in brain volume. The accelerated growth in brain volume and increased head circumference noted in younger children with autism was slowed to an average of 1 to 2% increase in older children. Brain volume in adolescents and adults with autism appears to show a slight decrease while that of normal controls continues to increase. 
COMMENT. Autism is a spectrum of developmental disorders, grouped under the term ‘pervasive developmental disorder’ (PDD), with impairments of social interaction, communication, and repetitive or restricted behaviors. While autistic disorder is characterized by all three behavior patterns, Asperger’s syndrome is milder than autistic disorder and is distinguished by normal language development. Within these categories the clinical manifestations show wide heterogeneity. The cause of autism is unknown and the clinical variation in the degrees of autistic spectrum disorder complicate the evaluation of neuroimaging studies. As pointed out by Mink JW and McKinstry RC, in their editorial , despite some contradictory results, imaging studies may provide a noninvasive in vivo mathod of investigating the neurobiology of autism.
The above study by Aylward and associates and that of Sparks et al, in the same issue , found increases in brain volume of young children with autism. Sparks also found that cerebellar, amygdalar, and hippocampal volumes increased in proportion to growth of the cerebral volume of autistic children. The average cerebral volume is 5 to 10% larger in autistic compared to normal children. After the age of 12 years, brain growth shows a plateau, and brain size in adolescents and adults is similar in autistic and control patients. Future studies will require careful selection of subjects and measurement of the growth of specific brain regions. The neurobiological and functional significance of these structural changes in the brain remains to be determined.