Researchers at Stanford University, CA, and University of North Carolina, Chapel Hill, used voxel-based morphometric analyses, and multivariate pattern and clustering analyses to compare brain MRI studies in 165 young boys, aged 1.57-4.15 years, diagnosed with idiopathic autism (iAUT) or fragile X syndrome (FXS) and idiopathic developmentally delayed and normal controls. Frontal and temporal gray and white matter regions of brains of boys with iAUT and FXS were developmentally aberrant as compared with controls. The differences were in opposite directions in iAUT and FXS relative to controls; iAUT brains had greater volume compared to controls, and FXS brains had smaller volume than control brains. Pattern of brain structure in iAUT generally resembled that of controls more than FXS, with or without autism. 
COMMENT. The authors conclude that idiopathic autism and fragile X syndrome have distinct neuroanatomical patterns, a finding that supports the neurobiological heterogeneity of idiopathic autism. They suggest that significant differences in aberrant brain morphometry may be found in other ASD-associated genetic disorders, eg Angelman syndrome, Rett syndrome. ASD has been diagnosed in more than 20 genetically determined syndromes, most prominently, FXS. (Harris JC. Editorial 2011; Moss J et al. 2009).
Brain size in autism. Cerebral cortical enlargement is reported in children with ASD examined by MRI at both 2 and 4-5 years of age . A disproportionate enlargement in temporal lobe white matter was observed in ASD children (n=38) cf controls (n=21). No significant difference from controls was observed in the rate of brain growth between 2 and 4 years of age. Brain enlargement observed in children with ASD is a consequence of an increased rate of brain growth with onset postnatally, before age 2 years and probably in the latter part of the first year of life. This early brain enlargement with increased cortical volume, but not cortical thickness, may be associated with increased cortical surface area and a distinct pathogenic mechanism. The early cortical overgrowth in young children with ASD is followed by a period of cortical thinning during adolescence and adulthood.