Structural MRI in autism: Findings and future directions

Structural MRI studies of the brain in autism have yielded inconsistent results until recent years. Studies over the past decade have revealed several exciting new findings and have fostered novel hypotheses about the onset and etiology of this disorder. The most consistent MRI finding in autism is that the brain is enlarged. Studies have suggested that brain overgrowth may be most robust early in development, but increased brain volume has been observed throughout adolescence and early adult life. Retrospective head circumference studies have indicated that the onset of brain enlargement may occur during the latter part of the first year of life and does not appear to be present at birth. Recent studies of infant siblings of children with autism suggest that the onset of the core behavioral features of autism also occur during the latter part of the first year of life and may not be present by 6 months of age. The coincident timing of the onset of brain and behavioral abnormalities in autism suggests that these features may be related. Future longitudinal MRI studies of infant siblings of children with autism will help elucidate this relationship and potentially delineate the pathogenesis of this disorder. Additional findings from structural MRI studies of autism have begun to map patterns of brain overgrowth across cortical lobes and tissue types (gray and white matter). These studies are somewhat inconsistent, but suggest generalized overgrowth affecting both cortical gray and cortical white matter, as well as several subcortical structures. The diffuse network of regions affected has shifted research attention from hypotheses about specific regions and structures to more widespread mechanisms involving neural circuits and diffuse mechanisms at the neuronal level. These findings, their implications for our understanding of the pathogenesis of autism, and future directions for structural MRI studies of autism are discussed.