Left hemisphere diffusivity of the arcuate fasciculus: influences of autism spectrum disorder and language impairment.

BACKGROUND AND PURPOSE: There has been much discussion whether brain abnormalities associated with specific language impairment and autism with language impairment are shared or are disorder specific. Although white matter tract abnormalities are observed in both specific language impairment and autism spectrum disorders, the similarities and differences in the white matter abnormalities in these 2 disorders have not been fully determined. MATERIALS AND METHODS: Diffusion tensor imaging diffusion parameters of the arcuate fasciculus were measured in 14 children with specific language impairment as well as in 16 children with autism spectrum disorder with language impairment, 18 with autism spectrum disorder without language impairment, and 25 age-matched typically developing control participants. RESULTS: Language impairment and autism spectrum disorder both had (elevating) main effects on mean diffusivity of the left arcuate fasciculus, initially suggesting a shared white matter substrate abnormality. Analysis of axial and radial diffusivity components, however, indicated that autism spectrum disorder and language impairment differentially affect white matter microstructural properties, with a main effect of autism spectrum disorder on axial diffusivity and a main effect of language impairment on radial diffusivity. CONCLUSIONS: Although white matter abnormalities appear similar in language impairment and autism spectrum disorder when examining broad white matter measures, a more detailed analysis indicates different mechanisms for the white matter microstructural anomalies associated with language impairment and autism spectrum disorder.