Localization of white matter volume increase in autism and developmental language disorder

TLDR

Autism and developmental language disorder both show increased brain volume driven mainly by unexplained white matter enlargement, particularly in radiate white matter that myelinates later, consistent with postnatal head circumference increases. The study aims to localize this white matter enlargement to guide research into its cause, tissue basis, and functional implications. The authors used a white matter parcellation method that divides cerebral white matter into an outer radiate compartment and an inner sagittal/bridging compartment. Both high‑functioning autism and DLD exhibit radiate white matter enlargement—across all lobes in autism and all but parietal in DLD—with greater increases in later‑myelinating regions, while inner white matter, cortex, corpus callosum, and internal capsule volumes are unchanged, suggesting an intrinsic postnatal white‑matter process that links the two disorders on a spectrum.

Abstract

Abstract Increased brain volume in autism appears to be driven mainly by an unexplained white matter enlargement, and we have reported a similar phenomenon in developmental language disorder (DLD). Localization of this enlargement would strongly guide research into its cause, tissue basis, and functional implications. We utilized a white matter parcellation technique that divides cerebral white matter into an outer zone containing the radiate compartment and an inner zone containing sagittal and bridging system compartments. In both high‐functioning autism and DLD, enlargement localized to the radiate white matter (all lobes in autism, all but parietal in DLD), whereas inner zone white matter compartments showed no volume differences from controls. Furthermore, in both autism and DLD, later or longer‐myelinating regions showed greater volume increases over controls. Neither group showed cerebral cortex, corpus callosum, or internal capsule volume differences from control. Radiate white matter myelinates later than deep white matter; this pattern of enlargement thus is consistent with striking postnatal head circumference percentile increases reported in autism. These findings suggest an ongoing postnatal process in both autism and DLD that is probably intrinsic to white matter, that primarily affects intrahemispheric and corticocortical connections, and that places these two disorders on the same spectrum.