Diffusion tensor imaging of cerebellar peduncles and association tracts in a case of primary complete callosal agenesis

The corpus callosum is the largest white matter tract in the brain, connecting the left and right cerebral hemispheres. Callosal agenesis is a condition where this tract fails to form during neurodevelopment. Despite this, individuals with callosal agenesis can present as cognitively and behaviorally asymptomatic, retaining some alternate form of interhemispheric communication. Afferent and efferent pathways linking the cortex to the contralateral cerebellum, such as the corticopontocerebellar tract (CPCT) and dentatorubrothalamic tract (DRTT), could serve as compensatory routes in individuals with callosal agenesis, and thus display adaptive changes in white matter structure. To investigate this possibility, we examined a 9-year-old boy with complete callosal agenesis, who performed in line with his peers on IQ and social communication measures, and compared his cerebellar and association tractography to 12 age-matched controls. Diffusion tensor imaging (DTI) metrics were calculated for a whole-brain seed, the cerebellar peduncles, the pontine decussations, the arcuate fasciculus, and the uncinate fasciculus. Whole-brain metrics showed typical fractional anisotropy (FA) but increased mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) in the callosal agenesis child compared with controls. This pattern was also observed in the right superior cerebellar peduncle. Dorsal pontine fibers exhibited low FA and high RD values in callosal agenesis. The left uncinate and right arcuate fasciculi exhibited a similar pattern of low FA and high RD compared with controls. Overall, this case study characterizes microstructural differences in cerebellar and association tracts in a child with complete callosal agenesis and intact cognitive abilities.