Atypical Hemispheric Asymmetry of the Corticospinal Tract In Autism Spectrum Disorder

Background:  

Most previous diffusion tensor imaging (DTI) studies in ASD have focused on corticocortical tracts (such as corpus callosum), whereas less is known about subcortical tracts supporting sensorimotor functions. We examined the corticospinal tract, which contains motor axons connecting cerebral cortex and spinal cord and is crucial for voluntary motor control. Any corticospinal abnormalities may therefore be related to motor impairments observed in previous studies.

Objectives:  

To examine the white matter integrity and hemispheric asymmetry of the corticospinal tract in children and adolescents with ASD, using probabilistic white matter fiber tracking.

Methods:  

DTI data from 11 children with ASD and 12 typically developing (TD) children were acquired on a 3T MRI scanner, using single-shot diffusion-weighted EPI pulse sequence with two degrees of diffusion weighting (b=0 and 1000 s/mm², 61 non-linear directions, 1.875x1.875 mm² in-plane resolution, 2 mm slice thickness). ASD and TD participants were matched for gender, age, verbal and nonverbal IQ, and handedness. Geometric distortions due to local magnetic field inhomogeneities were corrected using field maps. Diffusion tensor tractography was performed using a probabilistic tracking approach from two regions-of-interest (ROIs) placed in the primary motor cortex and the cerebral peduncle to derive the corticospinal tracts. Bayesian estimation of diffusion parameters using Markov Chain Monte Carlo sampling techniques and trilinear interpolation of the probability density functions were employed to determine the streamline between the given ROIs. Mean diffusivity (MD), and radial diffusivity of the corticospinal tracts were calculated. Asymmetry of corticospinal tract was determined for each index using the formula [2*(R-L)/(R+L)]*100.

Results:  

A marginally significant increase of mean diffusivity (MD) for corticospinal tract in the right hemisphere was found in the ASD group compared to the TD group (TD: MD = .95 +/- .04*10^-3; ASD: MD .98 +/- .05*10^-3 TD mm²/s, p=.07). There was an atypical rightward shift for MD (F=5.70, p=.04) and radial diffusivity (F=5.05, p=.02) in the ASD group.

Conclusions:  

Using probabilistic white matter tractography, we were able to delineate the corticospinal tracts linking primary motor cortex and cerebral peduncles. The results suggest an atypical corticospinal tract asymmetry in ASD, with greater impairment in the right hemisphere. Corticospinal tracts play an important role in motor and somatosensory functions. Our findings may relate to sensorimotor impairment and atypical hemispheric dominance in ASD.