ObjectiveDamage to the corpus callosum (CC) in amyotrophic lateral sclerosis (ALS) patients has been confirmed via electrophysiological, neuroimaging, and autopsy studies. Additionally, the CC is hypothesized to serve as a pathway for the spread of pathological information. This study aimed to investigate whether the CC plays a mediating role in the symptomatic spread of ALS.MethodsIn this observational study, diffusion tensor imaging (DTI) data were acquired from 45 individuals with the upper motor neuron-dominant (UMN-D) phenotype of ALS. The UMN-D ALS patients were categorized into two groups based on the direction of symptom spread, including 25 patients with horizontal spread (group H) and 20 patients with vertical spread (group V). Diffusivity indices were derived through whole-brain analysis and probabilistic fiber tracking.ResultsAccording to the voxel-based analysis and tract-based spatial statistics, differences in axial diffusivity (AD) in the CC were observed between disease subgroups, with patients in group H showing higher AD values than those in group V. Fiber tracking analysis revealed persistent differences in the AD indices of CC-primary motor cortex (PMC) fibers between the two disease subgroups.ConclusionIn UMN-D ALS patients, the direction of symptom spread may be related to the degree of CC involvement. The AD metric may be a more specific indicator of CC damage.