This study aimed to identify the white matter tracts contributing to the development of AHS in the patients with ischemic stroke. Our results suggest that damage to most of sections of CC, especially in the parietal and temporal section of the body of CC is associated with development of AHS after stroke.
In this study, we enrolled patients with corpus callosum lesions resulting from ACA territory stroke. CC consists of the primary commissural fibers connecting both cerebral hemispheres and plays a crucial role in the interhemispheric transfer of information.(Goldstein et al., 2024) Damage to the CC is a key factor in the development of AHS, which is a common neurological manifestation of callosal disconnection syndrome .(Chan & Ross, 1997; Hassan & Josephs, 2016; Jang et al., 2013a; Molko et al., 2002) Hassan et al proposed 3 types of AHS: frontal, callosal and posterior.(Hassan & Josephs, 2016) Among these, the anterior variant AHS, which includes the frontal and callosal variant, is associated with CC. Frontal AHS, affecting the dominant hand, is typically caused by a lesion in the dominant medial frontal lobe, the supplementary motor area, or the anterior portion of the CC. Callosal AHS, primarily caused by lesions in the CC, is characterized by intermanual conflict affecting the non-dominant (interhemispheric disconnection theory).(Feinberg et al., 1992; Goldberg & Bloom, 1990) Previous DTI study reported decreased FA value of the CC in a AHS patient (Jang et al., 2013a; Jang et al., 2013b; Sugawara et al., 2023), and our results also revealed decreased FA values in the CC, similar to previous findings. Thus, a lesion in the CC appears to be a necessary condition for the development of AHS.
In our study, the FA vale of the posterior portions of the CC body, specifically the parietal and temporal section, only AHS group was significantly lower than that in the control group. This result suggests that these two sections of the CC may play a more critical role in the manifestation of AHS during callosal disconnection syndrome. Previous studies on patients with CC lesions have documented a high variability in callosal disconnection syndrome, dependent on the location and extent of CC damage in each case.(Jang et al., 2013a) Previous studies reported that the lesions in the posterior part of the CC were linked with various symptoms, including tactile anomia, agraphia, somatosensory deficits, alexia, neglect, visual anomia, and impairment of visual recognition. (Koch et al., 2011) These symptoms, characterized by strong feelings of estrangement from the affected limb and parietal sensory deficits, significantly influence the manifestation of AHS features.(Hassan & Josephs, 2016)
In patients without AHS, the parietal and temporal sections of the CC body may serve as compensatory fibers for interhemispheric connections following damage to the anterior CC. In a case study with AHS after hemorrhage of CC, the patient’s AHS resolved after 7 weeks. Tractography revealed disrupted callosal fibers and unusual neural connections through temporal lobe fibers, suggesting compensatory mechanisms for recovery following damage to the CC fibers.(Jang et al., 2013b)
Neuroimaging and clinical studies implicated the supplementary motor area (SMA), pre-SMA, and their network connections in the development of the anterior variant of AHS.(Hassan & Josephs, 2016) The SMA, located on the mesial surface of the frontal lobe, is renowned for its role in motor planning, initiation, and even inhibition.(Hassan & Josephs, 2016) And the FAT is a white matter tract that connects the SMA and pre-SMA to the inferior frontal gyrus, which has also been identified as an emerging locus for motor inhibitory control in several studies.(Dick et al., 2019; Hampshire et al., 2010) SLF I originates from the superior parietal lobe and terminates within the SMA and premotor areas in the frontal lobe, playing a crucial role in proprioception and motor movement.(Schmahmann et al., 2008) The SLF II, which plays a role in visual-spatial awareness and attention, originates from the posterolateral parietal lobe.(Schmahmann et al., 2008) It extends through the postcentral, precentral, and middle frontal gyri, terminating within the dorsolateral prefrontal cortex.(Schmahmann et al., 2008) And the SLF III originates from the supramarginal gyrus and extends towards the ILF, terminating within the dorsal prefrontal cortex.(Schmahmann et al., 2008) This bundle is instrumental in processing somatosensory input and facilitating fine movements of the hand.(Schmahmann et al., 2008) These findings underscore that damage to the FAT and SLF can lead to a loss of inhibitory control over motor execution, contributing to the development of AHS. However, the FA values of FAT and SLF showed a significant difference from the control, with no difference between AHS and N-AHS groups. While the functions of the FAT and SLF are not yet completely understood, the causal relationship between damage to these tracts and the occurrence of AHS warrants further analysis in future studies.
The interpretation of studies on AHS is currently limited by the small number of subjects. However, none of the previously reported studies have analyzed all white matter tracts using DTI for studying AHS. Therefore, our results will provide additional insights regarding the neural pathways associated with AHS, and its underlying neural mechanisms.