The transcallosal highway: the ipsilateral silent period as a neural biomarker for impaired corpus callosum communication and gait asymmetry in people with multiple sclerosis
Date
2023
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Abstract
Multiple sclerosis is a neurodegenerative disease that damages the myelin sheath within the central nervous system. Axonal demyelination, particularly in the corpus callosum, impacts communication between the brain's hemispheres in persons with multiple sclerosis (PwMS). Changes in transcallosal communication impairs the coordination of gait which requires constant communication across the corpus callosum to excite and inhibit specific muscle groups. To further evaluate the functional role of transcallosal communication in gait and mobility, this study assessed the ipsilateral silent period (iSP), an indirect marker of transcallosal inhibition in PwMS. This study utilizes transcranial magnetic stimulation (TMS) to assess the inhibitory capacity between the brain's hemispheres. There is a lack of research analyzing directionality data between the more and less affected hemisphere in PwMS. Therefore, we evaluated outcome metrics dependent upon the individual's more affected hemisphere calculated from the subject's more affected limb observed during walking assessments and self-report. We hypothesize that the iSP may serve as a neural biomarker for transcallosal impairments evaluated by directionality differences between the hemispheres and highlight transcallosal inhibition as an underlying neural mechanism for gait asymmetries in PwMS. From twenty-nine PwMS, metrics such as depth iSP% average, duration, depth iSP% max, and onset latency were collected. No statistically significant differences were found between the two hemispheres. This suggests that PwMS may be able to preserve their interhemispheric inhibitory capacity irrespective of their more affected hemisphere. Additionally, another component of the study investigated gait coordination utilizing a split-belt treadmill training paradigm. Limb excursion asymmetry (LEA) measures, pre and post-training, were analyzed for spatial coordination and as a measurement of locomotor adaptability in PwMS. The relationship between LEA change and dSP% average highlighted a significant correlation (r=0.46, p= 0.02). Thus, showing that less interhemispheric inhibition corresponds with more spatial adaptability leading to a more symmetric gait. These findings may help determine the potential of iSPs as a neural biomarker to address gait asymmetries and stratify participants into mobility rehabilitation protocols.
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multiple sclerosis