P79-F Disrupted muscle cortical representations in amyotrophic lateral sclerosis studied by navigated TMS mapping

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Background The search for biomarkers in amyotrophic lateral sclerosis (ALS) can provide pathophysiological insights and is important for early diagnosis and monitoring of the disease progression. Mapping muscle cortical representations using navigated transcranial magnetic stimulation (nTMS) is a noninvasive method for probing corticospinal excitability. We aimed at studying nTMS map parameters as potential biomarkers in ALS. Methods We examined 12 ALS patients (4 women, age median 49, quartiles 40, 55) and 7 healthy controls (5 women, age median 37, quartiles 35, 41). Navigated TMS mapping was performed at the intensity of 110% of the resting motor threshold (RMT) with motor evoked potentials (MEPs) recorded from the abductor pollicis brevis muscle (APB). We used a 7 × 7 stimulation point grid, with 5 stimuli per cell in a pseudorandom order. We analyzed the RMT, the area of cells with at least one suprathreshold MEP, the area weighted by the probability of a suprathreshold MEP, and the area weighted by the mean MEP amplitude. Results The groups were not significantly different in age. The RMT was higher in ALS (p = 0.01, Mann-Whitney). Among the studied representation parameters, only the area weighted by the mean MEP amplitude showed a significant difference, with lower values in ALS patients than in controls (p = 0.02). Conclusions The results suggest that the extent of the APB motor representation in ALS patients is not significantly altered, whereas the excitability and amplitude-weighted area are reduced. Thus, it is important to study motor representation parameters other than the extent as potential ALS biomarkers. Funding RSF17-75-10062.