Assessing upper motor neuron dysfunction in ALS: from TMS-EEG and EMG neurophysiology to a combined tFUS-TMS...

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by the progressive loss of upper motor neurons (UMNs) and lower motor neurons (LMNs). Despite significant advances in molecular and neuroimaging biomarkers, the initial site of pathology and the causal contribution of UMN dysfunction to disease progression remain undetermined. Accumulating neurophysiological evidence points to cortical hyperexcitability as an early and potentially upstream mechanism, raising the possibility that UMN pathology drives LMN degeneration through an anterograde dying-forward process. In this review, we synthesize findings from noninvasive brain stimulation (NIBS) studies, with particular emphasis on transcranial magnetic stimulation (TMS)-based neurophysiological markers of UMN dysfunction. We review evidence from TMS-electromyography (TMS-EMG) and TMS-electroencephalography (TMS-EEG) paradigms demonstrating cortical disinhibition and excitatory-inhibitory imbalance in ALS, consistent with impaired GABAergic interneuronal dysfunction and supportive of a cortical onset hypothesis. Finally, we propose integrating transcranial focused ultrasound (tFUS) with TMS as a novel experimental and translational framework to directly examine and modulate cortical hyperexcitability and test the causal role of UMN dysfunction in ALS. The combination of targeted neuromodulation with sensitive neurophysiological readouts in controlled experimental designs offers a promising avenue to advance mechanistic insight, refine biomarkers, and inform mechanism-based therapeutic strategies. Together, these approaches position noninvasive neurophysiology as a powerful tool for elucidating UMN dysfunction in ALS.