Cortical hyperexcitability in amyotrophic lateral sclerosis: from pathogenesis to diagnosis

Identification of upper motor neuron involvement remains a critical component of a diagnosis of amyotrophic lateral sclerosis (ALS), although supportive clinical signs are often not easily appreciated, particularly in the early symptomatic stages of the disease. Although diagnostic criteria have been developed to facilitate improved detection of lower motor neuron impairment through electrophysiological features that have improved diagnostic sensitivity, assessment of upper motor neuron involvement remains problematic.

Recent evidence has emerged about pathophysiological processes, particularly glutamate-mediated excitotoxicity, which has resulted in the development of novel diagnostic investigations and uncovered potential therapeutic targets. Advances in genetics, including the C9orf72 gene, have changed concepts of ALS, from being classified as a neuromuscular disease to a disease that forms a continuum with other primary neurodegenerative disorders, particularly frontotemporal dementia. Transcranial magnetic stimulation has been utilized to provide pathophysiological insights, leading to the development of diagnostic and therapeutic biomarkers, which are now being introduced into the clinical setting.

Specifically, the advent of cortical hyperexcitability has been consistently identified as an early and intrinsic feature of ALS. With greater accessibility of TMS techniques promoting clinical utilization, TMS measures of cortical function may develop as a diagnostic biomarker, with further potential utility in the clinical trial setting for monitoring of neuroprotective and genetic-based therapies.