Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disorder of the motor neurons, characterized by upper motor neuron (UMN) and lower motor neuron (LMN) dysfunction. There have been significant technological advances in the development of neurophysiological biomarkers of UMN and LMN dysfunction in ALS. In this review, we discuss major advances in development of neurophysiological biomarkers in ALS, critiquing their potential in diagnosis and prognosis of ALS, as well as utility in monitoring treatment effects.
The threshold tracking transcranial magnetic stimulation (TMS) technique has established cortical hyperexcitability as an early and specific biomarker of UMN dysfunction in ALS, and associated with neurodegeneration. In addition to establishing cortical hyperexcitability as a pathophysiological mechanism, threshold tracking TMS has enabled an earlier diagnosis of ALS and provided a means of monitoring effects of therapeutic agents. Biomarkers of LMN dysfunction, including motor unit number estimation, the neurophysiological index, electrical impedance myography and axonal excitability techniques, have all exhibited utility in monitoring disease progression.
In addition to enhancing ALS diagnosis, the development of novel neurophysiological biomarkers has implications for clinical trials research and drug development, enabling the assessment of biological efficacy of agents in early stages of drug development.
aSydney Medical School, University of Sydney, Sydney, New South Wales
bDepartment of Neurology, Westmead Hospital, Westmead, New South Wales, Australia
cBeth Israel Deaconess Medical Center
dHarvard Medical School, Boston, Massachusetts, USA
Correspondence to Professor Steve Vucic, Department of Neurology, Westmead Hospital, Cnr Hawkesbury and Darcy Road, Westmead, NSW 2145, Australia. Tel: +61 2 9845 6097; fax: +61 2 9635 6684; e-mail: firstname.lastname@example.org