British Society for Clinical Neurophysiology

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Abstract Details

Threshold-tracking transcranial magnetic stimulation (TMS) is a quick and reliable method for point estimates of resting motor threshold (RMT) and uninterrupted monitoring of its fluctuations

RMT is a baseline characteristic of corticospinal excitability commonly used to adjust stimulation parameters for other TMS protocols. Conventionally, RMT is defined as stimulus intensity required to obtain a response in 50% of consecutive trials, and currently two probabilistic approaches – relative frequency and adaptive maximum-likelihood – are recommended by the International Federation of Clinical Neurophysiology [1].
Alternatively, motor threshold can be defined as stimulation intensity required to maintain a response of a specific size and obtained by threshold-tracking procedures [2]. Here, we compare threshold-tracking and its reliability to well-established RMT estimation methods.
RMT with conventional cut-off value of 0.05 mV was measured in 24 healthy volunteers (11 men; median age 22 years, range 18-55 years; all self-reported right-handed) from first dorsal interosseous muscle of the dominant hand using three methods: i) relative frequency (RF, 10/20 positive trial rule); ii) best PEST (parameter estimation by sequential testing) – an adaptive maximum-likelihood procedure; and iii) threshold-tracking (TT). Measurements were repeated twice by a single operator on the same day. Data is presented as mean ± standard deviation.
No difference in mean group RMT estimates was observed between the methods (RF 50.6±8.3, PEST 50.3±8.2, TT 50.9±9.1 % maximum stimulator output, p>0.05), but the duration of procedure differed considerably (number of stimuli required: RF 60±26, PEST 19±2, TT 12±4, p<0.05). All methods had excellent reproducibility (intraclass correlation coefficients: RF 0.89, PEST 0.90, TT 0.88) and similar test-retest repeatability (smallest detectable change: RF 7.8%, PEST 7.3%, TT 8.9% maximum stimulator output).
In conclusion, all methods provide the same value of RMT, but threshold-tracking offers an improved speed without fundamentally compromising the reliability of repeated measurements. While probabilistic methods provide point estimates only, threshold-tracking allows uninterrupted monitoring of RMT and therefore opens new avenues in TMS research and its clinical application.

TitleForenamesSurnameInstitutionLead AuthorPresenter
DrGintauteSamusyteUCL Institute of Neurology
MissNadaIbrahimNational Hospital for Neurology and Neurosurgery
ProfJohn CRothwellSobell Department
ProfHughBostockSobell Department
ProfMartinKoltzenburgNational Hospital for Neurology & Neurosurgery
Reference
[1] Rossini, P.M. et al. (2015) 'Non-invasive electrical and magnetic stimulation of the brain, spinal cord, roots and peripheral nerves: Basic principles and procedures for routine clinical and research application': Clinical Neurophysiology, 126(6), pp. 1071-1107
[2] Fisher, R.J., et al. (2002) 'Two phases of intracortical inhibition revealed by transcranial magnetic threshold tracking': Experimental Brain Research, 143(2), pp. 240-248.