Assessment of central motor conduction to intrinsic hand muscles using the triple stimulation technique: normal values and repeatability

Objective: To establish the triple stimulation technique (TST) for recordings from the first dorsal interosseus (FDI) and the abductor pollicis brevis muscles (APB), and to analyse the test–retest repeatability of the TST measurements in APB. Methods: The recently developed TST was slightly modified for recordings from small hand muscles to account for volume conducted activity from surrounding muscles. The TST combines transcranial magnetic stimulation (TMS) with a peripheral collision technique [Magistris et al. Brain 121 (1998) 437]. In contrast to conventional motor-evoked potentials (MEPs), it quantifies the number of conducting central motor neurons (expressed by the TST amplitude ratio, TST–AR). MEPs and TST were performed in 30 sides of 25 healthy subjects (target muscle FDI), and in 29 sides of 21 healthy subjects (target muscle APB). All APB recordings were repeated after 25±5.9 days. Results: The TST–AR averaged 97.4±2.5% in FDI and 95.9±4.7% in APB. There was a mean difference of the TST–AR ratio of 2.9±3.1% between the repeated APB recordings (95% limits of agreement±6.3%). Conclusions: TMS allows activation of virtually all motor neurons supplying FDI and APB, when effects of volume conduction are eliminated. Its test–retest repeatability is excellent. Significance: The TST is well suited for follow-up examinations of central motor conduction failures. The greater number of established target muscles widens its clinical applicability