Advanced Technology or Manual Skills: Evidence That The Accuracy of Corticospinal Responsiveness Is Not Ameliorated by Neuronavigation
Authors: Barbi Chiara , Vernillo Gianluca , Giuriato Gaia , Laginestra Fabio G. , Martignon Camilla , Emadi Andani Mehran , Schena Federico , Venturelli Massimo 
Published: May 2022

Purpose

Transcranial magnetic stimulation (TMS) is a noninvasive technique to assess corticospinal excitability and inhibition. TMS over the motor cortex during a voluntary contraction elicits short‐latency excitatory (motor‐evoked potential, MEP) and inhibitory (silent period, SP) responses in the target muscle. However, the technique for searching and determining the optimal site to elicit MEPs (“hotspot”), an essential component in order to correctly place the TMS coil, is not unanimously standardized. This can be a problem since changes in coil position and/or orientation may influence the magnitude of MEP and SP responses. Usually, the “hotspot” is marked either on a swim cap or on the scalp and the investigator holds the coil manually during the entire testing session. However, the capacity to maintain the coil placement within and between sessions may be limited. To reduce variability in coil position and orientation in space and over time, neuronavigated TMS (nTMS) is used. However, no studies investigated the accuracy of the assessment of corticospinal excitability and inhibition using nTMS versus a traditional manual approach for the maintenance of coil positioning.

Methods

18 healthy participants (10W/8M) volunteered in two identical and randomized sessions ~30 days apart. Maximal and submaximal neuromuscular evaluations were performed with TMS six times before (PRE), and at the end (POST) of a 2‐min sustained maximal voluntary isometric contraction (MVIC) (100%, 75%, 50% MVIC and 25%isoEMG). The “hotspot” was maintained either with a grid demarcated on the participants’ scalp or with visual feedback provided by nTMS on a computer screen. In both sessions, “hotspot” was considered as the site where the largest MEPs in the rectus femoris were elicited.

Results

There were not a “hotspot” technique × contraction intensities × time points interactions for both MEP (P = 0.217) and SP (P = 0.627). Bland‐Altman plots display adequate agreements between the difference in the two “hotspot” techniques and the means of the two “hotspot” techniques. The slopes of the resulting regression lines were not significantly different from zero (horizontal to x‐axis) for both MEP (all r ≤ 0.335, all P ≥ 0.162) and SP (all r ≤ 0.382, all P ≥ 0.131), indicating a uniformity of systematic error at all contraction intensities and time points.

Conclusions

Spatial stability of the TMS coil position over the motor cortex did not influence corticospinal excitability and inhibition responses in unfatigued and fatigued knee extensors. This is because the variability in MEP and SP responses is due to spontaneous fluctuations in corticospinal and motoneuron excitability and inhibition; and this variability is not affected by spatial accuracy. Therefore, nTMS or a traditional manual approach could be interchangeably use in order to maintain the TMS coil position in the exact “hotspot”.