Transcranial magnetic stimulation of the trigeminal nerve: Intraoperative study on stimulation characteristics in man

Abstract

We studied responses from the masseter and nasalis muscles following magnetic stimulation (magStim) and compared these responses with those obtained by direct electrical stimulation of the trigeminal (NV) and facial (NVII) nerve near the root exit zone during microvascular decompression operations of NVII. We found that (1) magStim threshold to excite the nerve is high for NV and low for NVII; (2) excitation of all motor fibers is impossible for NV, and easy for NVII; (3) optimal coil placement is critical for NV, but not critical for NVII; and (4) between and within subjects, the excitation site is variable on NV, but stable on NVII. We estimated that the anatomical location of magStim to be either within or outside the cerebrospinal fluid for NV, and to be in the labyrinthine segment of the facial canal for NVII. Physical models explain and clinical lesion models support these differences found between NV and NVII.

Transcranial magnetic stimulation of the brain: What is stimulated? – A consensus and critical position paper

“…The induced electric field does not only excite neuronal structures in the central nervous system. Peripheral co-stimulation of sensory and motor axons in the face or neck region and intracranial sensory and motor axons at the base of the skull may also be effectively excited by TMS ( Schmid et al, 1995 ). TMS induces eddy current in the cerebrospinal fluid, which can lead to excitation of all motor (and probably also sensory) fibers of the facial nerve close to the foramen ovale ( Schmid et al, 1992 , 1995 ).…”

“…Peripheral co-stimulation of sensory and motor axons in the face or neck region and intracranial sensory and motor axons at the base of the skull may also be effectively excited by TMS ( Schmid et al, 1995 ). TMS induces eddy current in the cerebrospinal fluid, which can lead to excitation of all motor (and probably also sensory) fibers of the facial nerve close to the foramen ovale ( Schmid et al, 1992 , 1995 ). Foraminal excitation of myelinated motor axons of the facial nerve occurs already at low stimulus intensities with threshold intensities ranging between 20 and 40% of maximal stimulator output, when using a standard round coil and a Magstim 200 device ( Schmid et al, 1995 ).…”

“…TMS induces eddy current in the cerebrospinal fluid, which can lead to excitation of all motor (and probably also sensory) fibers of the facial nerve close to the foramen ovale ( Schmid et al, 1992 , 1995 ). Foraminal excitation of myelinated motor axons of the facial nerve occurs already at low stimulus intensities with threshold intensities ranging between 20 and 40% of maximal stimulator output, when using a standard round coil and a Magstim 200 device ( Schmid et al, 1995 ). Foraminal motor responses of the facial nerve showed orientation dependency and were readily elicited at many lateral stimulation positions across the scalp, when the center of a round stimulation coil was positioned at electrode positions C3 (the approximate location of M1-HAND), P3 and T3 of the international 10–20 system for EEG electrode placement ( Schmid et al, 1995 ).…”

Inferring Causality from Noninvasive Brain Stimulation in Cognitive Neuroscience

“…Although the E-field is more focal for multielectrode montages with small electrodes as compared to classical two-electrode montages with large electrodes (Datta et al, 2009), the effective E-field will not be confined by the borders of the targeted brain area. In addition, both TMS and TCS also induce effective E-fields in the cranial periphery (green boxes), with magnitudes in the skin being inevitably larger than those in the brain (Asamoah, Khatoun, & McLaughlin, 2019), exciting efferent fibers of the facial nerve innervating the facial muscles (Chen, Chauvette, Skorheim, Timofeev, & Bazhenov, 2012), or afferent fibers of the trigeminal nerve innervating the scalp, face, and meninges (Siebner, Auer, Roeck, & Conrad, 1999;Schmid, Møller, & Schmid, 1995). In particular, the TCS-induced E-field, shunted via highly conductive skin tissue, also extends to peripheral neuronal structures in the retina (Lorenz et al, 2019;Schutter, 2016) and the vestibular system (Kwan, Forbes, Mitchell, Blouin, & Cullen, 2019).…”

The non-transcranial TMS-evoked potential is an inherent source of ambiguity in TMS-EEG studies

“…The time-varying electric field induces action potentials in myelinated axons in the extracranial tissue as well. Eddy currents evoked in the cerebrospinal fluid may also effectively stimulate proximal cranial nerves passing through foramina at the base of the skull (Schmid et al, 1995). Orthodromic action potential propagation in peripheral motor axons results in twitches of cranial muscles, which not only causes muscle potentials and electrode movement artifacts in the TEP recordings (Mutanen et al, 2013), but also a twitch-induced sensory input to the brain.…”