[Supplements to Clinical Neurophysiology] Transcranial Magnetic Stimulation and Transcranial Direct Current Stimulation, Proceedings of the 2nd International Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (tDCS) Symposium Volume 56 || Chapter 35 Transcranial magnetic stimulation in brainstem lesions and lesions of the cranial nerves

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<ul><li><p>Transcranial Magnetic Stimulation and Transcranial DirectCurrent Stimulation (Supplements to Clinical Neurophysiology, Vol. 56)Editors: W. Paulus, F. Tergau, M.A. Nitsche, le. Rothwell, U. Ziemann, M. Hallett 2003 Elsevier Science B.V. All rights reserved</p><p>Chapter 35</p><p>Transcranial magnetic stimulation in brainstem lesions andlesions of the cranial nerves</p><p>Peter P. Urban</p><p>Department of Neurology, University of Mainz; Langenbeckstr. 1, D-55IOI Mainz (Germany)</p><p>341</p><p>1. Introduction</p><p>Transcranial magnetic stimulation (TMS) representsa non-invasive, safe, and painless method of motorcortex activation for the functional assessment of therapid conducting corticobulbar and corticospinalprojections. TMS delivered to different levels of themotor system can provide information on theexcitability of the motor cortex, the functionalintegrity of intracortical neuronal structures, theconduction along corticospinal, corticonuclear, andcallosal fibres, as well as on the function of nerveroots and the peripheral motor pathway to themuscles. Brain stem lesions may lead to corticonu-clear and/or corticospinal tract involvement. It istherefore useful to examine both tracts, which can beidentified following the accurate selection of therespective recording and stimulation site.</p><p>The focus of this chapter is not only on TMSapplications in brainstem pathology, but also on thedescription of TMS techniques for the evaluation ofmotor cranial nerve function, which is an essential</p><p>* Correspondence to: Dr. Peter P. Urban, Departmentof Neurology, University of Mainz, Langenbeckstr. I,0-55101 Mainz, Germany.Tel.: +49-6131-175162, Fax: +49-6131-173271;E-mail: urban@neurologie.klinik.uni-mainz.de</p><p>measure in the diagnostic workup of brainsternlesions. Applications of TMS to the cranial nerveinnervated muscles have been the objective ofnumerous investigations, ranging from basic neu-roanatomic studies to determine the central course ofcorticonuclear projections to clinical applicationscarried out to determine the location of lesions,investigate the pathophysiology of ischemicdysarthria, detect clinically silent lesions in multiplesclerosis, obtain prognostic information regardingpersistent motor deficits following cerebral ischemia,and to identify corticonuclear tract involvement inmotor neuron diseases. TMS is also of clinicalrelevance in the evaluation of those peripheral cranialnerve lesions which most frequently affect the facialnerve.</p><p>2. Methods</p><p>Brainstem lesions may affect the corticospinal and/orcorticobulbar projections. A description of thestimulation and recording technique from the limbsis given in the IFeN recommendations (Rossiniet al., 1994; Rothwell et al., 1999), as well as by anumber of recently published reviews (Rossini andRossi, 1998; Weber and Eisen, 2002; Kobayashi andPascual-Leone, 2(03). In contrast to their application</p></li><li><p>342</p><p>in limb muscles, fractionated examinations of theprojections to the cranial nerve muscles are lesscommonly performed, and frequently require specialrecording and stimulation techniques, which aredescribed in greater detail in this review. Recordingsfrom the facial muscles and the tongue areparticularly useful in the assessment of corticonuclearfunction due to the relatively large size of the corticalrepresentation area of these structures.</p><p>3. Masticatory muscles (N. V)</p><p>Motor evoked potentials (MEP) of the masticatorymuscles by TMS have been described for healthysubjects and patients with hemiplegia, trigeminalneuralgia, amyotrophic lateral sclerosis (ALS), andmultiple sclerosis (e.g. Cruccu et al., 1989; TUrket al., 1994; Trompetto, 1998). However, reliablerecordings of TMS evoked MEPs of the massetermuscle are more difficult to achieve than those ofthe facial, tongue or intrinsic hand muscles dueto methodological problems associated with therecording and stimulation technique.</p><p>3.1. Recording technique</p><p>Recordings from the masseter muscle are performedmost frequently, although recordings from othermasticatory muscles as, e.g. the anterior digastricmuscle (Gooden et al., 1999), and the medial pterygoidmuscle (TUrk et al., 1994) have been reported. Arelevant methodological problem is the presence ofvolume conducted potentials from the overlying facialmuscles which are also activated in the course ofcortical stimulation. To avoid recording of volumeconducted facial muscle activity, both an enoralrecording technique (Turk et al., 1994) and a veryshort distance between active and reference electrodeshave been recommended (Guggisberg et al., 2(01).Furthermore, selective preactivation of the mastica-tory muscles is required to focus on the target muscleand to reduce the relative influence of the relaxedneighboring muscles.</p><p>3.2. Stimulation technique</p><p>3.2.1. Stimulation of the motor cortexDue to the relatively small cortical representation areafor masticatory muscles, the correct placement of themagnetic coil is essential (Guggisberg et al., 2001).Themasticatory muscles also have a higher motor thresholdcompared to facial muscles and the tongue. This maybe accounted for by the presence of fewer corticalconnections to the pyramidal cells (Guggisberg et al.,2(01). The location of the optimal stimulation siteusing a circular coil, has been described as 2-4 emparamedially to the vertex (TUrk et al., 1994). For thefigure-of-eight coil, a coil orientation of 1200 relative toa fronto-dorsal line in an area 4-10 em lateral to thevertex and 0-4 em frontal to the bi-auricular line hasbeen shown to have the lowest motor threshold, thehighest amplitudes, and shortest MEP latencies fromthe masseter muscle (Guggisberg et al., 2(01).</p><p>3.2.2. Peripheral nerve stimulationThe proximal part of the trigeminal motor nerve can beactivated with an ipsilateral coil position 6 em lateral tothe vertex. The responses occur at a mean latency ofabout 2 ms (Cruecu et al., 1989; Guggisberg et al.,2(01). Comparing the responses of direct electricaltrigeminal nerve stimulation during microvasculardecompression operations with TMS evoked responsespreoperatively, it has been shown that TMS activatesthe trigeminal nerve distal to the root entry zone withinthe cerebrospinal fluid near the porus trigerninale.However, the activation site was not constant andmigrated distally at increasing stimulus intensities(Schmid et al., 1995). Electrical stimulation of thedistal trigeminal nerve at the zygomatic arch evokesperipheral compound muscle action potentials(CMAP) with a latency of about 1.5 ms. However, dueto the small distance between stimulation and recordingsite, the presence of a substantial stimulus artefact ofteninterferes with the determination of the CMAP onset.</p><p>4. Facial muscles (N. Vll)</p><p>Recordings from the facial muscles allow reliableconduction measurements across the central and</p></li><li><p>peripheral motor pathways. Clinical applicationsinclude supranuclear and infranuclear facial palsiesof different etiologies, and the detection of aclinically silent involvement of the supranuclear tractin multiple sclerosis and ALS.</p><p>4.1. Recording technique</p><p>There are no firm recommendations regarding therecording site, and CMAPs have been recorded fromthe nasalis, mentalis, orbicularis oculi, frontalis,triangularis, levator labii, and buccinator muscles. Asystematic comparison of these recording sites as tothe elicitation of a CMAP, stimulus artefact,interferences with the Rl-component of the blinkreflex, cross-talk from the contralateral face, intersidedifferences of the amplitudes and intraindividualreproducibility showed clear advantages for thebuccinator and triangularis muscles under facial nervestimulation, and for the buccinator and levator labiimuscles under motor cortex stimulation (Urban et al.,2oo2a). These findings are consistent with results ofrecently published neuroanatomic studies demon-strating that only the lateral facial nucleus, whichcontains neurons for the orofacial muscles, receivescontralateral projections from the contralateralprimary motor cortex, while the upper face musclesreceive projections from the supplementary motorcortex and rostral parts of the gyrus cinguli(Morecraft et al., 2001). Thus, the orofacial musclesare especially suitable for the evaluation of the centralfacial motor pathway.</p><p>We prefer the use of an enoral technique forrecordings from the buccinator muscle, because it ischaracterized by a high CMAP amplitude caused bythe large muscle volume, no volume conduction fromthe contralateral side as a result of the lateral position,no relevant stimulus artefact associated with theenoral recording technique, no Rl-component of theblink reflex resulting from the caudal position, and adistinct negative deflection of the CMAP due to theclearly defined nerve entry zone (motor point). Forthis purpose, pairs of AglAgCI-surface disc elec-trodes are embedded at a distance of 18 mm in aspecially designed fork-shaped metacrylate device</p><p>343</p><p>adapted to the oral vestibulum (Urban et al., 1997a).The electrodes are in contact with the insides of thecheeks and a slight contraction of the buccinatormuscles during motor cortex stimulation is achievedby pursing the lips.</p><p>4.2. Stimulation technique</p><p>4.2.1. Stimulation of the motor cortexThe center of a circular coil is positioned tangentially2-4 em (buccinator muscle) lateral of Cz for motorcortex stimulation. Using a figure-of-eight coil, themost effective site for stimulation has been describedas 8-10 em lateral to the vertex, with a coil orienta-tion allowing the current induced into the brain toflow in a posterior to anterior direction (Meyer et al.,1994). Stimulation intensity is increased stepwiseduring slight preactivation until stable latencies (totalmotor conduction time: TMCT) are achieved.Recording of volume-conducted activity from theadjacent masticatory muscles is negligible due to theirhigher motor threshold, as may be observed in thepresence of absent activity in complete peripheralfacial palsy.</p><p>4.2.2. Peripheral nerve stimulationThe proximal peripheral facial nerve is stimulatedmagnetically at the extra-axial intracranial segment.A comparison of the responses to direct electricalfacial nerve stimulation during microvascular decom-pression operations with preoperatively recordedTMS findings showed that TMS activates the facialnerve at the end of the labyrinthine segment (canalic-ular stimulation) on leaving the low-resistancecerebrospinal fluid and entering the high resistancepetrous bone (Schmid et al., 1991). To achieveexcitation of the proximal facial nerve, the circularcoil is placed in a parieto-occipital position ipsilat-erally to the facial nerve. For stimulation of the left(right) peripheral nerve, side "B" ("A") of the circularcoil is viewed from the outside. The peripheralmotor conduction time (PMCT) is about 5 msand serves to calculate the central motor conductiontime (CMCT) (TMCT - PMCT =CMCT). Only arelatively low stimulation intensity (about 30-50%</p></li><li><p>344</p><p>of the maximal stimulator output) should be appliedfor magnetic stimulation to avoid stimulation of thedistal nerve at the stylomastoid foramen. Inadvertentfacial nerve stimulation at the stylomastoid foramencan be identified by comparing the PMCT with thedistal motor latency (DML), which is equal tothe conduction time on electrical stimulation of thedistal facial nerve at the stylomastoid foramen.The normal value for the difference between PMCTand DML (i.e. the transosseal conduction time)is about 1-1.5 ms (Rosler et al., 1989; Urbanet al., 1997a), although it is lower in the presenceof inadvertent distal nerve stimulation. In the lattercase, the stimulation intensity should be reducedand the coil position ought to be moved to a moreparietal site.</p><p>4.2.3. InterpretationBecause stimulation of the motor cortex yieldsvarying ipsilateral responses in healthy subjects, onlythe contralateral responses should be considered(Urban et al., I997a). A supranuclear lesion of thecorticofacial pathways is assumed, when: (1) noresponses are obtained on motor cortex stimulation(no reproducible response at four consecutive trialswith a gain of 200 (V/div); (2) the amplitudecorrelation between motor cortex response andM-wave amplitude with electrical stimulation is (10%(MEPIM-wave-ratio); and (3) a delayed CMCT orinterside difference of the CMCT (&gt; 2.5 SD from thenormal mean) is present. An infranuclear facial nervelesion may be assumed when the interside amplitudedifference of the M-wave is (50% compared to thenormal side) 10 days after the acute lesion.</p><p>5. Sternocleidomastoid muscle and trapeziusmuscle (N. XI)</p><p>A clinical application of motor evoked potentials(MEP) of the sternocleidomastoid (SCM) andtrapezius muscles evoked by TMS has recently beendescribed for the differentiation of amyotrophiclateral sclerosis and cervical spondylotic myelopathy(Truffert et al., 2000).</p><p>5.1. Recording technique</p><p>Recordings from both the sternocleidomastoid andtrapezius muscles have been reported in the literature.However, using surface electrode recordings, addi-tional volume conducted activity from the overlyingplatysma (SCM) (Thompson et aI., 1997), and theunderlying neck muscle activity (trapezius muscle)(Berardelli et al., 1991) has to be considered duringmotor cortex stimulation.</p><p>5.2. Stimulation technique</p><p>5.2.1. Stimulation of the motor cortexThe results of a number of studies using magneticand electric transcranial stimulation (Gandevia andApplegate, 1988; Berardelli et al., 1991; Odergrenand Rimpilainen, 1996; Thompson et al., 1997;Strenge and Jahns, 1998), and clinical observationsduring hemispheric suppression of one hemisphereby amytal during the Wada test (DeToledo and Dow,1998) have shown that stimulation of the motorcortex evokes both bilateral, but predominantlycontralateral responses in the SCM and trapeziusmuscle. The center of a circular coil is positionedtangentially 3-4 em lateral of Cz and 1-2 cm anteriorto the interaural line for motor cortex stimulation.Mapping studies using a figure-of-eight coil formotor cortex stimulation and dual monopolarshielded needles for muscle activity recordingsdemonstrated that the SCM is represented at thecerebral convexity medial to the upper limb repre-sentation, leading to short latency (mean 2.2 ms)responses in the contralateral SCM and longer latency(mean 9.3 rns) responses in the ipislateral SCM(Thompson et al., 1997).</p><p>5.2.2. Peripheral nerve stimulationThe accessory nerve may be magnetically stimulatedby placing the centre of the coil below the mastoid(Priori et al., 1991). No comparative intraoperativeand preoperative studies on the most suitablestimulation site have been publisheded thus far. Thedistal accessory nerve is generally stimulated electri-cally at the midpoint of the posterior border of the</p></li><li><p>SCM, and the CMAP is recorded from the trapeziusmuscle (e.g. Petrera and Trojaborg, 1984).</p><p>6. Tongue muscles (N. xm</p><p>Recordings from the tongue enable reliable conduc-tion measurements across the central and peripheralmotor pathways. Clinical applications include theidentification of supranuclear lesions of the...</p></li></ul>

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