Intracranial stimulation of facial nerve in humans with the magnetic coil

Somatosensory input in the context of transcranial magnetic stimulation coupled with electroencephalography: An evidence-based overview

The transcranial evoked potential (TEP) is a powerful technique to investigate brain dynamics, but some methodological issues limit its interpretation. A possible contamination of the TEP by electroencephalographic (EEG) responses evoked by the somatosensory input generated by transcranial magnetic stimulation (TMS) has been postulated; nonetheless, a characterization of these responses is lacking. The aim of this work was to review current evidence about possible somatosensory evoked potentials (SEP) induced by sources of somatosensory input in the craniofacial region. Among these, only contraction of craniofacial muscle and stimulation of free cutaneous nerve endings may be able to induce EEG responses, but direct evidence is lacking due to experimental difficulties in isolating these inputs. Notably, EEG evoked activity in this context is represented by a N100/P200 complex, reflecting a saliency-related multimodal response, rather than specific activation of the primary somatosensory cortex. Strategies to minimize or remove these responses by EEG processing still yield uncertain results; therefore, data inspection is of paramount importance to judge a possible contamination of the TEP by multimodal potentials caused by somatosensory input.

TMS-induced blinking assessed with high-speed video: optical disruption of visual perception

It is known that TMS can induce blinking, but it is unknown to what extent and at what time TMS-induced blinking can cover the pupil. We applied single-pulse TMS with a leftward and rightward monophasic current through a round coil over the occipital pole in 8 healthy subjects, using high-speed video to monitor left or right eye with a spatial resolution of 0.1 mm and a temporal resolution of 2 ms. We plotted eyelid position relative to upper and lower pupil borders as a function of time after TMS for each subject and current direction. We found 2 blinks in every subject, an isolated late blink with one current direction and a superimposed early and late blink with the other current direction, in accordance with our previously reported association between a leftward and rightward lower coil rim current and an early blink in right and left eye, respectively. Blink extent varied, but 4 subjects showed total pupil covering with both current directions. Blink timing varied, but pupil covering was initiated as early as 32 ms after TMS and pupil uncovering was completed as late as 200 ms after TMS. We found no saccades. We conclude that TMS can cause an important optical disruption of visual perception.

Activation of middle-ear muscles by transcranial magnetic stimulation

OBJECTIVES

To evaluate the reliability of transcranial magnetic stimulation in eliciting admittance changes due to activation of middle-ear muscles.

METHODS

Admittance changes induced by transcranial magnetic stimulation at the inion were evaluated in eight normal subjects, two subjects with prelingual deafness and 22 patients suffering from other otological disorders characterised by absence of acoustic reflex.

RESULTS

Responses showed a predominant negative peak in normal ears. Two small positive components, one preceding and the other following the negative deflection, were less consistently elicited. Only a positive wave was detected in otosclerotic subjects. Patients with tympanic membrane perforation or previous tympanoplasty with ossicular discontinuity did not show any response.

CONCLUSIONS

Transcranial magnetic stimulation is able to activate both stapedius and tensor tympani muscles. In conjunction with admittance audiometry, it may represent a method of exploring the mechanics of the middle ear when acoustic reflex testing is not reliable. It can be helpful in the confirmation of stapes fixation when a severe to profound hearing loss is present.

Magnetic Brainstem Stimulation of the Facial Nerve

Electrophysiological evaluation of cranial nerves provides information on its functional aspects and may be a valuable adjunct to imaging. In 10 normal subjects, we used transcranial magnetic stimulation with a double cone coil at the posterior head region to obtain orbicularis oris motor evoked potentials. Our findings suggest activation of descending facial fibers proximal to brainstem motoneurons. This method is advocated as an adjunct in the electrodiagnostic workup of facial nerve dysfunction.

Electrophysiologic identification and evaluation of stylohyoid and posterior digastricus muscle complex

PURPOSE

To identify the function of stylohyoid and posterior digastricus (STH-PD) muscle complex by the EMG techniques.

METHODS

Unaffected sides of the faces of 30 patients with facial paralysis or hemifacial spasm were investigated. A concentric needle electrode was inserted to the STH-PD muscle complex and another concentric needle electrode was inserted to the orbicularis oris (OO) muscle. Simultaneous recording were obtained from two muscles using electrical stimulation (ES) (in 25 cases) and magnetic coil stimulation (MS) (in 15 cases); and both in 10 cases. Afterwards, the function of STH-PD was studied such as whistling, lip pursing, swallowing, jaw opening and closing.

RESULTS

(1) The motor latency of compound muscle action potential (CMAP) of the STH-PD muscle was shorter than that of OO. (2) When the facial nerve was stimulated more distally than the stylomastoid foramen, the CMAP elicited from the STH-PD muscle complex immediately disappeared. (3) Ipsilateral MS was able to elicit the motor evoked potential (MEP) from STH-PD either at intracranially (half of cases) or at the extracranially. While OO muscle was always stimulated intracranially by MS. (4) The STH-PD muscle complex could not be basically recruited by the mimicry except lip pursing. The main recruitment were provided by swallowing and jaw opening. Cortical MS were facilitated during swallowing (5) Late reflex responses appeared in the STH-PD muscle complex during infraorbital-trigeminal and facial nerve ES.

CONCLUSION

The STH-PD muscle complex is identified electrophysiologically. Although it is innervated by the facial nerve, its functions are mainly related with jaw opening and oropharyngeal swallowing. However, it is activated by the lip pursing.

Corticonuclear innervation to facial muscles in normal controls and in patients with central facial paresis

Recently it has been proposed that corticobulbar innervation of the lower facial muscles is bilateral, that is from both right and left sides of the motor cortex. The objectives of this study were, i) to evaluate the corticonuclear descending fibers to the perioral muscles and, ii) to determine how central facial palsy (CFP) occurs and often recovers rapidly following a stroke. Eighteen healthy volunteers and 28 patients with a previous history of a stroke and CFP (mean ages: 51 and 61 years) were investigated by TMS (transcranial magnetic stimulation) with a figure of eight coil. Intracranial facial nerve and cortical motor evoked potentials (MEPs) were recorded from the perioral muscles. The periorbital MEPs were also studied. The absence of MEPs in both perioral muscles with TMS of the affected hemisphere was the most obvious abnormality. Also, central conduction time was significantly prolonged in the remaining patients. The mean amplitude of the affected hemisphere MEPs was diminished. The amplitudes of the unaffected hemisphere MEPs recorded from the intact side were enhanced especially in the first week following the stroke. During TMS, only the blink reflexes were elicited from the periorbital muscles due to stimulus spreading to trigeminal afferent nerve fibers. It is concluded that perioral muscles are innervated by the corticobulbar tract bilaterally. CFP caused by a stroke is generally incomplete and mild because of the ipsilateral cortical and multiple innervations out of the infarction area, and recovers fast through cortical reorganisation.

A facial nerve study using transcranial magnetic paired stimulation

OBJECTIVE

The purpose of this study was to evaluate the refractory period of the facial nerve by transcranial magnetic paired stimulation (TMPS) with as short interstimulus interval (ISI) as possible. Also, by applying TMPS, the long latency response obtained at the same time was recorded and its neurophysiological characteristics were studied.

METHODS

(Experiment 1) The subjects comprised of 30 normal volunteers and 19 patients with Bell's palsy. The experiments were carried out using two sets of Magstim model 200 and Bistim modules, a large coil measuring 90 mm in diameter with a maximal output of 2.0 tesla (T), and a Neuropack 8 (Nihon Kohden Co., Japan) to control the stimulation and record the electromyographic findings. The amplitude of compound muscle action potentials (CMAP) to the orbicularis oris muscle was studied by TMPS at the parieto-occipital region. The ISI was set at 0.9, 1.1, 1.3, 1.5, 1.7, 1.9 and 2.1 ms in order to measure the refractory periods of the nerve. (Experiment 2) The subjects comprised of ten normal volunteers. The same method as in experiment 1 was carried out. However, this time, the lead electrodes were placed on the orbicularis oculi muscle, similar to that of the blink reflex. The ISI was set at 40, 60, 80, 100, 200, 300, 500, 800 and 1000 ms, and the effects of the facilitation and inhibition were studied.

RESULTS

(Experiment 1) In normal subjects, when the ISI was less than 1 ms, a significant decrease in the amplitude was noted. In severe palsy cases with House-Brackmann Grade IV-V TMS yielded no response. In two of the cases of House-Brackmann Grade III, the CMAP was obtained. (Experiment 2) The long latency response with TMPS was most strongly inhibited when ISI was 80 ms.

CONCLUSION

We were able to investigate the refractory periods, the reflex pathway of the facial nerve and the trigeminal nerve including the pons and the medulla oblongata by TMPS.

Basic mechanisms of TMS

Transcranial magnetic stimulation (TMS) is now established as an important noninvasive measure for neurophysiologic investigation of the central and peripheral nervous systems in humans. Magnetic stimulation can be used for stimulating peripheral nerves with a similar mechanism of activation as for electrical stimulation. When TMS is applied to the cerebral cortex, however, some features emerge that distinguish it from transcranial electrical stimulation. One of the most important features is designated the D and I wave hypothesis, which is now widely accepted as a mechanism of TMS of the motor cortex. Transcranial electrical stimulation excites the pyramidal tract axons directly, either at the initial segment of the neuron or at proximal internodes in the subcortical white matter, giving rise to D (direct) waves, whereas TMS excites the pyramidal neurons transsynaptically, giving rise to I (indirect) waves. There are still other phenomena with mechanisms that remain to be elucidated. First, not only excitatory effects but also inhibitory effects can be elicited by TMS of the cerebral cortex (e.g., the silent period and intracortical inhibition). The inhibitory effect may also be used to investigate cerebral functions other than the motor cortex, such as the visual, sensory cortices, and the frontal eye field, from which no overt response like the motor evoked potential can be elicited. Second, there is an abundance of intraregional functional connectivities among different cortical areas that can also be revealed by TMS, or TMS in combination with neuroimaging techniques. Last, repetitive transcranial stimulation exerts a lasting effect on brain function even after the stimulation has ceased. With further investigation of the neural mechanisms of TMS, these techniques will open up new possibilities for investigating the physiologic function of the brain as well as opportunities for clinical application.

Transcranial magnetic stimulation in acute facial nerve injury

OBJECTIVE/HYPOTHESIS

Available electrodiagnostic tests that are used to evaluate facial nerve injury examine the nerve distal to the stylomastoid foramen; because most facial nerve injuries are within the temporal bone, the tests cannot evaluate the nerve at or across the injury site. The interpretation of these tests depends on the predictability (or unpredictability) of distal degenerative process. Transcranial magnetic stimulation may be able to stimulate the nerve proximal to the injury site. The hypothesis of the present study is that in cases of mild traumatic facial nerve injury where axonal integrity is maintained, proximal stimulation of the nerve using higher than normal stimulus intensities to "overcome" the block at the injury site result in recordable facial nerve activity.

STUDY DESIGN

A prospective controlled animal study comparing response to transcranial magnetic stimulation of the facial nerve in the following groups: mild injury, severe injury/transection, and control.

METHODS

We studied 44 facial nerves in 22 cats. Fifteen nerves were subjected to mild trauma. Five nerves were severely crushed, 2 nerves were completely transected, and 22 nerves were not traumatized. All nerves were examined with the transcranial magnetic stimulation system before the trauma, immediately after the trauma, and at 3, 8, and 12 weeks after trauma.

RESULTS

All nerves in the mild and severe trauma groups showed complete clinical paralysis immediately after trauma. The nerves in the mild trauma group showed significant increase in threshold as well as significant increase in latency for recordable facial muscle response to transcranial magnetic stimulation. Thresholds and latencies decreased gradually within 3 to 12 weeks and returned almost to preinjury levels. This paralleled the return of clinical facial muscle movement. In the severe trauma/transection group, the nerves had no facial muscle response to transcranial magnetic stimulation after trauma. Neither facial muscle response to transcranial magnetic stimulation nor facial muscle movements recovered.

CONCLUSIONS

In cats transcranial magnetic stimulation can assess the integrity of the facial nerve after trauma and predict its potential for regeneration. This technique can excite the nerve proximal to the injury site and may play a role in the clinical evaluation of the acute traumatic facial nerve paralysis. It can be used immediately after trauma, because it does not depend on wallerian degeneration to occur.

Transcranial cortical magnetic stimulation of lower-lip mimetic muscles: effect of coil position on motor evoked potentials

The effect of the coil position along the interaural line on motor evoked potentials of lower-lip muscles to cortical transcranial magnetic stimulation was investigated in 17 healthy subjects. Using a figure-8-shaped coil, we observed ipsi- and contralateral middle-latency motor evoked potentials in all subjects, when the coil was centered within an area between 4 and 13 cm lateral to the vertex. Maximal responses (mean amplitude 1.4 +/- 0.8 mV contralateral, 0.7 +/- 0.5 mV ipsilateral) with shortest mean onset latencies (11.3 +/- 1.6 ms contralateral, 12.1 +/- 2.8 ms ipsilateral) were obtained at a stimulus position of 10 cm lateral to the vertex. Cortical maps of mean amplitude and mean response duration showed inverse U-shaped configuration. Furthermore, we observed an additional polyphasic and mostly bilateral response in 16 of the 17 subjects.

A clinical study on the magnetic stimulation of the facial nerve

OBJECTIVES

A clinical study on the usefulness of magnetic stimulation of the facial nerve, with special attention paid to the selection of the coil shape and stimulation procedures.

STUDY DESIGN

The subjects consisted of 55 patients with Bell's palsy, 1 patient with a cerebellopontine angle (CPA) tumor, 1 patient with multiple sclerosis (MS), and 30 normal subjects. Three types of coils were used in this study; a 90-mm large single coil, a 40-mm small single coil, and a 20-mm small double coil.

METHODS

The compound muscle action potentials (CMAPs) and long latency response were evoked by transcranial magnetic stimulation (TMS) with a 90-mm large single coil. The 40-mm small single coil was used to test blink reflex by aiming it at the supraorbital nerve as the target site. The subcutaneous activation of the infra-auricular facial nerve was performed with the 20-mm double coil.

RESULTS

The reproducible CMAP and long latency responses were obtained from normal subjects with TMS. However, responses were observed only in patients with relatively mild Bell's palsy. The magnetic stimulation-evoked responses reflected the brainstem function in the patients with a CPA tumor and MS.

CONCLUSION

Although magnetic stimulation remains inferior to conventional electric stimulation in some sense and requires further study, this method is potentially useful because it can stimulate the facial nerve continuously from the cortex to the periphery and can effectively evoke responses reflecting the brainstem function.

Perioperative lesions of the facial nerve: follow-up investigations using transcranial magnetic stimulation

Peripheral facial palsy can occur after aural surgery and neurosurgery. Routine neurophysiological investigation (utilizing electrical stimulation and the blink reflex) does not allow the direct assessment of the site of a lesion. In the present study transcranial magnetic stimulation (TMS) was applied in order to evaluate the usefulness of this method for prognosis. Twenty-three patients with postoperative facial pareses (after removals of an acoustic neuroma in 12 patients and parotid tumors in 11) were investigated. Ipsilateral short-latency and contralateral long-latency responses (after cortex stimulation) were elicited. At the first examination (11.7 +/- 9 days after onset of the palsy) the components of the blink reflex were absent in all cases. Responses to electrical stimulation were abnormal in 80%. Ipsilateral short-latency responses after TMS could be obtained in 7 patients. Pathological long-latency TMS responses were elicited in 17 patients. Follow-up investigations up to 2 years revealed no prognostic aspects from peripheral electrical stimulation, the blink reflex and the short-latency TMS response. The absence or extent of delay in long-latency responses at first examination was strongly correlated with final clinical outcomes. As improvements of the responses preceded clinical regressions of the paresis, TMS proved to be an important neurophysiological method for an early prognosis of recovery after perioperative lesions of the facial nerve.

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

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.

On the site of transcranial magnetic stimulation of the facial nerve: electrophysiological observations in two patients after transection of the facial nerve during neuroma removal

The site of stimulation of the facial nerve after transcranial temporo-occipital magnetic stimulation is being controversially discussed, particularly whether the nerve is stimulated in the root exit zone in the cerebellopontine angle or whether stimulation originates within the bony canal of the facial nerve. In two case reports, the neurophysiological findings after the surgical transection of the facial nerve during the extirpation of a large acoustic and a facial nerve neuroma are presented. In both cases, transcranial magnetic stimulation of the facial nerve produced compound muscle action potential 4 and 2 days after the dissection of the facial nerve at the internal auditory canal and in the supralabyrinthine portion. These findings indicate that the site of stimulation in transcranial magnetic stimulation can be located to the course of the facial nerve within its bony petrosal canal distal to the external genu.

Assessment of motor pathways to masticatory muscles: an examination technique using electrical and magnetic stimulation

To study motor pathways to masticatory muscles, a new recording technique using surface electrodes was developed. The recording electrode was mounted on a spatula and inserted enorally into the pterygomandibular plica over the belly of m. masseter. Using this technique, mean latencies/amplitudes of the compound action potentials (CMAPs) in 18 healthy subjects were 1.2 ms/4.9 mV after electrical stimulation of the trigeminal nerve below the zygomatic arch, and 5.5 ms/1.1 mV after magnetic stimulation of the cortex. In 15 patients with unilateral lesions of the facial nerve, masticatory CMAPs had virtually symmetrical configuration, latency, and amplitude, excluding a major contribution of volume conducted activity from other cranial muscles. The technique was evaluated in patients after surgical treatment for trigeminal neuralgia. Patients with retrogasserian thermocoagulation and central demyelinating lesions were consistently identified.

Origin of the facial long latency responses elicited by magnetic stimulation

With magnetic stimulation (MS) it is possible to elicit bilateral long latency facial motor responses (LLRs). Due to a relatively wide magnetic field, the site of neural activation may take place in many different structures. The purpose of this study was to determine the site of origin of facial LLRs. The motor long latency responses were recorded bilaterally on the naso-labial folds (NLFs) with reference electrodes on the nose, and on some subjects also with reference electrodes on the chin. The stimulating coil was placed in the right parietal area. LLRs obtained with MS were compared to LLRs elicited electrically at the right stylomastoid foramen, supraorbital foramen, as well as cutaneous sensory area V1 of the trigeminal nerve. In addition, right sided high intensity electrical stimuli, paired magnetic stimulation and electrical stimulation with interstimulus intervals ranging from 0 to 80 msec were also applied for comparison. LLRs recorded with reference to the nose were always elicitable with MS as well as with the other stimulation procedures. The responses elicited with MS did not differ from those elicited electrically at various extracranial stimulation sites. With paired stimuli the second LLRs were inhibited by the preceding stimulation, whether given magnetically or electrically. In subjects with elicitable LLRs with chin references, the responses were always bilateral. Based on the similar characteristics with extracranial electrical stimuli, bilateral distribution of the responses, and inhibition of the second response with paired stimuli, it is concluded that the neural origin of LLRs to MS is in the extracranial trigeminal or facial nerve branches.

The polarity of the induced electric field influences magnetic coil inhibition of human visual cortex: implications for the site of excitation

Human perception of 3 briefly flashed letters in a horizontal array that subtends a visual angle of 3 degrees or less is reduced by a magnetic coil (MC) pulse given, e.g., 90 msec later. Either a round or a double square MC is effective when the lower windings or central junction region, respectively, are tangential to the skull overlying calcarine cortex and symmetrical across the midline. The modeled, induced electric field has peak amplitude at the midline, but the peak spatial derivatives lie many centimeters laterally. Thus, the foveal representation near the midline is closer to the peak electric field than to its peak spatial derivatives, i.e., excitation of calcarine cortex differs from excitation of a straight nerve. With an MC pulse that induces an electric field which is substantially monophasic in amplitude, the lateral-most letter (usually the right-hand letter) in the trigram is preferentially suppressed when the electric field in the contralateral occipital lobe is directed towards the midline. Inferences from using peripheral nerve models imply that medially located bends in geniculo-calcarine or corticofugal fibers are the relevant sites of excitation in visual suppression; end excitation of fiber arborizations or apical dendrites is considered less likely. This conclusion is supported by the fact that the induced electric field polarity in paracentral lobule for optimally eliciting foot movements is opposite to that for visual suppression, the major bends occurring at different portions of the fiber trajectories in the two systems.

Electrophysiological characterization of pre- and postoperative facial nerve function in patients with acoustic neuroma using electrical and magnetic stimulation techniques

Facial nerve function was examined in patients who underwent posterior fossa surgery for unilateral acoustic neuroma. Examinations took place prior to surgery (n = 47 patients), early after surgery (0-12 days, n = 16 of 47 patients), and late after surgery (187-1505 days, n = 29 of 47 patients). Clinical signs of facial palsy were present to a variable extent in 13 of 47 patients before, in 12 of 16 patients early, and in 18 of 29 patients later after surgery. Electrophysiologically, the facial nerve was stimulated electrically at the stylomastoid fossa and magnetically at its proximal intracanalicular segment. In addition, the face-associated motor cortex was stimulated magnetically. In patients with facial palsy, any of these stimulation methods resulted in a decreased amplitude of the response in the nasalis muscle. The decrease showed a linear relationship to the clinical grade of palsy, pre- and postoperatively. Corticomuscular latencies remained unchanged. We conclude that: (i) the electrophysiological characteristics of facial nerve lesions due to compression by acoustic neuromas or due to a complication of neuroma removal are those of a purely axonal neuropathy; (ii) the three stimulation techniques have a similar diagnostic yield, thus making the use of all three of them redundant; and (iii) the electrophysiological techniques allowed no prediction of the final facial nerve function.

Magnetic stimulation in hemifacial spasm and post-facial palsy synkinesis

The facial nerve was stimulated trascranially with a magnetic stimulator in 14 normal controls, 14 hemifacial spasm patients, and 16 post-facial-palsy synkinesis patients. Magnetic stimulation in normal controls revealed muscle responses which had latencies with a mean value of 4.99 +/- 0.49 ms and amplitudes of 2.41 +/- 1.08 mV. In the same group, transosseal conduction time was calculated to be 1.20 +/- 0.13 ms. In the hemifacial spasm group, the amplitudes of the responses on the affected sides were lower as compared to the unaffected sides (mean values 1.78 vs. 2.41 mV, P = 0.01). Also, the threshold to magnetic stimulation was elevated on the affected sides. These findings are suggestive of the presence of a hypoexcitability to magnetic stimulation in the root entry zone. In the post-facial-palsy synkinesis patients, magnetic stimulation of the affected sides resulted in responses with long latencies and low amplitudes (mean latency 6.34 ms, mean amplitude 0.90 mV). In the recordings made with magnetic stimulation, the difference of the latencies between the two sides was larger as compared to those obtained by electrical stimulation. The transosseal conduction time was also remarkably prolonged on the affected side. These findings may suggest that magnetic stimulation can be an effective method of showing intracranially located lesions of the facial nerve.

The site of impulse generation in transcranial magnetic stimulation of the facial nerve

The facial nerve can be stimulated in its intracranial course through transcranial magnetic stimulation (TMS). We studied the site of impulse generation produced by TMS by comparing the latencies of the muscle evoked potentials (MEPs) elicited with TMS and intracranial electrical stimulation (IES) of the facial nerve during neurosurgical posterior fossa procedures. In a series of 25 patients, the mean latency of the TMS elicited MEPs, recorded in the orbicularis oris muscle, was 5.0 ms (SD 0.58). Also IES of the distal part of the facial nerve in the internal acoustic meatus showed a mean latency of 5.0 ms (SD 0.68). Proximal IES in the root entry zone of the facial nerve, and intermediate IES between root entry zone and meatus, produced MEPs with significantly longer latencies compared to TMS and distal IES (p < 0.05). The findings suggest that the TMS induced facial nerve activation, leading to a MEP response, takes place within the internal acoustic meatus.

Transcranial magnetic stimulation excites the root exit zone of the facial nerve

The actual site of excitation of the facial nerve by transcranial magnetic stimulation was investigated in five patients with hemifacial spasm who underwent microvascular decompression. The facial nerve was stimulated preoperatively and intraoperatively by transcranial magnetic stimulation and intraoperatively by electrical stimulation at its root exit zone with a minimum of surgical invasion of the facial nerves. The onset latency of compound muscle action potentials recorded from the nasalis muscle was 5.06 +/- 0.44 ms by magnetic stimulation and 5.08 +/- 0.43 ms by electrical stimulation. The latency difference was 0.06 +/- 0.08 ms. Therefore, transcranial magnetic stimulation was basically the same as electrical stimulation in onset latency. From this study, it appears that the root exit zone of the facial nerves is stimulated by transcranial magnetic stimulation.

Analysis of the coil generated impulse noise in extracranial magnetic stimulation

An intense impulse noise artifact is generated by the coil used in extracranial magnetic stimulation (EMS) of the brain and cranial nerves. In this study we measured and analyzed the sound pressure level (SPL), spectral content, wave form, and time course of the magnetic coil acoustic artifact (MCAA) impulse noise in the sound field and in the ear canal of life-size models of the human cranium. Two different clinical magnetic stimulators and coils were used. Sound field measurements from both coils showed the MCAA to be a transient impulse noise with a rapid rise-time, brief duration, broad acoustic spectrum, and high intensity. Measurements made on models of the human head with the magnetic coils positioned at selected standard clinical positions for EMS, particularly the peripheral facial nerve, auricle and mastoid areas, indicated that the MCAA may reach sound pressure levels that exceed noise damage-risk criteria limits for sensorineural hearing loss. The maximum peak energy in the acoustic spectrum of the MCAA measured in the ear canal of the model heads was from 2 to 5 kHz, the range of highest sensitivity in human ears. Ear protectors were found to attenuate the SPL of the MCAA, reaching the ear canal of the model heads by 15-22 dB SPL, and were recommended for use by patients and subjects exposed to EMS.

Motor potentials of inferior orbicularis oculi muscle to transcranial magnetic stimulation. Comparison with responses to electrical peripheral stimulation of facial nerve

Magnetic stimulation at the vertex evoked a motor potential (MP) in the inferior orbicularis oculi muscle of 10 healthy subjects with an onset latency of 8-13 msec. Its amplitude increased and its latency decreased when the muscle was contracted: the latency measured 9.5 +/- 1.3 msec with an intensity of stimulation 10-15% above threshold in the contracted muscle. This MP is secondary to excitation of the motor cortex. With the coil placed over the occipital scalp and the same stimulation intensity, an MP was recorded with an onset latency at 4.5 +/- 0.6 msec. This response reflects the activation of the facial nerve root. The peripheral electrical stimulation of the facial nerve at the mandible angle elicited an MP with an onset latency at 3.5 +/- 0.4 msec. Most records showed the presence of late components at about 30 msec for all types of stimulation.

Transcranial magnetic stimulation of the facial nerve: intraoperative study on the effect of stimulus parameters on the excitation site in man

Magnetic stimulation (magStim) of the intracranial facial nerve is performed in clinical and research settings, but the activation site is a matter of controversy. Latencies of nasalis muscle responses to magStim were, therefore, compared with those obtained by direct electrical stimulation of the facial nerve (a) at the root exit zone (REZ); (b) at the porus of the facial canal; and (c) in the stylomastoid fossa during microvascular decompression operations in the cerebellopontine angle (CPA). Measurements of latencies of the nasalis muscle response, obtained while the stimulating coil was placed over the parieto-occipital area of the scalp, indicated that it was the labryinthine segment of the facial canal, 5 to 16 mm distal to the CPA, that was activated. This would be in agreement with studies of physical models reported in the literature that showed (a) the strength of the electrical current generated by a magnetic field is particularly high close to a nerve foramen; and (b) excitation to magStim is most likely to occur where the induced electrical field changes rapidly over distance, i.e., at anatomical boundaries between media of high and low specific resistance. These characteristics are found at the end of the labyrinthine segment of the facial canal, where the facial nerve leaves the low-resistance cerebrospinal fluid and enters the high-resistance petrous bone. The site of neural excitation is robust and unaffected by stimulus intensity and current direction within a wide range, or by large changes in location of the coil.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative analysis of enflurane anesthesia on primate motor and somatosensory evoked potentials

The effect of increasing enflurane concentration on magnetic-induced myogenic cranial (Cr) and peripheral (Pr) motor evoked potentials (MEPs), and electrically induced median (MN) and posterior tibial (PTN) somatosensory evoked potentials (SEPs) was studied in 10 monkeys. MEP, recorded from abductor pollicis brevis and abductor hallucis muscles, and SEP (short- and long-latency scalp recorded potentials) variables were examined at 0.25, 0.5, 0.75, 1.0 MAC enflurane concentrations. Cr-MEPs progressively attenuated (P less than 0.01) with 0.25 MAC and were abolished (greater than or equal to 0.75 MAC) by graded enflurane concentration. Stimulation threshold for Cr-MEP was progressively elevated (P less than 0.01), and eventually reliable responses were lost (greater than or equal to 0.75 MAC). Marked scalp zone reduction to obtain Cr-MEP responses was noted with increasing enflurane concentration. Pr-MEPs and most SEP peaks maintained good replicability but showed significant amplitude reduction (P less than 0.01). MEP and SEP latency values were not significantly delayed as long as the wave form remained identifiable. We conclude that enflurane has a differential influence on Cr-MEPs and SEPs. Administration of enflurane should be discouraged while monitoring myogenic Cr-MEPs since even a subanesthetic concentration is profoundly detrimental.

Magnetic facial nerve stimulation in normal subjects. Three groups of responses

Magnetic stimulation provides a method to stimulate the facial nerve transcranially. With this method, the stimulation can be directed to the intracranial part of the facial nerve, whereas conventional electric stimuli are delivered to a more peripheral part of the nerve. In 40 healthy subjects, ipsilateral responses with latencies of 4.5 +/- 0.4 ms were recorded on the nasolabial folds. The latencies were 1.1 ms longer than those elicited at the stylomastoid foramen by electric stimulation. Furthermore, a response with a mean latency of 12 ms (range 10-16 ms) appeared in 6 out of 10 healthy subjects and a polyphasic response with a mean latency of 32 ms in 9 out of 10 of these subjects. Transcranial magnetic stimulation seems to allow the examination of motor conduction through the proximal part of the facial nerve. In addition, the method may give further information concerning the facial activation mechanisms possibly by other central pathways.

Sound perception induced by extracranial magnetic stimulation in deaf patients

Two profoundly hard-of-hearing and deaf patients were examined by non-invasive extracranial magnetic stimulation (EMS) in an effort to determine whether EMS could evoke auditory sensations. The patients were fitted with standard earplugs and were stimulated at the auricle, the mastoid and the temporal lobe area. The threshold of auditory sensation (TAS) was determined at each stimulus position and found to be approximately 20-40% of the maximum EMS level (2.0 Tesla). The TAS was generally lowest in mastoid stimulation, but was variable, and dependent on the angle and position of the stimulating coil relative to the skull. Middle-ear muscle reflex (MEMR) tests performed by EMS of the auricle, mastoid and temporal lobe area contralateral to the probe ear were negative. It was concluded that EMS of the auditory system, particularly the mastoid area, can evoke auditory sensations in cochlea-deaf ears, and that this technique deserves further study as a non-invasive procedure for evaluating potential cochlear implant patients in conjunction with electrostimulation.

Transcranial magnetic stimulation excites the labyrinthine segment of the facial nerve: an intraoperative electrophysiological study in man

The site where transcranial magnetic stimulation (magStim) depolarizes the facial nerve was investigated in 6 patients who underwent surgery of the cerebellopontine angle (CPA). The facial nerve was stimulated (1) magnetically prior to craniotomy, (2) electrically near the brainstem (elREZ), (3) at the exit from the CPA into the facial canal (elPorus), and (4) in the stylomastoid fossa (elStylo). The range of latency differences (delta) of compound muscle action potentials (CMAPs) recorded from the ipsilateral mentalis muscle were as follows: delta elREZ-magStim: +0.5 to +1.1 ms (P less than or equal to 0.03, Wilcoxon test); delta elPorus-magStim: +0.2 to +0.5 ms (P less than or equal to 0.03); delta elStylo-magStim: +0.8 to +1.0 ms (P less than or equal to 0.03). On the basis of anatomical data and a facial nerve conduction velocity of 33-46 m/s in these patients, it was concluded that transcranial magnetic stimuli depolarized the facial nerve at a location 10-15 mm distal to its entrance into the facial canal. This corresponds to the end of the labyrinthine segment of the facial nerve, i.e. the transit zone where the nerve ceases to be surrounded by cerebrospinal fluid (CSF) with its high electrical conductivity and enters the high-resistance tissue of the petrous bone.

Acoustic trauma in extracranial magnetic brain stimulation

The effects of the magnetic coil acoustic artifact (MCAA) associated with extracranial magnetic field stimulation (EMFS) of the brain were studied in normal hearing rabbits. Spectral and intensity analyses showed that the MCAA is a high intensity transient signal with peak energy between 2 and 5 kHz, and peak amplitudes in the first 100-200 mu sec. At EMFS levels of 50-100% of maximum output (2.0 Tesla), the corresponding MCAA levels were 131-142 dB sound pressure level (peak hold) at the outer ear and amplified by the external meatus to reach 145-157 dB sound pressure level (SPL) at the position of the tympanic membrane in rabbits. Measurements of the acoustic middle ear muscle reflex (AMR) in non-anesthetized rabbits indicated that exposure to EMFS levels of 50-100% resulted in correspondingly increasing compound threshold shifts (CTS) and permanent threshold shifts (PTS) in the unprotected ears of the experimental animals. Auditory brain-stem responses (ABR) measures on the same and additional animals corroborated these findings. Morphological studies showed evidence of substantial cochlear trauma at EMFS levels as low as 50%, with increasing severity up to 100% EMFS. Morphological examination of inner ear structures following exposure to the MCAA in the acute preparation (fixed within hours after exposure) showed ruptures between pillar cells and a detached organ of Corti. Preparations examined 3 or more weeks after exposure showed damaged pillar cells, a widespread loss of outer hair cells, fused and fractured inner hair cell stereocilia, and kinocilium outgrowth on inner hair cells. Although this extremely short impulse contains approximately 2 orders of magnitude less acoustic energy than a continuous noise exposure of 131 dB for 15 min, it is substantially more injurious to the cochlea. The present findings suggest that the acoustic artifact produced by the EMFS coil in some clinical instruments may pose a potential risk for temporary and permanent hearing loss in patients and clinicians when held in close proximity to the unprotected ear. Initial studies suggest that the magnetic field alone did not appear to cause permanent hearing impairment. We recommend the use of ear protectors for the patient and clinician during EMFS as a precautionary measure to prevent possible hearing loss from the MCAA.

Transcranial magnetic stimulation in peripheral facial nerve palsy of central origin

We describe a 24-year-old woman with a peripheral facial nerve palsy associated with one-and-a-half syndrome due to multiple sclerosis. Transcranial magnetic stimulation located the facial nerve dysfunction in the brainstem.

AAEM minimonograph #35: Clinical experience with transcranial magnetic stimulation

We elicited motor evoked potentials (MEPs) using transcortical magnetic stimulation in 150 control subjects aged 14 to 85 years and 275 patients with a variety of diseases. There were no significant side effects. Cortex-to-target muscle latencies measured 20.2 +/- 1.6 ms (thenar), 14.2 +/- 1.7 ms (extensor digitorum communis), 9.4 +/- 1.7 ms (biceps), and 27.2 +/- 2.9 ms (tibialis anterior). Central motor delay between the cortex and the C-7 and L-5 measured 6.7 +/- 1.2 ms and 13.1 +/- 3.8 ms, respectively. Mean spinal cord motor conduction velocity measured 65.4 m/s. MEP amplitude expressed as a percentage of the maximum M wave was never less than 20% of the M wave. A value of less than 10% is considered abnormal. MEP latency increases linearly with age and central motor delay is longer in older subjects. Compound muscle action potentials and absolute MEP amplitudes decreased linearly with age. In multiple sclerosis (MS), MEP latency and central delay were often very prolonged. The MEP was more sensitive than the SEP in MS. In amyotrophic lateral sclerosis, MEP latencies were only modestly prolonged; the characteristic abnormality was reduced amplitude. When pseudobulbar features predominated MEPs were often absent. The MEP was of normal latency in Parkinson's disease, but age-related amplitude was often increased. MEP latency and amplitude were normal in Huntington's disease. Abnormal MEPs persisted several months after stroke despite good functional recovery. The MEP could be used to advantage to demonstrate proximal conduction slowing and block in demyelinating neuropathies. In plexopathy, ability to elicit an MEP several days after onset of paresis was good evidence of neuronal continuity in motor fibers.

Magnetic transcranial and electric direct stimulation of the facial motor cortex in dogs

Magnetic stimulation may allow noninvasive study of the entire course of the facial nerve. Our goal was to determine if evoked muscle action potentials can be obtained in facial musculature using electric direct cortical and noninvasive transcranial magnetic stimulation of the canine motor cortex. Thirty-four dogs were studied with electric direct cortical stimulation through a craniotomy and magnetic transcranial stimulation of the facial motor cortex. Facial nerve stimulation in the cerebellopontine angle allowed comparison to cortical responses. Latencies of 6.08 and 9.52 msec for orbicularis oculi and levator nasolabialis, respectively, were determined with magnetic transcranial stimulation, compared with 4.22 and 5.78 msec with electric direct cortical stimulation. In conclusion, magnetic stimulation of the facial motor cortex is possible in dogs, with longer central conduction times than with electric direct stimulation.