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

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Interaction between vestibulo-spinal and corticospinal systems: a combined caloric and transcranial magnetic stimulation study

We investigated the interaction between vestibular and corticospinal stimuli in 8 healthy volunteers. Vestibular stimulation was induced with unilateral ear caloric irrigation (30°C) with subjects supine. Single transcranial magnetic stimulation (TMS) pulses were delivered (double-cone coil, intensities 60-75% maximal output) every 10-20 s during vestibular activation and during baseline. Bilateral surface electromyography (EMG) from splenius capitis, sternocleidomastoid (SCM), obliquus externus abdominis, vastus lateralis, biceps femoris (BF), tibialis anterior and peroneus longus was obtained. During whole-body maximal rotatory voluntary isometric contraction (MRVC), only SCM and BF displayed EMG activation/inhibition patterns indicating axial rotatory action. TMS-induced motor evoked potentials (MEPs) after caloric irrigation revealed that only SCM showed consistent vestibular-mediated excitation/inhibition responses, i.e. an increase in MEP area contralateral to the irrigation and a decrease in MEP area ipsilaterally (+12.7 and -6.3% of the MRVC, respectively). A putative head turn induced by this SCM activity pattern would be in the same direction of the slow-phase eye movement. EMG in the 100 ms preceding TMS showed muscle tone values of approximately 10% of MRVC. After caloric irrigation, these values increased by ca. 2% for all muscles bilaterally and hence cannot explain the direction-specific SCM MEP changes. Thus, SCM MEPs show caloric-induced amplitude modulation indicating that SCM is under both horizontal semicircular canal and corticospinal control. This vestibular modulation of corticospinal SCM control likely occurs at cortical levels. The direction of the MEP modulation indicates a directional coupling between vestibularly induced head and eye movements.

Intraoperative facial motor evoked potential monitoring with transcranial electrical stimulation during skull base surgery

OBJECTIVE

To address the limitations of standard electromyography (EMG) facial nerve monitoring techniques by exploring the novel application of multi-pulse transcranial electrical stimulation (mpTES) to myogenic facial motor evoked potential (MEP) monitoring.

METHODS

In 76 patients undergoing skull base surgery, mpTES was delivered through electrodes 1cm anterior to C1 and C2 (M1-M2), C3 and C4 (M3-M4) or C3 or C4 and Cz (M3/M4-Mz), with the anode contralateral to the operative side. Facial MEPs were monitored from the orbicularis oris muscle on the operative side. Distal facial nerve excitation was excluded by the absence of single pulse responses and by onset latency consistent with a central origin.

RESULTS

M3/M4-Mz mpTES (n=50) reliably produced facial MEPs while M1-M2 (n=18) or M3-M4 (n=8) stimulation produced 6 technical failures. Facial MEPs could be successfully monitored in 21 of 22 patients whose proximal facial nerves were inaccessible to direct stimulation. Using 50, 35 and 0% of baseline amplitude criteria, significant facial deficits were predicted with a sensitivity/specificity of 1.00/0.88, 0.91/0.97 and 0.64/1.00, respectively.

CONCLUSIONS

Facial MEPs can provide an ongoing surgeon-independent assessment of facial nerve function and predict facial nerve outcome with sufficiently useful accuracy.

SIGNIFICANCE

This method substantially improves facial nerve monitoring during skull base surgery.

Facial nerve dysfunction in hereditary motor and sensory neuropathy type I and III

Facial nerve function was studied in 19 patients with hereditary motor and sensory neuropathy type I (HMSN I) and 2 patients with hereditary motor and sensory neuropathy type III (HMSN III, Déjérine-Sottas), and compared to that in 24 patients with Guillain-Barré syndrome (GBS). The facial nerve was stimulated electrically at the stylomastoid fossa, and magnetically in its proximal intracanalicular segment. Additionally, the face-associated motor cortex was stimulated magnetically. The facial nerve motor neurography was abnormal in 17 of 19 HMSN I patients and in both HMSN III patients, revealing moderate to marked conduction slowing in both the extracranial and intracranial nerve segments, along with variable reductions of compound muscle action potential (CMAP) amplitudes. The facial nerve conduction slowing paralleled that of limb nerves, but was not associated with clinical dysfunction of facial muscles, because none of the HMSN I patients had facial palsy. Conduction slowing was most severe in the HMSN III patients, but only slight facial weakness was present. In GBS, conduction slowing was less marked, but facial weakness exceeded that in HMSN patients in all cases. We conclude that involvement of the facial nerve is common in HMSN I and HMSN III. It affects the intra- and extracranial part of the facial nerve and is mostly subclinical.

Facial palsy in Heerfordt's syndrome: electrophysiological localization of the lesion

Heerfordt's syndrome is characterized by fever, uveitis, swelling of the parotid gland, and facial nerve palsy and represents a variety of neurosarcoidosis. Since the first description of the syndrome, discussion about the lesion site has been controversial and has included the assumption of direct nerve compression by parotid gland swelling or a lesion within the facial canal in light of observations of accompanying taste disturbance. We report on a 26-year-old man with typical Heerfordt's syndrome who developed bilateral facial nerve palsy. Electrical and magnetic stimulation of the whole facial motor path provided strong evidence for a pathological process that: (i) began in the cerebellopontine angle; (ii) spread distally into the facial canal; and (iii) could be characterized by proximal demyelination. The patient recovered completely within 6 weeks under immunosuppressive therapy with steroids.

Leprosy affects facial nerves at the main trunk: neurolysis can possibly avoid transfer procedures

The predilective sites of lesions in leprous peripheral nerves are well established, and their surgical decompression is common practice when sensorimotor disorders persist after medication. By contrast, the precise localization of leprous facial neuropathy still remains unclear, and musculofascial transfers have been the only type of surgical treatment. The goal of this study was to clarify where leprosy affects facial nerves and to determine whether neurolysis might suffice to restore facial function. In five Indian and two Egyptian patients suffering from leprous facial neuritis, the nerves were stimulated transcranially at the brainstem to evoke efferent motor nerve action potentials, which were recorded from the exposed nerves. Lesions were detected at the main trunk proximally from the first bifurcation in all cases. Epineuriotomy revealed fibrosis of the interfascicular epineurium in all instances, as an indication for interfascicular neurolysis. One patient was able to close his eye and showed a better smile soon after surgery. After 16 and 21 months, respectively, one patient had improved distinctly, two patients slightly, two patients showing no progress, and two patients were lost to follow-up. It is concluded that (1) leprous facial neuropathy is located at the main trunk close to the first bifurcation and not exclusively at the peripheral zygomatic branches, (2) microsurgical neurolysis can be considered in leprous facial neuropathy before transfer procedures as long as voluntary or spontaneous activity is present in the affected muscles, and (3) intraoperative transcranial electrical stimulation is an effective means of localizing the site and proximal extent of leprous facial neuropathy.

Sternocleidomastoid muscle responses to transcranial magnetic stimulation in patients with cervical dystonia

Ten cervical dystonia (CD) patients, with involuntary head rotation to one side and contralateral sternocleidomastoid muscle (SCM) hypertrophy, were investigated with transcranial magnetic stimulation, and the results were compared to those of 10 healthy subjects. Monopolar needle electrodes with isolated shafts were used for bilateral electromyographic recordings in the SCMs of the motor evoked potentials (MEPs) elicited by the magnetic stimulator. The latencies of ipsilateral SCM MEPs were shorter in the CD patients than in the control subjects (P < 0.001). The latencies of SCM activity suppression by TMS were longer in the CD patients than in the control group when stimuli were given on the contralateral side (P < 0.05). Both the clinically dystonic and the contralateral SCM of the CD patients exhibited significantly abnormal latencies of the ipsilateral SCM MEPs (P < 0.01) and of the SCM suppression (P < 0.05). Three CD patients also had consistent activity in the SCM counteracting the direction of head rotation during the suppression experiments. The latencies of the suppression of this abnormal activation were shorter (P < 0.05), than the latencies of the suppression in the SCM during normal voluntary activation by these CD patients (i.e. rotation of the head in the contrary direction). The results suggest bilaterally enhanced motoneuronal excitability and disturbed inhibitory regulation in patients with CD.

Prognostication of Bell's palsy using transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) provides a method to noninvasive excitation of the facial nerve in its intracranial segment close to the internal acoustic meatus. Thus, the site of facial nerve activation with TMS is proximal to or within the site of the lesion in Bell's palsy. To evaluate the prognostic capability of TMS in unilateral Bell's palsy we examined 137 patients with this method, and compared the results with electroneuronography (ENoG). Within 0-4 days from the onset of palsy, the patients with elicitable TMS responses recovered better than those in whom TMS responses were not elicitable. If TMS was performed 5-9 days or 10-28 days after the onset of palsy, it did not provide any prognostic information. Based on amplitude side-to-side differences, ENoG did not contribute prognostic information during the first 9 days from the onset of palsy. Later on, 10-28 days after the onset of palsy, ENoG showed an increased capability to discriminate the patients with poor prognosis. Thus, elicitable facial motor response with TMS predicts good prognosis of Bell's palsy at an early stage whereas poor response with ENoG predicts less favorable prognosis at a later stage.

Electrophysiological characteristics of lesions in facial palsies of different etiologies. A study using electrical and magnetic stimulation techniques

Using magnetic stimulation techniques in addition to conventional electrical stimulation, the entire facial motor pathway can be assessed electrophysiologically. To study the diagnostic yield of these examinations, 174 patients with facial palsies of a variety of etiologies were examined (85 Bell's palsies, 24 Guillain-Barré syndrome (GBS), 19 Lyme borreliosis, 17 zoster oticus, 12 meningeal affections, 10 brain-stem disorders and 7 HIV-related facial palsies). The facial nerve was stimulated electrically at the stylomastoid fossa and magnetically within its canalicular portion. Additionally, the face-associated contralateral motor cortex was stimulated magnetically. Recordings were from the nasalis or mentalis muscle, or both, using surface electrodes. Bell's palsy patients showed typically a unilateral local hypoexcitability of the facial nerve to canalicular stimulation. In GBS, bilateral latency prolongations were frequent, as expected for a myelinic disorder. In contrast, in zoster, predominant axonotmesis was unilateral, and in HIV infection sometimes bilateral. The method was very sensitive to detect subclinical dysfunctions in meningo-radiculitis and malignant meningeal diseases, either prior to the onset of palsy, or on the contralateral (clinically unaffected) side. It also distinguished reliably between central and peripheral facial motor pathway lesions. In our experience, these inexpensive and non-invasive electrophysiological techniques contribute substantially to the differential diagnosis of facial palsies.

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.

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.

Electromagnetic stimulation of the auditory system of deaf patients

Electromagnetically induced auditory perception was investigated in 18 deaf patients who were candidates for cochlear implants. In the extracranial magnetic stimulation (EMS) procedure, patients were stimulated with time-varying magnetic field brief pulses from a coil positioned at the i) auricle, ii) the mastoid, and iii) the temporal lobe area. EMS elicited auditory sensations in 26 ears (of 14 patients/subjects). The lowest threshold of auditory sensation (TAS) was found to be at the 20% EMS level, with a range of 20-50% of the maximum level (2.0 Tesla), and approximately equal sensitivity in each coil position. Eleven of the subjects hearing EMS-induced sound perceived changes in pitch while 6 heard "clicks" or clicks and tones. Spearman Rho correlation analysis showed a mild negative correlation between the EMS/TAS and the pre-implant FFA, best tone threshold (BTT), and direct promontorial electrical stimulation (ES) thresholds at 250 Hz and 500 Hz. No correlation was found between EMS or ES and performance on the pre-implant or post-implant psychacoustic tests (MAC VIII or 3-Digit speech tests) or the measurements of the thickness of cutaneous and osseous tissue from the stimulation sites at the mastoid and ear canal to the cochlear and 8th nerve. A fair positive correlation was found between the EMS/TAS and the post-implant (6 months) ES threshold when the electrodes allocated the 500 Hz frequency range were stimulated. A mild positive correlation between the pre-cochlear-implant promontorial electrical stimulation (ES) at 250 Hz and the four frequency tone average (FFA: 0.5, 1, 2, 4 kHz) was also found.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory brainstem and cortical responses following extensive transcranial magnetic stimulation

The long term effects of transcranial electromagnetic stimulation (TEMS) on auditory brainstem and cortical evoked responses and on neuroanatomical structures in the auditory tract were investigated over a 12 month period in rabbits exposed to 1000 stimuli at 100% maximum stimulation level (2.0 tesla instrument output) with a clinical magnetic coil positioned over the cranium. (1) The tone and click audiograms of the pre and post TEMS-exposed plugged ears were normal and did not differ significantly, suggesting that the protected cochlea is unaffected by TEMS. (2) The mean absolute and interwave latencies of auditory brainstem evoked responses (ABR) and the peak amplitudes of the vertex positive waves P1, P3, and P4 in the exposed rabbits were within normal limits, and comparable those of the normal, pre-exposed animals. Wave P5 in the exposed animals was more variable and significantly different from the normal data in mean latency and amplitude. (3) The mean latencies and amplitudes of the post exposed cortical (late) auditory evoked responses (CAER) were not significantly different from the non-exposed ears. Light microscopic examination of sections of the cochlear nucleus and inferior colliculus, possible sources of waves P2 and P5, respectively, of the ABR, showed no EMS-related changes in cellular organization or histological damage. In conclusion, no deleterious effects of TEMS were observed on the protected ear or the peripheral and central auditory system of rabbits after extensive exposure to long term, high intensity, low frequency time-varying magnetic field stimulation with a clinical instrument.

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.

Limitations of electromyography and magnetic stimulation for assessing laryngeal muscle control

The development of new phonosurgical techniques has increased the level of interest in the field of neurolaryngology. This field requires valid techniques for determining if muscle activation is normal. Laryngeal electromyography is being used more frequently to assess muscle innervation and synkinesis. Further, magnetic stimulation has been introduced as a noninvasive technique for nerve stimulation. Technical limitations that affect the clinical utility of both these techniques are reviewed: 1) difficulties obtaining selective and accurate electromyographic laryngeal muscle recordings, 2) normal variation in movement and muscle activation patterns within and between normal individuals when producing the same speech syllables, and 3) variation in laryngeal muscle response latencies between and within normal subjects during peripheral magnetic stimulation. Given the normal variation in laryngeal electromyography and magnetic stimulation response latencies, these techniques may not yet be reliable or accurate for assessing reinnervation or synkinesis following recurrent laryngeal nerve injury.

Neurobiological effects of extensive transcranial electromagnetic stimulation in an animal model

The effects of transcranial electromagnetic stimulation (TEMS) on the cellular morphology of the cortex, cerebellum, and brain-stem were systematically investigated in rabbits exposed to 1000 pulsed stimuli at 100% maximum stimulation level (2.0 Tesla at the skull) over a 12 month period with a 5 cm circular magnetic coil positioned over the cranium. Also, the acute effects of TEMS on heart rate and respiration were examined. (1) T1 and T2 weighted magnetic resonance images (MRI) of 1-3 mm sections in both sagittal and axial planes revealed no evidence of gross morphological changes or subtler tissue damage to the cerebrum, cerebellum, or brain-stem. (2) Light microscopic examination of 60 microns hematoxylin-eosin/Cresyl Violet Luxol Fast Blue stained sections of the brain-stem, cerebellum, and cerebral cortex showed no TEMS-related changes in cellular organization or histological damage. (3) Autonomic activity as reflected by heart rate was also unaffected by high intensity TEMS. Normal heart rate was maintained during repeated TEMS at 100% of maximum. (4) Respiration rate was briefly altered at the time of the stimulus, but returned to normal immediately after the stimulus. These findings in experimental animals revealed no biohazardous effects on the brain following extensive exposure to high intensity, low frequency time-varying magnetic field stimulation from the coil of a clinical instrument.