Transcranial magnetic stimulation of the facial nerve]

It was the object of the present study to determine whether transcranial facial nerve stimulation using a magnetic coil can be clinically applicable, and to find the site where the facial nerve is best stimulated. A magnetic coil was placed over the parieto-occipital skull of the subjects for stimulation, and the facial nerve was electrically stimulated in its intracranial and peripheral courses. Then an electromyogram was recorded from the nasalis muscle of the face on the stimulated side. In 9 healthy volunteers, 18 facial nerves received magnetic and electric stimuli in the peripheral region, and the actual site of stimulation was estimated from the conduction velocity of the nerve. The conduction velocity was 56.6 +/- 4.8 m/s, and the latency between CMAPs for electric at the magnetic stimuli to the posterior tragus was 1.23 +/- 0.21 ms. Therefore, the position stimulated by magnetic coil was estimated to be 70.0 +/- 11.4 mm central to the posterior tragus, i.e., near the root exit zone. In two patients undergoing surgery in the cerebellopontine angle, transcranial magnetic stimulation and electrical stimulation of the intracranial facial nerve were compared intraoperatively. The CMAP produced by transcranial magnetic stimulation coincided closely with that produced by direct electrical stimulation of the root exit zone. Thus, the facial nerve was stimulated at the root exit zone, and this method could be expected to be useful for evaluation of disorders of the intracranial facial nerve.     

Contralateral reinnervation of midline muscles in facial paralysis.

We report on a patient with recovery of activity of the left orbicularis oris and nasalis muscles 3 months after a complete left facial palsy. Stimulation of the affected facial nerve evoked no responses, whereas contralateral facial nerve stimulation showed polyphasic responses with very long latencies in the nasalis and orbicularis oris muscles. Needle electromyography (EMG) revealed abnormal spontaneous activity in the left orbicularis oris muscle. The motor unit action potentials on the left side of the face could be recruited only during marked contraction of the corresponding muscles on the right and were of low voltage and polyphasic ("nascent potentials"). Contralateral reinnervation is probably due to sprouting of terminal branches crossing the midline of the face and innervating bundles of muscle fibers on the affected side. This phenomenon seems unfamiliar to most clinicians. Whether the activity is due to conduction along nerve fibers or muscle fibers crossing the midline is discussed.     

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–facialpalsy 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. © 1993 John Wiley & Sons, Inc.     

Intracranial stimulation of facial nerve in humans with the magnetic coil

Using ourselves as subjects, maximal compound motor action potentials (CMAPs) were evoked in ipsilateral nasal and orbicularis oculi muscles (onset latency 4.9-5.4 msec) by a magnetic coil (MC) tangentially oriented over parieto-occipital scalp. The facial nerve was also electrically stimulated sequentially at the posterior tragus near the stylomastoid foramen, anterior tragus and 3 cm more distally. Onset latency of the CMAP elicited at posterior tragus ranged from 1.0 to 1.3 msec less than that elicited by the MC over scalp. Because the measured distal facial nerve motor conduction velocity was 50-60 m/sec, the locus of impulse generation induced by magnetic coil stimulation was estimated to be approximately 6.5 cm proximal to the site of electrical stimulation at the posterior tragus, i.e., closer to the exit of the facial nerve from the brain-stem than to its entrance into the internal auditory meatus. This non-invasive technique should be useful in evaluating patients with peripheral facial nerve disorders including Bell's palsy.     

The recruitment order of electrically activated motor neurons investigated with a novel collision technique.

OBJECTIVE: The development of a novel collision technique for assessment of the activation order of electrically activated nerve fibers, which is an important question in functional electrical therapy or for interpretation of results of motor unit number estimates. METHODS: Compound muscle action potentials were recorded with the belly-tendon configuration from the abductor digiti minimi. A novel modified Hopf's collision technique was applied on ten healthy male subjects to determine the distributions of conduction velocities (DCV) of all ulnar nerve fibers and of the fibers activated by electrical stimuli eliciting 20%, 50%, and 80% of the maximal muscle response. RESULTS: The maximum nerve conduction velocity was (means+/-SE) 64.1+/-0.85m/s. The median conduction velocity of estimated DCV was 58.9+/-0.97m/s (stimulus at 20%), 58.0+/-0.98m/s (50%), 57.2+/-0.91m/s (80%), and 56.5+/-0.84m/s (whole nerve) (all different between each other, P<0.001). CONCLUSIONS: The proposed collision technique allows the assessment of nerve conduction velocity distributions at maximal and sub-maximal stimulation levels and provided evidence for an inverse activation order of nerve fibers with electrical stimulation. SIGNIFICANCE: The excessive fatigue seen with nerve electrical stimulation can be explained by a preferential activation of large diameter nerve fibers. The motor units first activated with electrical stimulation are likely not representative of the motor unit pool in the muscle, which poses limitations in the reliability of some of the proposed methods for motor unit counting.     

Chronic vascular irritation of the facial nerve causes facial spasm in rats

Abstract

The abnormal muscle response, elicited by electrical stimulation of one branch of the facial nerve and recorded from muscles innervated by another branch, has been used previously as an objective sign of hemifacial spasm in the development of animal models of this disorder. In the present study we recorded spontaneous electromyographic activity from the orbicularis oculi muscle from both sides in rats in which a demyelination of the peripheral portion of the facial nerve and vascular contact had been made previously. The root mean square value of the electromyographic activity on the affected side was significantly larger than that on the unaffected side in all rats in which the vascular irritation had caused the abnormal muscle response to appear. The results support our earlier finding that vascular contact together with demyelination of the peripheral facial nerve can cause the development of signs of hemifacial spasm, including involuntary muscle contractions. [Neurol Res 1994; 16: 284-288]     

神经肌电图对特发性面瘫治疗及预后评估的研究

目的探讨神经肌电图(神经电图electroneurography,ENG和肌电图electromyography,EMG)在特发性面瘫治疗及预后评估中的价值。方法采用丹麦生产的keypointⅣ肌电图仪对96例确诊为特发性面瘫的患者进行患侧与健侧ENG、EMG检测,分别记录患侧面神经颞支、颧支、颊支的运动传导潜伏期、波幅以及所支配的额肌、眼轮匝肌、口轮匝肌的肌电图情况,并与自身健侧作对比。结果特发性面瘫患者患侧面神经运动传导潜伏期延长、M波波幅降低,与自身健侧相比,差异有统计学意义(P<0.05)。波幅下降<70%、NCV(神经传导速度)减慢<20%、EMG大致正常的轻度患者,3个月内面肌完全恢复,治愈率100%;波幅下降70%～90%、NCV减慢20%～50%、EMG呈部分神经源性损害的中度患者,3个月内大部分可完全恢复,治愈率87.8%;波幅下降>90%、NCV减慢>50%、EMG呈部分或完全神经源性损害的中重度患者,3个月内面肌完全恢复者为50%;诱发电位消失、NCV引不出、EMG呈完全神经源性损害的重度患者,3个月内面肌完全恢复者为25%。结论 ENG和EMG检测对特发性面瘫的神经损伤、面肌恢复等预后评估有重要价值,能对临床治疗提供客观依据。     

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 new method for measuring compound muscle action potentials in facial palsy: a preliminary study

To establish a simple, reproducible procedure for studying facial motor nerve conduction (MNC), we determined the optimal electrode position to record evoked compound muscle action potentials (CMAPs) from perioral muscles in normal subjects. We examined three new electrode positions in which the electrode connected to the one input of the amplifier was placed on the mental protuberance, and the one connected to the other input was placed on the skin over the orbicularis oris muscle (the philtrum, mouth angle, or lower lip). We then compared the morphology and amplitudes of the CMAPs, right-left differences, and the reproducibility of CMAP amplitudes with recordings taken from the standard electrode position in which one electrode was placed on the nasolabial fold closely lateral to the ala nasi, and the other was placed on the skin over the orbicularis oris. Percutaneous supramaximal electrical stimulation was applied to the main trunk of the facial nerve. All three of the new recording positions showed greater amplitudes and more obvious biphasic CMAPs than the standard method. Positioning the electrode connected to the negative input on the philtrum was optimal in terms of right-left differences and the reproducibility of CMAP amplitudes. Therefore, this midline recording is a simple, reproducible method for calculating the CMAP amplitude ratio. However, prior to clinical use of this procedure, analyses of patients with facial palsy are required.     

Investigation of facial motor pathways by electrical and magnetic stimulation: sites and mechanisms of excitation.

A refined technique is described for non invasive examination of the facial motor pathways by stimulation of the extra- and intracranial segment of the facial nerve and the facial motor cortex. Surface recordings from the nasalis muscle rather than from the orbicularis oris muscle were used, since the compound muscle action potential (CMAP) from this muscle showed a more clearly defined onset. Electrical extracranial stimulation of the facial nerve at the stylomastoid fossa in 14 healthy subjects yielded a mean distal motor latency of 3.7 ms (SD 0.46), comparable with reported latencies to the orbicularis oris muscle. Using a magnetic stimulator, transcranial stimulation of the facial nerve was performed. The mechanism of transcranial magnetic facial nerve stimulation was studied using recordings on 12 patients who had facial nerve lesions at different locations, and with intraoperative direct measurements in four patients undergoing posterior fossa surgery. The actual site of stimulation could be localised to the proximal part of the facial canal, and a mean "transosseal conduction time" of 1.2 ms (SD 0.18) was calculated. The cerebrospinal fluid (CSF) played an important role in mediating the magnetically induced stimulating currents. Finally, with transcranial magnetic stimulation of the facial motor cortex, clearly discernible CMAPs could be produced when voluntary activation of several facial muscles was used to facilitate the responses. From this, a central motor conduction time of 5.1 ms was calculated (SD 0.60, 6 subjects).     

Subthreshold continuous electrical stimulation facilitates functional recovery of facial nerve after crush injury in rabbit

We sought to determine whether electrical stimulation (ES) with subthreshold, continuous, low-frequency impulses is a viable clinical method for improving functional recovery after facial nerve crush injury. In 10 rabbits, bilateral crush injuries were made on the facial nerve by compression for 30 s with mosquito forceps, causing complete facial paralysis. Subthreshold continuous direct current ES with 20-Hz square-wave pulses was applied to the proximal stump on one side for 4 weeks. Vibrissae movement returned significantly earlier on the ES side, with a less variable recovery time. Electrophysiologically, the stimulated side had a significantly shorter latency, longer duration, and faster conduction velocity. Light and transmission electron microscopy revealed that the electrical stimulation also markedly decreased Wallerian degeneration. The average numbers of fluorescent, double-labeled nerve cells were significantly different between the ES and non-ES sides. This study shows that subthreshold, continuous, low-frequency ES immediately after a crush injury of the facial nerve results in earlier recovery of facial function and shorter overall recovery time.     

Compound sensory action potentials evoked by tactile and by electrical stimulation in normal median and sural nerves

The compound sensory action potential evoked by electrical stimulation provides a measure of the number and physiological properties of myelinated fibers in the nerve but does not allow evaluation of the most distal part of the sensory nerve. This study compares the compound sensory action potential, evoked by electrical and tactile stimuli, and recorded through needle electrodes placed close to the median and sural nerves of 22 normal males aged 16–51 years. The tactile probe, with a slight preindentation, delivered an indentation of the skin of 200 μm at a rate of 400 μ/ms at the tip of digit III and the dorsolateral side of the foot. The responses were recorded from the median nerve at wrist and elbow and from the sural nerve at the lateral malleolus and midcalf. The amplitudes of the responses averaged 0.5 μV and 0.7 μV in the sural and the median nerves (P < 0.02), respectively, which was only 5–10% of the amplitude evoked by electrical stimulation. The mean maximal conduction velocity determined by tactile stimulation was 54 m/s in the sural nerve compared with 65 m/s in the median nerve and similar to that calculated after electrical stimulation. In the median nerve the sensory conduction velocity was 8% faster than the motor conduction velocity. These findings indicated that only a fraction of the fibers in the nerve were activated by the probe and that the response was conducted along large myelinated sensory fibers. The latency of the tactile response was longer than that of the electrically evoked response due to the receptor delay and conduction along thin distal fiber portions. The delay at the mechanoreceptors was about 1 ms in the sural and 0.65 ms in the median nerve (P < 0.01). © 1994 John Wiley & Sons, Inc.     

建立人工面神经反射弧恢复失神经支配眼轮匝肌的闭眼功能

目的：建立人工面神经反射弧,恢复面瘫兔的闭眼功能。 方法：试验动物为新西兰兔12只。采用手术切断面神经制备单侧周围性面瘫模型,患侧行肌电图检查确诊。术后第5天患侧植入刺激电极,健侧植入采集电极并留置1月。患侧分别于术后第7天、第28天给予电流刺激;而健侧采集电极则连续采集2周的肌电信号。建立健侧眼轮匝肌肌电信号采集、中枢信号处理模式识别、患侧电流刺激眼轮匝肌系统。当健侧眼轮匝肌采集的肌电信号经信号识别、提取以及电脑分析判断,符合其闭眼刺激阈值时,即对人工电刺激器发出指令,由刺激电极直接作用于患侧眼轮匝肌,引起眼睑完全闭合。结果：刺激方式为正负方向矩形波,电流强度为0.40—0.70mA之间可引起患侧眼轮匝肌收缩,眼睑完全闭合。当健侧眼睑闭合时,其肌电信号电压大于50uV,触发电刺激器,电流刺激即引发患侧眼轮匝肌收缩,完成眼睑闭合。结论：利用MEMS技术,在面瘫兔模型建立“人工面神经反射弧”,恢复患侧眼轮匝肌闭眼功能。     

Diagnostic relevance of transcranial magnetic and electric stimulation of the facial nerve in the management of facial palsy.

OBJECTIVE: Earlier investigations have suggested that isolated conduction block of the facial nerve to transcranial magnetic stimulation early in the disorder represents a very sensitive and potentially specific finding in Bell's palsy differentiating the disease from other etiologies. METHODS: Stimulation of the facial nerve was performed electrically at the stylomastoid foramen and magnetically at the labyrinthine segment of the Fallopian channel within 3 days from symptom onset in 65 patients with Bell's palsy, five patients with Zoster oticus, one patient with neuroborreliosis and one patient with nuclear facial nerve palsy due to multiple sclerosis. RESULTS: Absence or decreased amplitudes of muscle responses to early transcranial magnetic stimulation was not specific for Bell's palsy, but also evident in all cases of Zoster oticus and in the case of neuroborreliosis. Amplitudes of electrically evoked muscle responses were more markedly reduced in Zoster oticus as compared to Bell's palsy, most likely due to a more severe degree of axonal degeneration. The degree of amplitude reduction of the muscle response to electrical stimulation reliably correlated with the severity of facial palsy. CONCLUSIONS: Transcranial magnetic stimulation in the early diagnosis of Bell's palsy is less specific than previously thought. While not specific with respect to the etiology of facial palsy, transcranial magnetic stimulation seems capable of localizing the site of lesion within the Fallopian channel. SIGNIFICANCE: Combined with transcranial magnetic stimulation, early electrical stimulation of the facial nerve at the stylomastoid foramen may help to establish correct diagnosis and prognosis.     

Facial reflexes evoked by electrical stimulation of peripheral facial nerve branches in the cat: experiments of the auriculotemporal--facial nerve anastomoses.

A reflex action potential in the orbicularis oculi muscle, in addition to the direct M response, has been evoked by electrical stimulation of the zygomatic branch of the ipsilateral facial nerve in decerebrate cats. The afferent part of the reflex arc runs via the auriculotemporal-facial nerve anastomoses to the mandibular nerve. This reflex potential has a lower threshold than the M response. Its waveform and its relatively small latency fluctuation are suggestive of a monosynaptic reflex arc. However, the mechanically evoked orbicularis oculi stretch reflex has a much smaller latency and has its afferent pathway via direct trigeminal fibers. The M reflex and the reflex action potential were likewise evoked in the lip musculature by electrical stimulation of the upper and lower buccal branch of the ipsilateral facial nerve. The clinical aspects of this reflex are discussed.     

Correlation of motor nerve conduction velocity and number of innervated muscle fibers

Distal and proximal motor (M) responses were recorded from the "Abductor Pollicis Brevis" (APB) muscle by using the collision method; median motor nerve was stimulated at distal (elbow) and proximal (wrist) regions in a concurrent manner. The delay between two stimuli (ISI: Inter-stimulus Interval), beginning at 9 ms, was decreased by 0.1 ms steps, until the proximal potential completely disappeared. Areas of M responses recorded for each ISI were calculated. Because the area difference between two subsequent ISIs is proportional to the number of muscle fibers innervated by the conduction velocity group at that interval, the relative numbers of muscle fibers for each velocity group were calculated. The results show that the motor nerve conduction velocities belonging to the innervating APB muscle vary between 38 m/s and 57 m/s; the conduction velocity of the group innervating the greatest number of muscle nerves was found to be 55-57 m/s, which comprised 10% of all fibers.     

CORRELATION OF MOTOR NERVE CONDUCTION VELOCITY AND NUMBER OF INNERVATED MUSCLE FIBERS

Distal and proximal motor (M) responses were recorded from the "Abductor Pollicis Brevis" (APB) muscle by using the collision method; median motor nerve was stimulated at distal (elbow) and proximal (wrist) regions in a concurrent manner. The delay between two stimuli (ISI: Inter-stimulus Interval), beginning at 9 ms, was decreased by 0.1 ms steps, until the proximal potential completely disappeared. Areas of M responses recorded for each ISI were calculated. Because the area difference between two subsequent ISIs is proportional to the number of muscle fibers innervated by the conduction velocity group at that interval, the relative numbers of muscle fibers for each velocity group were calculated. The results show that the motor nerve conduction velocities belonging to the innervating APB muscle vary between 38 m/s and 57 m/s; the conduction velocity of the group innervating the greatest number of muscle nerves was found to be 55-57 m/s, which comprised 10% of all fibers     

Changes in a rat facial muscle after facial nerve injury and repair

This study describes changes in a rat facial muscle innervated by the mandibular and buccal facial nerve branches 4 months after nerve injury and repair. The following groups were studied: (A) normal controls; (B) spontaneous reinnervation by collateral or terminal sprouting; (C) reinnervation after surgical repair of the mandibular branch; and (D) chronic denervation. The normal muscle contained 1200 exclusively fast fibers, mainly myosin heavy chain (MyHC) IIB fibers. In group B, fiber number and fiber type proportions were normal. In group C, fiber number was subnormal. Diameters and proportions of MyHC IIA and hybrid fibers were above normal. The proportion of MyHC IIB fibers was subnormal. Immediate and delayed repair gave similar results with respect to the parameters examined. Group D rats underwent severe atrophic and degenerative changes. Hybrid fibers prevailed. These data suggest that spontaneous regeneration of the rat facial nerve is superior to regeneration after surgical repair and that immediacy does not give better results than moderate delay with respect to surgical repair. Long delays are shown to be detrimental.     

Signs of hemifacial spasm created by chronic periodic stimulation of the facial nerve in the rat

We show in this paper that daily electrical stimulation of the facial nerve near its entrance to the brain stem in rats results in abnormalities in the electromyographic response of the facial muscles that resemble those seen in patients with hemifacial spasm. After about 4 weeks of daily electrical stimulation of the facial nerve for 1 min/day, stimulation of the temporal branch of the facial nerve resulted in an abnormal EMG response from the mentalis/orbicularis oris muscles, consisting of an initial deflection with a latency of about 6 ms, followed by a burst of EMG activity lasting 10 to 50 ms. We found such "lateral spread" of antidromic activity in all 10 rats that were subjected to chronic stimulation, but not in normal rats and not in rats that had electrodes implanted but which had not been stimulated. Measurements of neural conduction times suggest that the location of the cross transmission that gives rise to this lateral spread response is central to the site where the facial nerve exits the brain stem, probably in the facial motor nucleus. We take these results to support the hypothesis that chronic stimulation of the facial nerve can change the facial nucleus in such a way that it becomes hyperactive, and cross transmission between neurons that are innervating different facial muscles is facilitated.     

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)