F-wave in patients with hemifacial spasm: observations during microvascular decompression operations

Lateral Spread Response: Unveiling the Smoking Gun for Cured Hemifacial Spasm

Hemifacial spasm (HFS) is a rare disorder characterized by involuntary facial muscle contractions. The primary cause is mechanical compression of the facial nerve by nearby structures. Lateral spread response (LSR) is an abnormal muscle response observed during electromyogram (EMG) testing and is associated with HFS. Intraoperative monitoring of LSR is crucial during surgery to confirm successful decompression. Proper anesthesia and electrode positioning are important for accurate LSR monitoring. Stimulation parameters should be carefully adjusted to avoid artifacts. The disappearance of LSR during surgery is associated with short-term outcomes, but its persistence does not necessarily indicate poor long-term outcomes. LSR monitoring has both positive and negative prognostic value, and its predictive ability varies across studies. Early disappearance of LSR can occur before decompression and may indicate better clinical outcomes. Further research is needed to fully understand the implications of LSR monitoring in HFS surgery.

Significant Correlation between Delayed Relief after Microvascular Decompression and Morphology of the Abnormal Muscle Response in Patients with Hemifacial Spasm

Although microvascular decompression (MVD) is a reliable treatment for hemifacial spasm (HFS), postoperative delayed relief of persistent HFS is one of the main issues. In patients with hemifacial spasm, stimulation of a branch of the affected facial nerve elicits an abnormal response in the muscles innervated by another branch. Several specific types of waves were found in the abnormal muscle response (AMR). This study aimed to confirm the relationship between the initial morphology of the AMR wave and delayed relief of persistent HFS after MVD. We retrospectively analyzed and compared the data from 47 of 155 consecutive patients who underwent MVD for HFS at our hospital between January 2015 and March 2020. Based on the pattern of the initial AMR morphology on orbicularis oculi and mentalis muscle stimulation, patients were divided into two groups, namely, the monophasic and polyphasic groups. The results of MVD surgery for HFS were evaluated 1 week, 1 month, and 1 year postoperatively, by evaluating whether or not the symptoms of HFS persisted at the time of each follow-up. There were significantly higher rates of persistent postoperative HFS in patients with the polyphasic type of initial AMR at 1 week and 1 month after the surgery (p < 0.05, respectively), as assessed using Yates chi-squared test and Fisher's exact test. A significant correlation was observed between delayed relief after MVD and polyphasic morphology of the AMR in electromyographic analysis in patients with hemifacial spasm.

REVIEW ■ : Axonal Hyperexcitability: Mechanisms and Role in Symptom Production in Demyelinating Diseases

Some of the symptoms associated with demyelinating disorders are believed to originate from spurious impulses arising ectopically in axons at the site of demyelination. This review describes such "positive" symptoms and the patterns of impulses that may be associated with them, including continuous trains of impulses, as well as spontaneous and triggered impulse bursts. The mechanisms underlying the generation of such trains by individual axons are described, including the roles of sodium and potassium currents, the composition of the extracellular fluid, impulse "reflection" at demyelinated sites, and stretch-sensitive ion channels. The contribution of ephaptic transmission to symptom production and its potential role in the generation of ectopic impulses are discussed. The factors involved in the generation of massed discharges are also examined as a basis for certain paroxysmal clinical phenomena. NEUROSCIENTIST 3:237-246, 1997

Electrophysiologic investigation during facial motor neuron suppression in patients with hemifacial spasm: possible pathophysiology of hemifacial spasm: a pilot study

Objective

To evaluate the pathophysiological mechanism of hemifacial spasm (HFS), we performed electrophysiological examinations, such as supraorbital nerve stimulation with orbicularis oris muscle recording and lateral spread tests, after suppressing the patient's central nervous system by administering intravenous diazepam.

Methods

Six patients with HFS were recruited. Supraorbital nerve stimulation with orbicularis oris muscle recording and the lateral spread test were performed, followed by intravenous application of 10 mg diazepam to achieve facial motor neuron suppression. Subsequently, we repeated the two electrophysiological experiments mentioned above at 10 and 20 minutes after the patients had received the diazepam intravenously.

Results

Orbicularis oris muscle responses were observed in all patients after supraorbital nerve stimulation and lateral spread tests. After the diazepam injection, no orbicularis oris muscle response to supraorbital nerve stimulation was observed in one patient, and the latencies of this response were evident as a slowing tendency with time in the remaining five patients. However, the latencies of the orbicularis oris muscle responses were observed consistently in all patients in the lateral spread test.

Conclusion

Our results suggest that ectopic excitation/ephaptic transmission contributes to the pathophysiological mechanisms of HFS. This is because the latencies of the orbicularis oris muscle responses in the lateral spread test were observed consistently in the suppressed motor neuron in our patients.

Discovery of a new waveform for intraoperative monitoring of hemifacial spasms

BACKGROUND

Surgeons often rely on intraoperative electrophysiological monitoring to determine whether decompression is sufficient during microvascular decompression surgery for hemifacial spasms. A new monitoring method is needed when an abnormal muscle response is occasionally not available or is unreliable. This study was an observational clinical trial exploring a new waveform recorded from the facial muscles while the offending artery wall was electrically stimulated.

METHODS

Thirty-two patients with typical hemifacial spasm and 12 with trigeminal neuralgia as a control were included. The facial muscle response was recorded during microvascular decompression surgery while the offending artery wall was stimulated (2 mA × 0.2 ms). The latency, amplitude, and effective refractory period were analyzed.

RESULTS

A waveform was recorded from the facial muscles of patients with hemifacial spasm when the offending artery wall was stimulated and was named the "Z-L response." The latency was 7.3 ± 0.8 ms, the amplitude was 0.08 ± 0.02 mV, and the effective refractory period was 3.5-4 ms. The Z-L response disappeared immediately after microvascular decompression. No waveform was recorded from the facial muscles of patients with trigeminal neuralgia while the anterior inferior cerebellar artery, which adheres to the facial nerve, was stimulated (2 mA × 0.2 ms).

CONCLUSION

We found a new waveform for intraoperative monitoring of hemifacial spasm. The Z-L response was useful when the abnormal muscle response was absent before decompression or persisted after all vascular compressions were properly treated. Particularly, the Z-L response may help neurosurgeons determine the real culprit when multiple offending vessels exist.

Treatment with botulinum toxin improves the hyperexcitability of the facial motoneuron in patients with hemifacial spasm

Botulinum toxin type A (BTX) injection into the orbicularis oculi muscle is an effective treatment for patients with hemifacial spasm (HFS). The objectives of this study were to investigate the effect of this treatment on HFS, in particular the associated hyperexcitability of the facial motor nucleus, and to discuss the potential mechanism of HFS. F waves in the mentalis muscle were examined before, 2 and 6 weeks after the BTX treatment of only the orbicularis oculi muscle in ten patients with HFS. F/M ratio, duration of F waves and frequency of F waves decreased significantly after the BTX treatment compared with those before the BTX treatment. These findings demonstrate that the excitability of the facial motonucleus decreases after BTX treatment of the orbicularis oculi muscle. From these results, we hypothesize that the trigeminal afferent input and the cortical control contribute to the hyperexcitability of the facial motor nucleus in patients with HFS. This warrants further investigation into the pathophysiology of HFS.

Experimental study on the correlation between abnormal muscle responses and F waves in hemifacial spasm

OBJECTIVES

To investigate the correlation between abnormal muscle response (AMR) and F wave by establishing an animal model of hemifacial spasm (HFS).

METHODS

Both demyelination in the main trunk of the facial nerve just distal to stylomastoid foramen and vascular compression were used to duplicate animal model of HFS in ten New Zealand white rabbits. AMR and F waves were elicited from the orbicularis oculi and mentalis muscles respectively by stimulating marginal mandibular branch of the facial nerve 6 weeks post-operatively. Correlation analyses were used to compare the relationship between AMR/M and F/M amplitude ratio and between the duration of AMRs and F waves.

RESULTS

There was a linear correlation between the mean values of the AMR/M and F/M amplitude ratio (r=0.8602, p<0.01), which can also be found between the duration of AMRs and F waves (r=0.7702, p<0.01).

DISCUSSION

Enhanced F waves and AMRs may have the same origin. The F wave can be regarded as a more direct index in the diagnosis pre-operatively, monitoring intraoperatively and follow-up post-operatively in patients with HFS.

Electrophysiological study of ephaptic axono-axonal responses in hemifacial spasm

One of the classic features of hemifacial spasm (HFS) is spread of the blink reflex responses to muscles other than the orbicularis oculi. The pathophysiological mechanisms underlying the generation of such abnormal responses include lateral spread of activity between neighboring fibers of the facial nerve and hyperexcitability of facial motoneurons. In this report we present evidence for another mechanism that can contribute to the generation of responses in lower facial muscles resembling the R1 response of the blink reflex. In 13 HFS patients, we studied the responses induced in orbicularis oris by electrical stimuli applied at various sites between the supraorbital and zygomatic areas. We identified responses with two different components: an early and very stable component, with an onset latency ranging from 10.5 to 14.8 ms, and a more irregular longer-latency component. Displacement of the stimulation site away from the supraorbital nerve and towards the extracranial origin of the facial nerve caused a progressive shortening of response latency. These features indicate that, in our patients, the shortest latency component of the orbicularis oris response was likely generated by antidromic conduction in facial nerve motor axons followed by axono-axonal activation of the fibers innervating the lower facial muscles. Our results suggest that motor axono-axonal responses are generated by stimulation of facial nerve terminals in HFS.

A normative study on human facial F waves

F waves from the nasalis muscle were obtained bilaterally after transcutaneous constant-current stimulation of the facial nerve in 37 of 42 (88%) healthy volunteers examined. F waves were of varying shape and latency. Standard parameters (latencies, amplitudes, ratios, chronodispersion) of F-wave analysis were assessed. Of these, minimum and mean F-wave latencies, the F ratio, and F-wave and peripheral conduction times (PCT) were approximately normally distributed, with a low standard deviation. Latencies and conduction times exhibited a significantly positive correlation with body height, as has been demonstrated for the F-wave latencies in limb muscles. Side and sex differences were present but without statistical significance. Chronodispersion and chronodispersion range, F-wave amplitudes, and F-wave frequencies were broadly scattered. It is concluded that F-wave latencies and conduction times are best suited to define a range of reference values. Moreover, facial F waves bear characteristics similar to those described for F waves recorded from limb muscles.

Long-latency response to transcranial magnetic stimulation in patients with hemifacial spasm

OBJECTIVE

We studied the long-latency response of the orbicularis oris muscle elicited with transcranial magnetic stimulation in patients with hemifacial spasm (HFS) and evaluated the excitability of the facial nucleus.

METHODS

We compared the thresholds on both sides in 8 normal volunteers and 7 patients with hemifacial spasm. The thresholds were determined as the lowest intensity required to produce motor evoked potentials with an amplitude of at least 50 microV in the orbicularis oris muscle. Average values were given as means +/- standard deviation. Wilcoxon's rank sum test was used for comparisons between the sides of normal subjects and of patients with HFS with respect to the threshold stimulus.

RESULTS

There was no significant difference between the thresholds on the two sides of the normal subjects (mean 1.88+/-5.30%, P > 0.05). In patients with HFS, there was a significant difference between the thresholds on the spasm side and the normal side (mean 20.7+/-13.0%, P < 0.05) In one patient studied after MVD, the difference between both sides disappeared.

CONCLUSION

The difference between the thresholds in patients with HFS and the normalization in threshold after MVD suggested that the mechanism of HFS was hyperexcitability of the facial nucleus.

Electrophysiological investigation of hemifacial spasm after microvascular decompression: F waves of the facial muscles, blink reflexes, and abnormal muscle responses

In patients with hemifacial spasm, it has been said that the spasm is due to cross compression of the facial nerve by a blood vessel and that microvascular decompression (MVD) of the facial nerve is an effective treatment. The F waves, which result from backfiring of antidromically activated motor neurons of the facial motor nucleus, are indices of the excitability of the facial motor nucleus and are enhanced in patients with hemifacial spasm. Measuring blink reflexes and abnormal muscle responses (lateral spread), a characteristic sign of hemifacial spasm, has been used to investigate the mechanism of hemifacial spasm pathophysiologically. Thus the authors measured F waves of the facial muscle, blink reflexes, and abnormal muscle responses before and after MVD in patients suffering from hemifacial spasm to investigate the excitability of the facial motor nucleus and the course of the cure of hemifacial spasm after MVD. The authors obtained facial nerve-evoked electromyograms in 20 patients with hemifacial spasm before and after the MVD procedure. On the spasm side, the F waves and blink reflexes were enhanced preoperatively compared to those on the normal side and abnormal muscle responses were recorded in all patients. In 12 patients whose hemifacial spasm had not disappeared completely for 5.1 +/- 1.7 (mean +/- standard error) months following the MVD procedure, F waves were still enhanced significantly and abnormal muscle responses were still recordable, albeit at lower amplitude. Within 1 month after the hemifacial spasm had disappeared completely. F waves were still significantly enhanced in 17 patients and abnormal muscle responses were recorded in seven of 15 patients. Subsequently, the enhanced F waves and abnormal muscle responses disappeared completely. The authors' study supports the hypothesis that the cause of hemifacial spasm is hyperexcitability of the facial motor nucleus and suggests that additional surgery should not be performed for at least 2 years after MVD, because that period is necessary for the disappearance of the hyperexcitability of the facial motor nucleus.