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• Response to Marcus A Acioly
  Masafumi Fukuda
  Published on: 13 April 2016
• Is there a role for facial motor evoked potentials in the microsurgical treatment of hemifacial spasm?
  Marcus A Acioly
  Published on: 13 April 2016

• Published on: 13 April 2016

  Response to Marcus A Acioly

  • Masafumi Fukuda, M.D.
  • Other Contributors:
    • Makoto Oishi, Tetsuya Hiraishi, and Yukihiko Fujii

  We thank Marcus A Acioly for his interest in our research paper.

  We agree with Acioly's suggestions regarding the term "facial motor evoked potential (FMEP)". We are intrigued by the concept of FMEP waveform morphology. We think that the FMEP complexity referred to by Acioly probably includes the duration of FMEP. In HFS patients, the decrease in the duration of FMEP after MVD is conceivable as being reflective o...

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  We thank Marcus A Acioly for his interest in our research paper.

  We agree with Acioly's suggestions regarding the term "facial motor evoked potential (FMEP)". We are intrigued by the concept of FMEP waveform morphology. We think that the FMEP complexity referred to by Acioly probably includes the duration of FMEP. In HFS patients, the decrease in the duration of FMEP after MVD is conceivable as being reflective of the lower complexity of FMEP waveform and normalisation of facial nerve excitability. In future, we propose to investigate whether the waveform complexity of FMEP might be independent of the amplitude of FMEP in both HFS and skull base tumour patients.

  Lateral spread response (LSR) is still a standard monitoring tool to predict the postoperative outcomes in HFS patients [1]. However, as described in our paper [2], especially in patients in which LSR data were lacking (could not be recorded, disappeared before MVD, remained even after MVD), FMEP monitoring could help us confirm when MVD was completed. Therefore, we routinely use FMEP monitoring as well as LSR monitoring during MVD. In Acioly's series, the orbicularis oculi FMEP amplitude did not change, while the orbicularis oris FMEP amplitude increased. In some patients in our series also, the orbicularis oris FMEP amplitude increased after MVD. On the other hand, in our investigation, the orbicularis oculi FMEP amplitude decreased in all HFS patients. We think, therefore, that the orbicularis oculi FMEP is a more reliable marker [2]. The disparity between our results and Acioly's results may arise from the differences in the stimulation methods. We think that a stable FMEP is obtained by using supramaximal intensity stimulation.

  Indeed, the usefulness of FMEP monitoring during MVD for HFS was concluded from the results for a small population in this study. As Acioly says, we have to determine whether the monitoring is useful for predicting the postoperative outcome in a larger population of HFS patients.

  1.Yamashita S, Kawaguchi T, Fukuda M, et al. Abnormal muscle response monitoring during microvascular decompression for hemifacial spasm. Acta Neurochir (Wien) 2005;147:933-8.

  2.Fukuda M, Oishi M, Hiraishi T, et al. Facial nerve motor-evoked potential monitoring during microvascular decompression for hemifacial spasm. J Neurol Neurosurgery, Psychiatry 2010; 81:519-23.

  Conflict of Interest:

  None declared

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  Conflict of Interest:
  None declared.

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• Published on: 13 April 2016

  Is there a role for facial motor evoked potentials in the microsurgical treatment of hemifacial spasm?

  • Marcus A Acioly, Clinical and Research Fellow
  • Other Contributors:
    • Florian Roser, Paulo H.P. de Aguiar, Marcos Tatagiba

  Marcus Andre Acioly, MD1,2 Florian Roser, MD1 Paulo Henrique Pires de Aguiar, MD2 Marcos Tatagiba, MD1 1 Department of Neurosurgery, University of Tuebingen, Germany 2 Department of Neurology, University of Sao Paulo, Brazil

  Dear Editor,

  The recent article by Fukuda et al.[1] documents a continuing interest in the study of a novel art of facial nerve (FN) monitoring. There are some important issues that...

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  Marcus Andre Acioly, MD1,2 Florian Roser, MD1 Paulo Henrique Pires de Aguiar, MD2 Marcos Tatagiba, MD1 1 Department of Neurosurgery, University of Tuebingen, Germany 2 Department of Neurology, University of Sao Paulo, Brazil

  Dear Editor,

  The recent article by Fukuda et al.[1] documents a continuing interest in the study of a novel art of facial nerve (FN) monitoring. There are some important issues that we would like to discuss, however. The term facial nerve motor evoked potential is used to describe elicited recordings in response to transcranial electrical stimulation.[1] According to the American Association of Electrodiagnostic Medicine glossary of definitions, motor evoked potential is defined as, a compound muscle action potential produced by either transcranial magnetic stimulation or transcranial electrical stimulation.[2] As waveforms were recorded from facial muscles using paired sub-dermal needle electrodes and not directly over the FN itself,[1] the elicited recordings are better defined as facial motor evoked potentials (FMEP).[3] FMEPs then represent compound muscle action potential (CMAP) recordings and should be evaluated accordingly.[4] Quantitative EMG was initially explored for clinical use during the 1950s and is currently applied for techniques involving a detailed analysis of CMAP characteristics such as amplitude, latency, complexity, duration and area under the curve.[5] Amplitude is by far the most used quantitative parameter for intraoperative FMEP monitoring.[1,3] Additionally, Fukuda et al.[1] evaluated FMEP duration in their study. Nonetheless, duration of the action potential is a function of amplitude and waveform morphology so that shorter duration is observed in recordings of lower amplitude and lower complexity.[6] Conversely, waveform morphology is an underestimated CMAP parameter for intraoperative MEP monitoring. In this regard, we have conducted a prospective study to analyze intraoperative electrophysiological changes in FMEP waveform morphology and their correlation to postoperative facial function during vestibular schwannoma resection (submitted). Analysis of correlation coefficients revealed a significant statistical negative correlation with the immediate and late postoperative FN outcome, such that greater FMEP complexity predicted better FN function during all surgical stages directly related to tumour manipulation, especially for orbicularis oris recordings. These results in facial function prediction indicate that waveform complexity seems to be independent of FMEP amplitude serving as an intraoperative electrophysiological indicator of FN integrity. The remaining respect deals with the additional role of FMEP over lateral spread response (LSR) monitoring during microsurgical treatment of HFS. We have developed a retrospective cohort study including two groups of patients operated on for HFS treatment, one group monitored exclusively with LSR (4 patients) and other group monitored with LSR and FMEP (5 patients). Surprisingly, final orbicularis oculi FMEP amplitude had no variation across surgery and orbicularis oris FMEP amplitude was significantly increased after adequate decompression compared to baseline FMEP recordings (p=0.043). Besides, the unique patient with incomplete HFS remission in Group 2 had significant amplitude reduction (greater than 50%) in both orbicularis oculi and oris muscles right after decompression. Based on our results, the use of FMEP did not bring any additional information as measured by outcome (complete remission Group 1 [75%] and Group 2 [80%]; p=0.866). We share authors opinion that further study in a larger population is required to evaluate FMEP variation after FN decompression. FMEP is a promising method for FN monitoring during microvascular decompression for HFS and a still evolving technique, however HFS pathogenesis is multifactorial with a potential negative influence on FMEP intraoperative interpretation.

  References

  1. Fukuda M, Oishi M, Hiraishi T, et al. Facial nerve motor-evoked potential monitoring during microvascular decompression for hemifacial spasm. J Neurol.Neurosurg.Psychiatry 2010;81:519-23.

  2. AAEM glossary of terms in electrodiagnostic medicine. Section I: Alphabetic list of terms with definitions. Muscle Nerve 2001;24:S5-S28.

  3. Acioly MA, Liebsch M, Carvalho CH, et al. Transcranial electrocortical stimulation to monitor the facial nerve motor function during cerebellopontine angle surgery. Neurosurgery 2010;66:354-61.

  4. Beric A. Transcranial electrical and magnetic stimulation. Adv.Neurol. 1993;63:29-42.

  5. Phongsamart G, Wertsch JJ. Quantitative electromyography. Phys.Med Rehabil.Clin N.Am. 2003;14:231-41.

  6. Buchthal F, Guld C, Rosenfalck P. Action potential parameters in normal human muscle and their dependence on physical variables. Acta Physiol Scand. 1954;32:200-18.

  Conflict of Interest:

  None declared

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  Conflict of Interest:
  None declared.

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