Central or Peripheral Activations of the Facial Nerve?

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To the Editor: We read with great interest the article by Wilkinson and Kaufmann (Monitoring of facial muscle motor evoked potentials during microvascular decompression for hemifacial spasm: evidence of changes in motor neuron excitability. J Neurosurg 103:64–69, 2005). The authors have tried to prove the hypothesis that facial motoneurons become less excitable after MVD in patients with HFS. We strongly believe that this hypothesis can be proved; however, we are deeply concerned about the method used in this study.

First, the authors elicited MEPs in facial nerve innervated muscles using a single stimulus applied

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    Facial MEPs responses in the left orbicularis oris muscle elicited by a train of four transcranially applied stimuli (S1 at C4/Cz points), followed by a single transcranial stimulus (S2) in the patient with HFS under general anesthesia. It is obvious that only a train of four stimuli, but not a single stimulus, elicits facial MEPs. After MVD, the threshold for eliciting facial MEPs significantly increases while the amplitude decreases.

References

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Rösler KMHess CWSchmid UD: Investigation of facial motor pathways by electrical and magnetic stimulation: sites and mechanisms of excitation. J Neurol Neurosurg Psychiatry 52:114911561989

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