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Year : 2020  |  Volume : 5  |  Issue : 1  |  Page : 1-6

The role of transcranial magnetic stimulation in acute Bell's palsy

1 Department of Neuropsychiatry, Assiut University, Assiut, Egypt
2 Department of Physical Medicine, Rheumatology and Rehabilitation, Assiut University, Assiut, Egypt

Date of Submission30-Dec-2018
Date of Decision18-Apr-2019
Date of Acceptance10-Jun-2019
Date of Web Publication05-Feb-2020

Correspondence Address:
Noha Abo El Fetoh
Department of Neuropsychiatry, Assiut University Hospital, Assiut
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/JCMRP.JCMRP_156_18

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This review discusses the reported data in studies using transcranial magnetic stimulation (TMS) in acute Bell's palsy (BP). These studies displayed the role of TMS in confirming the diagnosis and providing prognostic information about outcome of acute unilateral BP. In summary, two TMS roles have been discussed in these reviewed studies when employing TMS in patients with BP: firstt, the role of TMS as a diagnostic tool for BP when a lesion is of peripheral and lower motor in nature either applied over the ipsilateral parieto-occipital region (canalicular stimulation) or over the contralateral facial area of the motor cortex (cortical stimulation), and the second role of TMS is providing data about its prognostic value in recovery and BP outcome. The overall TMS studies are valuable in prognosis of BP regarding recovery and sequelae.

Keywords: Bell's palsy, diagnosis, onset of symptoms, outcome, prognosis, transcranial magnetic stimulation

How to cite this article:
El Fetoh NA, Fathi NA, Gamal Eldein RM, Shehetta MS. The role of transcranial magnetic stimulation in acute Bell's palsy. J Curr Med Res Pract 2020;5:1-6

How to cite this URL:
El Fetoh NA, Fathi NA, Gamal Eldein RM, Shehetta MS. The role of transcranial magnetic stimulation in acute Bell's palsy. J Curr Med Res Pract [serial online] 2020 [cited 2021 Apr 19];5:1-6. Available from: http://www.jcmrp.eg.net/text.asp?2020/5/1/1/277492

  Introduction Top

Bell's palsy (BP) is an acute, peripheral facial paresis of unknown cause[1]. It is the most common form of peripheral facial palsy in adults[2],[3], with annual incidence range of 20–30 per 100 000 in most epidemiological studies [3–5]. However, other studies recorded higher incidence rate ranged from 51.89 to 107/1 000 000 [6–8].

In at least 85% of affected cases, a complete or near-complete recovery can occur within 5 months without treatment[2]. Up to 30% of patients with BP fail to completely recover of facial function, with the result that thousands of these patients had permanent, potentially disfiguring facial weakness each year[9]. Therefore, these patients want to know the probability and duration of recovery[10],[11].

Neurophysiological methods such electrical stimulation (ES) and transcranial magnetic stimulation (TMS) of the facial nerve are presumed to be tools to confirm the diagnosis and obtain information concerning the prognosis of the palsy at the onset of symptoms [12–14].

  The Role of Conventional Electroneurophysiological Tests in Bell's Palsy Prognosis Top

Electroneurography (ENoG) is the most frequently used test and has been claimed to be the most reliable test to assess facial nerve degeneration in BP [15–19]. During ENoG study, the nerve is stimulated percutaneously over the stylomastoid foramen and the compound muscle action potential (CMAP) is recorded in the affected facial muscle and reported as a percentage of the CMAP amplitude nonaffected side. Facial nerve degeneration of more than or equal to 90% has been shown to predict long-term outcome of facial weakness[20],[21]. Recently, Khedr et al.[22] reported that when the degeneration rate of affected frontalis muscle exceeds 50% of unaffected side, it indicates poor predictors of recovery in BP.

However, the ENoG test cannot be used in the early stage of BP as  Wallerian degeneration More Details of nerve fibers takes at least 72 h to become apparent after an acute injury to the facial nerve. It is therefore recommended that ENoG should not be performed until at least 3 days after the onset of facial palsy[23]. Others choose ENoG as a prognostic test between 5 and 14 days after onset[24],[25].

Prognostic procedures in objective electrophysiologic examinations include the nerve excitability test, ENoG, electromyography, and stapedial reflex measurements. Each of these has advantages and limitations in practice, but nerve excitability test and ENoG are the most widely used. The facial nerve conduction test has been used for the prognosis of the disease. The recovery rate varies with the degree of denervation or degeneration rate[11],[22],[26],[27]. However, ENoG responses are variable based on the branch of the facial nerve and electrode placement in patients with BP, even within the same patient[11].

The blink reflex is an another neurophysiological test that allows assessment of the entire efferent peripheral pathway of the facial nerve[28], but the specificity and sensitivity of this parameter increases over time after BP onset, not in early onset[29].

The role of transcranial magnetic stimulation in Bell's palsy

The distal part of the facial nerve is accessible to ES but a large part of the nerve is located within the cranium. In a series of reports, TMS can excite the facial nerve and the facial representation in motor cortex painlessly[12],[30],[31],[32],[33]. TMS can be applied over the ipsilateral parieto-occipital region, with the base of the coil over the mastoid (canalicular stimulation) or at labyrinthine segment and performed over the contralateral facial area of the motor cortex (cortical stimulation)[30],[31],[32],[33].

In BP, evoked response of the labyrinthine segment of the facial nerve by TMS is significantly reduced or even completely absent within hours after symptom onset[34],[35], where conduction block within the canalicular proportion to TMS within 3 days from symptom is thought to be specific for BP diagnosis[30],[31],[32],[33],[34],[35],[36]. Therefore, after 3 days from symptom onset, the diagnostic value of TMS vanishes owing to progressive axonotmesis[37],[38]. Thus, TMS allows identification of a conduction failure at the canalicular portion of the facial nerve which is not accessible by ES, and thus helps to identify the location of the lesion[39].

The facial muscles are considered to have bilateral cortical innervation[40]. There are controversial observations in old studies with TMS, where some authors recorded responses in the upper and lower facial muscles with TMS of the facial M1 area. The central delay is significantly longer for facial muscles compared with that of limb muscles or muscles innervated by other cranial nerves[41],[42]. However, another study found no contralateral upper facial responses from stimulation of the facial M1 area but there were low amplitude responses from stimulation of the mesial frontal region, suggesting that the upper facial movements are controlled by the medial frontal cortex rather than by the M1[43].

Transcranial cortical magnetic stimulation (TCMS) of the motor cortex can be used to evaluate corticospinal excitability and function[44],[45], whereas the intracranial course of the facial nerve prevents more proximal stimulation by ES. With the introduction of TMS, it became possible to excite the intracranial segment of the facial nerve and its cortical motor representation area, and thus conduction measurements can be performed across the entire peripheral and central facial motor pathways[46].

The topographic presentation of the motor area to facial muscles was reported in the study by Rimpilainen et al.[47]. The cortical motor evoked potential (MEP) recording response was differentiated from the magnetic peripheral motor responses, as the later had shorter latencies, constant shape, and appeared only at high stimulation intensities as previously reported[12],[30],[35]. In addition, the intensity of TMS on the motor cortex to evoke cortical response was never strong enough to stimulate peripheral facial nerve[32],[48].

Many studies[13],[33],[47],[49],[50],[51],[52],[53],[54],[55],[56],[57],[58] using TMS early in acute BP within the firstt week of onset from the presenting symptoms are illustrated in [Table 1], which confirm the diagnosis of BP as a peripheral lesion of facial nerve and provide valuable information about prognosis for BP recovery either used alone as investigatory tool or combined with ENoG studies. Most of these studies reported that good prognosis with better and early recovery among patients with BP who were giving a positive response early within the firstt week of symptoms to TMS than those who were eliciting no response to TMS [Table 1].
Table 1: Transcranial magnetic stimulation studies in Bell's palsy

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Alternatively from another aspect, the evoked cortical (MEP) response of affected facial muscles by TCMS in BP is confirming that the lesion is peripheral and lower motor in nature and intact central pathway as observed by Rosler et al.[30],[35]. Therefore, according to the aforementioned data, TMS to the facial nerve could be used to conduct impulses along the whole of its course following transsynaptic excitation of the motoneurons, although its proximal segment could not be excited directly by ES. The decreased magnetic excitability by canicular stimulation at labyrinthine segment had been proved to be one of the most sensitive indicators for inflammation or compression of the facial nerve in BP.

Similarly, investigation of patients with BP using TCMS technique (cortical stimulation of facial muscles) could provide a useful assessment tool in confirming the diagnosis and providing prognosis as previously reported by others[34],[48]. However, Rosler et al.[30] concluded that patients with BP who evoked cortical MEP response by TMS early in the firstt week were suspected to have better prognosis and recovery than who did not elicit MEP response with TMS of facial muscles.

In practice, Rimpilainen et al.[47] found that the degeneration process in facial nerve had an effect on MEPs amplitude to TMS, and it was progressively more difficult with time, and after the firstt 4 days became impossible, to predict clinical outcome and recovery from the BP with this method[47]. However, conduction time (CT) is a valuable parameter in the detection of conduction along the whole facial nerve pathway up to corticomotor areas of facial muscles, including measurement of peripheral and central motor CT by the prolongation or side difference of CT[59].

From another point of view, Glocker et al.[34] observed a relationship between cortical MEP amplitude of affected facial muscle and CMAP of amplitude of affected to unaffected muscle amplitude ratios of facial nerve in ENoG studies. When they performed stimulation of the facial nerve electrically, and magnetically in the labyrinthine segment, as well as the face-associated motor cortex magnetically stimulated in patients with facial palsy, they found a marked reduction of the amplitudes of MEP evoked by magnetically in the labyrinthine segment, which was more pronounced than the amplitude reduction to stimulation of the facial nerve electrically early during the disease at the firstt 4 days. These changes persisted for several months, although facial nerve function had recovered to normal. Inspite of having an inflammatory lesion and lower motor neuron in nature, these data suggest that the cortical areas of facial muscles had a role in BP recovery. This hypothesis was investigated recently in the study Lee et al.[60]. However, they considered that severity of BP in acute phase and duration for recovery might have an influence on the cortical reorganization based on findings of the study by Klingner et al.[61], using fMRI in follow-up of patients with BP for recovery.

  Conclusion Top

In this review, we presented the reported data of different studies that have used TMS as a diagnostic tool for acute BP within the firstt week of presenting symptoms from the onset and providing valuable information about prognosis of BP recovery. These reported data are promising to conduct future studies that should address many questions, including (i) the optimum timing of TMS assessment in acute BP and follow-up and (ii) comparison between magnetic excitability parameters evoked at labyrinthine segment and cortical segment at faciomotor cortex of facial nerve and its prognostic value.

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Conflicts of interest

There are no conflicts of interest.

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