Facial nerve electrodiagnostics for patients with facial palsy: a clinical practice guideline

Key points

• Electroneurography (ENoG) and needle electromyography (nEMG) are the standard facial electrodiagnostic tests; blink reflex testing, surface EMG, transcranial magnetic stimulation (TMS) and facial nerve mapping are reserved for selected cases.
• Most patients with a facial nerve disorder should undergo facial electrodiagnostics.
• ENoG is most valuable between 72 hours and 21 days after onset; a side difference of 30% or more is pathologic, and a difference of 70-95% is considered relevant for prognosis.
• EMG is most valuable from 2-3 weeks to 3 months after onset and can detect reinnervation about 2-3 months before clinical movement returns.
• A strong consensus (agreement among more than 95% of participants) was reached for all recommendations.

Overview

This clinical practice guideline from members of the International Head and Neck Scientific Group and the Multidisciplinary Salivary Gland Society describes the equipment, practical use and interpretation of facial nerve electrodiagnostics in peripheral facial nerve disorders. Its main message is that ENoG and needle EMG, repeated during the acute phase and interpreted together, help assess lesion severity and predict recovery. It is based on a systematic literature search (2015-2019, 32 articles), and because high-quality evidence is lacking, the recommendations reached grade B.

Nerve injury types and timing

Peripheral facial nerve injury is classified as neurapraxia, axonotmesis or neurotmesis, and electrodiagnostics try to tell these apart, which is not always possible depending on the time since injury. After an intratemporal lesion such as Bell's palsy, Wallerian degeneration takes about 72 h to reach the nerve beyond the stylomastoid foramen, so ENoG can be normal during that window. Signs of muscle degeneration cannot be seen on EMG before 10-14 days, and nerve degeneration is complete at 21 days.

Electroneurography

The main trunk of the facial nerve is stimulated at the stylomastoid foramen and the compound muscle action potential is recorded, typically in the nasolabial fold, first on the healthy side and then on the affected side. The result is the amplitude of the affected side as a percentage of the healthy side; the test has a re-test variability of about 20%. The first ENoG is done about 3 days after onset and repeated every 3-5 days. ENoG alone cannot reliably distinguish neurapraxia from a more severe lesion.

Needle electromyography

The frontalis, orbicularis oculi, orbicularis oris and zygomaticus muscles give a good overview of facial nerve function. A standard sequence assesses insertion activity, pathologic spontaneous activity at rest, activity during voluntary movement and, in chronic palsy, synkinetic activity. Fibrillation potentials and positive sharp waves are signs of degeneration and generally indicate a poor prognosis, while polyphasic reinnervation potentials, seen after 4-6 weeks, indicate reinnervation. nEMG cannot differentiate axonotmesis from neurotmesis.

Other tests and documentation

If ENoG and nEMG are both performed, blink reflex testing adds little, but it may help when a brainstem lesion is suspected. TMS is not recommended for routine use. Multichannel surface EMG is not used routinely in acute palsy but helps analyse mimic expression and synkinesis in chronic disorders. Transcutaneous facial nerve mapping, which takes about 25 minutes, can support surgical planning in complex cases. Equipment should perform ENoG, at least 2-channel EMG and blink reflex testing, and results should be documented on a standardized form.

Frequently asked questions

When should ENoG be done after acute facial palsy?

The first test is done about 3 days after onset and repeated every 3-5 days; ENoG is most valuable between 72 hours and 21 days after onset and should be interpreted together with EMG.

Can electrodiagnostics always tell how severe the nerve injury is?

Not always. The answer depends on the time since injury, and nEMG alone cannot separate axonotmesis from neurotmesis, so repeated testing increases the value of the results.

Is transcranial magnetic stimulation useful?

It is not recommended for routine use because even patients with minimally affected nerves can lose excitability on TMS; it may help in highly selected cases of intratemporal pathology.

Source

Guntinas-Lichius O, Volk GF, Olsen KD, Mäkitie AA, Silver CE, Zafereo ME, et al. Facial nerve electrodiagnostics for patients with facial palsy: a clinical practice guideline. European Archives of Oto-Rhino-Laryngology 2020;277(7):1855-1874. DOI: 10.1007/s00405-020-05949-1. Open access under a Creative Commons Attribution 4.0 licence. This page is a summary prepared by Medpresso from the original publication and is not a substitute for the full text or for medical advice.