Clinical Forum

J Am Acad Audiol 4 : 109-115 (1993)

Electroneurography : Electrical Evaluation of the Facial Nerve Douglas L. Beck* James E. Benecket

Abstract

This article provides an introduction, anatomic considerations, and description of technique for the performance of electroneurography . Two case studies are provided as illustrations .

Key Words: Electroneurography (ENoG), evoked electromyography (EEMG), facial nerve, paralysis

E

lectroneurography is used to determine the physiologic status ofthe facial nerve. The term electroneurography (ENoG)

refers to the electrical stimulation of the facial nerve. Another term used to describe the same test is evoked electromyography (EEMG). This term refers to the motoric response resulting from electrical stimulation of the facial nerve.

ENoG is easily accomplished in the office and requires no anesthesia . It is noninvasive, quick, efficient, and highly reliable . ENoG is an objective measure of facial nerve function and is interpreted within the context of the clinical presentation and the patient's relevant history.

The purpose of ENoG testing is to quantify the percentage of facial nerve fibers that are electrically stimulable . The patient serves as the control because the weak side is compared to the normal side . ENoG accurately approximates the percentage of functioning nerve fibers based on the amplitude of the motoric response . ENoG helps the physician to differentiate between facial nerve diagnoses, and to implement an

*Saint Louis University Medical Center, Department of Otolaryngology--Head and Neck Surgery, St . Louis, Missouri ; f Private Practice, St Louis, Missouri

Reprint requests : Douglas L . Beck, Department of Otolaryngology-Head and Neck Surgery, 3660 Vista Avenue, Suite 312, St . Louis, MO 63110

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effective management strategy designed to fa-cilitate facial motion as soon as possible (Hughes, 1990).

The facial nerve is familiar to the audiolo-gist as it is a prominent factor in the impedance audiometry reflex arc. A loud sound presented to either ear elicits a bilateral acoustic reflex, which is measured and can contribute to the differential diagnosis.

The reflex arc is dependent on. the patient's hearing, auditory nerve, brain stem, and facial nerve for the normally present stapedial muscle contraction. With normal hearing and a facial nerve lesion peripheral to the stapedial branch of the facial nerve, the reflex will be normal even though the patient's face may be paralyzed (Moller, 1987).

In otology, neurotology, and otolaryngology, patients present with facial nerve injury, weak-ness, or paralysis. A paralyzed face is debilitat-ing (Pulec,1974; Luxford and Brackman,1985) . Diagnosis and treatment of facial nerve disor-ders are most successful when approached in a timely manner .

Bell's palsy is only one possibility in the differential diagnosis of facial paralysis. Other causes include: temporal bone fracture, facial nerve neuroma, herpes zoster oticus, otitis me-dia, mastoiditis, chickenpox, mumps, chole-steatoma, glomus jugulare, meningioma, and iatrogenic paralysis .

Journal of the American Academy of Audiology/Volume 4, Number 2, March 1993

WHEN TO TEST

T he timing of the test relative to the onset of facial paralysis is important (Gantz,

1984). There is only a small window of opportu-nity during which the ENoG offers reliable and useful information.

The test must not be performed too early or too late . Wallerian degeneration of the facial nerve requires 72 hours to completely deacti-vate the nerve (Gantz, 1987). Testing prior to completion of Wallerian degeneration may lead to an incorrect (usually overly optimistic) con-clusion.

An ENoG on the first day following onset of facial paralysis is likely to show a high percent-age of intact fibers, despite total facial paraly-sis. In this situation, the ENoG quantifies the insult before total damage has been done. It is likely that an ENoG performed on the second or third day would show further deterioration. A test after 72 hours would be more representa-tive of the complete injury . Similarly, testing after 21 days is of limited use. Even a good (50%) ENoG obtained after the 3-week window offers questionable prognostic information.

ENoG protocol requires a timely, repeat-able test, and an immediate report to the treat-ing physician. The audiologist and physician must work quickly to test, interpret, and man-age the patient with a facial nerve disorder .

REPORT GUIDELINES

A Ithough the audiologist does not make the diagnosis or surgical decisions, these deci- sions are often based on the information pro-vided via the ENoG report . It is important to be aware of the information that our medical col-leagues need to appropriately and effectively manage the patient.

In reporting the ENoG result, the quantity of electrically stimulable fibers of the paralyzed side, as a percentage of the normal side is provided . .

When comparing both sides of the face, most normal adults produce ENoGs that are within 20 percent of each other.

For example, suppose a patient is referred for an ENoG evaluation, l week post onset ofleft facial paralysis . The ENoG reveals the healthy (normal) response to be 3500 microvolts and the paralyzed side to be 700 microvolts . This is reported as "Left = 20% Right." The left side ENoG amplitude was determined to be 20 per-cent of the right side . An alternative notation

would be that the left side is 80 percent denervated .

In the above example, the test may be re-peated 4 or 5 days later. The second test reveals if the nerve has remained stable, is more stimulable, or less stimulable. If the second test shows that the weak side has decreased in function to less than 10 percent of the healthy side, this is a "red flag ." The physician must be notified immediately. It has been shown that a quickly scheduled facial nerve decompression, on an appropriate patient, is of far greater benefit than the identical procedure performed at a later date (Fisch, 1974). If the ENoG shows that a higher percentage of fibers are stimulable (e .g., 1750 microvolts, or 50%), a good prognosis is anticipated and the patient is followed.

ENoGs may be ordered every 4 or 5 days until a plateau is seen, voluntary motion is initiated, or as ordered by the physician. It is important to follow these patients to resolution .

FACIAL NERVE GRADING SCALE

A ttempts to categorize facial nerve function have varied greatly. In essence, normal

facial motion is defined by voluntary control of all the muscle groups on the ipsilateral side of the face . Facial paralysis is the total inability to control muscles on the ipsilateral face . Facial weakness refers to the degrees of disability between normal motion and paralysis. Although these three basic categories give a gross impres-sion of facial nerve disorder, they are inexact categorizations and should not be used.

The House-Brackmann (HB) facial nerve grading system (House and Brackmann, 1985) is an effective and efficient tool used to describe facial presentations (Table 1) .

TYPES OF FACIAL NERVE INJURY

ifferent types of facial nerve injury are D consistent with various ENoG results. Pro-fessor Ugo Fisch has provided extensive analy-sis and descriptions of these injuries (Fig . 1) (Fisch, 1980).

Neurapraxia describes a paralysis with-out peripheral degeneration . It is usually re-versible . Neurapraxia is the phenomenon asso-ciated with Bell's palsy.

Axonotmesis describes an inner nerve dis-order. Individual inner nerve fibers are dam-aged extensively and complete peripheral de-generation occurs . The outer casing (epineurium)

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Electrical Evaluation of the Facial Nerve/Beck and Benecke

Table 1 Fa

Grade Description

I Normal

II Mild dysfunction

III Moderate dysfunction

IV Moderately severe dysfunction

V Severe dysfunction

VI Total paralysis

Normal facial function in all areas

Gross : slight weakness noticeable on close inspection ; may have very slight synklnesis

At rest: normal symmetry and tone Motion

Forehead : moderate to good function Eye : complete closure with minimum effort Mouth : slight asymmetry

Gross : obvious but not disfiguring difference between two sides ; noticeable but not severe synkinesis, contracture, and/or hemifacial spasm

At rest : normal symmetry and tone Motion

Forehead : slight to moderate movement Eye : complete closure with effort Mouth : slightly weak with maximum effort

Gross : obvious weakness and/or disfiguring asymmetry At rest : normal symmetry and tone Motion

Forehead : none Eye : Incomplete closure

Gross : only barely perceptible motion At rest : asymmetry Motion

Forehead : none Eye : Incomplete closure Mouth : slight movement

No movement

From House JW, Brackmann ED . (1985) . Facial nerve grading system . Otolaryngol Head Neck Surg 93:146 .

remains intact and may serve as a conduit for regenerating fibers . Recovery can be expected and may lead to a good quality outcome.

Neurotmesis is a total separation of the nerve. These are complete anatomic separa-tions and the prognosis is poor .

ENoG can detect neurapraxia. ENoG can-not differentiate between axonotmesis and neurotmesis.

PATIENT PREPARATION

B efore testing, the patient is informed as to the nature of the test . The test is described

in detail to the patient and a demonstration of the electrical stimulation may be offered (the first author has stimulated his own facial nerve hundreds of times by way of demonstration) .

If the patient has a pacemaker or any medi-cal condition that may contraindicate an electri-cal test, the appropriate medical personnel must

cial Nerve Grading System

Characteristics

be consulted, and a written authorization ob-tained . We do not know of any patient being harmed by ENoG testing.

STIMULATION PARAMETERS

T he stimulation system is electrically cali-brated to present an electrical stimulus of known parameters . As the goal here is to assure that all viable neural fibers are being electri-cally stimulated, the stimulation protocol is often approached by a "stimulate, evaluate, increase, repeat" paradigm . If the evaluation phase reveals a plateau in the response ampli-tude (defined as no further increase in response amplitude with an increase in stimulation cur-rent), a 10 to 20 percent increase in current is applied and used as the final stimulation level.

In our practice, when testing most alert, adult patients we use a hand-held bipolar stimulator. Our nominal starting current is 30

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Journal of the American Academy of AudiologyNolume 4, Number 2, March 1993

Peripheral Facial Paralysis

Elec trodiagnosis and Prognosis of Facial Nerve Lesions

Reversible Conduction Block (Neurapraxia)

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(OTM-Ma Very Good Prognosis

Degeneration

1. Endoneural Tube Intact (Axonotmesis)

2. Endoneural Tube Severed (Neurotmesis)

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Good Prognosis Bad Prognosis

to 35 milliamps with a 150 microsecond pulsewidth. After the initial evaluation is com-pleted, we increase the current by 5 milliamps and look for a response plateau. We do not test above 40 milliamps.

STIMULATION PREPARATION

A nonconductive (nonmetal) chair is provi-ded for the patient. The stimulus sites are cleaned using an alcohol pledget. Make-up, per-spiration, and dirt may increase electrical im-pedance. Although a clean stimulus site is de-sired, we do not abrade the skin at the stimula-tion site .

A drop of electrolyte gel is placed on the anode and the cathode of the stimulator. Too much gel can cause a salt bridge, or a path for shunting . Insufficient gel renders high skin impedance, potentially activating more pain receptors and resulting in an unsatisfactory result.

We instruct patients to close their eyes and teeth gently . We tell them that the first pulse will feel like a small bee sting. We request that they allow -their face to twitch, that is, not to fight the response . If the patient is unable to tolerate the stimulation, the current is reduced to 30 milliamps and the pulse width is reduced to 100 microseconds . Typically, pain is more highly related to pulse width than to current level.

RECORDING ELECTRODE MONTAGE

A three-electrode system is used to record the ENoG. The nasal ala serves as the

112

Figure 1 Electrodiagnosis and prognosis of facial nerve lesions. (From Fisch U. (1980) . Maximal nerve excitability testing vs electroneuronography . Arch Oto-laryngol 106:353 . Copyright 1980, American Medical Association.)

ipsilateral active (-) recording site . The active electrode is referenced to a noncephalic C7 (+) reference site . The contralateral nasal ala serves as the common (or ground) site .

RECORDING PREPARATION

T raditionally, the nasolabial fold has been used as the recording site for bipolar re- cordings (Smith et al, 1988). Research and our own verification has demonstrated that cleaner, more highly repeatable waveforms are obtained using the above described nasal ala protocol (Gutnick et a1,1990 ; Kelleher et a1,1990) . As the ENoG response is robust, it is not necessary to average these recordings . However, we recom-mend two or three repetitions of each trace to ensure repeatability .

Prior to attachment of the recording elec-trodes, the skin is cleaned with an alcohol pledget, and minimally abraded with Omni-prep . Cup electrodes (standard ABR type) are filled with electrolyte gel and are securely taped to the prepared recording sites (Fig. 2) . Smaller electrodes may yield a better response .

RECORDING ANALYSIS

T he latency of the ENoG has not proven to be a reliable indicator of facial nerve sta-

tus. Amplitude is the primary measure describ-ing the waveform . By convention, ENoG is dis-played and measured with the first peak up-ward . The peak-to-peak amplitude is measured and recorded from the early positive peak to the subsequent negative peak .

'101 . f I If 111

Electrical Evaluation of the Facial Nerve/Beck and Benecke

Standard Nasal Ala Recording Stimulation Site Electrodes

CASE STUDIES

Case 1: Bell's Palsy

The patient is a 39-year-old female . She presents with a left facial paralysis, 5 days post onset. The otologist assigned her a grade VI (House-Brackmann) facial presentation, and reported the rest of her otolaryngologic exami-nation as normal . The patient was referred for a complete audiometric evaluation and electroneurography .

The audiometric evaluation revealed nor-mal hearing bilaterally . The patient demon-

Right Ear

250 1000 4000

Right Ear

Left Ear

Pure Tone Audiometry

Speech Audiometry i 100 t 80

0 U

80 a 40

20

0 0 40 80

HL in dB 0 40

HL in dB

C

BC

Ker ro srateots

Pure Tone Audiometry Unmasked Masked

0 49 A

Acoustic Reflex Crossed Uncrossed ® 13

Speech Audiometry Unmasked Masked

dB

0

20 40 so so

100

250 1000 4000

Left Ear

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Figure 3 Case 1 : Audiometric data .

80

Non-Cephalic C-7 Recording Electrode

Figure 2 ENoG stimulation and recording sites.

strated speech reception thresholds and pure-tone averages within normal limits bilaterally. Word recognition scores were excellent bilater-ally with masking in the nontest ear (Fig . 3) .

Impedance audiometry revealed type A tympanograms bilaterally . Acoustic reflex test-ingrevealed normal ipsilateral and contralateral responses when measuring from the right ear. When measuring from the left ear, the ipsilateral response was elevated at 2000 Hz, contralateral stimulation showed an elevated reflex at 500 Hz and no response at 1000 and 2000 Hz (see Fig. 3) .

The patient was instructed and prepared . It was confirmed that the patient did not have any electrical devices surgically implanted (such as a pacemaker) . Electroneurography was performed.

Electroneurography revealed that the nor-mal (right side) response was 3559 microvolts . This response was repeated several times and found to be repeatable (Fig . 4) . The left side revealed a response of 2825 microvolts . This response was repeated several times and found to be reliable . The ENoG was reported as, "AS (left) = 79% AD (right)." The patient was re-ferred back to the otology department for follow-up and management . The otologist determined that this response was consistent with

Figure 4 Case 1 : Electroneurographic data. The top tracing was for the left (weak) side (2825 pV). The bottom tracing was for the right (normal) side (3559 pV). Diagno-sis was Bell's palsy.

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Journal of the American Academy of Audiology/Volume 4, Number 2, March 1993

neurapraxia and the diagnosis of Bell's palsy was made. A repeat ENoG was ordered for 7 days later. The repeated ENoG was similar to the original test . The patient began to regain facial motion on the left side 6 weeks post onset (typically repeated ENoGs are not ordered after visible improvement has occurred).

Case 2: Facial Nerve Trauma

The patient is a 24-year-old male with blunt trauma to the left side of his skull. The patient presented to us 7 days post trauma with facial paralysis and hearing loss . A CT scan demon-strated a left temporal bone fracture .

The audiometric evaluation revealed es-sentially normal hearing in the right ear with a small decrease in sensitivity at 6000 Hz (30 dB). The left ear presented with a mixed hearing impairment .

The speech reception thresholds were con-sistent with the pure-tone findings . The word recognition scores were good to excellent bilat-erally with masking in the nontest ear. Imped-ance audiometry revealed a type A (normal) tympanogram on the right side, and a type B (flat) tympanogram on the left side . The only reflexes present were the right ipsilateral (Fig . 5) .

Right Ear Left Edr

Pure Tone Audiometry

250 1000 4000

Right Ear

0 U e

dB 0

20 40 60 80 100

250 1000 4000

Speech Audiometry

0

Left Ear

0 40 80 0 40 80 HL in dB HL in dB

KEY TO SYMBOLS

100 80

60

40

20 0

Acoustic Reflex Crossed Uncrossed

Pure Tone Audiometry ® 0 Unmasked Masked

AC O 40 Speech Audiometry BC 0 Unmasked Masked

O 40

Figure 5 Case 2 : Audiometric data .

Left no response

Figure 6 Case 2: Electroneurographic data .

Electroneurography revealed that the nor-mal (right) side had a robust, repeatable re-sponse measured as 3303 microvolts .

The left side was not stimulable using our standard recording and stimulation protocol . An attempt was made to stimulate the indi-vidual branches of the facial nerve using a distal stimulation site, and by individually stimulat-ing the upper and lower branches of the facial nerve. No responses were obtained on the left side (Fig . 6) .

The patient was referred back to the otology department for consultation and management.

A repeat of the ENoG test was ordered and performed 4 days later. The second test con-firmed the findings of the original test . No response was obtained on the injured side . The otologist discussed the findings and treatment options with the patient. A combined middle fossa/mastoid surgical decompression was elected.

At the time of surgery, the otologist found that the geniculate ganglion had been crushed by a small bony fragment. This was removed and the patient's postoperative course was un-eventful . Facial function slowly returned over the next 8 months and the patient did achieve an excellent (grade I) result .

SUMNURY

E lectroneurography is part of the arma-mentarium used to assess facial nerve

integrity. ENoG is noninvasive, quick, and effi-cient in determiningthe percentage ofstimulable fibers of the facial nerve . ENoG helps to estab-lish a differential diagnosis and a likely progno-sis. ENoG is well established as the "gold stand-ard" for facial nerve analysis .

REFERENCES

Fisch U. (1974) . Facial paralysis in fractures of the petrous bone . Laryngoscope 84:2141-2154 .

Fisch U. (1980) . Maximal nerve excitability testing vs electroneuronography . Arch Otolaryngol 106:352-357 .

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Electrical Evaluation of the Facial Nerve/Beck and Benecke

Gantz BJ. (1987) . Idiopathic facial paralysis. In : Current Therapy in Otolaryngology-Head and Neck Surgery. Toronto: BC Decker, 62-66.

Gantz BJ . (1984) . Traumatic facial paralysis. In : Gates GA, ed . Current Therapy in Otolaryngology-Head and Neck Surgery. Toronto: BC Decker, 112-115.

Gutnick HN, Kelleher MJ, Prass RL . (1990) . A model of waveform reliability in facial nerve electroneurography. Otolaryngol Head Neck Surg 103:344-350 .

House JW, Brackmann ED . (1985) . Facial nerve grading system . Otolaryngol Head Neck Surg 93:146-147 .

Hughes GB . (1990) . Practical management of Bell's palsy. Otolaryngol Head Neck Surg 102:658-663 .

Luxford WM, Brackmann DE . (1985) . Facial nerve sub-stitution: a review of sixty-six cases . Am J Otol 6 (Suppl):55-57.

Moller MB. (1987) . Otoneurological evaluation of pa-tients with posterior fossa tumors . In : Sekhar LN, Schramm VL, eds. Tumors of the Cranial Base : Diagnosis and Treatment. Mount Kisco, NY : Futura Publishing, 553-562.

Pulec J. (1974) . Bell's palsy: diagnosis, management and results of treatment. Laryngoscope 84:2119-2140 .

Smith IM, Murray AM, Prescott RJ, Barr-Hamilton R. (1988) . Facial nerve electroneurography, standardiza-tion of electrode position . Arch Otolaryngol Head Neck Surg 114:322-325 .

Kelleher MJ, Gutnick HN, Prass RL . (1990). Waveform morphology and amplitude variability in facial-nerve electroneurography .Laryngoscope 100:570-575 .