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Enog) by Douglas L Evaluation of the facial nerve via electroneuronography (ENoG) By Douglas L. Beck and James W. Hall III Facial nerve paralysis is a debilitating condition.1 Patients measurement of the acoustic reflex during immittance with the condition often experience severe emotional and test batteries. psychological impacts because of facial disfiguration and the resulting physical limitations and difficulties associ- Alternative tests of facial nerve function ated with speaking, drinking, eating, and making facial Many other tests of facial nerve function have been, and expressions. For many of these patients, socialization is continue to be, used. These include, among others: the extraordinarily limited and difficult. Hilger test, electromyography, acoustic reflex testing,5 The evaluation of facial nerve viability by means of elec- evoked accelerometry, antidromic nerve potentials, MRI troneuronography (ENoG) is critically important in the and CT radiologic evaluations, maximal nerve stimulation management of facial nerve disorders. Depending on the tests, minimal nerve stimulation tests, transcranial mag- outcome of the ENoG evaluation, the physician may choose netic stimulation,6 and blink reflex tests.7 to “watch and wait” or may decide to intervene surgically. Surgery is by no means trivial, and its utility is often directly THE HOUSE-BRACKMANN SCALE determined via the EnoG evaluation. The most commonly used tool for grading facial nerve Facial nerve disorders have a variety of etiologies includ- function is the House-Brackmann (HB) scale.8 The HB ing: Bell’s palsy (BP), iatrogenic (surgically induced) injury, scale is used to approximate the quantity of volitional trauma to the temporal bone secondary to motor vehicle motion the patient has based on clinical facial presenta- accidents (MVA), otitis media, herpes zoster oticus, mul- tion. Although the HB scale is derived from clinical obser- tiple sclerosis, Melkersson- vation, and variation among Rosenthal syndrome, mastoi- observers exists, this scale does ditis, mumps, chicken pox, ...The evaluation of facial allow us to grossly describe Guillain-Barré syndrome, the characteristics and degree central nervous system dis- nerve viability by means of of facial nerve motion. orders (e.g., stroke glomus The HB scale has six jugulare, meningioma, and electroneuronography (ENoG) grades. Each grade is reported facial nerve neuroma.2 as a fraction (e.g., 1/6 = grade is critically important in the one). A grade one presenta- AUDIOLOGISTS AND tion is perfectly normal. ENOG management of facial Grade two indicates a slight ENoG is used by audiolo- or mild weakness. Grade gists to evaluate the integrity nerve disorders...” three is a moderate weakness of the facial nerve. This pro- with good (or normal) eye cedure involves electrical closure. Grade four is a mod- stimulation of the facial nerve at or near the stylomastoid erate weakness with no volitional eye closure. Grade five foramen and the subsequent measurement and interpre- is a severe weakness. Grade six is a total facial paralysis. tation of the motoric response as recorded at or near the In some respects, one might argue that only grade six nasio-labial fold. Although other procedures are used as (6/6) presentations require EnoG testing. After all, the pur- well (see below), the ENoG test is the only relatively objec- pose of the test is to determine if the facial nerve is neu- tive measure of facial nerve integrity. rophysiologically intact. And, since it can be inferred that The ENoG compares the neurophysiologic response in a patient who has any volitional motion (as would be of the normal side of the face to that of the abnormal side. indicated by grades one through five) the facial nerve is The findings help in determining (1) whether or not sur- indeed intact, then there is no need for EnoG testing. gical intervention is recommended and (2) the probable Nonetheless, it is useful to chart the progress of facial prognosis. nerve disorders via EnoG even in cases without a grade six This procedure falls within the scope of practice of presentation. Additionally, it is sometimes difficult to dis- audiologists as defined both by the American Academy cern if an apparent grade five presentation is really a grade of Audiology and the American Speech-Language-Hear- five or if it is actually a grade six. In some cases, the extremely ing Association.3,4 Audiologists have been evaluating facial limited motion seen is from the masseter, or from muscula- nerve function for many decades, starting perhaps with ture motion from the contralateral (normal) side. In such 36 The Hearing Journal Electroneuronography March 2001 • Vol. 54 • No. 3 cases, the motion seen does not indicate an ness worse than HB grade 4/6. end of the timing window is the 21-day intact facial nerve. The etiology of BP is unknown. Viral maximum. EnoG evaluation allows a subclinical neuropathy, bacterial infections, genetics, Specifically, if ENoG and subsequent analysis which may impact medical or sur- environmental and many other causes surgical intervention are delayed past a gical decisions. However, once a visible have been suggested. It is likely that there 21-day post-onset window, the test and improvement is apparent regarding facial are a number of reasonable etiologies. Var- possible surgical intervention are of ques- nerve reanimation, further ENoG testing ious treatment options are available for tionable value. In essence, the ENoG provides little information. BP, including surgical intervention, “wait- should be performed for the first time at ing and watching,” acyclovir treatment,11 about 72 hours post-onset and again at PHYSIOLOGY OF and other medical options. intervals of 3 to 5 days until a trend is THE FACIAL NERVE observed and confirmation can be Humans have 12 pairs of cranial nerves, Wallerian degeneration and obtained. but our discussion will be limited to the timing of ENoG If the trend and confirmation are seventh nerve, the facial nerve. Each facial In cases of BP and other facial nerve determined prior to 21 days post-onset, nerve has some 10,000 fibers.9 About two- injuries, it takes some 72 hours for Wal- surgical intervention may be an option. thirds of the fibers are motor fibers, and lerian degeneration (WD), which is the However, if the ENoG is not conducted about one-third are sensory. The sensory denervation of the neural fibers, to occur.12 until more than 21 days post-onset, it is portion of the facial nerve is the nervus That means that if a patient is found to of little clinical use. Specifically, an ENoG intermedius. It is estimated that only half have an HB grade six facial presentation result obtained 8 weeks after onset of of the motor fibers need to be function- 1 hour after the onset of BP, administer- facial paralysis is difficult, if not impos- ing for a person to have essentially nor- ing ENoG would be likely to produce a sible, to interpret. mal facial nerve function. somewhat normal result. That is, because As the facial nerve exits the brainstem, the facial nerve would not yet have not TYPES OF FACIAL it traverses the cerebellopontine angle completed WD, the fibers would still be NERVE INJURY (CPA) to the medial end of the internal physiologically intact, although non-func- Fisch has used three terms—neuropraxia, auditory canal (i.e., the porus acousticus). tional volitionally. The resultant ENoG neurotmesis, and axonotmesis—for the Progressing distally to the stylomastoid would produce a false negative result if the three primary types of facial nerve injury.13 foramen, the facial nerve includes the test was performed prior to complete WD. It should be noted that ENoG cannot dif- labyrinthine segment, the tympanic seg- Therefore, it is important to wait ferentiate between neurotmesis and ment, the pyramidal bend, the mastoid approximately 72 hours before perform- axonotmesis. portion, and, finally, the stylomastoid fora- ing the first ENoG. However, at the other Neuropraxia is the most common find- men, where the nerve exits the skull and is readily positioned within the parotid gland. Stimulus As the main trunk of the facial nerve O Transducer pair of electrodes enters the parotid region, it separates into O Site stylomastoid foramen region two main divisions—superior and infe- O Orientation horizontal with cathode (negative) posterior rior. The superior division then divides O Type electrical pulse (shock) of constant current or voltage into three sections—temporal, zygomatic, O Mode continuous and buccal divisions. The inferior divi- O Duration 0.2 msec (200 µsec) sion divides into the buccal, mandibular, O Rate 1.1/sec and cervical branches.10 O Laterality unilateral, assess uninvolved side first O Intensity milliamperes (mA) sufficient to produce a supramaximal response BELL’S PALSY: (usually 15 to 25 mA) Bell’s palsy (BP) is probably the single most common cause of facial nerve dis- Acquisition orders. In the general population, BP O Amplification X 5000 (or less if response exceeds 1000 µV) occurs in about 15 of every 100,000 peo- O Filter setting 3 to 5000 Hz ple (0.00015%). The condition will recur O Notch filter none in 5% to 10% of patients. O Analysis time 20 msec The time from onset of BP to total, O Pre-stimulus time 1 msec unilateral facial paralysis is usually about O Number of sweeps 1 to 20 24 to 48 hours. Spontaneous recovery is O Electrodes Channel 1-Nasolabial fold (corner of mouth to base of nose) common, occurring among some 75% to on side ipsilateral to stimulation 80% of all patients, usually in about 3 to Ground (common)-forehead 4 weeks. However, about 15% to 20% of O Interelectrode impedance less than 5000 ohms patients maintain a lifelong residual weak- ness following resolution of the BP. About Table 1. Summary of ENoG measurement parameters. 5% of all patients have permanent weak- 38 The Hearing Journal Electroneuronography March 2001 • Vol.
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