HSC Section 3 - Trauma, Critical Care and Sleep Medicine

Surgical Management of Acute Facial Palsy

INDICATIONS FOR SURGICAL INTERVENTION Principles of Electrophysiologic Testing

Electrophysiologic testing provides an objective means of assessing nerve function and is indicated for patients with complete paralysis. Those with incomplete paralysis carry a favorable prognosis and electrophysiologic testing is not indicated. Electro- physiologic testing offers prognostic value for the likelihood of poor recovery (House-Brackmann [HB] grade 3 or higher) in patients with complete paralysis, thereby identifying those who may be candidates for surgical decompression. ENoG and electromyography (EMG) are the two most accurate and reliable electro- physiologic tests currently in use. ENoG estimates the relative proportion of nerve fibers that have undergone waller- ian degeneration and is most useful 4 to 14 days after the onset of complete paraly- sis. 26,27 ENoG testing is not performed before the fourth day of paralysis because it takes approximately 3 days for wallerian degeneration to occur. Testing is not per- formed after 2 weeks of paralysis because patients who have not reached the critical degeneration threshold by that time have a high likelihood of good recovery (HB 1 or 2) and are not candidates for surgical decompression. 18 ENoG uses an evoked, supramaximal electrical stimulus to activate the facial nerve as it exits the skull at the stylomastoid foramen ( Fig. 1 A). The technique of performing ENoG must be standardized in order to provide reliable data regarding the prognosis of facial nerve function and recovery. 26,28,29 Surface electrodes are used to record the evoked biphasic compound muscle action potential (CMAP) ( Fig. 1 B), which depends on the synchronous discharge of multiple viable nerve fibers. The maximum amplitude of the CMAP correlates with the number of remaining nerve fibers that are responsive to stimulation. 30 The CMAP from the affected side is then compared with the CMAP of the normal side, which serves as a control, and a percentage of degenerated nerve fibers is calculated. 29 Voluntary EMG measures motor activity with needle electrodes placed in the orbi- cularis oris and orbicularis oculi muscle while the patient is asked to make forceful facial contractions ( Fig. 1 C). In the acute phase of paralysis, EMG is used when ENoG shows 90% or greater neural degeneration, 31 to confirm absence of muscle function. Presence of active motor units on EMG in the setting of severe degeneration on ENoG indicates a phenomenon termed deblocking, which is the asynchronous discharge of regenerating nerve fibers that fail to produce a measurable CMAP on ENoG. 31 Deblocking is a sign of nerve regeneration and portends a favorable prog- nosis. Therefore, patients with deblocking should not proceed to surgical decompres- sion despite severe degeneration on ENoG. Bell Palsy: Prognosis and Assessment of Surgical Candidacy In the setting of acute facial nerve paralysis, the utility of surgery depends on the severity of paralysis. Approximately 90% of patients with incomplete facial nerve pa- ralysis (HB 5 or less) show complete resolution of their symptoms with steroid ther- apy alone. 1,32,33 However, only w 60% of patients with complete paralysis (HB 6) recover to HB 1 or 2 without surgical intervention. Electrophysiologic testing is there- fore used in this group of patients to identify those at high risk for poor recovery, who may be candidates for surgical intervention. Neural degeneration of greater than 90% on ENoG testing has been associated with increased likelihood of poor outcome. 2,18 ENoG testing is performed serially between days 4 and 14 to assess both the extent and rate of degeneration. Rapid progression to the 90% level indicates more severe neural injury (eg, neurotmesis) and less likelihood of return to normal facial function. 34

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