What is included the preprocedural evaluation for facial nerve repair?

Updated: Nov 28, 2018
  • Author: Tang Ho, MD, MSc; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

The history reveals the cause of the facial paralysis and is extremely important for treatment planning. The signs and symptoms of facial paralysis are obvious. The House-Brackman grading scale for facial paralysis is used to provide an objective description of the degree of paresis or paralysis. Patients demonstrate a lack of tone in addition to no movement on the affected side.

Of particular importance is the inability to protect the eye adequately. This should be assessed by careful inspection for signs of exposure keratitis, ability to close the eye, and the presence or absence of a Bell phenomenon. Questioning of partners and family members can reveal the adequacy of eye closure during sleep. Measures to protect the eye (eg, lubricants, artificial tears, or eye taping at night) must be instituted as appropriate.

In selected cases in which either the location or site of injury is unknown, both computed tomography (CT) and magnetic resonance imaging (MRI) may be helpful. High-resolution CT of the temporal bone provides the best imaging of the bony confines of the facial nerve and may reveal the pathologic site at any point along its course. MRI is superior for delineating the details of the soft tissues, including the nerve itself. Accordingly, it is the study of choice for diagnosing lesions such as acoustic neuromas and facial nerve schwannomas.

Several electrodiagnostic tests are used to evaluate the status of the facial nerve. These may be helpful in instances when the continuity of the nerve is unknown (eg, after penetrating trauma in which the function of the nerve immediately after injury is not documented).

More often, nerve testing is helpful in cases where transection is not a concern but the viability of the nerve is questionable. For example, testing would be helpful for patients with complete paralysis after acoustic neuroma surgery in which the nerve was maintained and patients with idiopathic facial nerve paralysis. Of course, these tests are useful for cases of complete facial nerve paralysis.

The tests available for evaluating the status of facial nerve function include nerve excitability testing (NET), maximal stimulation testing (MST), electromyography (EMG), and electroneurography (ENoG). Of these, the 2 that are currently considered most helpful are ENoG and EMG.

ENoG is an objective quantitative measurement of nerve function. It measures the summation potential of the response on the normal side of the face upon stimulation and compares the amplitude of the response to that on the paralyzed side. The measurement is thought to correspond to the number of remaining functional nerve fibers and thus to be predictive of the likelihood of spontaneous recovery. For example, in Bell palsy, patients who demonstrate greater than 90% degeneration within 14 days of onset have a decreased chance of recovery.

It should be kept in mind that ENoG (like MST and NET) is not useful in the first 3 days after injury, because Wallerian degeneration has not yet occurred to a significant enough extent. Tests performed during this timeframe overestimate the level of function.

EMG is often complementary to ENoG. It can be used to determine if the nerve in question is in fact in continuity (as evidenced by recorded volitional activity), shows evidence of Wallerian degeneration (as reflected by fibrillation potentials), or exhibits signs of reinnervation (as reflected by polyphasic innervation potentials). Fibrillation potentials typically arise 2-3 weeks after injury, and polyphasic reinnervation potentials may precede clinical signs of recovery by 6-12 weeks.


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