Facial Nerve Repair Periprocedural Care

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.

Currently, the following 3 options are available for surgical repair of the facial nerve:

• Direct nerve repair

• Cable nerve grafting

• Nerve substitution or techniques

Primary facial nerve repair

Direct repair of the facial nerve is the best method for rehabilitating the paralyzed face. It provides a chance of restoring spontaneous emotional expression to the face. Restoring continuity of the nerve through coaptation of the 2 nerve ends is indicated whenever the available nerve length is sufficient to allow this to be accomplished without undue tension.

Primary nerve repair may be performed virtually anywhere along the course of the nerve. An exception is the most proximal part of the nerve in the cerebellopontine angle if the length is inadequate for grafting. In addition, if the nerve has been transected distal to the lateral canthus, repair is usually unnecessary, because the nerve typically regenerates spontaneously in the more medial areas of the face.

If the nerve repair is to be successful, functional motor units must be available to receive reinnervation. That is, the facial musculature must not have atrophied excessively, and the motor endplates must be functional without significant fibrosis that would prevent reinnervation.

There remains some controversy as to how long after injury these conditions may still be met. Most authors consider reinnervation by direct repair to be possible up to 1 year after the injury. Some, however, argue that less than or more than a year is reasonable. In selected cases, EMG may be helpful to determine whether the nerve and muscle can be stimulated distal to the site of injury. If stimulation is possible, a functioning motor endplate muscle unit is likely.

The optimal timing of repair has been discussed extensively in the literature. The commonly accepted belief today is that an injured nerve should be repaired as soon as possible, not in a delayed fashion. This belief was initially promoted in the 1970s on the basis of experimental data suggesting that repair ideally should be performed 3 weeks after injury. Subsequent data, however, reinforced the idea that the best long-term results are achieved when the repair is performed as soon as possible.

Cable nerve grafting

When the length of facial nerve available is insufficient to permit primary repair, the best option is cable grafting. The same general principles of microsurgery that apply to direct repair also apply to cable grafting. Preoperative considerations must include a discussion with the patient regarding the options for donor nerves for repair. Commonly used nerves include the great auricular nerve and the sural nerve.

The great auricular nerve has the advantages of proximity to the operative field and ease of harvest. The diameter is usually an appropriately sized match for the facial nerve. Because up to 10 cm may be harvested, this nerve is a good choice for most cable nerve grafts. The main disadvantage is the numbness of the ear that inevitably results.

In addition, use of the great auricular nerve has been discouraged in cases of malignant disease because of the possibility of microscopic involvement of the nerve. Those who are concerned about this possibility suggest using an alternative donor out of the field of involvement in cases resulting from facial nerve sacrifice secondary to malignancy, particularly when the tail of the parotid is involved or metastases to the neck are present.

The sural nerve has some advantages over the great auricular nerve, in that it is longer (up to 40 cm) and has a greater number of neural fascicles. Use of this nerve results in lateral foot numbness.

Another nerve that may be used for cable nerve grafting is the medial antebrachial cutaneous nerve of the upper arm. This can provide 15 cm of length and has a branching pattern that can be used to graft to multiple facial nerve branches.

Nerve substitution techniques

Cranial nerve substitution techniques include hypoglossal-facial (XII-VII) anastomosis and cross-face grafting. The spinal accessory–facial anastomosis is another well-described procedure, but it is largely relegated to historical status or situations in which other options are not available.

Hypoglossal-facial anastomosis

The hypoglossal-facial (XII-VII) anastomosis is the preferred approach to reanimating the face when the proximal end of the facial nerve is not available and the peripheral system is still viable. This situation may arise after surgery of the skull base, as in resection of an acoustic neuroma.

If sacrifice of the facial nerve is known at the time of tumor removal, hypoglossal-facial anastomosis is best performed at that time. In cases in which paralysis becomes evident postoperatively, the same principles of timing apply. If no return of function becomes apparent, performing the cranial nerve substitution procedure is appropriate. A successful result is possible many months later, but after 12-18 months, the likelihood of such an outcome is small.

Essentially, 2 options exist for this procedure. Either the hypoglossal nerve is completely transected and connected to the facial nerve, or a partial transection of the hypoglossal nerve is attached to the facial nerve. The latter may be accomplished by means of an interposition cable graft. An alternative is to mobilize the facial nerve via a mastoidectomy and nerve dissection, allowing the facial nerve to be transected more proximally. The facial nerve can then be brought to the partially transected hypoglossal nerve for repair.

The first option, a classic end-to-end anastomosis, involves sectioning the hypoglossal nerve distal to the takeoff of the descendens hypoglossi. This produces reliably improved facial tone and symmetry in more than 90% of patients. The improvement occurs over a period of 4-6 months; it is largely observed in the midface, is less noticeable in the lower face, and is even less significant in the upper face.

On the other hand, hemitongue mobility is impaired. This loss of function must be weighed against the expected gain in facial tone, symmetry, and movement. Spontaneous mimetic function is not expected to return, and some training is necessary to teach the patient to smile by stimulating the hypoglossal nerve. Nevertheless, this procedure is generally well tolerated and fairly successful.

The second option involves the use of jump grafts, in which the hypoglossal nerve is only partially transected and a graft (eg, the great auricular nerve) is used to connect the transected portion of the hypoglossal nerve to the end of the facial nerve. The advantages of this technique include minimization of tongue weakness and a purported decrease in synkinesis. The disadvantages include the need for 2 anastomoses and the availability of fewer nerve cells to “drive” the face. ^[7]

Cross-face grafting

A facial nerve cross-face anastomosis attempts to connect branches of the facial nerve of the normal side with corresponding branches of the paralyzed side. This procedure may be chosen in cases where the proximal facial nerve of the involved side is unavailable for repair. It is made possible by the redundancy of facial nerve innervation that is often present in many areas of the face, particularly the midface.

Alternatively, a donor nerve with less associated morbidity (eg, the marginal mandibular nerve) may be sacrificed for the sake of the other side. Some iatrogenic weakness of the donor side is expected, and the patient must be well informed of this likely result before consenting to undergo the procedure.

Some facial nerve surgeons believe that donor side weakness can contribute to success, noting that many patients are happier with a more symmetric-appearing face, even at the cost of some function. This is a matter of debate. As in all reinnervation procedures, success depends on the presence of functioning motor end plates in the paralyzed side. Thus, when paralysis has lasted longer than 1 year, the likelihood of success may be compromised. The cross-face technique has had variable success in different reports and remains controversial.

Early follow-up relates to typical wound care. Long-term follow-up is very important. With most procedures, the results are delayed for several months to a year. In some cases, results are delayed for as long as 2 years, particularly with nerve transposition repairs. ^[8] For this reason, adjunctive procedures (eg, gold weight eyelid implant and brow lift) may be considered early.

If no improvement is observed 1 year after the repair, reexploration of the site of anastomosis is acceptable, and revision is performed if the repair is found to be inadequate. Another option is to consider adjunctive functional and cosmetic procedures or even alternative measures such as dynamic or static slings and free tissue transfer techniques.