Historical Perspective

The first facial nerve decompression for Bell palsy was described by Balance and Duel in 1932.2 They advocated decompression of only the distal 1 cm of the mastoid segment for cases of Bell palsy. The prevailing sentiment at the time was compression of the vascular supply to the nerve at the stylomastoid foramen resulted in the facial paralysis; therefore, further exposure of the nerve was not necessary to improve the vascular supply to the nerve. They did not speculate on the cause of the vascular compression. Interestingly, these authors also remarked on inflammation of the geniculate ganglion seen in some of their cases but attribute this finding only to cases of Ramsay Hunt syndrome. They did recommend decompression of the geniculate ganglion in patients with Ramsay Hunt syndrome when faradic stimulation was lost, but cautioned that outcomes are guarded. They did not recommend decompression of the geniculate ganglion in Bell palsy.

Between the 1930s and the 1950s, surgical decompression involved more proximal portions of the mastoid segment but did not involve the entire facial nerve. In Kettel′s 1963 monograph on “Pathology and surgery of Bell palsy,” he ascribed the facial weakness to autonomic dysregulation of the vascular supply of the nerve at the stylomastoid foramen.3 He recommended distal decompression of the nerve when the patient demonstrated a complete paralysis and electromyography (EMG) showed no voluntary activity. He also noted that surgical intervention for Bell palsy was a controversial issue, as indeed it remains today.

There was much debate about both the cause of Bell palsy and the location of the lesion between the 1930s and the 1970s. Several authors felt the lesion was located at the origin of the chorda tympani.4,5 Blatt and Freeman favored chorda tympani neurectomy as treatment for Bell palsy, although this procedure was not widely adopted.6 Another group of surgeons advocated for transmastoid decompression.7,8 This decompression extended from the geniculate ganglion or distal labyrinthine segment to the stylomastoid foramen. It did not involve the meatal foramen or address the full length of the labyrinthine segment. In the procedure described by Yanagihara et al in 1979, the incus is removed and the supralabyrinthine air cells are removed to gain access to the geniculate ganglion and the tympanic segment of the nerve. The incus is then replaced at the end of the decompression and secured in place with fibrin glue.8 In one of the largest series of surgically treated patients, Yanagihara and colleagues evaluated 101 patients with Bell palsy who met the following criteria: age > 16 years, House-Brackmann (HB) grade V or VI, > 95% degeneration on evoked EMG, failed medical therapy, and with no severe systemic illness.9 Patients were offered surgery within 3 months of the onset of paralysis. Of these 101 patients, 58 opted for transmastoid decompression and the remaining 43 patients refused surgical intervention. In general, those patients who underwent surgery achieved normal or near-normal facial function at a higher rate than those who only received steroids. There were two factors that negatively impacted on long-term facial function: age > 50 and prolonged time to surgical intervention. However, the authors noted that although patients with these factors had worse outcomes in general, surgical intervention improved the long-term outcome when compared with steroids alone. In the patients > 50 years, the overall recovery rate to HB grades I or II was > 25% for those who underwent surgery and < 20% for those who used steroids alone. Although patients operated on after 30 days of paralysis had worse overall recovery rates, those who underwent surgery between 31 and 60 days had a better chance of HB grade I or II function (38%) compared with those who did not have surgery (23%). Unfortunately, no statistical analysis was provided for these subgroups.

May also performed transmastoid decompression of the facial nerve7 but failed to find significant benefit. Although he originally advocated for facial nerve decompression, after evaluating his case series, May retracted his endorsement of surgery.10,11 He felt the risks of the operation did not outweigh the benefits because he had found no difference in outcomes between surgical and nonsurgical patients. May′s retraction was highly influential, and many surgeons stopped recommending operative intervention to patients with Bell palsy. However, a few case series have been published since then, most notably the case controlled series by Gantz et al as described subsequently.12 Several other groups have reported on small numbers of patients operated within variable time intervals and with mixed results.13,14

The electrical testing of the facial nerve has also undergone a variety of changes since the 1920s. Prior to the 1960s, electrical testing relied on faradic (short duration direct current or alternating current) and galvanic (high voltage direct current of long duration) stimulation. Response to the stimulation was assessed by visual inspection. Routine use of EMG began in the 1960s and provided clinicians with a more sensitive, although still indirect, method to evaluate the integrity of the facial nerve. Fisch and Esslen popularized the term electroneuronography (ENoG), which has been shortened to electroneurography in the literature. Whereas EMG uses penetrating needle electrodes, ENoG uses surface electrodes and an external electric stimulus to assess the status of the nerve. Despite the fact that ENoG may be somewhat less sensitive when compared with EMG, ENoG is the preferred method of facial nerve testing in the literature. ENoG demonstrates degeneration of the nerve fibers within 3 to 5 days. However, the presence of fibrillation potentials, diagnostic of severe denervation, take between 10 and 25 days to develop on EMG. Prior to the development of fibrillation potentials, the percentage of denervated motor action units is not clear on EMG.15

Other testing used in the past to identify the site of the lesion of the nerve include Schirmer lacrimation testing, electrical taste testing, and stapedial reflex testing. Many patients present with normal lacrimation testing and abnormal stapedial reflex testing or abnormal taste testing. Prior to routine electrical testing, this topognostic testing was used by many authors to justify distal decompression. Later work revealed that topognostic testing is insensitive to identify the site of the lesion. Gantz et al performed intraoperative evoked EMG (EEMG) and compared the results to preoperative Schirmer lacrimation testing in 13 patients undergoing subtemporal decompression.16 They found that 4 of 12 patients with normal Schirmer tests had lesions proximal to the geniculate ganglion and that the 1 patient with a lesion of the mastoid segment of the nerve had an abnormal lacrimation test. Schirmer test correctly identified the site of lesion in only 61% of patients.

Rationale for Subtemporal Decompression

In 1961, House described the middle fossa approach to the internal auditory canal.17 He advocated use of this procedure in several situations: acoustic tumor removal, facial nerve surgery, and vestibular nerve section for intractable Ménière disease. House and Crabtree in 196518 and Pulec in 196619 described total facial nerve decompression using the middle fossa transmastoid approach. The middle cranial fossa (MCF) approach provides access to the facial nerve from the brainstem to the tympanic segment of the facial nerve. The MCF approach allows for auditory and vestibular function preservation. Prior to 1961, full access to the nerve required a translabyrinthine approach, sacrificing hearing and balance function, neither of which is acceptable for a patient with a 50% chance of good functional recovery.

As more subtemporal decompressions were performed, information regarding pathologic findings in the facial nerve was reported. In 1972, Fisch and Esslen reported on a series of 12 patients who underwent total facial nerve decompression via the middle fossa approach. In 11 of the 12 patients, the nerve was found to have “pronounced [edema], red swelling… with marked vascular injection… proximal to the geniculate ganglion.”20 Eight of these eleven patients had involvement of the nerve within the internal auditory canal. Five of the eleven patients had swelling within the nerve distal to the geniculate ganglion. To identify the site of the conduction abnormality, Fisch and Esslen performed serial intraoperative EEMG starting from the stylomastoid foramen and ascending to the internal auditory canal. In the three patients tested in this manner, the conduction abnormality was located proximal to the geniculate ganglion and within the internal auditory canal (IAC).

Gantz et al also reported this proximal conduction abnormality.16 Eighteen patients with between 90 and 98% degeneration on preoperative ENoG underwent decompression. In two patients, intraoperative EEMG could not be performed for technical reasons. Of the remaining 16 patients, 15 (94%) demonstrated conduction blocking proximal to the geniculate ganglion. One patient had a conduction block at the origin of the chorda tympani. According to these data, the vast majority of patients will have a conduction block proximal to the geniculate ganglion. Decompression should therefore involve the bony canal proximal to the geniculate ganglion.

There is anatomic data to corroborate the intraoperative conduction findings. The fallopian canal in the labyrinthine segment is particularly narrow: 0.69 mm in diameter on average.21 There is also a tight arachnoid band at the opening of the fallopian canal in the fundus (meatal foramen).21,22 These two anatomic “bottlenecks” do not allow room for edema of the nerve without compromise of the nerve fibers.

Taken together, the anatomic findings with the intra-operative electrical testing suggest the pathology in Bell palsy is localized to the distal IAC or labyrinthine segment of the nerve. If the patient is to be offered a decompression surgery, the operation should involve the entire labyrinthine segment with lysis of the arachnoid band within the fundus. Exposure to this area is best achieved via the MCF approach.

Indications for Surgical Intervention

While there are many clinicians who endorse surgical intervention in a selected patient population,9,12,22 there are others who feel surgery is never indicated in Bell palsy.10,23 At the present time, the evidence in the literature regarding facial nerve decompression is primarily from case series. There are no meta-analyses of surgical intervention, and a Cochrane Review of surgical therapy for early treatment of Bell palsy has been proposed but has not been completed.24 Nor is there any formally randomized controlled trial for any surgical intervention for Bell palsy. May et al calculated the number of patients needed to adequately power a randomized trial for surgical intervention.11 Using a Chi-squared test with type I error of 0.05, 716 patients would need to be enrolled.11 Given the rarity of surgical necessity and the ethical dilemmas surrounding randomization in a surgical study, a truly randomized trial is unlikely to occur. A decision for surgery is therefore based on the available information regarding patient outcomes and a frank discussion with the patient.

Patients presenting within 14 days of the onset of Bell palsy who have complete facial paralysis are encouraged to undergo electrical testing. Electrical testing should not be performed in the first 3 to 5 days because wallerian degeneration of the severely affected nerve fibers has not occurred. Electrical testing is used to determine the percentage of nerve fibers that have undergone conduction blockade. Nerve fibers that remain intact but have axonoplasm flow blockade (neuropraxia) can be electrically stimulated distal to the site of conduction blockade. Fibers undergoing wallerian degeneration as a result of injury to the axon (axonomesis) or the entire neural tubule (neurotmesis) cannot be stimulated distal to the site of injury. When the neural tubule remains intact, the axon can regenerate to its original motor unit; in neurotmesis, axonal regrowth can result in aberrant innervation and synkinesis. Current testing can distinguish neuropraxia from axonomesis/neurotmesis but cannot differentiate between types of wallerian degeneration. However, the rate of degeneration can give insight as to the degree of axonal injury; rapid denervation is associated with neurotmesis whereas slower degeneration is seen in axonomesis ( Fig. 11.1 ). Currently, two electrical tests are in use to determine the extent of nerve degeneration and surgical candidacy: ENoG and voluntary EMG.

EMG allows for evaluation of spontaneous motor unit potentials of the facial muscles. During a voluntary EMG, no external electric stimulus is applied to the nerve. However, the patient may stimulate the muscle voluntarily. Transcutaneous needle electrodes measure electric activity within the muscle. The presence of a compound motor action potential response on voluntary EMG indicates early deblocking of neuropraxic fibers. ENoG measures the compound muscle action potential of the nerve at the nerve terminus after a maximal electrically evoked stimulus. ENoG allows for a comparison between the two sides of the face. The response of the weakened side of the face is reported as a percentage of the response of the normal side. Whereas EMG records the action potential in a small number of muscle fibers around the needle electrode, the ENoG records activity from the whole muscle because the electrodes are placed over the surface.

Patients with < 90% nerve degeneration on ENoG have a good prognosis (recovery to HB I or II). Those with > 90% nerve degeneration have a less optimistic prognosis. When ENoG demonstrates > 95% degeneration within 2 weeks of the onset of the paralysis, there is only a 40 to 50% chance of recovery to HB I or II.22 Patients who progress to 90% degeneration will have further degeneration to the 95% level 90% of the time. Therefore, to prevent the progression to the 95% degeneration level, Fisch recommended decompression when the patient reaches 90% degeneration on ENoG.22

Fisch advocated daily testing between the sixth and fourteenth day or 90% degeneration is reached. Serial testing provides information about the rate of denervation; those that rapidly progress to the 90% level have more severe neural injury (neurotmesis) and thus are less likely to have return to normal facial function.25 Because testing on a single day does not provide information about the velocity of the degeneration, patients with complete paralysis should undergo serial testing.

Patients who demonstrate no function on ENoG should also undergo voluntary EMG. The addition of voluntary EMG allows identification of motor unit potential with voluntary movement of the facial muscles. The presence of voluntary motor unit potentials indicates a good prognosis for return of facial function. The presence of voluntary motor unit potentials excludes the patient from surgery. Many studies in the literature do not use voluntary EMG and rely solely on ENoG to assess for surgical candidacy.10,11,23 These authors saw no difference in recovery of function following surgery in patients with ENoG denervation > 90% when compared with steroids alone. It is likely that these patients who had full recovery of function despite the poor ENoG findings had early deblocking of the nerve and would have had a positive voluntary EMG, and thus a good prognosis for recovery. These patients should have been excluded from surgical decompression in the first place.

Intervention is required before day 14 after the onset of complete paralysis. In the controlled series published by Gantz et al, those patients operated on after day 14 had no improvement in facial nerve function when compared with nonsurgical control patients who received steroids.12 Fisch also advocated early surgical intervention based on the presumed pathophysiology of nerve damage.25 Neurotmesis (disruption of both the axon and the endoneurium) results in rapid loss of neural response. With loss of both the axon and the endoneurium, regrowth of the nerve results in aberrant innervation with resulting weakness and synkinesis. Axonomesis (loss of the axon with preservation of the endoneurium) progresses more slowly, so that although there is near total denervation, the neural pathways remain. In the latter situation, as the axon regrows, it can innervate the appropriate muscle and results in good functional recovery ( Fig. 11.2 ).

Fisch noted that patients who progress to > 95% degeneration after 4 weeks of paralysis have good spontaneous return of function (93%). Those with degeneration within the first 3 weeks did not (64% with good recovery). The cause of the difference in recovery rates relates to the degree of the neural injury.22

Finally, the patient must be willing to undergo a surgical procedure. The operation is performed via a middle fossa approach and has associated risks. Decompression of the labyrinthine segment requires drilling near the cochlea and superior semicircular canal, and there is a risk of hearing loss or vestibular dysfunction. Other risks include meningitis, temporal lobe edema resulting in temporary aphasia, seizure, stroke, and death. Given these risks, some patients choose not to undergo surgery. However, other patients are willing to accept those surgical risks for the opportunity to maximize their recovery potential. A detailed and thorough discussion of the procedure and its risks and benefits should be had with any surgical candidate.

These surgical criteria were validated in the case- controlled study by Gantz et al.12 Thirty patients were enrolled over a 15-year period at the University of Iowa. To qualify for the surgical arm of the study, patients had to present within 14 days of the onset of complete paralysis (grade VI) as measured by the HB scale, have > 90% degeneration on ENoG, and no voluntary EMG activity. Patients were allowed to choose between the treatment groups: surgical (n = 19) or nonsurgical (n = 11). Patients electing oral steroid treatment were considered nonsurgical controls. A second group of patients (n = 7) was operated on early in the study period between days 14 and 28 of paralysis, as originally recommended by Fisch. This group of patients represented a surgical control group as they were treated past the 14-day criteria. Surgical patients underwent decompression of the distal IAC, meatal foramen, labyrinthine segment, geniculate ganglion, and tympanic segment via a middle fossa exposure. No patient in the steroid control group achieved a grade I at 7 months of follow-up. In the nonsurgical control group, 4 of 11 patients achieved a grade II facial weakness and 7 of 11 (64%) patients were grade III, considered a poor outcome. In the surgical control group, two patients had good outcome (HB grade II), whereas the remaining five had a poor outcome (HB grade III). Among patients undergoing decompression within 14 days, 18 of 19 patients had a good outcome (HB I or II), and only 1 patient had a grade III outcome. The statistical comparison of these three groups was significant in favor of early surgical decompression (p = 0.0001). To date, this is the only study in the literature with rigorous inclusion criteria and both medical and surgical controls. The decisive results of this study validate the following criteria for decompression (see Fig. 11.2 ):

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  Complete facial paralysis, grade VI on the HB scale26

  
  
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  Decompression must occur within 14 days of the onset of complete paralysis

  
  
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  ENoG findings of > 90% nerve degeneration

  
  
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  No voluntary EMG motor action potentials

  
  
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  Patient desires to undergo surgical intervention