Acoustic Neuromas and Facial Paralysis
Due to its proximity, lesions of the eighth cranial nerve have significant impact upon the integrity and function of the facial nerve. Because of this, treatment decisions for acoustic neuromas (more correctly named “vestibular schwannomas”) are guided by a desire to reduce patient symptoms, while sparing residual facial nerve function and minimizing complications from treatment. These decisions have become complex as radiological advances have increased our ability to detect small, minimally symptomatic lesions.
Only 5% of facial nerve weakness is related to the presence of a tumor, and facial paralysis rarely presents as an initial symptom of an acoustic neuroma. In fact, facial paralysis is typically only a manifestation of a large, compressive lesion that is situated in a narrow region of the internal auditory canal (IAC). These tumors can cause nerve dysfunction by inducing nerve ischemia, obstructing cerebrospinal fluid (CSF), or direct compression. In fact, the finding of facial nerve dysfunction should alert the clinician that the lesion may not be an acoustic neuroma, but may instead represent a facial nerve neuroma or other cerebellopontine angle (CPA) neoplasm. Still, progressive facial paralysis can be a late symptom of an acoustic neuroma. In a 1992 study, most patients with acoustic neuromas had nearly 4 years of hearing loss, tinnitus, or vertigo prior to their diagnosis. If facial nerve paralysis developed, the diagnosis was made in the same year.1
Classically, slowly progressive or recurrent facial paralysis heralds the presence of either a facial nerve tumor or an extrinsic lesion compressing the facial nerve. Facial twitching should suggest to the examiner that the facial nerve is in a state of irritation and heighten suspicion regarding the presence of a tumor. In general, motor fibers of the facial nerve are tolerant to gradual external pressure secondary to its rich blood supply and dense fibrous covering, but the nerve may be splayed on the surface of the tumor and have little resilience.
Hearing loss, tinnitus, imbalance, headache, and facial numbness are many times more common than facial nerve weakness at the time of diagnosis. Facial pain, while frequently seen in disorders such as Ramsay Hunt syndrome or malignant neoplasms involving the facial nerve, is uncommonly associated with acoustic neuromas. Numbness of the ear canal and conchal bowl, however, can be a clinical finding associated with an acoustic neuroma. This was first described by Hitselberger and House in 1966 as hypesthesia to pin prick testing and, in the context of limited imaging capability, was used as an early clinical sign of the presence of a tumor.2 Additional studies have confirmed that somatic sensory afferents carried by the facial nerve, via the sensory auricular branch, are found in posterior external auditory canal and inferior conchal bowl.3 As imaging capabilities have advanced, the utility of “Hitselberger sign” has diminished, but it still helps confirm radiological findings.
Facial schwannomas are relatively rare, slow- growing tumors that may present with facial weakness and should be in the differential for a CPA or IAC neoplasm. Typically, these tumors are mistaken for acoustic neuromas when they are primarily confined to the IAC and do not enter the meatal foramen or fallopian canal. A series by McMenomey et al described 32 primary facial nerve tumors, of which 38% were believed to be acoustic neuromas preoperatively.4 Unfortunately, there are no tests at the clinician′s disposal (other than preoperative imaging) to differentiate intracanalicular acoustic neuromas from facial schwannomas. The most important imaging sign is presence of tumor in the labyrinthine segment of the facial nerve, which differentiates a facial nerve tumor from an acoustic neuroma.
Electroneuronography involves active stimulation of the extratemporal facial nerve and has not proven a reliable diagnostic test for these lesions. The intratemporal facial nerve is particularly susceptible to dysfunction secondary to the growth of these tumors due to the narrowness of the meatal foramen. Facial twitching and spasm should heighten the clinician′s suspicion of a facial nerve neuroma, and patients should be counseled about intra-operative findings that may warrant decompression procedures versus resection.
Limiting Risk of Facial Paralysis
There is considerable controversy and variable practice patterns for the management of acoustic neuromas. Treatment decisions must consider a constellation of factors, such as the age of the patient, the overall health of the patient, the symptoms caused by the tumor, and the progression of symptoms attributable to the tumor.5 If a patient and surgeon elect microsurgical management, treatment-specific considerations must be made regarding the position of the tumor, preoperative hearing, and anatomic features. The patient must be well informed regarding the risks of surgery, which may be substantial in view of these factors. As with all benign tumors, management decisions must be made with emphasis toward minimizing complication and morbidity, while achieving favorable outcome.
For many patients, observation is appropriate for acoustic tumors. Multiple meta-analyses have demonstrated that, particularly in the IAC, a large proportion of acoustic neuromas are quiescent, and symptoms are not related to size or patient age.6 Recent studies cite that over long-term follow-up, approximately less than one-third of intracanalicular tumors demonstrate radiological evidence of growth, although hearing deterioration occurs regardless of growth.7–10 From a series of 70 patients older than 65 with acoustic neuroma followed for a mean duration of 4.8 years, 42% of tumors did not grow or regressed.11 In patients who are asymptomatic from an audiologic, vestibular, and facial nerve standpoint, the main impetus toward intervention may be a patient′s desire to address the anxiety associated with an intracranial mass. It is important to be knowledgeable about the natural history of these tumors, and the patient must be informed that if symptoms develop, or radiographic characteristics change, intervention may become warranted.
Observation is not always innocuous, however, and may lead to sudden and complete hearing loss, imbalance, or facial nerve weakness. Facial weakness is an ominous sign that a patient may suffer the sudden onset of facial paralysis from tumor growth, and it should be treated aggressively. Still, the rate of facial nerve weakness as a presenting symptom of an acoustic neuroma is ∼5%, and although this rate is low, subsequent facial nerve paralysis can have devastating and permanent effects. Despite multivariate analyses on the natural history of acoustic neuromas, no clinical or radiological factors have been identified as predictive of tumor behavior.
Radiosurgical management of acoustic neuromas can be a favorable option for many reasons. The treatment is brief, does not require general anesthesia, and sidesteps multiple complications that are nearly exclusively associated with surgical resection. Initially, it was described as an adjunct for incompletely resected acoustic neuromas, but more recently, it is offered as a primary modality for treatment. Treatment goals with radiosurgical techniques are inherently very different from surgical aims. There is an expanding body of literature suggesting that radiation may arrest vestibular schwannoma tumor growth, possibly through radiation-induced fibrosis and damage to tumor vasculature. The patient must be made aware that the tumor will remain after treatment, and that those symptoms such as hearing loss, imbalance, and tinnitus may progress.12
The effectiveness of radiation for management of these tumors is a particularly difficult subject of research secondary to the slow-growing nature of these tumors and heterogeneity in outcomes measures (i.e., radiographic criteria for growth, audiometry, vestibular symptoms, and tinnitus handicap). There is considerable variability in the description of the natural history of these tumors, and the application of radiation is transitioning to those tumors that have radiographic evidence suggesting growth. There is a 6- to 12-month period of transient tumor expansion after radiation in ∼16% of treated patients, suggested to be a result of the edema associated with central tumor necrosis, which further confounds reported measures.13 Radiosurgery methods vary from center to center, based on available equipment and personnel. Investigations are ongoing comparing the long-term advantages and effects of different administration techniques such as Gamma Knife (Elekta, Stockholm, Sweden), Cyberknife (Accuray, Sunnyvale, California), LINAC (linear accelerator radio-therapy), and fractionated radiotherapy. Published results are promising and suggest a high (> 95%) 10-year tumor control rate, with a limited immediate side-effect profile.
The marginal dose of radiation administered is a critical predictive factor for facial paresis with radiotherapy. Earlier studies described a rate of facial nerve paresis of ∼21% when the average marginal tumor dose was 16 Gy using a Gamma Knife system.14 Currently, most centers use a marginal tumor dose between 12 and 14 Gy; facial nerve outcomes have improved considerably. The rate of facial nerve weakness after radiotherapy is now cited at < l% for both Gamma Knife and LINAC systems.14,15 Likewise, the rate of trigeminal neuropathy has fallen from 16% to 4.4% as the marginal doses were decreased, and the trigeminal nerve is rarely affected by radiation for intracanalicular tumors.16 Vertigo, tinnitus, hydrocephalus, xerophthalmia, sudden hearing loss, risk of radiation-induced malignancy, and accelerated vertebrobasilar atherosclerosis are described complications of radiotherapy, although their incidence in the setting of low-dose radiotherapy is unclear.
If the tumor continues to grow despite radiotherapy, literature suggests that surgical resection becomes more difficult and is associated with poorer facial nerve outcome. In a recent review by Friedman and colleagues, 46% of patients treated with salvage microsurgery had complete facial weakness at their first postoperative visit.17 Histologically, this is supported by studies that have demonstrated significant tumor fibrosis and facial nerve adherence after radiotherapy.18,19 It is possible that as the technology for radiotherapy administration changes, its effect on the facial nerve and surrounding structures will be further reduced.
Microsurgery
There are three commonly used approaches for acoustic neuroma resections: the translabyrinthine craniotomy, the middle fossa craniotomy, and the retrosigmoid craniotomy. Each affords a different view of the CPA and has advantages and disadvantages that will be reviewed. Facial nerve outcomes have improved as imaging and monitoring have made advances. Magnetic resonance imaging has improved the clinician′s ability to detect small, minimally symptomatic acoustic neuromas. Intraoperative facial nerve monitoring has had dramatic effects on dissection technique.
Since its introduction, intraoperative facial nerve monitoring has played a vital role in improving facial nerve integrity from 67%, to current studies that cite facial nerve anatomic integrity of > 98%.20 Multiple studies have suggested intraoperative parameters that aid with prediction of both immediate and long-term facial nerve outcomes.21,22 These can further inform intraoperative decision making and guide rehabilitative procedures that may be done to prevent exposure keratitis. If facial nerve stimulation, proximal to the site of tumor resection, remains 0.04 mA or less, than there is a 77% probability that the patient will have House-Brackmann (HB) grade I/II facial nerve function at 8 days postoperatively.23 Electrophysical facial nerve monitoring has proven more sensitive that observation or video capture technology, and remains a useful tool for minimizing morbidity in acoustic neuroma surgery.24
Translabyrinthine Craniotomy
Since the development of the translabyrinthine approach for CPA tumors by Dr. William F. House in the early 1970s, there have been relatively few modifications to this approach, and it remains a workhorse in the surgical management of acoustic neuromas. Facial nerve monitoring and auditory brainstem response monitoring have made tumor dissection safer for the patient and have improved clinical outcomes. An advantage of this approach is its wide exposure of the IAC and brainstem with very limited cerebellar retraction. It affords excellent hand position at a relatively shallow depth of field, making it a comfortable procedure for the surgeon. The facial nerve can be visualized throughout its entire intratemporal course, affording perspective during tumor dissection.
Early studies suggested that the translabyrinthine craniotomy was the gold standard in facial nerve preservation microsurgery of the IAC, although more recent studies suggest that the experienced surgeon can achieve similar outcomes through the middle fossa craniotomy.25,26 Studies performed at the House Clinic indicated that 98.5% of patients treated with this approach between 1984 and 1989 had an anatomically intact facial nerves postoperatively.27 This approach permits identification of the proximal and distal facial nerve during tumor dissection, has consistent intraoperative landmarks, and provides a panoramic view suitable for removal of large tumors ( Figs. 20.1, 20.2, 20.3, 20.4, 20.5, and 20.6 ).
While facial nerve transection is extremely rare in this approach, injuries to the sensory contribution of the facial nerve are common ( Fig. 20.7 ). A review of 224 patients who underwent acoustic neuroma resection revealed that few (2–6%) patients noted tearing while eating, xerophthalmia, or taste abnormalities preoperatively. When questioned postoperatively, 44% of those patients described tearing while eating. Reduction in tearing was noted in 72%, and taste alterations were noted in 48%. Recovery was variable in this study, and the authors suggested nervus intermedius dysfunction should be included in reporting criteria.28
A high-riding jugular bulb or an anteriorly positioned, dominant sigmoid sinus are not contraindications to translabyrinthine approach. The presence of active chronic otitis media is, however, considered a contraindication and may require a staged procedure with ear canal overclosure.
The most noteworthy disadvantage exclusive to the translabyrinthine craniotomy is the passage through inner ear structures and subsequent sacrifice of hearing. While there are reports of hearing preservation through a partial labyrinthectomy, with meticulous waxing and resurfacing of the semicircular canals intraoperatively, this practice is not commonplace.29 Also, with cochlea and cochlear nerve preservation, descriptions of cochlear implantation have been published, although circumstances warranting this approach are rare.30 Frequently, placement of an osseointegrated bone conduction hearing device can be performed at the time of tumor resection, although considerations must be made toward avoiding communication with the mastoid cavity and subsequent development of a CSF leak. This permits hearing perception bilaterally, but patients must be counseled that this ability does not afford sound localization, as sound is simply rerouted to the contralateral, functional cochlea.
Early after introduction of the translabyrinthine craniotomy, the medical community expressed concerns regarding the risk of meningeal and intracranial contamination with otogenic bacteria. Instead, meningitis is described as extremely rare in most series, at ∼1%. A CSF leak, however, increases the risk of meningitis to roughly 14%.31 A large review of 600 cases cited the rate of CSF leak after translabyrinthine approach, either through the incision or the eustachian tube and presenting as rhinorrhea, as 1.8%.32 Abducens paralysis, lower cranial nerve injury, major intracranial vessel injury, or parenchymal injuries are extremely rare. The mortality rate from complications of this approach is extremely low (< 1%), although medical comorbidities, as with any major surgery, must be fully addressed preoperatively. Special consideration must be made toward the period of vestibular rehabilitation postoperatively, which may involve several days of nausea, visual tracking difficulty, and bed rest.
At the authors’ institution, patients are observed in the intensive care unit for one evening postoperatively, and early ambulation is encouraged. Facial nerve monitoring is prerequisite; perioperative antibiotics are continued for 24 hours postoperatively; abdominal fat grafting and Eustachian tube plugging are routine. Lumbar drains are reserved for patients at high risk for CSF leak, such as pre-operative hydrocephalus or intracranial hypertension.
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