Interest developed in the possibility of saving hearing in patients with acoustic neurinomas. In 1984, one group published the results of a series of 22 patients in whom an attempt was made to save hearing. Subsequent publications have updated this series. In 1988, Gardner and Robertson reviewed the reports published in the English literature from 1954 to 1986 on hearing preservation following the removal of an acoustic neuroma. However, in spite of extensive clinical interest in this subject and the use of intraoperative monitoring, the results of hearing preservation have not improved over the past two decades.
When Can Useful Hearing Be Saved?
The chance of saving useful hearing in the patient with a unilateral acoustic neuroma and no evidence of neurofibromatosis has a direct relationship to the size of the tumor and the preoperative level of hearing. In reports of patients in whom it has been possible to preserve some hearing, the tumor has usually been less than 2 cm in diameter. This would not be unexpected from the observation that in larger tumors the cochlear nerve usually merges into the tumor. In one series of patients with unilateral acoustic neurinomas, it was found that when the tumor is intracanalicular or extends up to 0.5 cm into the posterior fossa, the chance of saving useful hearing is 60 percent; and when it extends 0.5 to 1.5 cm, the chance is 35 percent. With tumors that are 2.0 to 3.0 cm in diameter there is a low probability of hearing preservation, and when the tumor is 3,0 cm or larger, only a few cases of preserved hearing have been reported.
The second criterion for preservation of useful hearing is satisfactory preoperative hearing. Rarely does the hearing improve from a non-useful to a useful category. The definition of useful hearing has been discussed by several authors, with the most common criterion being a speech reception threshold (SRT) less than 50 dB with a speech discrimination score (SDS) of 50 percent or more (50-50 rule). Although an SDS of 70 percent or better has been proposed as a definition for serviceable hearing, many patients benefit from a lower level of hearing. Most of patients fall under the 50-50 rule, but it is better to use an SDS of 35 percent or better to define useful hearing.
Etiology of Hearing Loss during Tumor Removal
There are several factors that may cause hearing loss during the operation. These include involvement of the cochlear nerve by the tumor, interruption of the vascular supply to the cochlea or nerve, injury to the labyrinth. and extension of the tumor into the cochlea.
When the acoustic neuroma is intracanalicular or the extension into the posterior fossa is 1,5 cm or less, the cochlear nerve is usually found on the tumor capsule as a separate bundle, allowing the possibility of nerve preservation. As the tumor enlarges, the cochlear nerve tends to be incorporated into the tumor so that in large tumors no more than 10 percent have a cochlear nerve that is on the tumor capsule. There is also considerable variation in the adherence of the nerve to the tumor and the amount of dissection required to separate it.
The ability to maintain the vascular supply to the nerve and cochlea is one of the most difficult problems in preserving hearing. The location of the internal auditory artery is variable, and there may be more than one arterial vessel entering the internal auditory meatus. When the artery is on the anterior aspect of the nerve complex and does not need to be dissected extensively, the chances of saving hearing are better. In an occasional patient, an artery within the tumor or a small, apparently insignificant vessel in the arachnoid going to the internal auditory meatus area seems to be the important blood supply for hearing. Angiography is of no help in determining the relationship of the internal auditory artery to the tumor.
Injury to the labyrinth can also cause hearing loss. However, if this injury is not extensive, is recognized, and is sealed with bone wax, hearing may be preserved.
When the tumor extends far laterally and invades the cochlea, complete removal of the tumor cannot be done without destroying hearing. A gadolinium-enhanced magnetic resonance imaging (MRI) scan may allow a preoperative diagnosis of such cochlear invasion.
Electrophysiologic monitoring of the facial nerve is done during the operation and has become an established procedure. This type of monitoring assists the surgeon in the identification and preservation of the facial nerve. A click-evoked potentials recorded by a needle electrode placed through the inferior part of the tympanic membrane on the cochlear promontory (electrocochleography) and by electrodes in the scalp (brain stem auditory evoked potentials: BAEPs).
Electrocochleography (ECochG) records near-field potentials and provides rapid feedback of the compound action potential of the auditory nerve probably generated near the cochlea and cochlear microphonic potentials, which are generated by the hair cells of the inner ear. BAEPs are far-field potentials that have a slower feedback. In practice only wave V, which is generated within the brain stem, is monitored because the other potentials are much smaller and often undetectable.
The short-latency ECochG potentials are the most useful for monitoring during operation because they are generally not affected by anaesthesia, they are almost always detectable and they have immediate feedback. On the other hand BAEPs, while useful, are undetectable in some patients even when there is useful hearing: in addition, it may take up to a minute or more to obtain satisfactory recordings because of the small amplitude of the potentials.
By monitoring both ECochG and BAEPs, the entire portion of the auditory system at risk during an acoustic neurinoma operation can be monitored. The presence of N1 indicates the integrity of the auditory nerve peripheral to the tumor: wave V is an indication of auditory nerve activity central to the tumor: and the cochlear microphonics indicate the status of the cochlea, which is at risk from interruption of blood supply or from damage to other structures essential for cochlear function.
The waveforms of the electrocochleogram is stable and reproducible in most patients. There is no problem with the eardrum. The potential problems with the recording include dislodgment of the electrode in the ear, blood or fluid entering the middle ear and blocking sound transmission (either from the electrode trauma or from opening the mastoid air cells), the inability to recognize the cochlear microphonic waveforms in some patients because of their small amplitude, the possibility that direct trauma to the cochlear nerve may not cause immediate changes in the electrocochleogram and the theoretical possibility that potentials will be generated from a site distal to the tumor.
When the status of N1 and wave V at the end of the operation are correlated with the hearing outcome, it is found that if N1 and wave V are lost there is no hearing. If wave V and N1 were present, most patients have useful hearing: if N1was present and wave V was lost or never detected, the results were not predictable. A fundamental limitation of the monitoring is related to how the individual nerve fibers react to the injury. They may stop conducting completely, there may be too few fibers left to generate a gross potential that can be detected or they may conduct a modified or desynchronized impulse.
The hope is that monitoring will give an indication of early hearing compromise that is reversible and will allow the surgeon to alter the dissection. This is the case in some patients in whom a change occurred that recovered when the dissection was stopped or altered. Monitoring has not made a definite difference in the outcome when there has been abrupt loss of function without warning that does not recover, presumably due to interruption of vascular supply: when gradual loss of function occurs and when there is no change in any waveform during the operation. However, monitoring helps to better understand the problems in preserving hearing function.
Steroids are usually started 48 h prior to surgery and a higher dose (methylprednisolone 80 mg IV) is given just before the operation. The blood glucose level is monitored carefully. The high steroid dose is continued every 6 h during the operation and then is gradually tapered over 5 to 10 days depending on the size of the tumor and the degree of facial nerve function.
After anaesthesia is induced, an indwelling Foley catheter is inserted and 10 to 20 mg of furosemide is given intravenously. During the preparation and the exposure of the dura. a 20% solution of mannitol is given intravenously in a dosage of 1 to 1.5 g/kg over 20 to 30 min.
Selection of Approach
Hearing may be preserved with either a middle fossa or suboccipital transmeatal approach to the tumor. Usually the posterior fossa approach is used because the middle fossa exposure provides limited access to the posterior fossa and is reported to be associated with a higher incidence of facial weakness. The middle fossa approach has been used when the tumor is localized to the lateral end of the internal auditory canal.
Patient Positioning and Placement of Monitoring Equipment
The operating table is turned so the surgeon can sit behind the head with his or her feet under the table. The patient is placed in a supine position with the ipsilateral shoulder slightly elevated and the head turned nearly parallel to the floor and elevated and held with a skeletal fixation headrest. An armrest is placed for the surgeon's arm nearest the vertex of the patient's head. The other arm rests on the patient. In patients with an unrelated condition involving the neck, it may be necessary to elevate the shoulder more or use a lateral position. During the operation the line of sight to the brain stem may be changed by moving the microscope and/or rotating the operating table from side to side and hence the position has been called the supine-oblique position. Before preparation and draping of the surgical field, the equipment used for monitoring the facial and cochlear nerves is placed.
Incision and Exposure
A vertical incision is centered approximately 1.0 cm medial to the mastoid process. A graft of pericranial tissue, 3 to 4 cm in diameter, is taken from the occipital region and used in closing the cerebellar convexity dura at the conclusion of the operation.
Suboccipital muscles are incised in line with the incision and are separated carefully from their attachments to the bone using sub-periosteal dissection and electrocautery. The lateral two-thirds of the suboccipital bone is exposed. A burr hole is made, the dura separated and a craniotomy flap cut.
Further bone is removed to expose the transverse sinus, the turn of the sigmoid sinus and the edge of the petrous bone. If the mastoid air cells are entered, they are occluded immediately with bone wax. If fluid enters these cells and gets into the middle ear, there may be interference with the intraoperative monitoring.
The dura mater is opened vertically, with an area of medial dura kept intact to protect the retracted cerebellum. Stellate dural incisions provide superior, lateral and inferior flaps of dura that are held back with sutures. The cerebellum is then gently elevated, the arachnoid is opened and cerebrospinal fluid (CSF) is allowed to drain. This will usually relieve any bulging of the cerebellum and allows exposure of the cerebellopontine angle with minimal retraction. A catheter is placed in the subarachnoid cistern to allow CSF to continue to drain during the operation.
Retraction of the cerebellum must be done slowly and carefully because, on occasion, this retraction has altered the auditory evoked response. Changing the direction or degree of retraction usually causes the potentials to recover. Any bridging veins from the cerebellum to the dura are divided carefully. The self-retaining retractors are then placed. and the operating microscope positioned.
Microsurgical Removal of the Tumor
The arachnoid over the posterior capsule of the tumor is opened. The petrosal vein is usually divided. The retractors are repositioned. The eighth nerve complex will usually be seen coming into the inferior medial corner of the tumor. If it is not immediately visible, the cerebellar tissue next to the tumor may be shrunk with bipolar coagulation to expose the inferior medial aspect of the tumor and the nerve complex.
When an attempt is being made to save hearing, the next step is usually exposure of the tumor in the internal auditory canal. The dura is removed over the region of the internal auditory canal and the bone is carefully removed with an air drill, using constant suction and irrigation for cooling. Removal should extend for a distance of no more than 10 mm, since more lateral bone removal runs the risk of entering the semicircular canals. Usually the lateral end of the internal auditory canal is not exposed. In the drilling of the canal, it is necessary to pause at frequent intervals to check the potentials and to allow further cooling and irrigation of the area.
The surgeon has a choice of beginning the dissection medially or laterally. The preservation of hearing does not seem to relate to whether the dissection is carried along the nerve in a medial or lateral direction. However, medial traction on the cochlear nerve must be avoided. An attempt is made to preserve any arterial vessels going into the auditory meatus.
Dissection then depends on an assessment of the relationship of the tumor to the vestibular and cochlear nerves. If possible, it is better to begin the dissection from medial to lateral. The vestibular nerve fibers entering the medial edge of the tumor are divided using sharp dissection: the cochlear and facial nerves are identified and the dissection proceeds from medial to lateral. In some patients it may be difficult to define the cochlear nerve medially. The tumor is then rotated carefully near the lateral end of the canal for identification of the seventh nerve anterosuperiorly and the cochlear nerve anteroinferiorly. It is important to avoid stretching or putting tension on these nerves. The position of the seventh nerve is confirmed with stimulation. An internal decompression of the tumor may be done using sharp dissection and bipolar coagulation to facilitate the exposure. Dissection along the facial and cochlear nerves is done with fine straight or curved microdissectors, canal knives and sharp dissection with microscissors. Dissection is alternated from different directions, depending on what seems to give the best exposure, the easiest plane of dissection and the least traction on the nerves. When the cochlear and facial nerves have been clearly defined. the vestibular nerves coming into the tumor are divided on both the medial and lateral aspects of the tumor. In some patients the lateral end of the tumor may not be completely exposed because of the limitation in bone removal, which often does not reach the lateral end of the canal. In these patients the tumor is transected near the end of the canal and the lateral extent of the tumor is removed with a small angled ring curette.
During the dissection there may be intermittent bleeding along the nerves. A fine regulated suction will keep the field clean and will not damage the nerves. Most of the bleeding will stop spontaneously. As stated previously. when the surgeon is trying to save hearing, an attempt is made to preserve any significant arterial vessel entering the internal auditory meatus.
After removal of the tumor, any mastoid air cells that have been entered while drilling to expose the internal auditory canal are occluded with bone wax and an adipose tissue or muscle graft is placed in the area where the bone was removed. After careful inspection of the haemostasis, the dura is closed with the graft of pericranial tissue, the bone flap is replaced and an acrylic cranioplasty is done if needed,
Long- Term Results
Gross total removal of the tumor is achieved in most patients in whom an attempt was made to save hearing. Concern about recurrence following removal of an acoustic neurinoma with preservation of the cochlear nerve has been discussed in the literature. Thedinger et al. emphasize that inadequate exposure of the lateral end of the internal auditory canal may be associated with leaving a remnant of tumor. Neely reported that in patients in whom all of the tumor appeared to have been removed, residual tumor was found in the cochlear nerve, and he concluded that "histologic data suggest that complete tumor removal in attempts to preserve hearing may be beyond our surgical capabilities. However, Samii et al. reported no recurrence in 16 patients who had removal of intracanalicular acoustic neurinomas with anatomic preservation of the cochlear and facial nerves who had been followed 1 to 8 years. In one series an attempt to preserve hearing was done in 119 patients with tumors less than 2.0 cm in diameter. Follow-up computed tomography (CT) and MRI have shown no definite recurrence. A few patients have an area of gadolinium enhancement in the internal auditory canal on MRI. Whether this represents residual tumor or postoperative scar is unknown but follow-up scans have shown no change.
The long-term results of hearing preservation have been evaluated. In the report of Shelton et al., 14 (56 percent) of 25 patients who underwent removal of an acoustic neurinoma by the middle fossa approach suffered a significant loss of hearing in the operated ear over a mean follow-up of 8 years (range. 3 to 20 years). On the other hand, Palva et al. reported a significant loss in only 2 of 13 patients during the first 4 years following suboccipital removal. Rosenberg et al. did not observe a significant decline in nine patients followed for 1.3 to11 years. McKenna et al., reporting a series of 18 patients with follow-up ranging from 3.4 to 10.4 years (mean. 5.4 years, found 4 patients (22%) with a significant decline in hearing. Changes did not correlate with tumor size, preoperative level of hearing, intraoperative changes in hearing. the interval between initial symptoms and surgery, sex or age.
The attempt to save hearing does not change the ability to save facial nerve function. This function also relates to tumor size, as it does when no attempt is being made to save hearing.
Management of Tumor in the Only Hearing Ear
This problem will occasionally be encountered in patients with a unilateral tumor when the opposite ear is deaf because of previous infection or trauma. The following guidelines for treatment of these patients are suggested:
1. If hearing is stable, the patient is followed carefully with audiograms and MRI scans.
2. If the tumor is under 1.5 cm and there is progressive hearing loss, the chances of hearing preservation based on the size of the tumor, the patient's age and audiogram findings are discussed with the patient and a decision is made between total and subtotal removal.
3. If the tumor is over 1.5 cm and there is progressive hearing loss, it is recommended an internal auditory canal decompression and subtotal removal of the tumor using intraoperative monitoring.
Pensak et al. reviewed this problem and reported two patients in whom unilateral acoustic neurinomas in the only hearing ear (1.0 cm and 2.0 cm in size. respectively) were treated with complete removal and retention of usable hearing.
Bilateral Acoustic Neurinomas
Surgery for bilateral acoustic neurinomas occurring in a patient with little or no expression of neurofibromatosis is similar to that for unilateral tumors. The tumor usually has a smooth capsule and displaces the facial and cochlear nerves, allowing the possibility of preserving function in these nerves with removal of the tumor. In contrast, the bilateral acoustic tumors associated with extensive expression of neurofibromatosis are often multi lobulated, suggesting that such a tumor may form from multiple sites or that there may be multiple separate tumors in the cerebellopontine angle (neurofibroma or meningioma).
When the patient has bilateral acoustic neurinomas with useful hearing in both ears, complete removal should be considered on one side if one or both tumors are small. This will give the best opportunity to preserve hearing.
The patient who is already deaf in one ear and shows progressive hearing loss in the only hearing ear presents a difficult problem. In several patients, a planned subtotal removal of the tumor using intraoperative monitoring with internal decompression of the tumor and decompression of the internal auditory canal to try to stabilize hearing for as long as possible, is recommended.
If both tumors are already of medium or large size when the patient is first seen, treatment is guided by the clinical symptoms. Usually one tumor will need to be removed. The extent of removal of the second tumor is based on evaluations of the clinical status. Patients who have no symptoms or have only mild symptoms that seem to be stable, are followed carefully with periodic audiometric and radiologic studies.