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CloseTreatment Options for Acoustic Neuromas
Tumor Excision by Microsurgery for Acoustic Neuroma
At a 1991 Consensus Conference of the National Institutes of Health (NIH) regarding treatment options for patients with acoustic neuroma, surgical excision of the tumor was recommended as the therapy of choice for symptomatic acoustic neuromas due to its high rate of control, low rate of complications, low rate of tumor recurrence, and very low rate of mortality.
There are several important issues that must be considered in deciding to undergo surgical removal of an acoustic neuroma. These include:
- What is the status of hearing levels in the affected and unaffected ear?
- What level of hearing preservation (considered successful if postoperative hearing is within 15 dB of preoperative levels) can be expected after surgery?
- Which type of surgical approach is most appropriate for the patient?
- What is the age and overall health of the patient?
There are two types of surgical procedures for acoustic neuromas: partial tumor removal and total tumor removal. Both types are performed by neurosurgeons highly trained in microsurgery, a delicate type of surgery that is aided by specialized instruments and an operating microscope.
Partial Tumor Removal
While the ideal goal of surgery for acoustic neuroma is the complete removal of the tumor with a minimum of complications, there are cases in which there may be overriding circumstances which may prevent this goal from being achieved. When a patient is at high risk for postoperative complications, a health care provider may initially suggest partial tumor removal with the understanding that additional surgery or radiotherapy may be required in the future. The goal of partial tumor removal is to reduce the size of the tumor in a way that reduces the risk of complications to the patient. This may also reduce the risk of facial nerve complications that can often result from surgery. There is, however, still a risk of hearing loss with this type of surgery.
The surgeon may elect to perform a near-total resection in which a small part of the tumor may be left due to inaccessibility or other reasons. Alternatively, the decision may be made to perform a subtotal resection which leaves a larger segment of the tumor. The tumor recurrence rate following a subtotal resection is considerably higher than that following a near-total resection. Recurrences can usually be seen within the first 5 years post-surgery. If a subtotal resection is necessary, the surgeon may also consider supplementary stereotactic radiosurgery in order to reduce the risk of recurrence.
Following partial tumor removal, the health care provider will typically order periodic MRI's to monitor the potential growth rate of the remaining tumor.
Total Tumor Removal
Thanks to the development of new instrumentation and operating techniques, the risks associated with the total surgical removal of acoustic neuromas have been greatly reduced. The main goals of the surgeon are to remove the tumor without causing any damage to the patient's facial nerve (which can result in facial paralysis) or the patient's hearing ability. The surgery may last 8 to 12 hours.
In order to minimize the risk for facial nerve injury during surgery, the 1991 Consensus Conference of the National Institutes of Health recommended intraoperative facial nerve monitoring as the standard of care. Intraoperative monitoring of the facial nerve provides continuous feedback to surgeons as they approach the facial nerve during resection. Electrodes are placed on the face and responses to facial stimulation throughout the surgery (e.g., heat, cold, touch) are tracked by auditory and visuals feedback provided by a special machine. The NIH also noted that the surgery and monitoring must be done by highly qualified neurosurgeons.
Ongoing intraoperative monitoring techniques for auditory nerve response to stimulation such as clicking sounds include the ABR (auditory brainstem response) test which is most widely used and the cochlear nerve action test. If the waveforms are intact at the end of surgery, it is a sign that hearing is most likely preserved.
There are three primary approaches that surgeons use for resecting (removing) acoustic neuromas. These surgical approaches include:
- Middle fossa approach
- Suboccipital (retrosigmoid) approach
- Translabyrinthine approach
The surgical approach selected is based upon the size of the tumor, the location of the tumor, preoperative level of hearing, importance of hearing preservation, patient preference, and the professional skills of the surgeon. Patients should carefully discuss all the advantages and disadvantages of each approach with their surgeons.
When considering the various surgical approaches, the rates of efficacy and complications as reported in the medical literature are highly variable due to several factors that include:
- Differences in terminology used to describe the same surgical procedures
- Modifications of the surgical approaches that are not described in the studies
- Lack of consistency in reporting the size of the tumor
- Differing criteria for inclusion of patients
- Some studies cite results for "surgery" without defining which approach(es) were included
- Some articles report results for all types of surgery for acoustic neuromas without specifying a particular approach.
Middle Fossa Approach
The middle fossa approach to acoustic neuroma surgery is considered to be very difficult and demanding but offers the surgeon good visualization of the internal auditory canal. A small piece of cranial (skull) bone is removed above the ear canal, providing the surgeon with a good view of small tumors that are situated in the internal auditory canal. The piece of bone is put back at the end of the surgical procedure (craniotomy). The middle fossa approach is typically used if the tumor is small (less than 5 mm. extension into the cerebellopontine angle) and the patient has good preoperative hearing and speech discrimination. During this approach, the surgeon may have to manipulate the facial nerve which may increase the risk of potential complications such as facial nerve injury.
Advantages of the middle fossa approach include:
- Good visualization of the internal auditory canal
- High rates of hearing preservation
- Low rates of facial nerve injury
Difficulties involved with the middle fossa approach include:
- The facial nerve passes through this area increasing the risk of injury which may result in temporary or permanent facial paresis
- Limited exposure of certain structures in the surgical field
- Technically demanding surgery
- May cause injury to the temporal lobe
Suboccipital Approach
The suboccipital approach, also called the retrosigmoid approach can be performed for almost any size tumor. It is more versatile than the cranial middle fossa approach and offers good access to parts of the internal auditory canal and the cerebellopontine angle, a space located behind the internal auditory canal. It is performed most commonly for small tumors when preoperative hearing is good and hearing preservation is a goal of surgery.
The surgeon creates an opening in the cranium behind the mastoid part of the ear (craniotomy) or removes parts of the skull (craniectomy) over the cerebellar portion of the brain. The dura (membrane) is cut which allows drainage of cerebrospinal fluid that allows the cerebellum to move away from the tumor. With this approach, the surgeon is able to view the posterior surface of the tumor in relation to the patient's brainstem and remove the tumor. The dura is then sewn up to prevent further leakage of cerebrospinal fluid. This approach is typically used in the presence of small tumors to reduce the risk of hearing loss. While it offers better chances for hearing preservation, it may decrease the surgeon's ability to view the facial nerve, thus increasing the potential risk of facial nerve complications.
Advantages of the suboccipital approach include:
- Possibility of hearing preservation
- Tumors of all sizes can be removed
Some of the difficulties involved with this surgical approach include:
- May carry a higher risk of cerebrospinal fluid leakage
- May be associated with a higher rate of long-lasting postoperative headaches. It is thought that this may be due to bone dust which may enter the spinal fluid space during drilling.
- May be associated with a higher risk of tumor recurrence than other surgical approaches
- May require manipulation of the cerebellum during surgery
- Facial nerve preservation appears to correlate with tumor size
Translabyrinthine Approach
The translabyrinthine approach for acoustic neuroma surgery is typically considered only if hearing preservation is not an objective due to poor preoperative hearing levels. The translabyrinthine approach may be used to remove tumors of any size including those that are large with significant extension of the tumor into the cerebellopontine angle. The translabyrinthine approach is the most direct approach to access the cerebellopontine angle. An incision is made behind the patient's ear. The mastoid bone and some parts the inner ear (semicircular canals) are removed which provides easier access to the internal auditory canal and the cerebellopontine angle. The translabyrinthine approach rarely involves the manipulation of the brainstem, thereby reducing associated risks and it provides good visibility of the side of the brain that faces the tumor. A benefit of this approach is that it makes it easy for the surgeon to locate the facial nerve in the temporal bone before the tumor is removed, thus increasing the surgeon's chances of preserving facial nerve function. Unfortunately, hearing is always sacrificed with this approach.
Advantages of the translabyrinthine approach include:
- Tumors of all sizes can be visualized and removed
- No part of the brain needs to be manipulated
- Good preservation of facial nerve function
The major disadvantage of this surgery is:
- There is always a complete loss of hearing on the operated side
- Some portions of the cerebellopontine angle and cranial nerves are not well visualized
- A fat graft is required where fat is surgically removed from the abdomen and grafted to the site of the incision.
Efficacy of Surgical Treatment and Prognosis
The control rate (rate of tumor recurrence) of all three surgical approaches is similar. Complete removal of acoustic neuromas is achieved in 95% of patients. Because there is a small chance that the tumor will come back again, the health care provider will typically order a follow-up MRI within 1 to 5 years. The NIH Consensus Statement noted that follow-up visits may take place every three months following surgery to every 1-2 years as the duration from the time of surgery increases, depending on the clinical presentation of the patient. Mortality rate following surgery is less than 1%. It is important to note that the successful outcome of surgery for acoustic neuromas continues to improve as surgical techniques and equipment become more sophisticated and precise. For this reason, it is very important to choose a neurosurgeon who has extensive experience with the surgical approach to be performed and to ask the surgeon about their rate of success and incidence of complications. There are differing opinions regarding the degree to which intraoperative monitoring of the auditory nerve accurately predicts hearing preservation. While in select groups of patients 80% hearing preservation has been reported, overall approximately 50% of patients retain functional hearing in the affected ear following surgical excision. There have been only a few reports of patients experiencing an improvement in hearing following acoustic neuroma microsurgery. It is believed that removal of the tumor and subsequent decompression of the cochlear nerve may explain any improvement in hearing. Regarding the stability of long term hearing following surgery, results of studies are mixed and inconclusive.
In a study published in 2003 by researchers from the Department of Otolaryngology at the University of California, investigators analyzed the success rate of hearing preservation following excision of large acoustic neuromas partially situated in the cerebellopontine angle. The surgical approach studied was the suboccipital or retrosigmoid approach. Overall, only 6% of the patients retained good hearing postoperatively. Thus, patients who wish to attempt hearing conservation in the presence of a large tumor, it is important to choose carefully between the translabyrinthine approach, which has a lower rate of complications but where there's no chance of hearing preservation, and the retrosigmoid approach where there is an increased risk of persistent headaches, increased incidence of postoperative vestibular dysfunction and a slightly increased risk of tumor recurrence but still presents a chance for hearing conservation. To read more about this study, please click on this link: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&listuids=12806299&queryhl=4&itool=pubmed_docsum
The rates of preservation of facial nerve function continue to improve with more effective intraoperative monitoring. The degree of function of the facial nerve is evaluated by the House-Brackmann scale, a widely used facial nerve grading system that rates facial function on a scale from Level I (normal) to Level VI (no movement and complete loss of muscle tone). It is commonly employed to assess facial nerve function following surgery for acoustic neuroma. To learn more about the House-Brackmann Facial Nerve Grading System, please follow this link: http://www.entusa.com/bells_palsy.htm
It is estimated by some researchers that 5-10% of patients may experience facial paralysis or paresis following excision of small tumors. Some studies indicate that up to 60% of patients experience some residual facial weakness following excision for large tumors. Other studies estimate facial nerve functioning at approximately 80% following microsurgical excision of the tumor. Up to 25% of patients with poor facial nerve function at the time of discharge will eventually achieve normal function. The prognosis of patients with low facial nerve function at 6 weeks postoperatively (level III or higher) is poor. Facial palsy is reported to occur in 10-30% of patients, the majority of who make a full recovery.
Regarding the rates of the preservation of hearing and facial nerve function, the following results have been noted in the medical literature:
- Middle Fossa Approach - For the middle fossa approach, hearing preservation is considered successful (within 15 dB of preoperative hearing level) in up to 50% of patients undergoing this procedure. Up to 68% of patients retain functional hearing (less than the 15 dB designation of success). Facial nerve function is retained in up to 95% of patients.
A study published in 2003 evaluating the effect of age on the outcome of middle fossa surgery for acoustic neuromas noted that for older patients there is a lower chance for preserving hearing than for younger patients. However, older age does not affect the outcome of surgery regarding facial function. There was also a higher rate of cerebrospinal fluid leakage in older patients.
Suboccipital Approach - Following suboccipital or retrosigmoid surgery, hearing preservation is reported in up to 58% of patients regardless of tumor size. Facial nerve function preservation, however, is related to tumor size. If the tumor is small, up to 95% percent of patients retain facial nerve function at level I or II on the House-Brackmann grading scale. If the tumor is larger than 4 cm, however, the percentage drops to 38%.
Translabyrinthine Approach - The translabyrinthine approach sacrifices all hearing in the affected ear. The major advantage of this approach is the preservation of facial function. At one year post-surgery, one study showed that approximately 83% of patients maintained level I and II scores on the House-Brackmann scale. If the tumor was larger than 3 cm, level I and II function was noted in 52% of the patients. In a 5-year study using this approach, 70% of all patients had good facial nerve function. Another study following this surgery reported facial nerve preservation at 86%, facial paralysis at 31%, and delayed partial paresis (weakness) at 50%. At two years post-surgery, approximately 63% of patients were at the level I or II of the House-Brackmann scale for facial function.
You can read more about post-surgical preservation of hearing and facial function by clicking on the following links: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&listuids=12806302&queryhl=4&itool=pubmed_docsum
Postoperative headache, ranging in intensity from mild to severe, has been reported in approximately 33% of patients following acoustic neuroma surgery, though the range varies widely in the literature. Headaches usually resolve within 12 weeks but some patients experience persistent postoperative headaches for 1 year or longer. Most headaches resolve by 3 years post-surgery.
The risk of residual tumor following acoustic neuroma surgery is about 5-10% or less and is most commonly encountered following suboccipital surgery. This may be attributed to a small amount of tumor remaining in a section of the auditory canal which is very hard to visualize during surgery. Symptoms of recurring tumor may include headache, unusual facial sensation, or difficulty with speech pronunciation. Inflammation at the site from which the tumor was removed may persist and must be differentiated from recurrent tumor through MRI scans with gadolinium.
Complications of Acoustic Neuroma Surgery
After any type of microsurgery for excision of an acoustic neuroma, the patient typically will be observed for at least one night in the intensive care unit of the hospital. Patients may experience a range of immediate postoperative symptoms including headache, fatigue, nausea/vomiting, tinnitus, facial droop or palsy, balance problems, inability to close the eyes, or dryness of the eyes. The patient is typically discharged after 3-4 days. The total recovery time for surgical removal of an acoustic neuroma may take from 4 to 6 weeks or longer. After 3-6 months, an MRI may be ordered by the physician in order to visualize the site of the tumor for future reference. The MRI may be repeated at 5-10 years post-surgery.
The most common complications of acoustic neuroma surgery include:
- Cerebrospinal fluid (CSF) leakage
- Postoperative headaches
- Facial nerve complications
- Vestibular nerve dysfunction
Cerebrospinal Fluid Leakage
Leakage of cerebrospinal fluid through the surgical site or through the Eustachian tube and middle ear occurs in up to 20% of patients following acoustic neuroma surgery. It occurs most frequently following suboccipital surgery and the least following middle fossa surgery. Meningitis may occur in up to 5% of these patients. Cerebrospinal leakage may be corrected either by placing an extra stitch at the surgical site, by re-closure of the operative site, or by placement of a temporary lumbar drainage catheter.
Postoperative Headaches
Postoperative headaches are experienced by most patients who undergo acoustic neuroma surgery. They may be due to several factors, including:
- Pain from the incision
- Muscle pain
- Disturbances of cerebrospinal fluid pressure
In some patients, postoperative headaches persist for months or years after surgery. There is debate whether a relationship exists between tumor size and the incidence of postoperative headaches. It appears from some studies that the patient group at highest risk of suffering persistent postoperative headaches are those with small tumors (< 1 cm diameter) operated via a suboccipital approach for hearing conservation purposes.
Facial Nerve Complications
"Acceptable" long-term facial nerve function after surgery is reported to occur in 60% to 100% of patients with an average of 85%. Severe injury to the facial nerve can result in speech impairment, drooling, difficulty chewing and swallowing, and the loss of taste. The degree of facial nerve dysfunction following surgery is impossible to predict.
Limited paralysis or paresis of facial muscles may occur if the tumor is compressing the facial nerve, such as if the acoustic neuroma is attached to the facial nerve or wrapped around it. Under certain circumstances, the facial nerve can be surgically repaired at the time of surgery which delays considerably the recovery period of facial nerve function. If facial paralysis does occur, special attention must be given to the eye since the ability to blink may be affected.
With the presence of facial nerve weakness, the patient may also experience difficulty completely closing their eye resulting in exposure and drying of the cornea. The feeling most typically associated with this problem is that of sand or some other foreign particles in the eye and causing significant irritation. It is very important for the patient to keep their eyes well hydrated by using artificial teardrops as need and perhaps eye ointment at night.
Vestibular Nerve Dysfunction
Since an acoustic neuroma is situated directly on the vestibular nerve, any surgical procedure to remove the tumor will necessarily involve manipulation of the nerve to some degree or direct injury to the vestibular nerve. The severity of nerve damage preoperatively determines how quickly the brain will compensate in restoring equilibrium. If the patient had good preoperative vestibular function and the nerve on the affected side is functioning relatively well, then the symptoms following surgery will be more pronounced than if the tumor had already destroyed part of the nerve. In this case, the brain will have already compensated for the lack of vestibular information input from the affected ear and, postoperatively, vestibular dysfunction will be minimal.
Though impossible to predict, it is generally noted that it takes a longer period of time for older patients to recover vestibular function than for younger patients. An overall time frame for recovery, that may be different for each patient, is as follows:
- Patients will experience difficulty with equilibrium most strongly immediately following surgery.
- After approximately 2-3 days they are typically able to walk with assistance
- After 1-2 weeks, disequilibrium may occur following head motion
- After a few months, disequilibrium may result from sudden head motion
- After approximately 6 months, the brain has sufficiently compensated for the lack of complete vestibular information so that the patient no longer feels any dysfunction of balance.
Which Surgical Approach Should I Consider?
There are many factors that have to be considered regarding selecting the optimal surgical approach for acoustic neuroma surgery. These factors include:
- Hearing level
- Size of tumor
- Location of tumor
- Experience of the neurosurgeon
- Patient preference
It is essential for patients to discuss each of these issues with their surgeon before making a decision regarding which surgical approach would be most appropriate for their particular situation.
Hearing Level
If there is no hearing in the affected ear preoperatively, the translabyrinthine or suboccipital approach may be considered. It there is useful hearing, then the middle fossa approach or the suboccipital approach may be surgical options. The criteria for hearing preservation when undergoing surgery vary. Some institutions require a pure tone loss of less than 50 dB and no more than a 50% loss of speech discrimination skills. Other institutions require a maximum of a 30 dB pure tone loss with a minimum of 70% intact speech discrimination.
Size of the Tumor
Hearing preservation is easier to achieve with smaller tumors of 2 cm. or less in size. If the tumor is large, many surgeons may prefer the translabyrinthine approach in which there is no hearing preservation in order to minimize the risk of facial nerve injury. The larger the tumor, the lower the chance for success of hearing preservation following surgery and the higher the chance of complications, such as facial palsy.
Location of the Tumor
Each surgical approach offers good accessibility and visibility of different parts of the inner ear. Depending on the exact location of the tumor, the surgeon may favor one surgical approach more than another.
Experience of the Neurosurgeon
It is important to discuss with the neurosurgeon how much experience he/she has with each type of surgery. Studies clearly indicate that the risk of complications and the rate of recurrence of acoustic neuromas are closely tied with the experience of the surgeon and the hospital in which the surgery is performed. The experience of the staff taking care of the patient postoperatively is also very important.
Preference of the Patient
The patient must decide whether they have a strong preference for hearing preservation or facial nerve preservation since each surgical approach is associated with variable risks for each of these potential complications.