Acoustic Neuroma

Hearing loss right after surgery Immediately after

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Introduction:

An acoustic neuroma is a benign growth on the hearing and balance nerve, sometimes referred to as the 8th cranial nerve. The tumor itself arises from the Schwann cells, which cover the nerve. Acoustic neuroma may also be called acoustic or vestibular schwannoma, or even an acoustic or vestibular neurinoma. The 8th cranial nerve is actually composed of two separate nerves. One is the balance nerve, the other is the hearing nerve. Although the balance nerve may also be separated into two parts, a superior an inferior, it is easier just to think of it as a single nerve. Although unilateral hearing loss is the most common symptom associated with acoustic neuromas, the vast majority, in fact almost all acoustic neuromas, actually involve the balance nerve and not the hearing nerve.

The incidence of acoustic neuromas can be measured in a variety of ways. It has been said that the chance of developing an acoustic neuroma in one's life time ranges anywhere from 1 to 1000 to 2.5%. The higher number, 2.5%, derives from findings on autopsies; the tumors are usually quite small and also asymptomatic. Another way of looking at the incidence is to determine how many new cases are diagnosed per year. In the United States this can range anywhere from about 2000-3000 new cases per year.

Although acoustic neuromas are not the most common intracranial tumor, they do make up approximately 6% of all brain tumors. Of the tumors located in the cerebellar pontine angle and/or in the internal auditory canal, these growths make up 85% of all tumors. Benign meningiomas make up the other large percent of tumors in this location. The rest are a variety of malignant or benign growths. The vast majority of acoustic tumors are found in adults, usually between the ages of 30-60 and only rarely are they found in children.

Acoustic neuromas are, by definition, single-sided or unilateral in nature. However, there is an entity of bilateral acoustic neuromas, which is labeled as neurofibromatosis II. Over 95% of all patients will present with a classic single-sided acoustic neuroma. As discussed below, patients who suffer from neurofibromatosis II, have a hereditable condition and require separate treatment considerations than do those with single-sided acoustic neuromas. This will be discussed later.

Signs and Symptoms:

There is no one constellation of symptoms that is a hallmark of acoustic neuromas. However, there are a number of different symptoms that should spur the physician into obtaining an MRI with gadolinium to evaluate for an acoustic neuroma. Typically, the following symptoms can be associated with acoustic neuroma:

· Unilateral hearing loss

· Disequilibrium or imbalance

· Tinnitus – usually unilateral

· Vertigo

· Pressure or fullness sensation in one ear but usually with another of these symptoms

· Facial paresis, paralysis, or numbness

· Headaches

Hearing loss: Unilateral hearing loss or hearing loss on the side of the tumor is the most common symptom experienced by patients with acoustic neuroma. Indeed, over 90% of patients with an acoustic neuroma will have, as their first symptom, decreased hearing in the affected ear. The most common type of hearing loss is in the high frequency range. However, this pattern of loss is seen in only a half to two thirds of patients with acoustic neuroma. Other hearing loss patterns can include low frequencies, which can mimic Meniere's disease, or a "cookie bite" or “U” shaped pattern, sometimes associated with congenital loss.

Another form of unilateral hearing loss is that of sudden loss. Sudden loss can also be associated with acoustic neuroma in about a quarter of cases. Interestingly, however, even though sudden loss can be highly associated with an acoustic neuroma, because the prevalence of sudden loss is so high, the actual number of acoustic neuromas found in patients with sudden hearing loss is actually quite low. Many patients will regain all of the hearing they lose) is response to steroid or other treatment. This does not mean that an acoustic neuroma can be ruled out. Therefore, any patient who has suffered a sudden sensorineural hearing loss should undergo an MRI with gad usually within a month of the sudden loss.

Vertigo and disequilibrium: One would think that because these tumors almost always arise from the vestibular portion of the 8th nerve, dizziness or imbalance would be the most frequent complaint. However, the hearing portion of the 8th cranial nerve seems to be extremely sensitive to pressure changes and probably blood flow changes as well. That does not mean, however, that patients do not suffer from imbalance or vertigo with these tumors, it just is not the most common symptom. The most common vestibular complaints are unsteadiness or a problem maintaining balance. They may notice this to be more intense in a darkened room or with quick movements. Interestingly, there are some patients who have large tumors who can recall periods where they had some balance issues but over time these had gone away. Because these tumors tend to grow very slowly, the ability for the "good side" to compensate for the vestibular loss on the tumor side is quite high, thus lessening any chances of significant dizziness or disequilibrium later in the course of treatment. As tumors become larger and larger and start to press and compress the cerebellum, the disequilibrium symptoms can become more central in nature.

Tinnitus: Most patients with acoustic neuroma suffer from unilateral sensorineural hearing loss, and it is very common for them also to have associated tinnitus. Tinnitus is nothing more than an abnormal noise heard by the patient that is variously described aswhistles, bells, crickets, car engines, seashells, and beyond. Normally, this type of phenomena is central in nature; when the brain no longer receives normal hearing input it makes up its own noise. This is not unlike the patient who complains of pain, burning, or itching of a limb that has been amputated.

Ear fullness: Ear fullness is an annoying, but not uncommon complaint with hearing loss. This can occur either with sudden hearing loss, with gradual loss of hearing or an acoustic neuroma. The pathophysiology, or reason why the fullness is experienced, is not really known, but over time, it may dissipate.

Facial paralysis or numbness: Complaints involving the facial nerve or face are uncommon in small or medium sized tumors. It is not until the tumor has gained significant size, usually greater than 3 cm, that these symptoms may show themselves. Indeed, facial paresis or paralysis, even in very, very large tumors, is quite uncommon. Facial twitching, however, is not all that uncommon in large tumors and is said to be in the 10-15% range. The complaint of facial numbness also occurs only in the larger tumors and sometimes will respond to medical therapy. In addition, there are a small number of patients who experience odd taste sensations or loss of taste on half of the tongue. This is because the nerve for taste, chorda tympani, is part of the facial nerve.

Headache: Headaches are extremely rare and unusual in small or medium sized tumors. Again, those tumors which are quite large, pressing in on the cerebellum, do cause a much higher incidence of headache. The headaches are usually not constant, but rather fluctuate in intensity and time.

Diagnosis:

The symptoms of acoustic neuromas described above, as well as suspicion of Meniere's disease or other vestibular disorders, should lead the physician to evaluate for acoustic neuroma. All patients who have vestibular complaints or hearing loss ahould have a hearing test. It is probably the first objective test a patient will have. Unilateral hearing loss, whether it be high frequency, low frequency, cookie bite (“U”-shaped) or sudden, can all be harbingers of an acoustic neuroma. The audiogram should also include a word recognition test, the patient’s ability to repeat back spoken words. If a patient scores more poorly on word recognition when the words are presented at a louder level versus softer or quieter level a phenomena called "roll over" has occurred. This has a higher association with acoustic neuroma.

Once a unilateral hearing loss is documented or the patient continues to have unilateral vestibular complaints, the superior test to rule out an acoustic neuroma or other benign growth in the cerebellar pontine angle (CPA) is a T1 weighted MRI scan with and without gadolinium. Because other disease entities can cause unilateral hearing loss or vestibular symptoms, or fluctuating vestibular symptoms it is not recommended to do a limited MRI since this may miss diagnoses like MS or other tumors. (Pictures of MRIs here too).

There are patients who cannot tolerate an MRI scan. For them, an open MRI scanner can be used or heavy sedation/general anesthesia employed. However, there are still a subset of patients who cannot have an MRI scan due to foreign bodies, such as pacemakers or clips, or because of their size. For these patients, we rely on older tests that have fallen out of use with the advent of MRI. One of these tests is the auditory brain stem evoked response (ABR) study. This test simply measures the amount of time it takes for sound to go from the ear to the brain. In patients who have an acoustic neuroma, it will take longer for the sound to travel in the affected ear than it will for the opposite or normal ear. The reason that this test fell out of favor is that a third of patients who had normal ABRs, were found on MRI scan to have small tumors, located in the internal auditory canal. Even in larger tumors, the test had both high false positive and false negative rates. Testing of vestibular function with an electronystagography (ENG)_ test, usually will not provide useful diagnostic information. However, it may correlate with severity of damage to the vestibular systems, both immediate and long term, after surgery or gamma knife. One would expect patients who have already compensated for a loss to do much better after removal of the tumor than a patient who has not had any loss or compensation.

For those patients who cannot undergo an MRI with gadolinium study, a CT scan and ideally, an air CT scan, will provide some useful information. With this scan, the clinician should be able to diagnosis medium and large acoustic neuromas, although small tumors in the internal auditory canal may not be visible.

Once the diagnosis of an acoustic neuroma is made, it is usually classified as to location and size. It is generally accepted that a tumor up to 1.5 cm at its longest dimension (including the internal auditory canal portion) is classified as small. Those tumors that are just in the bony internal auditory canal are sometimes referred to as intracanicular acoustic neuromas. These tumors are classified as small, even if a small portion extends outside of the canal, into the space of the cerebellar pontine angle. Medium tumors are a little bit bigger, usually 1.5 to 2.5/3 cm in its longest dimension. They may abut the cerebellum or brain stem, but usually exert no significant compression. Large tumors are greater than 3 cm, with compression of the cerebellum and/or brain stem.

Patients diagnosed with NF II are classified according to the size of each individual tumor.

The diagnosis of an acoustic neuroma is classically made with MRI, with T1 enhancement using gadolinium contrast and no enhancement on non-gadolinium images. Other benign growths that can occur in this region, the most common of which is meningioma, have similar characteristics to an acoustic neuroma, except that the shape and involvement of the dura resulting in a “dura tail” is the hallmark of meningiomas and is not seen in acoustics. Other lesions, such as lipomas or epidermoids, can be diagnosed with the MRI scan results. Fat suppression studies are useful in ruling out a lipoma and epidermoids are bright regardless of the use of gadolinium.

Growth patterns: Before understanding what treatment may or may not be the best option for a particular patient, it is essential to understand the growth patterns of an acoustic neuroma. It should be noted that acoustic neuromas grow very slowly, usually on average of 1 to 2 mm per year. Because this growth is slow patients have the time and opportunity to research and evaluate which treatment option they would like to pursue.

As acoustic neuromas grow, they will exert pressure onto the internal auditory canal and actually enlarge or widen the canal.

The tumor itself will usually stay in its capsule and rather than invading the auditory portion of the 8th nerve, the 7th nerve, the cerebellum, or the brain stem, the tumor simply compresses these structures as it grows. However, as the tumor fills the internal auditory canal, the physiology of the 7 and 8th cranial nerve are affected. Instead of being a thick cable, these nerves are pushed up against the wall of the internal auditory canal (making the nerves a thin ribbon,) and are further stretched and ribbonized in the cerebellar pontine angle as the tumors grow into the medium and largesizes. Thus, although paralysis is usually not occur preoperatively, when the tumor is surgically removed the 7th nerve is at risk for bruising, further stretching, or tearing – resulting in temporary or permanent facial paralysis. Compression of the brain and/or brainstem may also lead to increased chance of central injuries such as stroke, seizure, headaches, other cranial nerve dysfunction, death, bleeds, and CSF leak during and after surgery.

Treatment:

There are currently three accepted treatment methods for acoustic neuromas:

· Observation with MRI monitoring

· Surgical excision

· Stereotactic radiosurgery (STR) or also referred to as stereotactic radiation treatments (STR) (most commonly gamma knife, Cyberknife, or "linear accelerator (LINAC)"

(see Table 1)

Observation: As noted, acoustic neuromas grow very slowly. There is increasing evidence that patients over the age of 65 may experience an arrest of growth of the tumor. Indeed, most patients who are diagnosed with acoustic neuroma over the age of 65 may be best served by just observing whether or not the tumor continues to grow, since approximately 50% will not grow. The size of the tumor may also be a factor as to whether observation is an adequate treatment option. Small tumors located in the internal auditory canal with no significant symptoms, or with mild unilateral hearing loss, mild fullness, and/or mild tinnitus may be candidates for observation. If the tumor were to grow, the chance of preserving hearing with surgery, or treating and preserving function with gamma knife are the same whether a tumor in the internal auditory canal is 5-6 mm, or 7- 8 mm.

Even in younger or middle-aged patients there is a chance that these tumors will not grow. There is little harm in following these tumors, to see if they continue to grow except that there is a chance of further hearing loss.

Another scenario that deserves observation, rather than surgery, is if a patient has lost all or most of their hearing, such that a hearing aid would be of little to no benefit. Regardless of whether the tumor is all intracanalicular or just protruding out of the porus or even into the CP angle without cerebellar or brain stem compression these patients may best be served by observation depending upon age and other symptoms.

Typically, a repeat MRI scan with gadolinium is performed 6-9 months after the original scan, and then every year thereafter.

It should be noted that although there are no “treatment risks” associated with observation, there is still a small risk of further hearing loss, tinnitus, or disequilibrium by doing nothing.

Tumors that are large, compressing the brain stem or cerebellum significantly, or patients with significant vestibular complaints or worsening hearing loss, may not be very good candidates for observation.

Surgical excision: Depending on the size of the tumor, hearing status, and surgeon preference, there are three, well recognized surgical exposures. In order to accomplish surgical excision of the lesion, the surgery is usually performed by a team of surgeons that consist of a neuro-otologist (ENT surgeon who has completed a fellowship in neurosurgery of these structures), along with a neurosurgeon. The three approaches that are used are the middle cranial fossa approach, the retrosigmoid or suboccipital approach, and the translabyrinthine approach.

Middle cranial fossa approach (MCF): Typically, the middle cranial fossa approach (MCF) is designed to remove tumors mainly from the internal auditory canal. Although it can be extended and facilitate removal of some tumors with a small involvement of the CP angle, it is basically used for patients with intracanalicular tumors. The advantages of the middle cranial fossa approach are that it facilitates visualization of the entire tumor in the internal auditory canal, allowing for greater chance of hearing preservation. The theoretical extra risks would be that one does have to retract on the temporal lobe (risking seizure, stroke, bleed, memory loss, dysphagia) and the facial nerve may be more exposed when drilling open the internal auditory canal. As with any surgical approach, there are risks for CSF leak, headaches, infection, meningitis, and bleeding. Depending on the incision to perform the middle cranial fossa approach, it may be more susceptible to some temparo-mandibular joint (TMJ) pain in the postoperative period. (Picture)

Suboccipital or retrosigmoid approach: This approach can be used for any size tumor - for hearing preservation or no hearing preservation. Basically a small craniotomy or hole in the skull is made behind the sigmoid sinus. Because the craniotomy is quite small and is repacked to prevent dura from attaching/scarring to the skull/neck musculature and soft tissue, there is significant decrease in the amount of postoperative headaches (3-5% rather than 20-30%). The internal auditory canal must still be drilled out, as with the middle cranial fossa or the translabyrinthine approaches. Some surgeons report a complication in preserving hearing with this approach, as it is sometimes more difficult to visualize and preserve the posterior semicircular canal while laterally approaching the internal auditory canal. However, from personal experience and review of the literature, this does not seem to bear out.

Translabyrinthine approach: With this type of approach, hearing will not be preserved. It offers the widest and largest exposure for removal of tumors and is usually reserved for those tumors that are quite large and may benefit from increased exposure. However, as stated above, the retrosigmoid approach can also be used to remove large tumors. The translab approach skeletonizes the facial nerve in the mastoid bone and removes all of the semicircular canals and then opens the internal auditory canal. The entire dura of the cerebellum is exposed and the sigmoid sinus is decompressed to maximize exposure of the tumor.

Risks of Surgery

Balance: Surgical excision of acoustic neuromas carries risks and unavoidable sequelae. Because most acoustic neuromas involve the vestibular nerve, sacrifice of this nerve in order to remove the tumor is necessary. When this happens, the vast majority of patients will wake up from surgery very dizzy. Dizziness will usually last approximately three days and then start to improve. The patient will have disequilibrium or imbalance that they will have to learn to compensate for afterward. This process can take up to two months and occasionally even longer. We recommend vestibular rehabilitation therapy to decrease the amount of time needed to compensate. Even with good compensation, patients will notice some lingering problems, such as a feeling of off-balance when they turn very quickly or stumbling and falling if they get up in the middle night and they cannot see.

Facial paralysis: The key of surgery is to preserve the facial nerve and facial nerve function. Thus, many surgeons may elect to leave a microscopic amount or a very minimal amount of tumor on the facial nerve if it is felt that removal of this would permanently injury the facial nerve. Typically, when this is done, these tumors do not seem to regrow, probably from lack of blood supply. If more is left behind, one may consider additional SRT, or may elect to simply follow the remaining tumor with yearly MRI scans.

Hearing Preservation: The ability to save hearing is closely correlated with the size of the tumor. Small tumors or tumors regulated to just the internal auditory canal have the highest rate of hearing preservation. This can be anywhere from 40-80%. Even if the hearing is preserved, hearing may decrease significantly from preoperative levels and hearing may still drop with time, i.e., months and years after surgery. The extended time during which hearing may decrease is believed to arise from decreased vascularity or scarring of the blood vessels that provide blood to the auditory nerve.

Other Risks: Other risks of surgery include CSF leak, infections, meningitis, pain or headaches, intracranial bleeds, seizures, strokes and death. All of these are uncommon except CSF leak (1-5%) and headaches (as discussed).

Headaches can occur in anywhere from 10-34% of patients, according to some reports. However, we have found that in the retrosigmoid approach, if the craniotomy is refilled with either bone dust or Gelfoam, or some other type of material, in order to prevent scarring onto the dura, the incidence of headaches, goes down significantly, to around 3%.

Tinnitus may occur after either stereotactic radiation therapy or surgery. It is a symptom that we really do not know much about.

Postoperatively, most patients are admitted to a room, where their neurological status will be checked quite frequently. They are usually ready for discharge on around day three or four post-surgery. Because of the compensation that is required, most patients are out of work for at least four and up to 6-8 weeks. During this recovery period patients may typically experience great days (full of vim and vigor) followed by a day of lethargy.

Stereotactic radiotherapy: Stereotactic radiotherapy has for many years been synonymous with gamma knife surgery or gamma knife radiation treatment. However, other forms include Cyber knife, the "LINAC" model and others. The gamma knife procedure is a same day, one treatment procedure. A patient arrives in the morning, a frame is attached to the skull with four small pins. The patient will then have repeat MRI and CT scans performed with the frame in place. The surgeon will then plan the treatment in order to deliver the radiation stereotactically to the tumor and nothing else. This is accomplished by having the patient attached to a head piece with hundreds of little holes in it that allow the radiation to be directed exactly to where the tumor is. Once the plan is made and certified by the radiation oncologist and the physicist, the patient undergoes the treatment, which can last anywhere from 15 minutes to 45 minutes, on average. When the patient is finished, the frame is removed and the patient returns home. Again, the patient is followed periodically with MRI with gadolinium scans to insure that the tumor is not growing. If the tumor were to continue to grow, the patient would have to consider surgical excision.

CyberKnife treatment is performed in a similar fashion to gamma knife, but without a head frame. In addition, the patient usually undergoes about 3 treatments rather than one.

In the short term, there are very few side effects from the stereotactic radiation treatment. Occasionally, a patient may have some dizziness or early onset hearing loss or facial paresis. However, most of the complications associated with gamma knife are longer term, meaning that they occur anywhere from 5-10-15 years post-treatment. This could include facial paralysis or hearing loss. However, since the advent of using lower doses than had been the earlier protocol, the risks of these complications have decreased significantly. Indeed, there are reports now of less than 1% facial paresis after 5 and 10 years and minimal hearing loss as well. An extremely rare occurrence, since there are only a few case reports of this, could be malignant transformation of a benign tumor. If the tumor does grow afterwards and must be removed surgically, the approach is more difficult because of increased scarring and scar tissue. This may have its greatest impact on facial nerve preservation. However, most neuro-otologists report that although removing the tumor from the nerve is indeed more difficult, patients have not had more significant facial paralysis with this surgery. Because the tumor, and hence the vestibular nerve “dies” at a slower rate than it would from simply cutting the nerve, as in open surgery, the patient usually compensates for the loss of vestibular function gradually, and in most instances does not perceive a loss of function, other than the same late effects that surgical excision causes.

Table 1.

COMPLICATIONS SURGERY GAMMA KNIFE

Death 0.5% Rare

CFS leak 5-10% Rare

Meningitis 2-5% Rare

Facial weakness/paralysis 5-10% Early less than 1%

Hearing loss right after surgery Immediately after

retrosigmoid 60% rare 5-10yrs out

middle fossa 20-40% is 10-50%

translab 100%

Headache 3% Rare

Disequilibrium 100% Less than 5%

Stroke Rare Rare

Post-operative Complication Treatment:

Facial paralysis: If facial weakness or paralysis does occur, a variety of treatments can ease some of the discomfort. At night, a lubricant will usually be used in the eye, and the eye will usually be taped closed or a moisture chamber used in order to prevent corneal abrasions. During the day, Natural Tears or other eye drops are used to keep the eye moist. This is especially important if the patient is outside or is in cold weather. If it appears that the weakness is going to be longstanding or take a long time to recover, a gold weight may be placed in the eyelid in order to promote good eye closure. Tarsorrhaphy or sewing the eyelids together is usually something that is not needed and is no longer recommended. If the facial paralysis is permanent, then a referral to a facial plastic reconstructive surgeon is usually obtained, as there are different procedures that can be used to enhance the facial aesthetics. Patients who have difficulty with the facial nerve may also complain of a metallic taste or an odd taste sensation on half of their tongue. That will usually improve over 6-9 months, as the other side may take over.

Fatigue: A patient’s level of energy frequently fluctuates for about a month after surgery. There are some days when they will feel invigorated and good to go and other days when they will feel like they cannot even make it out of bed. This is not unusual in someone who has undergone acoustic neuroma or other brain surgery.

Hearing Loss: Many patients will have no to almost no hearing after open surgery, and possibly after stereotactic radiation surgery as well. If the loss is such that a hearing aid does not help, the patient is probably a candidate an excellent candidate for a BAHA implant The BAHA consists of a small screw that integrates into the skull with a pin that protrudes slightly out of the skin A hearing processor attaches to this and can be removed just like a hearing aid. The system works by sending the sound waves through the skull and letting the good (opposite) ear hear what is coming from the hearing loss side. Another alternative could be a CROS hearing aid or a TransEar® aid but in our experience the BAHA gives the best results.

NF2:

NF2 is by definition bilateral acoustic neuromas. These neuromas may also be found at different sites in the brain or spinal column. The treatment is similar in that observation, surgery, and STR are all options. There is some evidence that SRT works well while others think that it may make the growths more aggressive. Most people recommend no treatment if the tumors are small and present minimal symptoms. Others recommend removal of the small ones surgically soon for the best chance of preserving hearing. For large tumors with no hearing an auditory brainstem implant may be an option to provide hearing after tumor removal. There are no right answers to the any of the questions the treatment of these tumors present. Rather all options must be discussed and information disseminated so that an informed decision can be made by the patient.

Questions to ask physicians:

When trying to determine what treatment protocol is appropriate or whether or not you are comfortable with the physician treating you, here are some questions to consider asking:

· For a tumor of my size, what have been your results with respect to facial nerve damage, hearing, and imbalance for surgery as compared with SRT?

· What is the likelihood that my hearing will remain the same after surgery or SRT?

· If I were to have surgery, do you anticipate total tumor removal or will I need further surgery or SRT?

· What are your rates of complications?

· Do you perform both surgery and SRT? What type of SRT do you perform and what are its advantages?

· How long have you been performing surgery or SRT on patients who have acoustic neuromas?

· Have you had any problems in any of your patients?

· Are you certified to perform SRT in patients with acoustic neuromas and/or are you a fellowship-trained board certified neuro-otologist?

· Do you work with a neurosurgeon on surgical cases, and if not why not?

· What are the long-term effects of SRT that you see in your patients?

· Are physicists and radiation oncologists involved in the planning of the SRT?

· When and how often should I schedule followup MRIs?

Conclusions: All in all, acoustic neuroma treatment and management should include a mutual understanding, discussion, and decision by the patient with the otolaryngologist and his/her team providing all of the necessary information for the patient to be able to make an informed decision.