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Acoustic Neuroma

A discussion of acoustic neuromas

ACOUSTIC TUMOR

GENERAL COMMENTS

Acoustic tumors are fibrous growths originating from the balance nerve and are not malignant. They do not spread to other parts of the brain, other than by direct extension. They constitute approximately 10% of all brain tumors. They are located between the brain and the inner ear, adjacent to vital brain centers. As they grow, they cause involvement of the surrounding nerves. If they are allowed to grow over a long period time, they press into the vital brain centers, and will eventually cause pressure on the brain and ultimately this could be fatal. In most cases, these tumors grow slowly. However, in other people the growth is quite rapid and multiple symptoms may develop. At times there may be bleeding into one of these tumors causing sudden new symptoms. The cause of most acoustic neuromas is unknown. There is a small group of patients who have acoustic tumors as a result of having neurofibromatosis Type II, which in genetically determined.

Acoustic tumors usually displace normal tissue as they grow, yet remain within their lining (encapsulated). An acoustic neuroma first distorts the eighth nerve, and then presses on the seventh (facial) nerve. The slowly enlarging tumor protrudes from the internal auditory canal and as mentioned will ultimately press into the brainstem. The tumor also may press on adjacent nerves, such as the fifth, or trigeminal nerve, which is the nerve providing facial sensation. Pressure on the brainstem, caused by tumor growth, can be life threatening. Many diagnostic procedures are used to be as certain as possible of an accurate diagnosis, and to determine the extent of the tumor growth. When treating these tumors, the preservation of life is the most important objective. When the tumors are small, the morality rate (i.e., loss of life) is less than 1%. When they are middle or large sized, the mortality rate is approximately 3%. The second objective of surgery is to preserve as many vital structures as possible. In many cases, a completely normal life results following surgery and in others, minimum or even maximum degrees of handicap may result.

The first symptom from the tumor is usually some disturbance in hearing, such as inability to hear on the telephone, fullness in the ear, and/or tinnitus. This is due to pressure on the cochlear portion of the eighth nerve but also due to involvement of the blood supply to the hearing structure. The cochlear nerve is more sensitive to pressure than most other cranial nerves. The ability to hear requires not only an intact nerve but adequate blood supply to the hearing structure. The blood supply to the cochlear nerve and to the cells receiving the sound in the ear comes from the internal auditory artery. This artery originates inside the head and passes through the internal auditory canal with the nerves, and therefore is involved with the tumor. Sudden deterioration or fluctuation in hearing may relate to pressure on this artery.

DIAGNOSIS OF ACOUSTIC NEUROMA

The diagnosis of an acoustic neuroma begins with a complete history, physical examination, and conventional audiologic evaluation. Following this, if there are any indications of tumor, an auditory brainstem response (ABR) test maybe done with a computerized audiometer. This uses a sophisticated computerized audiometer to analyze the electrical activity of the hearing nerves on both sides to determine if there is normal conduction of the signals to the brain. If the conduction is slowed down on one side, this may indicate a tumor. CAT scanning or Magnetic Resonance Imaging (MRI) scans are sophisticated imaging methods used to determine (by picture) the cerebellopontine angle region of the brain to determine if there is an acoustic neuroma present. At times, an electronystagmagram (test of the balance system) may be used to determine the involvement of the tumor with the balance nerves.

TREATMENT

At this time, the only treatment that can cure the patient with an acoustic neuroma is surgical removal. The main goal is the preservation of life, with a minimum of future physical disturbances. To accomplish this, a team consisting of a Neurotologist, Neurosurgeon, as well as an Internist, surgery as well as the pre and post operative care. The Neurotologist and the Neurosurgeon are co-surgeons during this surgical procedure.

Following surgery, the patient is usually admitted to the intensive care unit for 24-48 hours post operatively for close observation. After that period, the patient is transferred to the floor or to an intermediate unit. Patients are usually in the hospital for 7-10 days. Within the last several decades, microsurgical techniques have been pioneered and refined. With the use of an operating microscope and lasers, the Neurotologist and Neurosurgeon are able to remove the tumor with an extremely low mortality rate. Damage to the surrounding nerve tissue is markedly decreased. Routinely facial nerve function is monitored during surgery. In some situations the cochlear nerve is also monitored when it appears feasible to preserve hearing. The size of tumor, hearing status, patient’s age, and health determine which approach is utilized for removing the tumor, when surgery is contemplated. When the tumor is confined within the internal auditory canal, or just outside the canal, and there is useful hearing, the middle fossa or retrosigmoid approaches are generally preferred. Both approaches allow the possibility of hearing preservation. When the hearing is poor the translabyrinthine approach is preferred. The translabrythine approach is very effective for tumor removal and preservation of the facial nerve; however, there is no chance for hearing preservation. For larger tumors occasionally the retrosigmoid approach may be utilized, and occasionally a combined approach such as middle fossa and retrosigmoid approach have been utilized when one is trying to preserve hearing.

The treatment of vestibular schwanomas must be individualized to each patient by an experienced team of doctors. Microsurgical removal of these tumors remains the treatment of choice as determined by the National Institute of Health (NIH) Consensus Development Conference. Further research is needed on the advantages and risks of other management options, which include careful observation and stereotactic radiation.

The natural history of acoustic neuromas is not known. Each tumor has its own biology and growth characteristics. In general, acoustic neuromas are regarded to be slow growing averaging 2mm per year (range 1mm – 12mm/year). The critical question to be answered when observation is considered is whether the acoustic neuroma will cause any problems during the natural course of the patient’s remaining life span.

Another form of treatment which has recently developed is the use of
stereotactic radiation therapy (i.e., gamma knife). Stereotactic radiation
therapy is a technique based upon the principle that a single high dose of radiation delivered precisely to a small area of tumor could arrest the growth of the tumor and not damage surrounding brain tissue and/or function. In this technique, under a light general anesthesia, the head is secured in a special device to hold it in a proper position and a single dose of radiation is applied (stereotactically) by the therapist. The dose is calculated to injury/kill tumor cells yet minimize side effects or risks. There is no incision. Almost all patients are discharged from the hospital the same day or the following day. Life long follow up with MRI scans is currently recommended. Unlike microsurgery which would remove the tumor, stereotactic radiation therapy “controls” the tumor growth. The tumor does not disappear but persists, allegedly, in a “harmless” state. The risks of microsurgical removal are well defined, short term and consistently quantified in thousands of patients over 30 years. No such data exists for acoustic neuroma treatment by stereotactic radiation therapy. Short term risks of stereotatic radiation therapy appear comparable to surgery in many areas. The real risk of stereotatic radiation therapy may be ongoing, long term and, as yet, undetermined. Tumor regrowth has been seen in 6 – 24% of stereotactic radiation therapy patients. The long term growth control rates for stereotactic radiation therapy have not been established. Serious complications can also occur as a result of stereotactic radiation therapy. Cost for microsurgery includes the operation, approximately a seven day hospital stay, and a MRI scan at and three to five years. The patient is also cured; disease free. Cost for stereotactic radiation therapy includes the treatment and MRI scans are required at 6, 12, 24, 48, 96, etc. months as the tumor persists. There are medical costs associated with this follow up. When the tumor regrows, the substantial cost of salvage surgery must be factored. There is also evidence that suggest salvage surgery in a patient who has had prior stereotactic radiation therapy is much more difficult with increased risk to facial nerve preservation. Radiation therapy represents an alternative for patients who are elderly, or have medical problems such as cardiac or pulmonary disease and are at high risk for surgical removal.

SIZE OF TUMOR

Risks and complications of surgery vary with the size of the tumor. Larger tumors have more serious complications, and more likelihood of complications. In addition, the approach to remove the tumor may differ, depending upon the size the tumor, the patient’s medical status, age of the
patient, and the status of the hearing. The removal of an acoustic tumor is a major surgical procedure, entailing a craniotomy (opening into the skull), with possibilities of serious complications, even death, whether the tumor is large or small. Different approaches are used as indicated above to minimize the complications and, at the same time, remove the entire tumor. Tumors are classified as small, medium, or large canal extending from the inner ear toward the brain. Contained in this canal are the hearing and balance nerves, as well as the facial nerve, which enables the face to move on that side. In addition, there are numerous blood vessels which supply the inner ear.

The operation for acoustic tumors is performed under general anesthesia, using an operating microscopic. The surgical approach to a small tumor may be the middle fossa approach, through an incision in front of and above the ear; by the suboccipital approach, with an incision toward the back of the head; or a translabyrinthine approach may be used. The approach is determined by the amount and quality of hearing in each patient.


In the vast majority of cases of small tumors, the tumor is totally removed. Every effort is made to preserve the hearing in small tumors by one of the first two approaches, and still remove the tumor. However, it is imperative
that tumor removal take precedence over hearing preservation. In approximately 35% of the cases, serviceable hearing may be preserved. If
the tumor involves the hearing nerve, or an artery leading to the inner ear, total loss of hearing will result in the operated ear.

Medium Tumor

The treatment of vestibular schwanomas must be individualized to each patient by an experienced team of doctors. Microsurgical removal of these tumors remains the treatment of choice as determined by the National Institute of Health (NIH) Consensus Development Conference. Further research is needed on the advantages and risks of other management options, which include careful observation and stereotactic radiation.

In a medium tumor, this is an extension from the bony canal towards the brain, but there is not yet pressure on the brain itself.

Again, the surgery is performed under general anesthesia, utilizing an operating microscope and surgical team. A translabyrinthine approach is made through an incision behind the ear, over the mastoid bone. The mastoid and inner ear structures are removed to expose the tumor. The tumor is then totally removed, in the vast majority of cases. The mastoid bone defect is closed with fat taken from the abdomen. This approach sacrifices the hearing and balance nerves in the operated ear, and the patient is made permanently deaf on that side. This approach has been developed to provide maximum safety to the facial nerve, and allow complete removal of the tumor. The balance mechanism of the opposite ear will provide stability for the patient in approximately one to four months.

Large Tumor

The large acoustic tumor has extended out of the bony canal, and into the brain cavity, producing pressure on the brain and vital structures. The approach to a large acoustic tumor requires more extensive removal of the bone, to expose the tumor and control large vessels, which obstruct access to the tumor. Occasionally special vascular studies are required prior to surgery to help diagnose the location of larger vessels.

The surgical approach, the translabyrinthine-suboccipital approach, is done through an incision behind the ear. The mastoid bone, inner ear structures, and a portion of the skull are removed to expose the tumor. The tumor is then totally removed, unless vital sign changes prevent its removal from the brainstem. If there are changes in blood pressure, pulse rate, or respiratory rate, the surgery may have to be terminated before the tumor is totally removed. In this instance, it would be necessary to have a second operation to completely remove the tumor. The surgical defect is closed with fat taken from the abdomen. The translabyrinthine-suboccipital approach causes the patient to be permanently deaf in the operated ear. The balance mechanism is removed, but the balance mechanism of the opposite ear will provide stabilization of the patient in one to four months.

RISKS AND COMPLICATIONS OF ACOUSTIC TUMOR SURGERY

It is not possible to list every complication that may occur before, during, and following a surgical procedure. The following discussion is included to indicate some of the risks and complications peculiar to cerebellopontine angle tumor surgery. In general, the smaller the tumor at the time of surgery, the less risk of complications. However, all these operations are major and may have significant complications.

Hearing Loss

Occasionally in small tumors it is possible to remove the tumor and preserve hearing: however, in the vast majority, it is not. There must be good hearing and the tumor must be located in a precise position to allow hearing preservation. In the majority of the time, total hearing loss results in the operated ear from the surgery. Fortunately, the development of the CROS hearing aid has allowed the transfer of sound from the operated ear to the other ear, so that one may hear for social or business purposes from the operated side.

Ear Noises

Ear noise (tinnitus) usually remains the same after surgery as it did before surgery. In approximately 30% of patients the tinnitus may be less, but in 10% it may be more noticeable.

Taste Disturbance and Mouth Dryness

In approximately 5% of the patients, this disturbance may be prolonged. But in most, it lasts for a few weeks and then disappears.

Dizziness and Balance Disturbance

In the vast majority of cases of acoustic tumors, it is necessary to remove the balance nerve to remove the tumor. However, in many cases the balance nerve has been damaged by the tumor prior to surgery and is causing unsteadiness. In many of these patients, there is an improvement in the preoperative unsteadiness. Nevertheless, in most patients there is some temporary dizziness following surgery, which may be severe for days to a few weeks. Imbalance or unsteadiness on head motion is prolonged in 30% of the patients. Some patients notice unsteadiness when fatigued for several years. There are programs to enable the patient to overcome the dizziness and imbalance. In a few cases, the blood supply to the portion of the brain responsible for coordination (the cerebellum or brainstem) is decreased by the tumor or removal of the tumor. Difficulty in coordination and balance may, therefore, last in these patients for years.

Although acoustic tumors do not arise from the facial nerve, they are in intimate contact with it. This nerve controls the movement of the muscles to the face, including those that close the eye. It is very common to have a temporary paralysis of the muscles of the face following the removal of acoustic tumors. This weakness may persist for six to twelve months and occasionally, there may be permanent residual weakness or paralysis.

A facial paralysis may result from nerve swelling, or nerve damage. If it is merely nerve swelling, it may return in a short time (three weeks to three months). Swelling of the nerve is common due to the fact that the nerve is compressed and distorted by the tumor in the internal auditory canal. Tumor removal with the use of the operating microscope usually results in preservation of the nerve, but nerve stretching may result in swelling of the nerve, with subsequent temporary paralysis. Facial function is observed for approximately one year following surgery. If it becomes certain the facial function will not recover (approximately 15% of the cases), a second operation may be performed to connect the facial nerve to a nerve in the neck (facial-hypoglossal anastomosis).

Eye Complications

The major medical problem with facial paralysis following this surgery is that the eye may become dry and unprotected. It can then become infected or abraded. Care by an eye specialist (ophthalmogist) may be necessary. It may be necessary to use ophthalmic (eye) drops, or apply ointment to the eye frequently, insert a gold weight beneath skin in the upper eye lid to close the eye lids, or even to partially sew the eyelids closed. The purpose of these efforts is to keep the eye moistened, as well as provide comfort, and improve the appearance.

Other Nerve Weaknesses

Acoustic tumors may become involved with other nerves besides the hearing and balance nerves, and the facial nerve. They may be in contact with nerves which supply the eye muscles, mouth, neck and throat. These nerves may be injured with resultant double vision, numbness of the throat, and tongue, weakness of the shoulder, weakness of the voice and difficulty swallowing. In some cases these problems are permanent.

Brain Complications and Death

These tumors, as they grow, press on vital brain centers which control breathing, blood vessel and heart function. As the tumor enlarges it may begin to receive blood supply from the brain centers. Usually careful dissection under the microscope avoids complications. However, the blood supply to the vital brain centers may be disturbed in the removal of the tumor. If this occurs, serious complications result, including loss of muscle control, stroke, paralysis, or death. In our experience, death occurs rarely as the result of small tumors, less than 3% of medium sized tumors, and in about 5% of the large tumors.

Postoperative Spinal Fluid Leak

In all cases of acoustic tumor surgery, there is a temporary leak of cerebrospinal fluid (fluid surrounding the brain). This leak is closed prior to completion of surgery with fat from the abdomen. However, approximately 5% to 10% of the time this leak reopens and further surgery may be necessary to close it. Most of the time, this leak can be closed by placing a drain in the cerebrospinal fluid space through the back. While this drain is in place the patient has to remain in bed, but in the vast majority of cases this stops the leak without further surgery.

Postoperative Bleeding and Brain Swelling

Brain swelling and bleeding may develop after tumor surgery. If this occur, a subsequent operation may be necessary to open the wound, stop the bleeding, and allow the brain to expand. If the brain swelling is severe, it often results in paralysis or death.

Postoperative Infection

Postoperative meningitis occurs in approximately 3% of the patients. Meningitis is an infection of the fluid and covering surrounding the brain. In a very small percentage of cases, an infection may involve only the external portion of the wound. When these complications occur, hospitalization is prolonged, and treatment with high doses of intravenous antibiotics is indicated. Occasionally, these antibiotics may cause an allergic reaction, suppress the body’s blood forming tissues, or may produce hearing loss in the good ear. Fortunately, antibiotic complications are rare.

Transfusion Reaction

Rarely, it is necessary to administer a blood transfusion during surgery. Adverse reactions to transfusions are rare. An occasional late complication is a viral infection of the liver (hepatitis). An even more rare complication is AIDS. Recent innovations in blood banking, however, have made this an extremely rare complication. Blood transfusions, currently, are not given unless there is bleeding of over two units of blood. It is possible and advisable to donate blood for yourself or have suitable family members donate blood for you. This must be done in advance of surgery by at least two weeks.

PARTIAL VS. TOTAL REMOVAL OF ACOUSTIC TUMORS

The goal of acoustic tumor surgery is to remove the entire tumor and preserve as much function as possible. Partial tumor removal may be necessary if the patient’s response to surgery indicates disturbances of any vital brain centers, such as respiration, blood pressure, or heart function. Most of the time, if there is a disturbance in the brain center and surgery is stopped, function can be restored. Occasionally, however, once the vital brain centers are disturbed, they do not recover.
If termination of the operation is necessary without removal of the entire tumor, the remaining portion of the tumor may gradually enlarge to again
produce symptoms. A subsequent operation can often then be accomplished without a significant change in vital signs, as there is a change in the relationship of the tumor to the brain.

If the tumor is partially removed, you will be so informed. As mentioned above, reduction of the size of the tumor often allows it to separate from the vital brain centers, and it can be removed at a later date. In most cases a wait of two weeks to four months is indicated, depending upon the circumstances. In other cases, the course of continued observation is determined best for the patients. If this is necessary, x-ray imaging techniques will be used to evaluate the tumor from time to time for possible regrowth.

CONCLUDING REMARKS

There is no known “cure” for an acoustic neuroma, except surgical removal, at the present time. The earlier tumors are diagnosed and removed, the less likely the possibility of serious complications.

It is important to face the problem of an acoustic neuroma as it exists at the time of diagnosis, and accept whatever risks are necessary to remove the tumor. Maintaining a positive attitude before, during, and after the surgery is necessary for all members of the team, as well as the patient, for the best results.

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