More About Acoustic Neuromas

What causes a schwann cell to become a tumor?

The answer to this question remains unclear. There appear to be two regions within the genetic makeup of a schwann cell termed tumor suppressor genes. The function of these genes is to prevent the formation of tumors. Only one properly functioning gene is needed to prevent a tumor from occuring. A change or a mutation has to occur in the same schwann cell twice thereby affecting both tumor suppressor genes before a tumor can form.

Most acoustic neuromas arise from these aforementioned mutations which are acquired after birth. As such they are almost always unilateral and can not be transmitted from one generation to the next.The chance of having a tumor develop in the opposite ear is almost unheard of.

There is a condition which can be transmitted from generation to generation known as Type II neurofibromatosis where individuals are born with one defective tumor suppressor gene. These patients need only one mutation in any schwann cell during their lifetime to cause development of a tumor. At the time of detection, these patients are usually found to have bilateral acoustic neuromas. In addition these patients can have schwanomas of other cranial nerves.

How common are these tumors?

According to a Danish study, the frequency with which these tumors occur is 10-13 cases per million general population. Accurate data on prevalence and incidence in the United States is not available.

What are the symptoms that indicate the possibility of a tumor?

Hearing loss or tinnitus (ringing in the ears) especially if greater in one ear as compared to the other may be the first signs of the presence of a tumor. Mild dysequilibrium can occur as more of the vestibular nerve is destroyed. Numbness or tingling in the face may indicate that a larger tumor is pressing on the trigeminal nerve, the nerve that conveys facial sensation not motion.As the tumors get larger, they can press on the cerebellum and pons to cause more severe dysequilibrium.With even larger tumors spinal fluid pathways within the brain can become obstructed and cause hydrocephalus. Nausea, vomiting and headache are common with this condition. Progressive growth can lead to a comatose state and eventual death.

What is the growth rate of these tumors?

These tumors vary in regards to growth rate. The growth rate can vary from 0.1 mm to 10.0 mm per year. Growth does not necessarily follow a linear curve. Some tumors may remain relatively quiescent for some years and be followed rapid growth thereafter. Growth remains unpredictable.

What are the management options?

Acoustic neuromas can be managed in a number of different ways. Each method has its own inherent risks and benefits. Age, size of the tumor, history of growth, status of hearing and balance function and associated medical problems are all important factors in determining the method of management.

Some of these tumors can be followed with imaging done every six months to determine the growth rate of the tumor. It must be remembered that this growth rate may not truly follow a linear curve. A long period of no growth can be followed by rapid growth over a short period of time. The benefit of this approach is obviously the fact that surgery may not be needed right away. The risk is that over time you will probably lose hearing in the involved ear over time. Additional growth of the tumor may make it more difficult to surgically dissect the tumor from the facial nerve or brainstem.

Another approach to management involves radiation therapy, namely a way of delivering the radiation called the gamma knife or stereotactic radiosurgery. The latter two names are really misnomers because the radiation treatment involves no surgery. High dose radiation is delivered to the core of the tumor in an effort to slow down growth. Unlike radiation for malignant tumors elsewhere which involves a usual six week course, the stereotactic radiation treatment for acoustic neuromas is a one day treatment. The benefit of this treatment is that surgery is not required. The downside of this treatment is that if the tumor begins to grow after having had radiation, surgery may be required to excise the tumor. The complications from surgery are greater if prior radiation treatment has been rendered. It is surgically more difficult to dissect the tumor from the facial nerve or brainstem after having had radiation treatment. Another risk is the 15 % chance of facial paralysis with treatment with radiation alone. In general radiation does not cure the tumor. Not all tumors of all sizes are candidates for radiation treatment. We offer radiation treatment to patients with small tumors who can not undergo surgery because of other medical problems or to those patients with small tumors and advanced age.

Microscopic surgery offers the best chance for complete cure. Acoustic neuroma surgery is done under general anesthesia. Typically a patient will spend the first night after surgery in an intensive care unit and be discharged from the hospital two to five days following surgery depending on the postoperative course. Generally small and medium-sized tumors can be removed with one operation. Larger tumors may require two or more operations.

In surgery to preserve hearing, the hearing can be monitored during the course of surgery using auditory brainstem response monitoring. An auditory brainstem response is a special hearing test done while a patient is hearing clicks or discrete tone bursts. Electrodes around the scalp detect waves which are generated from all parts of the brain. The waveforms are filtered so that signals generated by each click or tone are amplified while all other extraneous signals are computer averaged or phased out. This produces a tracing showing five discrete waveforms, each of which corresponds to a specific part of the auditory pathway from the inner ear to the brainstem. Normally waves I through V occur one to five milliseconds after the click or tone is heard, respectively. The presence of an acoustic neuroma may slow the transmission of the click or tone so that these waves occur at much later times. The neuroma may also dampen the amplitude of the waveform to the point where it may completely disappear. During tumor dissection, the presence and timing of the waveforms is what is monitored. Generally if the waveforms are lost during surgery, the hearing has also been lost.

Just like hearing, the status of facial nerve functioning can also be monitored. While under general anesthesia recording electrodes are placed in the facial muscles. These electrodes detect contractions of the facial muscles, and the signals are transmitted to a speaker so the surgeon may hear the responses. Contractions can be generated by tumor dissection from the facial nerve. Facial nerve monitoring is also helpful in preventing facial nerve injury from manipulation of the tumor.

An operation to excise an acoustic neuroma involves three phases, an initial approach to allow visualization of the tumor, followed by tumor removal and then closure of the wound. There are three approaches or avenues around or through the inner ear that allow adequate visualization for tumor removal. Each has its own benefits, risks and consequences. They are the translabyrinthine approach, the suboccipital approach and the middle fossa approach.

In order to understand the surgery, one must comprehend some basic anatomy. Under the scalp is the bony skull. Under the bony skull is a tough fibrous layer of connective tissue called the dura mater, or dura for short. Within the dura is the brain which floats in a fluid medium called spinal fluid. Spinal fluid bathes the brain and circulates within the brain functioning to protect the brain from trauma. The spinal fluid is also responsible for providing nourishment for the brain.

The translabyrinthine approach is an approach which involves an incision behind the ear and a craniectomy, i.e. permanent removal of bone, of the mastoid bone and the semicircular canals of the inner ear. A consequence of this approach is the complete loss of hearing on the operated side. The major benefit of this approach is that the brain does not need to be retracted or pushed aside. Tumors of all sizes can be visualized and removed using this approach. At the end of the operation, fat is harvested from the abdomen and used to replace the bone which was removed. This helps prevent the possibility of spinal fluid leakage.

The suboccipital approach involves an incision further behind the ear and either a craniotomy or craniectomy over the cerebellar portion of the brain. The dura of the posterior fossa is then opened allowing egress of spinal fluid. As spinal fluid is evacuated, the cerebellum relaxes away from the tumor. A retractor is used to depress the cerebellum to allow adequate visualization of the tumor. The posterior aspect of the internal auditory canal is usually drilled to allow exposure of tumor within the canal. At the end of the procedure the dura is sewn shut to prevent spinal fluid leakage. The advantage of this approach is that it allows for the possibility of hearing preservation. The disadvantage of this approach is that the cerebellum has to be retracted to allow adequate visualization of the tumor. Also tumor in the lateral most aspect of the internal auditory canal cannot be directly visualized because a portion of the inner ear is in the way. Another disadvantage of this approach is that some patients can have prolonged headaches. This is thought to occur from the bone dust that enters the spinal fluid space. Tumors of all sizes can be removed using this approach.

The middle fossa approach involves an incision above the ear and a craniotomy over the temporal lobe of the brain. Like the suboccipital approach, this approach also allows the possibility of hearing preservation. The dura of the internal auditory canal is opened and the tumor is removed. This approach is commonly used for small tumors within the internal auditory canal. The advantage of this approach is that it allows complete visualization of the entire internal auditory canal.

The technique of tumor removal is the same in all three approaches. The central portion of the tumor is first removed either mechanically with instruments or with a special tool called a CUSA. The CUSA is an instrument whose tip vibrates at a very rapid rate. The tumor cells within the core of the tumor are vibrated loose. A cavity is created as the tumor cells are constantly irrigated and suctioned away. The capsule of the tumor is then dissected from surrounding structures like the facial nerve, cochlear nerve and brainstem. In most cases both the superior and inferior vestibular nerves are removed with tumor.

Which approach is used for which tumor?

Guidelines for which approach is used for each tumor vary according to the size of the tumor, the status of the hearing present and the degree of experience on the part of the surgeons. We prefer to use the middle fossa approach with small tumors within the internal auditory canal when the patient has good hearing. The translabyrinthine approach is preferred when the patient has no useful hearing. When tumors become larger than 2.0 cm., the chances of preserving hearing are almost nonexistent. Either the translabyrinthine or suboccipital approaches can be used for these tumors.

What are the consequences of losing hearing in one ear?

When you are unable to hear from one ear, the ability to localize sounds or tell where a sound is coming from is lost. It is the minute time difference between when a sound enters one ear and the other that allows the brain to perceive the direction from where the sound is originating.

There are certain types of hearing aids called CROS and BICROS aids which are helpful when hearing is lost in one ear. These hearing aids take sound that normally would have entered the non-hearing ear and route the signal to the hearing ear. It involves wearing at least an earmold in both ears.

What are the chances of saving hearing?

In the best of circumstances when there is good hearing present and good waveforms present on the auditory brainstem response and there is a small tumor involving the internal auditory canal, the chances of preserving any hearing is about 40%. In all other situations, the chances of hearing preservation are lower.

What happens to tinnitus?

It is attractive to believe that an acoustic neuroma involving the vestibular nerves which is affecting the cochlear nerve might be causing the tinnitus that many patients experience as an initial symptom. After an acoustic neuroma is removed and even when the cochlear nerve has been removed, tinnitus may persist. We now believe that the presence of an acoustic neuroma may permanently sensitize nerve cells or neurons along the auditory pathway within the brainstem. As yet there has been no treatment that is uniformly effective in the treatment of postoperative tinnitus.

What will happen to the balance function?

When an acoustic neuroma is excised, the superior and inferior vestibular nerves are usually removed. The balance information which was being transmitted from the inner ear to the brainstem comes to an immediate stop. As a consequence it is not unusual after surgery for patients to experience some dysequilibrium.

The degree of dysequilibrium experienced depends on how much of the balance information was reaching the brainstem prior to surgery. If the tumor had destroyed the vestibular nerves so that no balance information was reaching the brainstem, then the brain will have already compensated for the lack of balance information from that inner ear. In this case, the patient will not experience any dysequilibrium after the operation. If the tumor only partially destroyed the vestibular nerves so that some information was still reaching the brainstem, then the brain will have only partially compensated. In this case, the patient will experience some dysequilibrium after the operation.

Following surgery, the brain needs time to compensate for and adapt to the lack of balance information it is getting from the operated ear. The time this adaptation takes is variable and generally unpredictable. It is generally felt that older patients take longer to compensate than their younger counterparts. Patients experience the most dysequilibrium immediately after the operation. Usually 2-3 days after the operation they are able to walk with some assistance. Approximately 1-2 weeks after the operation, head motions may trigger transient dysequilibrium. Several months after the operation, only the most sudden head motion may lead to momentary dysequilibrium. About six months after surgery, this momentary disturbance usually resolves.

Facial nerve function following acoustic neuroma surgery

As mentioned previously, acoustic neuromas are benign tumors that arise from the vestibular nerves that may secondarily invade the cochlear nerve and impair hearing. These tumors usually push the facial nerve aside.

In most instances the tumor can usually be dissected off the facial nerve. In these cases some facial weakness seen soon after surgery will usually return to normal with time. The degree of facial nerve dysfunction after surgery can not be reliably predicted. Generally the larger the tumor, the greater the chance of facial nerve dysfunction after surgery. On occasion, the tumor can be found to invade the facial nerve. If this is found, our preference has been to leave a small fragment of tumor on the facial nerve. We have not seen growth of these small fragments left on the nerve over time. Perhaps this is because most of the blood supply to the tumor has been lost.

On rare occasions with large tumors, the facial nerve can become disrupted during the course of tumor dissection. The facial nerve can be repaired at the time of surgery, in which case facial nerve recovery would begin 18 months following surgery. Another option is to divert some of the nerve fibers headed for the tongue toward the facial nerve in a separate operation.

It is very important to prevent dryness of the eye. With facial nerve weakness, it might be difficult to completely close the eye leaving the cornea exposed. A feeling as though there is a particle of sand in the eye or redness of the conjunctiva indicates that the cornea is drying. Frequent use of artificial tear drops is a necessity. Lacrilube ointment can be used at nighttime. Occasionally it becomes necessary to implant a small gold weight in the upper eyelid. The weight is heavy enough to allow the eye to close completely but light enough to be able to open the eye and have useful vision. The gold weight can always be removed later once facial nerve recovery occurs.

The role of facial nerve rehabilitation involving exercises and electrical stimulation is controversial. The true efficacy has yet to be determined.

Spinal fluid leakage following surgery

An uncommon complication of acoustic neuroma surgery is leakage of spinal fluid. In all three approaches, spinal fluid can leak through the wound. This can be usually managed by placing another stitch under local anesthesia directly at the site of leakage. Spinal fluid may also enter the middle ear and drain down the eustachian tube and out the nose. When this occurs, a pressure dressing with a lumbar drain is used for several days. The lumbar drain is placed in the back similar to a spinal tap, however here a small plastic tube is left in place. Spinal fluid is drawn out periodically over several days and then the drain is removed. This causes lowered pressure in the spinal fluid space which allows time for healing at the operative site. On rare occasion, exploration of the wound may become necessary to close the site of leakage.

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