What Is An Acoustic Neuroma?

     An acoustic neuroma is a benign (non-cancerous) tumor that originates from the nerves of hearing (cochlear nerve) and balance (vestibular nerve). There are twelve cranial nerves that exit each side of the brainstem and serve various functions. The nerve of hearing and balance together form the eighth cranial (cochleo-vestibular) nerve. The acoustic nerve lies deep within the skull base and connects the inner ear to the brainstem. This area of the brain also controls important heart rate, blood pressure and respiratory (breathing) functions. The hearing and balance portions of this nerve are intertwined as they exit the brainstem, then split into their respective divisions as they enter the inner ear through a bony channel termed the internal auditory canal. This canal varies in length from approximately 1/4 to 3/4 of an inch. Acoustic neuromas usually arise from the outer sheath or layer of the hearing and balance nerve within this canal. Two other important structures within this canal include the seventh cranial (facial) nerve which controls facial movement and the blood supply to the inner ear (Figure 1). There is no known cause for acoustic neuromas in the vast majority of patients. Rarely, they may be due to an inherited disorder known as type II neurofibromatosis. In these patients, the tumors are generally in both ears (bilateral) and tend to occur earlier in life.

How Common Are Acoustic Neuromas?

Acoustic neuromas comprise approximately 6% of all intracranial tumors (tumors which arise inside the skull). This incidence has remained relatively consistent for many decades. Large studies in Europe and the United States report the occurrence of those tumors which cause symptoms at approximately 10 persons per one million population per year. With a U.S. population of 250 million, this translates to approximately 2500 new cases diagnosed per year in this country. If people who harbor an asymptomatic tumor are also considered, the incidence of acoustic neuroma in the general population is still well under 1%. A slightly increased occurrence in females has been noted in some studies. They tend to begin in mid-life and the average age at presentation has been shown to be 50 years in recent studies.

What Is The Growth Pattern Of Acoustic Neuromas? As noted above, these tumors generally start from the covering of the eighth cranial nerve within the internal auditory canal (Figure 2). As they expand, erosion of bone around the internal auditory canal is common. However, most of the growth is out of the canal into the intracranial cavity (Figure 3). This space between the inner ear and brain is known as the cerebellopontine angle. As it grows out of the inner ear, the tumor assumes a pear-shaped appearance.Further growth leads to compression of the brainstem and possible obstruction of the pathways for spinal fluid circulation (Figure 4). Although acoustic neuromas grow at a variable rate, they are generally considered slow-growing tumors. Recent studies, using sophisticated radiologic imaging (MRI), have shown these tumors commonly grow at a rate of less than 1/10 of an inch per year. However, approximately 10-15% may grow rapidly, up to 1/2 inch per year. As the tumor grows, it causes a pressure effect on surrounding structures which explains the progression of symptoms.

What Are The Symptoms Of An Acoustic Neuroma?

While it may be possible to describe the "typical" symptoms of an acoustic neuroma, there is a wide variation in the severity of these findings. Because of this, any person with complaints of inner ear problems, especially involving only one ear, should undergo a thorough evaluation to rule out an acoustic neuroma.

Hearing Loss -- This is the most frequent and usually earliest symptom in patients with an acoustic neuroma. Found in over 95% of patients with an acoustic neuroma, this symptom is due to pressure on the hearing portion of the eighth cranial nerve fibers or blockage of the blood supply to the inner ear. The hearing loss occurs on the side of the tumor. It initially involves the higher frequencies or higher-pitched sounds. The loss generally occurs gradually over a number of years, eventually leading to total deafness in the involved ear. However, rapid loss of hearing can occur, and has been reported in up to 20% of patients. This is thought due to sudden blockage of the blood supply to the inner ear. At the same time, a small percentage of patients (probably 5-15%) present with normal or near-normal hearing in the involved ear, and are evaluated because of other symptoms.

Tinnitus -- Tinnitus is the medical term for ear noise or ringing, and is a common symptom of acoustic neuroma. It is usually high-pitched, and occurs on the side of the tumor. In the vast majority of cases, it is accompanied by hearing loss.

Dizziness/Imbalance -- Before a discussion of this symptom, it is necessary to differentiate between the different types of dizziness. Vertigo refers to a sensation that the environment is moving or spinning. This is often made worse by a change in head position. Vertigo usually occurs in episodes. Disequilibrium, on the other hand, refers to complaints of imbalance or unsteadiness, especially with walking. This symptom is generally more continuous.

   True vertigo is not a common symptom of acoustic neuroma, occurring in probably less that 20% of patients. It is associated with smaller tumors. While rare with large tumors, patients may relate a history of this symptom some years before the diagnosis of their tumor. Disequilibrium is more common than vertigo, occurring in nearly half of all patients. Over 70% of patients with large tumors (greater than 1.5 inches in size) experience disequilibrium.

Facial Numbness and Pain -- Facial numbness is rare in small tumors, but occurs in approximately half of patients with tumors larger than 1 inch. It usually starts in the cheek region, then progressively spreads to the upper and lower parts of the face. The pain experienced by some patients is generally of a sharp, stabbing nature that is intermittent. These facial symptoms are due to pressure and irritation of the fifth cranial nerve (the nerve which provides feelings of touch and sensation to the face and eyes). This nerve is separate and distinct from the seventh cranial (facial) nerve which provides movement to the facial muscles (rather than feeling or touch sensations).

Facial Weakness or Spasms -- The seventh cranial (facial) nerve travels through the internal auditory canal and is closely associated with acoustic tumors. It is very resistant to the pressure effects of a slowly expanding acoustic neuroma. Facial weakness or paralysis from the effects of the tumor are very uncommon. The nerve may be quite compressed or stretched by the tumor and still exhibit normal facial movement. However, involuntary facial twitching or spasms occur in approximately 5% of patients.

Headache -- This symptom is very dependent on the size of the tumor. In small tumors (less than 1/2 inch), headache due to acoustic neuroma is quite rare. In medium-sized tumors (between 1/2 to 1-1/2 inches) approximately 20% of patients experience headaches, while for large tumors (greater than 1^1/2 inches) this figure is 40%.

Late Symptoms -- Extremely large tumors lead to obstruction of the outflow pathways for cerebrospinal fluid (termed hydrocephalus). This leads to increased pressure within the head which in turn can cause visual problems. Difficulty with hoarseness, swallowing, choking on liquids or food and shoulder weakness can occur from compression of the tenth, eleventh, and twelfth cranial nerves. Again, these symptoms are due to nerve compression from extremely large tumors.

What Are The Diagnostic Tests For The Identification Of Acoustic Neuroma?

Hearing Evaluation -- As hearing loss is the initial symptom in the majority of patients with acoustic neuroma, a complete hearing evaluation (audiogram) in a sound-proof booth is usually the first test obtained to screen for these tumors. As already mentioned, the hearing loss is one-sided or greater on the side of the tumor and generally affects the higher frequencies first. Speech discrimination scores, which assess the ability to understand the spoken word ("I can hear you speak, but I can't understand what was said"), are usually depressed also. An abnormality of any significant degree in one ear should prompt further investigation.

Brainstem Evoked Response (BER) Test -- The brainstem evoked response (BER) test can be considered a "specialized" hearing test used to specifically screen for the presence of an acoustic neuroma. A rapid click or tone sound is introduced into the ear and the electrical activity, or brain waves, are recorded from the ear through the brainstem. This electrical response is often slowed on the side of the tumor. While this test is abnormal in 95% of larger tumors, it may miss 20-30% of small tumors (less than 1/2 to 3/4 of an inch). An abnormality in this test on the side of a hearing loss should always prompt a radiologic x-ray study of the head.

CT Scan (Computed Tomography) -- A CT scan (computed tomography) with contrast dye injection was the study of choice throughout the 1970's and early 1980's. A tumor slightly over 1/2 inch in size or larger can be reliably detected by CT, and newer generation scanners have improved in their ability to pick up smaller tumors. However, they can still miss a small tumor or one confined strictly to the internal auditory canal. In addition, a generalized CT of the head without detailed attention to the area of the inner ear or one without contrast will commonly miss small and even medium-sized tumors.

MRI (Magnetic Resonance Imaging) -- This has largely replaced CT as the study of choice since the mid-1980's. When used with contrast dye injection (gadolinium), this radiologic study can detect a tumor as small as 1/10 of an inch in size.

What Are The Treatment Options For Acoustic Neuroma?

These consist of observation, microsurgical removal (partial or complete) and radiation therapy. Which option is recommended is based on a number of factors, including age and general health of the patient, size and exact location of the tumor, degree of residual hearing in the involved ear and impending complications or symptoms.

Observation
Acoustic neuromas may be incidentally discovered on an MRI scan done to evaluate another problem that cannot be attributed to the tumor. Since these tumors are benign and produce symptoms by a pressure effect on surrounding structures, observation may be appropriate in a select group of patients. This is usually seen in the clinical setting of an elderly patient with a small tumor. If there is a good chance the tumor will not need treatment during the patient's life expectancy, it may be followed initially with yearly MRI scans. If no growth is documented, and symptoms do not occur or progress, continued observation may be prudent. However, if tumor growth occurs, treatment may become necessary.
   
     The patient with a tumor in an only hearing ear, or better hearing ear, also presents a dilemma. Allowing the patient to benefit from residual hearing as long as possible in this setting is desirable, but must be weighed against possible complications of continued growth. This may be especially true if the tumor is large enough that hearing preservation from removal is unlikely. Once hearing is lost due to tumor growth, active treatment is often recommended.

Microsurgical Removal
     Significant advancements have been made in the last 30 years for microsurgical excision of acoustic neuromas. These include refinement in the surgical approach and instrumentation used for their removal. The operating microscope is universally employed to aid magnification of this complex area of the skull. The incidence of injury to surrounding critical structures has diminished, and the mortality (death) rate from acoustic neuroma surgery is extremely low. Facial nerve function is routinely monitored intra-operatively, which has decreased the frequency and severity of injury to this structure. In appropriate cases, hearing is also monitored in an attempt to preserve function. The operation is performed by a surgical team consisting of a neuro-otologist (ear, nose, and throat physician with advanced training in complex ear disorders) and a neurosurgeon. At the present time, the only treatment that can cure these tumors (meaning complete removal) is microsurgical removal. Total tumor removal can be accomplished by one of several surgical approaches. Which one is recommended depends on the exact location and size of the tumor along with whether an attempt to preserve residual hearing is warranted.

	Translabyrinthine Removal -- In this approach, the bone directly behind the ear (mastoid bone) is removed to expose the inner ear structures which are also removed. The internal auditory canal is exposed along its entire length and is then opened to remove the tumor. By removing the inner ear, all residual hearing is lost and thus this approach is used when the hearing loss is already severe or if hearing preservation is not a realistic goal. (Figure 5).
	Retrosigmoid Removal -- In this approach, bone behind the mastoid and inner ear is removed to expose the tumor and the posterior aspect of the internal auditory canal. The back wall of the canal is removed to allow access to this portion of the tumor. This method can allow for hearing preservation in certain cases and may be used for small and large tumors (Figure 6).
	Middle Fossa Removal -- In this approach, bone is removed above the ear to expose the top of the inner ear and internal auditory canal. Following removal of the roof of the canal, access to the tumor is gained. This method is reserved for small tumors with good residual hearing that is worth trying to preserve (Figure 7).

     Partial or incomplete removal of an acoustic neuroma may be necessary under certain circumstances with the understanding that further surgery may be needed in the future. This decision may be made during surgery if disruption of heart rate, blood pressure or other vital functions is caused by manipulation or attempt at tumor removal near important brain centers. Partial removal may also be advocated for large tumors causing neurologic deterioration in the elderly. Prevention of further deterioration is the goal of surgery and the residual tumor may not regrow significantly enough during the patient's anticipated lifetime to cause further problems. Detection of future growth is accomplished with follow-up MRI scans. Partial removal may also be necessary in patients who have a tumor in the only hearing, or better hearing, ear. This is done in an attempt to preserve residual hearing for as long as possible, while at the same time reducing the size of the tumor to prevent other complications. However, even partial tumor removal may result in further hearing loss.

Stereotactic Radiation Therapy
     Stereotactic radiation (commonly called radiosurgery), consists of a large dose of radiation delivered to the tumor in a single session. The radiation dose is delivered precisely to the tumor in an attempt to arrest growth while minimizing injury to the surrounding nerves or brain. Two sources of stereotactic radiation exist. One consists of radioactive cobalt (known as gamma ray or gamma knife) and the other source is a linear accelerator (known as LINAC). Either can deliver a precise dose of the radiation to the tumor. When felt necessary, our neurosurgical team can offer linear accelerator stereotactic radiosurgery for selected cases.
   
     Radiation therapy does not kill or injure tumor cells immediately following treatment. While some cells are destroyed in a matter of weeks, others die gradually over a period extending up to 18 months after treatment. With radiation therapy, the goal of treatment is to arrest the growth or shrink the tumor, not cause complete disappearance. Combining the results of the major published series, approximately 50-60% of tumors are reduced in size, 30-35% remain stable and 10-15% increase. However, it must be kept in mind that acoustic neuromas are generally slow-growing tumors and may show no detectable growth over several years time even without treatment. A tumor which continues to grow after radiation is frequently more difficult to surgically remove (secondary to scar formation) than a tumor not previously radiated.

   Radiation therapy for acoustic neuromas was first performed in Sweden in the late 1960's. Initially, it was restricted to patients considered a poor risk for surgical intervention or for advanced age. Some facilities now offer this form of treatment to all patients who are unable or unwilling to undergo surgery. However, long-term follow-up from multiple centers is not available at the present time to fully assess the risks/benefits of this form of treatment. The 1991 National Institutes of Health Consensus Development Conference on Acoustic Neuromas, consisting of a pool of internationally recognized experts, stated "For the majority of symptomatic patients with vestibular schwannoma (another term for acoustic neuroma), surgery remains the treatment of choice."

What Is The Expected Post-Treatment Course?

Patients undergoing surgical removal of their tumor are generally hospitalized for five to seven days, assuming no problems arise. A stay of 24 to 48 hours in the intensive care unit immediately following surgery is routine. Patients are generally advised to remain off work for a minimum of four to six weeks and possibly longer depending on the occupation. (With stereotactic radiation, patients often return home several hours after treatment and may return to work in several days.) Fatigue, difficulty concentrating and mild pain at the surgical site are common. Immediate post-operative pain is usually controlled with medication. Specific residual problems following surgery consist of the following:

Hearing Loss -- In patients with small tumors less than 1/2 to 3/4 inch in size, preservation of residual hearing may be attempted. Intra-operative monitoring of cochlear nerve function is often performed in order to minimize trauma to this delicate structure during tumor removal. Monitoring during surgery is usually performed with auditory brainstem evoked response (BER) testing or direct cochlear nerve electrodes. However, in medium-sized and large tumors (greater than 3/4 of an inch) hearing may have already been rendered useless on the side of the tumor. In the situation of a large tumor with good levels of hearing, less than 25% of patients are able to retain useful hearing. Due to hearing loss in the ear with the tumor, patients may have difficulty localizing the source of a sound (even though they hear it in the good ear) and may experience difficulty understanding conversations in the presence of background noise. A CROS-type hearing aid, which picks up sound on the side of the poor hearing and transfers it to the hearing ear may be of benefit for some patients. It must be emphasized that surgery has no effect on the hearing of the opposite ear.

Tinnitus -- In the vast majority of patients this symptom continues following surgery, but may increase, at least in the immediate post-operative period. In only a small number of patients is tinnitus eliminated following surgery, however some patients note that it is less in comparison to before tumor removal.

Facial Weakness or Paralysis -- All major centers performing acoustic neuroma surgery today employ intra-operative facial nerve monitoring. This important technological advancement has greatly reduced the frequency and severity of facial nerve injury secondary to tumor dissection. In only a small percentage of cases (less than 5%) is actual excision of this nerve necessary for adequate tumor removal. However, partial injury to the nerve or swelling may lead to temporary or permanent problems. As could be expected, the smaller the tumor, the greater the chance of preserving facial nerve function. With small tumors less than about 3/4 inch in size, there is generally an 85-90% chance of long-term normal or near-normal facial function. Patients with larger tumors often have some degree of at least temporary facial weakness following surgery. However, the degree to which the tumor is adherent to the facial nerve, and not just its size, is a major determinant of ultimate facial function. Occasionally a delayed paralysis may occur several days to one week following surgery. This is felt to be due to swelling of the nerve and has an excellent prognosis for recovery (in greater than 90% of patients).

   In the presence of a facial paralysis, final recovery may take up to 18 months. The more delayed the onset of recovery, the less chance near-normal levels of function will be obtained. Improvement is gradual and often is initially seen in the lower facial muscles and last in the eyebrow and forehead region. As recovery occurs, involuntary twitching and occasional spasm of the muscles is experienced. With more severe degrees of weakness or paralysis, following recovery there may be an inability to selectively contract only certain portions of the face without movement of other facial muscles. This phenomenon, known as synkinesis, may consist of involuntary eye closure with smiling, or with attempted eye closure the corner of the mouth may contract. This may improve with physical therapy treatments.

Eye Problems -- There are two types of eye problems. The first is the inability to close the eye voluntarily or loss of the blink reflex and is a consequence of facial paralysis. This can lead to excessive exposure and drying of the eye, as tears are no longer dispersed adequately over the surface of the eye. The second type of problem is more common and may occur with or without facial paralysis. This consists of decreased tear production. Small nerve fibers that partially travel though the ear and accompany the facial nerve serve to stimulate the tear gland and may be disrupted during surgery. Faulty regeneration of these fibers may lead to a phenomenon known as "crocodile tears." This consists of uncontrollable tear production while eating.

   Drying of the eye may require treatment with regular use of artificial tears or eye lubricants. This may be required temporarily or permanently depending on the degree of facial weakness. With facial paralysis, it is important to protect the eye from the effects of wind and dust with eyewear when outside.

   If it appears that facial weakness may be of a long-standing or permanent nature, several (minor) surgical procedures can be performed to help protect the eye. A small gold weight can be implanted in the upper eyelid under local anesthesia to help with eyelid closure. In another procedure, the upper and lower eyelids can be partially sewn together to protect the eye. These procedures are generally reversible, should facial function return. The upper and lower eyelids can also be "tightened" to reduce the portion of the eye that is exposed in patients with loose skin. Special plugs can also be inserted into the tear ducts to slow down drainage of tears from the eye.

   In some patients, double vision may occur after surgery, especially in larger tumors. This is usually temporary and resolves over time.

Cerebrospinal Fluid (CSF) Leak -- The area around the brain which contains spinal fluid must be opened during removal of acoustic neuromas. Often, small bony openings surrounding the ear (air cells) are unroofed and, if not sealed completely at the end of the surgical procedure, may allow this fluid to enter the middle ear space behind the eardrum. From here it may travel down a channel known as the eustachian tube to the back of the throat. The CSF then usually drains out of the nose when the head is tipped forward. The other possible route for CSF leakage is directly through the skin incision. Leakage of CSF occurs in approximately 15% of patients after surgery. When present post-operatively, it commonly prolongs the hospitalization by several days. Treatment may include head elevation, bed rest, medication to reduce CSF production, or placement of a drain from the lower spinal cord to reduce pressure at the operative site and allow for sealing at the origin of the leak. If these conservative measures fail, it may be necessary to re-open the operative site and repack the area of leakage. When this becomes necessary, usually the bone directly behind the ear (mastoid bone) is entered to plug the flow of fluid into the ear. When CSF leaks occur, the risk of meningitis (infection around the brain) increases.

Imbalance/Dizziness -- During acoustic neuroma surgery, the balance or vestibular portion of the eighth cranial nerve is, by necessity, removed with the tumor. As such, dizziness in the immediate post-operative period is common. The environment may seem to move or spin, however this sensation usually resolves within a few days. Patients will usually experience some imbalance with walking or quick head movements that improves over weeks to months. How quickly patients improve depends largely on how much function had been impaired before surgery from the effects of the tumor. Patients who already had significant loss of balance function (usually seen in larger tumors) before surgery have adjusted to some extent and may have fewer problems following surgery than patients with smaller tumors where balance was not significantly affected before the surgery. Pre-operative assessment of balance function is often performed to determine how much has been lost due to the effects of the tumor. This gives some indication of how much difficulty with imbalance and dizziness will be encountered after surgery. As the brain adjusts for the loss of balance function from the ear, these symptoms always lessen, but may never completely resolve. The majority of patients return to their pre-operative level of activity without significant imbalance problems. Generally, the younger the patient, the more completely and quickly the problems disappear. With fatigue or poor vision in a dimly-lit room (or darkness), patients may notice mild balance difficulties years after surgery.

Headaches -- Some degree of headache is inevitable following any type of intracranial surgery in the early post-operative period. Persistent headaches can be quite troublesome in some patients. This may take one of several forms. Some patients may experience pain localized to the skin incision or side of the surgery. Commonly, patients describe headache in the back of the head near the neck. Others may note headache mainly deep and between the eyes. Finally, the headache may be more generalized over the entire head. Headache is typically episodic and may be triggered by coughing or straining. Alcoholic beverages may also increase the intensity or frequency.

   The chance of developing persistent post-operative headache is somewhat dependent on the surgical approach for tumor excision. A higher incidence of headache is seen following the retrosigmoid approach than with translabyrinthine surgery. Interestingly, there may be a greater incidence of post-operative headache in patients with small tumors than with larger lesions. Overall, approximately 30% of patients consider headache a significant problem following surgery. This figure refers to the first one to two years after the operation. Beyond that time, headaches of a significant nature are quite rare. Recent research suggests that bone particles created from drilling on the ear bone may become trapped in the surgical site and lead to irritation of surrounding structures with subsequent development of headaches. Technical improvements are ongoing in an attempt to reduce post-operative headaches. Treatment of headache may include anti-inflammatory medications (such as ibuprofen or naproxyn), muscle relaxants, steroids, neck muscle massage, application of heat/cold or biofeedback therapy. Narcotic-type medications are usually reserved for the initial post-operative period and with severe flare-ups.

Fatigue -- Lack of energy and easy fatigability are almost universal in the first several months following surgery and may be noticeable even six to 12 months later. However, only 15% of patients are significantly affected on a long-term basis. The majority of patients return to work within eight weeks following surgery.

Psychological Effects -- A period of emotional stress or upset following surgery is not uncommon. This is usually self-limited and resolves. However, 20-30% of patients may experience depression of varying degrees. More than 85-90% of patients are able to resume their pre-operative social and occupational roles.

Support Groups
Support groups consisting of other patients who have been diagnosed with acoustic neuroma can be helpful. For further information about acoustic neuroma, a national newsletter, national meetings, and a support group in your area with local meetings, contact:

Acoustic Neuroma Association
P.O. Box 12402
Atlanta, GA 30355

Email: ANAUSA@AOL.COM

Home page: http://132.183.175.10/ana/

If you've been newly diagnosed with an acoustic neuroma, it helps to know you're not alone. Former patients have volunteered to talk about their treatment experience.

For more information, call Milwaukee Otologic at 414-529-3215.

Figure 5, 6, and 7 courtesy of Atlas of neuro-otology and Skull Base Surgery, Author: Robert K. Jackler, M.D., Editor: Roberty Hurley, Copyright © 1996, St.Louis, MO, Mosby-Year Book, Inc.