Acoustic Neuroma

General Information

Acoustic neuromas are benign tumors that grow on the nerve for hearing and balance.   The nerve for hearing and balance is more properly called the  vestibulocochlear nerve or the eighth cranial nerve.  This nerve exits from the brainstem along with the facial nerve (seventh cranial nerve) and travels through the cerebellopontine angle to enter the internal auditory canal.  The vestibulocochlear nerve then divides into a cochlear portion, which enters the cochlea and carries hearing, and a vestibular portion, which enters the labyrinth and carries balance information.  Acoustic neuromas should more properly be called vestibular schwannomas since they arise from the Schwann cells (cells that cover nerves in the central nervous system) and occur most commonly on the vestibular portion of the vestibulocochlear (eighth cranial nerve).

These tumors are rare.  They occur in 1 person per 100,000 people per year.  These tumors can occur either sporadically or as part of an inherited disorder.  The sporadic tumors happen on only one side.  The inherited disorder is associated with bilateral acoustic neuromas or a unilateral acoustic neuroma along with other brain or nerve tumors.  This inherited disorder is called neurofibromatosis type 2, or NF-2; and is inherited in an autosomal dominant trait.  Both NF-2 and sporadic cases of acoustic neuromas are associated with an abnormality on chromosome 22.

The most common symptom of acoustic neuroma is hearing loss.  This hearing loss is usually a unilateral, slowly progressive sensorineural hearing loss.  However, some patients experience a sudden (<3 days) onset of hearing loss; and yet other patients have little or no hearing loss.  On audiometric testing, pure tone hearing, speech discrimination and/or loss of acoustic reflexes might signify the presence of an acoustic neuroma.

Along with unilateral hearing loss, a ringing sound (also called tinnitus) in one ear is a common symptom of an acoustic neuroma.  However, acoustic neuromas do not produce a characteristic tinnitus.  This tinnitus is usually a constant sound in just one ear that has been present for more than two months.

Although these tumors occur on the vestibular portion of the eighth nerve, vertigo is an uncommon symptom of acoustic neuromas.  More often, patients complain of an off-balance sensation, especially if turning quickly.  It is theorized that since these tumors grow slowly that the brain compensates for the unilateral vestibular loss nearly simultaneously.  Vestibular testing (ENG) can sometimes show the vestibular loss due to an acoustic neuroma.

Since the facial nerve travels with the vestibulocochlear nerve into the internal auditory canal, facial paralysis might occur as these tumors grow larger and larger (>3 cm).

Other symptoms of acoustic neuroma are headache, blurred vision, numbness of the face, hydrocephalus, or even death (if allowed to grow unchecked).  These symptoms usually do not occur until the tumors have reached a large size (>4 cm).

Diagnostic Tests

Although these symptoms are found in patients with acoustic neuromas, none are specific enough to diagnose a tumor.  An audiogram is a necessary test in the evaluation of these symptoms and is an essential step in diagnosing an acoustic neuroma.  If there is a significant amount of hearing loss or other symptoms to suggest an acoustic neuroma, then an ABR (auditory brainstem response) might be used to evaluate further the reason for the hearing loss.  

ABR is a sensitive test for picking up tumors that are larger than 1 cm in size.  Smaller tumors might escape detection with ABR alone.  Abnormal ABR findings demand further evaluation by MRI.

Ultimately, an MRI (magnetic resonance imaging) with Gadolinium contrast is required to diagnose these tumors.  This type of imaging study gives an exquisite view of the brain and the cranial nerves.  In patients with an acoustic neuroma, the gadolinium contrast is picked up by the tumor cells and makes the tumor stand out from the surrounding normal brain and nerve tissues.  MRI can pick up tumors that are as small as 1 mm.  MRI scans permit a means of measuring the tumor's size, which can be used to monitor the tumor's growth, if desired.

CT (computerized tomography) scans with contrast can be used to evaluate patients who cannot otherwise have MRI scanning.  CT lacks the fine detail found on MRI, but can reliably demonstrate tumors larger than 1-2 cm.  

Treatment

In general, treatment can be divided into three options:

• Surgery

• Radiotherapy

• Observation

Surgery

The goal of surgery is to remove completely the tumor and preserve as much normal function as possible.  All of these procedures are performed under general anesthesia.  During every procedure, the function of the facial nerve is monitored.  In patients with measurable hearing, hearing can be monitored using intraoperative ABR.  All patients will spend the first night after surgery in the surgical ICU.  The patient is transferred to a regular room as his or her condition permits, usually on the day following surgery.   Patients should plan on spending one week in the hospital, but usually only 4 -5 days of total hospitalization are required.

The choice of surgical approach is based on several factors: (1) size of the tumor, (2) status of hearing, and (3) experience of the surgeon.  Other factors, such as facial or other cranial nerve status and patient preference, are also considerations.  The three surgical approaches for removal of acoustic neuromas  each have their own advantages and disadvantages:

1. Middle cranial fossa approach

2. Retrosigmoid/suboccipital approach

3. Translabyrinthine approach

Middle Cranial Fossa Approach

In this approach, an incision is made in the skin above and behind the ear.  Next, a small window of skull bone above the ear is removed, and the dura of the temporal lobe is elevated exposing the bone of the  middle cranial fossa.   Using a microscope and drill, this bone is removed and the internal auditory canal is exposed.  The dura of the internal auditory canal is incised, and the tumor is exposed.  The tumor is then separated from the facial nerve and, possibly, the cochlear nerve.  The tumor is then removed.  The operative site is then closed by using a small plug of muscle and a bone graft in the middle fossa defect.  The window of bone, removed at the start of the procedure, is replaced and sometimes affixed with microsurgical plates.  The soft tissues are then closed in layers.  A sterile pressure dressing is then applied.

Since this procedure leaves the inner ear intact, it is an approach for potential hearing preservation. 

Retrosigmoid/Suboccipital Approach

This approach goes through the bone behind the sigmoid sinus (the large vein that drains blood from that side of the brain) at the back of the head (the occiput).  Once the skin and soft tissue over the skull is opened, an opening in the occiput is created.  Using a microscope, the dura is opened and the lateral lobe of the cerebellum is moved toward the midline.  This movement exposes the tumor within the cerebellopontine angle; and this portion of the tumor is separated from the facial nerve and removed.  Oftentimes, the tumor extends into the internal auditory canal.  Using a fine drill, the bone of the canal is removed and the remaining portion of tumor is removed.  A plug of soft tissue is used to close the internal auditory canal.  The cerebellum is allowed to return to its normal position and the dura covering it is closed.  The soft tissues and skin are closed in layers, and a sterile dressing is applied.

Since this procedure leaves the inner ear intact, it is an approach for potential hearing preservation.

Translabyrinthine Approach

This approach goes through the mastoid and the labyrinth (the balance organs) in order to expose the internal auditory canal and the tumor.  The skin is incised about 4 cm behind the ear.  The bone of the mastoid and temporal bone is exposed and opened using a high speed drill and microscope.  The facial nerve is identified within the temporal bone and preserved; the labyrinth is removed.  The bone of the internal auditory canal and the posterior fossa is removed.  The dura covering these structures is  opened, exposing the tumor.  The tumor is separated from the facial nerve; the vestibular nerves are divided, and the tumor is removed.  Once the tumor has been removed, closure occurs by harvesting a portion of fat from the abdomen and using this tissue to plug the hole in the dura.   The soft tissues and skin are then closed in layers, followed by a sterile dressing.

Since this procedure removes the inner ear, it is not possible to save hearing in the operated ear. 

Risks and Complications of Surgery

All surgical procedures have potential problems and expected outcomes.  The most common potential problems and outcomes will be described as enumerated below:

1. Hearing loss.  Many patients with acoustic neuromas already have hearing loss.  However, a small number of patients have small tumors (<1.5 cm) and good hearing.  Hearing preservation can be attempted by using either the retrosigmoid approach or the middle fossa approach.  Success rates of hearing preservation, even in the best of circumstances, however, are only about 30%.  Even more uncommon is hearing improvement, although we have operated on patients who experienced this improvement.   Hearing loss from this surgery is more often an expected outcome from the surgery. 

2. Dizziness.  Due to the slow growth of these tumors, many patients have already adapted to the loss vestibular input from the tumor side.  Regardless of the approach taken, the loss of vestibular input from surgery will be complete.  Young patients (those less than 65 years of age) will usually recover their balance quickly (within one month).  Persistent imbalance can occur and usually vestibular rehabilitation will help to improve the patient's balance.

3. Tinnitus.  This is a common symptom of acoustic neuroma.  Removing the tumor does not necessarily change, improve, or diminish the tinnitus.  Even in patient with hearing preservation might continue to have tinnitus following surgery.  Most patients seem to accommodate to this tinnitus after the surgery.

4. Facial paralysis. Avoiding facial paralysis has been the major advancement in acoustic neuroma surgery over the last 10 - 15 years.  The use of facial nerve monitors has helped surgeons to identify the facial nerve, and thus preserve its function.  Large tumors (>3 cm in size) are harder to remove and preserve facial function.  Some smaller tumors can be difficult to separate from the facial nerve as well, especially if the tumor is hard and fibrous.  If the nerve can be anatomically preserved, frequently its function will return to near normal function.  Facial nerve grafts are sometimes performed at the time of operation in patients whose facial nerve cannot be saved.  Other techniques for facial reanimation include the hypoglossal to facial nerve graft, static facial slings and gold weight eyelid implants.  Please look at the facial paralysis page for more information regarding this problem.

5. Cerebrospinal fluid leak.  The brain is surrounded by fluid, called cerebrospinal fluid or CSF.  Any time the dura is opened, there is a potential for leakage of CSF.  Great care is taken at the time of closure to prevent this fluid from leaking either out of the incision site, the ear canal, or the nose (by way of the Eustachian tube).  Regardless of technique, CSF leak seems to occur in about 10% of cases.  Treatment for CSF leak can involve placing a drain in the back (called a lumbar drain) to remove spinal fluid and allow the repair site to heal.  Sometimes additional surgical procedures are necessary to stop the fluid leak.

6. Meningitis.  Meningitis is inflammation or infection of the coverings of the brain (the meninges).  Careful attention to sterile technique and the use of peri-operative antibiotics has minimized the occurrence of meningitis to 1-4%.  If meningitis does occur, a spinal fluid tap will be performed to find the organism that is causing the infection and to determine which antibiotics will be most effective at curing the infection.

7. Headache.  Headache following acoustic neuroma surgery can be a sign of meningitis.  Sometimes, patients who have had a retrosigmoid approach will have headache that persists for several months following surgery.  This headache is usually relieved by NSAIDs.

8. Other central nervous system complications, such as stroke, seizure, and death, have been reported in large surgical series; however, their occurrence is rare (<1%).

Radiotherapy

Radiation has been used to treat acoustic tumors for the past 20 or so years.  This procedure involves a special form of radiation therapy using a device commonly called the Gamma Knife.  More properly, it is a stereotactic radiotherapy that can deliver more than 200 beams of radiation directed to the tumor location.  This concentrated dose of focused radiation energy can destroy tumor cells.  This procedure is done using local anesthesia and sedation and is performed on an outpatient basis.  There is only one treatment given.

The advantage of this approach is the avoidance of surgery, the period of hospitalization, and the recovery period necessary following surgery.   Initially, hearing preservation and facial nerve function can be higher than surgery; however, long term results are probably no different than that for surgery.

The disadvantage of this approach is manifold.  Although the radiation does kill some tumor cells, it does not kill all cells, and it does not remove the tumor.  The remaining cells might grow, producing a recurrent tumor.  In those cases where a recurrent tumor was surgically removed, the procedure was technically more difficult; and the post-operative facial nerve function was poorer than in surgical patients with equivalent sized tumors without prior radiation therapy.  Facial nerve weakness or paralysis has been reported after Gamma knife therapy alone.  Sometimes this is due to swelling in the tumor following radiation.  Hearing loss does occur in patients who have Gamma Knife therapy.  Sometimes this hearing loss occurs suddenly after treatment or in several months after treatment.

In general, we have recommended Gamma Knife therapy to patients who could not otherwise have surgery and who did not have a large tumor causing central nervous system problems. 

Observation

These tumors are benign, meaning that they do not spread throughout the body, and slow growing.  These patients undergo serial MRI scans every 6 - 12 months, depending on the tumor's rate of growth.  Observation of the tumor might be used in patients that have very small tumors and who are otherwise not good candidates for the treatments listed above (e.g. patients > 75 years of age with complicating medical problems).  Surgery is usually required as tumors grow out of the internal auditory canal and into the cerebellopontine angle and brainstem. 

However, in patients that are in good health, have good hearing, and have small tumors (< 1 cm), surgery does offer the possibility of complete removal with preservation of hearing and facial nerve function.

Concluding remarks

Acoustic neuromas are rare tumors that grow on the vestibulocochlear nerve.  The symptoms of these tumors include hearing loss, tinnitus, imbalance, facial nerve weakness, headache, etc.  Surgical treatments are very successful at removing tumors, and preserving facial nerve function.  Hearing preservation can be attempted in appropriately selected patients.  Radiation therapy is available to patients, especially those who could not otherwise undergo surgery.  Some tumors are followed on serial MRI examinations, especially in elderly patients with small tumors.

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