Acoustic Neuroma is the name commonly applied to a benign tumor arising from the sheath cells of the vestibular component of the 8th cranial nerve (Vestibulocochlear nerve). The correct name is vestibular schwannoma. However, since the prominent early symptoms are unilateral hearing loss and tinnitus (ringing in the ears) due to compression of the auditory component of the 8th cranial nerve, the name acoustic neuroma appears to be here to stay. The tumors originate inside the bony canal through which the 8th cranial nerve passes on its way to the inner ear. The 7th cranial nerve (Facial nerve) which supplies the muscles of the face passes through the same canal. As the tumor enlarges, it extends into an intracranial space at the base of the brain referred to as the cerebello-pontine angle (CPA). Since other types of tumors can be seen in this location, tumors here are usually referred to as a cerebello-pontine angle tumor until a definite diagnosis is made. Once the tumor has extended into the cerebello-pontine angle, it may encroach on other cranial nerves such as the 5th cranial nerve, causing facial numbness, or compress the brain stem causing ataxia.
Acoustic Neuromas are most frequently diagnosed by MRI scan in a patient with unilateral hearing loss. Important information to be determined from the MRI scan are distance the tumor extends laterally in the auditory canal, the extent to which the tumor expands in the cerebello-pontine angle, and whether or not the brain stem is contacted or distorted. Other important diagnostic tests are the audiogram and the recordable brain stem audio evoked responses (BAERs) because these will provide indicators of the possibility of saving hearing.
Based on the MRI scan, Acoustic Neuromas fall into three classifications:
- Entirely intracanalicular, which means the entire tumor is no bigger than a bean and is completely within the bony canal.
- Intracranial extension without brain stem distortion, which means the intracranial portion of the tumor is small, i.e. 1-2 cm.
- Intracranial extension with brain stem distortion, which means the intracranial portion of the tumor is bigger than 2 cm and pressing on the stem of the brain.
A patient with an acoustic neuroma has two options: the patient can either wait and see what happens over time, or pursue treatment. When treatment is selected there are only two viable alternatives: microsurgical removal or stereotactic radiation, as there is no effective medical therapy. Fortunately, acoustic neuromas are slow growing, as well as benign (noncancerous), so decision making is not urgent.
There are multiple surgical approaches to the auditory canal and cerebello-pontine angle including retromastoid, transcochlear, and middle fossa. The surgeon should recommend which is best for any individual case based on the size and location of the tumor and the status of facial nerve function and hearing.
All surgical procedures for removal of acoustic neuromas should be performed with the operating microscope by experienced microsurgeons. Because most of these tumors involve both the internal auditory canal and the intracranial space they should be removed by a surgical team comprised of a experienced neurosurgeon and an experienced otologist.
Conventional radiation therapy has not proven effective for acoustic neuromas so the only effective non-surgical treatment is high intensity, single dose radiation, focused on the tumor. To accomplish this requires sophisticated equipment which can aim the radiation beam based on the MRI scan. Currently, there are two methods in common use: the multisource Cobalt-60 gamma radiation unit (Gamma Knife) or the stereotactically controlled linear accelerator. Either technique is capable of delivering a precise dose of radiation to the tumor but, there are some differences in selecting the size and shape of the beam. If stereotactic radiation is selected as the method of treatment, these issues should be discussed with the treating physician.
There are two disadvantages of stereotactic radiation. One is that since the radiation does not kill all the tumor cells there is a significant possibility that the tumor will grow in the future, which requires that follow-up MRI scans be performed indefinitely. The other, which is not possible to document, is the risk of radiation induced tumors developing sometime in the future. It is known that other forms of radiation therapy to the head have produced tumors 15-30 years later but no documented cases secondary to stereotactic radiation have been reported. Nevertheless, the younger the patient stereotactic radiation becomes less advisable.
For any given patient, the issues regarding treatment are different depending on the MRI classification and the hearing status. The discussion below examines the different considerations involved for each class.
Since these tumors are entirely within the bony canal they pose no threat to the brain or cranial nerves other than VII and VIII, which are in the canal. Therefore, the crucial questions are how severely affected are the nerves already and how fast is the tumor growing. Since the nerve most susceptible to pressure is the auditory component of nerve VIII, detailed hearing test and auditory evoked potentials should be performed. If useful hearing is present it is advisable to remove the tumor or treat it with stereotactic radiation before it becomes larger. With microsurgery, the tumor can be completely removed with minimal risk and a reasonable chance of preserving hearing. Facial nerve and auditory monitoring are, of course, required. With stereotactic radiation therapy, no actual surgery is required but the tumor will still be present and will need to be followed indefinitely by MRI scan. The outcome for preservation of hearing are similar in both techniques.
If useful hearing is not present it is reasonable to follow the tumor for one year with a repeat MRI scan to see if it is growing before making a decision regarding surgery, radiation, or more waiting. However, if knowing you have a tumor in your head and not doing something is unacceptable, it is appropriate to proceed with surgery or stereotactic radiation.
Intracranial Extension without Brainstem Distortion
These tumors have already demonstrated their propensity to grow but are not yet life threatening. They should be surgically removed if the general health of the patient permits, otherwise they should be treated with stereotactic radiation. The surgical approach depends on the status of the hearing and the preference of the surgeon but the primary objective other than total removal of the tumor should be to preserve the facial nerve. This requires facial nerve monitoring during the course of the operation. If useful hearing is still present an effort should be made to preserve by intraoperative auditory monitoring and selecting a surgical approach which does not damage the cochlea.
Intracranial Extension with Brainstem Distortion
Large tumors which distort the brain stem are potentially life threatening and should be surgically removed as soon as possible. The goal should be total removal but great care must be taken to ensure that neither the brain stem or its blood supply are injured in the process. The facial nerve also remains important and it may be necessary to leave a portion of tumor capsule to protect the brain stem, blood vessels or facial nerve. The possibility of preserving any hearing with large tumors is minimal.