By Gabriel E. Hunt, Jr., MD

The vestibular schwannoma, better known as the acoustic neuroma, is a benign noncancerous tumor that arises from the superior division of the vestibular nerve, which is part of the eighth cranial nerve complex. This nerve is responsible for equilibrium and hearing. This tumor derives its name from the particular site on the nerve from which it arises.

The reason for the name "schwannoma" is secondary to the fact that it arises from the schwann cells of the peripheral nervous system, which then grow in to the intracranial space and compress surrounding neural structures. The term "vestibular" comes from the nerve from which it originates. The vestibular nerve is part of the eighth cranial nerve complex consisting of the vestibular and cochlear portions.

The concern for this benign lesion stems from the surrounding structures, which can conceivably become compromised with continued unbridled growth of the tumor. The anatomy that is of greatest concern includes the cochlear portion of the eighth cranial nerve, the facial nerve, trigeminal nerve and brainstem. These are all extremely vital structures that can cause a myriad of symptoms from hearing loss to death.

Vestibular schwannomas are one of the most common intracranial tumors. They constitute approximately six percent of all primary intracranial tumors. The annual incidence is 0.80-1.20 cases per 100,000 population, and it is the most common tumor found in the cerebellopontine angle. The diagnosis of this tumor results in approximately 2,500 new cases per year. These tumors are usually very slow growing and, therefore, usually present with only subtle clinical signs.

There are specific signs and symptoms associated with the growth of acoustic neuromas; these include unilateral nonpulsatile tinnitus, unilateral hearing loss, cerebellar dysfunction, facial nerve dysfunction, swallowing difficulty and loss of unilateral facial sensation. If the tumor grows large enough, the patient can have symptoms secondary to brainstem compression and associated hydrocephalus.

The typical patient usually presents to the otolaryngologist with complaints of hearing loss or less commonly, complaints of dizziness. The type of hearing loss that occurs is a unilateral sensorineural loss that is caused by direct compression of the cochlear nerve or secondary to interruption of the blood supply. The sensation of vertigo is caused by vestibular dysfunction. This is usually described as a rotation of a patient's surroundings when he/she is standing still. Vertiginous symptoms usually occur in a larger percentage of tumors that are less than one centimeter (1 cm). The thought is that since these tumors are so slow growing, by the time they get to be large, the body is able to compensate for the unilateral vestibular dysfunction. As the tumors attain sizes greater than about three centimeters (3 cm), brainstem compression is associated with compression of the central vestibular nucleus and, therefore, can cause a higher incidence of disequilibrium. Disequilibrium occurs when signals from the other organ systems are compromised.

The facial nerve is in closest proximity to the superior vestibular nerve, but facial weakness is only appreciated in a small fraction of patients with these vestibular tumors. These tumors usually get remarkably large in size, and there is other cranial nerve dysfunction present before facial paralysis begins. This cranial nerve carries both motor and sensory functions, and it is more common to have sensory loss to the external ear canal than to have unilateral facial weakness. The other cranial nerves that are in close proximity that can become affected are the nerves that are responsible for swallowing, and these can also be compromised if the tumor is allowed to grow.

When patients have not presented with hearing loss or vestibular dysfunction, they present with headaches. The headaches that are commonly seen are not localized to any one region and, therefore, are hard to diagnose from this presenting symptom. The quality and duration of the pain also does not help tremendously with the diagnosis and is very nonspecific.

Click on images to enlarge:

  Intracanalicular tumor             Large acoustic neuroma             Intraoperative view

Historically, acoustic neuromas were diagnosed using primitive detection devices such as pneumoencephalograms and audiography. The first acoustic neuroma was described in Holland in 1777, and the first successful resection was performed in 1830. When our methods of diagnosis and surgical resection for these tumors were implemented, the morbidity rate was extremely high (70-80%). Since the advent of CT scans and magnetic resonance imaging, detection has become much easier. Utilizing these new modalities afforded neurosurgeons the ability to diagnose and treat these lesions earlier and with more precision and accuracy. These tumors have a characteristic appearance on MRI the helps to make the diagnosis easier. The tumors enhance with contrast administration and they widen the internal auditory canal, which is where they originate from. The tumors can range in size from a small homogeneously enhancing intracanalicular mass to a large inhomogeneously enhancing mass with severe brainstem compression.

The treatment modalities that are now available following diagnosis have become more sophisticated in the past decade. The neurosurgeon now has more weapons from which to choose in the evaluation and treatment of patients.

The gold standard of treatment has been surgical resection. Outcomes have improved over the years with the advent of better neurosurgical instruments, monitoring practices, and microscopy. These technological advances have allowed the surgeon to perform surgical approaches and resections with less complications and subsequently better patient outcomes.

Surgery is the gold standard for treatment of this disease process, and radiosurgery provides a noninvasive option for those patients for which surgery is not an option. The key to good surgical outcomes stems from the careful management and the wise utilization of the technological advances. The planning of the surgical approach to resect these tumors is the most important part of the whole process. There are three different approaches to resection that are used based on the symptomatology the patient is experiencing. The three approaches are the middle fossa, translabyrhinthine, and retrosigmoid suboccipital approach.

The retrosigmoid approach has been the most popular one used by neurosurgeons to treat this tumor. It provides the surgeon with a relatively larger exposure and direct visualization of the brainstem and associated nerve roots that are possibly compromised from compression by larger tumors. It allows for identification of the nervous structures as they exit the brainstem. This approach has been used in an effort to treat larger tumors and to attempt to preserve hearing even though the majority of patients with these large tumors lose their hearing anyway.

The middle fossa approach has the best record for hearing preservation of the three approaches for tumors that are less than 2 cm. The limits of this approach are the retraction incurred upon the temporal lobe and the difficulty encountered when resecting larger tumors that compress the brainstem. This approach is commonly performed in patients who have a tumor that lies medially in the internal auditory canal and who still have serviceable hearing in the affected ear.

The translabyrinthine approach is used for both intracanalicular and intracranial tumors. This approach is used when the patient does not have serviceable hearing in the affected ear and the tumor is not too large so there is not a large amount of brainstem compression. This technique provides for excellent visualization of the facial nerve in the internal auditory canal. The middle fossa approach is more commonly used when there is a tumor that lies more medially in the canal and allows for tumor resection with less risk of damage to the surrounding nervous structures.

The best surgical decisions are made through utilization of the team approach. When otolaryngologists and neurosurgeons work together, as they do at the Cedars-Sinai's Maxine Dunitz Neurosurgical Institute, the patient benefits from their expertise and has the opportunity to have the best surgical management offered.

Radiosurgery has become a new initial modality used in the treatment of acoustic neuromas over the past 10 years, and it provides a noninvasive treatment option. The evolution of the radiosurgical technology has provided the neurosurgeon with another viable first option for the management of this tumor. Some of the most recent studies that have been performed comparing surgery to radiosurgery have leaned toward using radiosurgical intervention for the first line treatment of appropriately sized lesions, but there is no long term data that shows effective long-term control. We know that radiation does not obliterate the tumor, and in some cases, the tumor actually increases in size before there is any reduction noted. The response rate for this modality also takes about two years to observe any significant effect. The use of this option is of paramount importance when a patient is not a good surgical candidate and surgery cannot be safely performed.

Even though radiosurgery has changed the way that this and other intracranial lesion are treated, there are still risks associated with this treatment. With these tumors in such close proximity to the brainstem and other cranial nerves, there is always a risk of causing damage to these structures and causing further neurological injury. Tumors that are less than 3cm are usually reasonable to treat with this modality, but the larger the tumor, the greater the risk of not obtaining the desired results. There is also the risk that the response to radiation might not provide substantial local control of the tumor growth and, therefore, necessitate surgical intervention. Surgical resection following radiosurgery for these lesions is not an easy task and is thought to be complicated by increased scar tissue formation and, therefore, an increased potential for damage to the surrounding neural structures. The changes in the morphology of the tumor after radiation present a formidable challenge for the surgeon. The main reason for surgical intervention in treating these tumors is not hearing preservation, because the first symptom on presentation is usually unilateral hearing loss. The goal of surgery is to decompress the brain stem in larger tumors and to preserve seventh cranial nerve function. When a tumor has been radiated initially, and then regrows, it becomes extremely difficult to remove the tumor completely at the time of surgery because the integrity of the seventh nerve is at an extremely high risk of being compromised.

The acoustic neuroma is a benign tumor that can be treated appropriately by an experienced team utilizing an aggressive management strategy. The advent of better technology has afforded the patient the ability to assist in the planning and implementation of their treatment, starting with the increased awareness and decision making provided by use of the Internet. The Internet has become an invaluable source of knowledge that has provided the patient with access to unlimited resources and information about this disease process, and often points them in the right direction for diagnosis and treatment. Other technological advances have also played a large role in the treatment and prognosis for patients afflicted with this tumor. Higher resolution MRI and CT has given the practitioner the ability to diagnose these tumors sooner and allowed their patients to seek intervention earlier when the tumors are smaller and easier to treat with less associated risks. Once the diagnosis has been made, advances in microscopy and surgical instrumentation over the years has given the surgeon the ability to resect these lesions with less associated complications. One of the important advances that has played a huge role in decreasing the morbidity associated with surgical resection is the continued evolution of neuromonitoring devices. The ability to accurately monitor the facial nerve function and brainstem auditory evoked responses has provided the surgeon with another tool to allow for more confident aggressive tumor resection. The advent of new radiosurgical technology also gives another treatment option that was not available in the past.