Acoustic neuromas (AN) are benign, generally slow growing tumors that intimately involve the seventh and eighth cranial nerve (CN VII and VIII) in a portion of the brain commonly referred to as the cerebellar-pontine angle (CPA). CN VIII has two distinct parts; the cochlear (hearing) nerve that transmits sound-induced nerve impulses between the inner ear and the brainstem, and the vestibular nerve that transmits balance information along similar pathways.

Meanwhile, CN VII, or the facial nerve, is responsible for the movement of half the face on the same side as the nerve. All these cranial nerves travel through a narrow bony opening termed the internal auditory canal. Acoustic neuromas most commonly arise from schwann cells, which make insulation (myelin) for the vestibular nerve. As a consequence these tumors are often appropriately referred to as vestibular schwannomas.

What is Neurofibromatosis Type II?

There are two types of acoustic neuromas: 1) Sporadic and 2) Neurofibromatosis Type II (NF-II), which is often aptly termed bilateral (affecting both sides) acoustic neuroma disease. The sporadic form of AN is responsible for 95% of cases, while NF-II causes the remainder. The bilateral nature of the disease in NF-II poses special challenges in terms of treatment.

NF-II is a rare, dominantly inherited genetic condition with a prevalence of 1 in 100,000 people. Virtually all patients with NF-II eventually manifest bilateral acoustic neuromas. Compared to sporadic AN, those associated with NF-II tend to grow faster, present at a younger age (oftentimes by the teenage years), and respond more poorly to virtually all forms of treatment. The implication of the latter is a greater likelihood of tumor recurrence and an increased risk of cranial nerve complications. Eventual complete (bilateral) deafness is not uncommon among patients with NF-II. By contrast, the sporadic form of AN presents in middle age, and patients have better outcomes with all types of surgical intervention.

What are the presenting symptoms for an acoustic neuroma?

Acoustic neuromas most commonly present with hearing loss. However a disturbance in balance, ringing in the ear (tinnitus), facial weakness, pain or even facial numbness, alone or in any combination, are other potential presenting symptoms. The fifth cranial (CN V), or trigeminal nerve, which is involved in transmitting facial sensation is often situated immediately adjacent to a moderately sized AN. As a consequence, pressure from an enlarging tumor can often cause facial numbness or pain.

In summary, the symptoms produced by acoustic neuromas include:

Hearing loss
Ringing in the ear (tinnitus)
Imbalance
Dizziness (vertigo)
Facial weakness
Facial numbness or pain
Discomfort (feeling of fullness/pressure) in or around the ear

Hearing Loss

The vast majority of patients with acoustic neuromas suffer some hearing loss as a result of injury to the cochlear division of CN VIII. This damage results from either direct infiltration or compression of the nerve by the tumor, or alternatively, interference with the normal blood supply. The hearing loss experienced by patients with AN is typically subtle, disproportionately affects the high frequency hearing range, and gradually progresses over years as the tumor increases in size. The damage to the hearing is such that it becomes ever more difficult for a patient to distinguish the texture of different sounds, thereby making it harder to identify discrete words when spoken at usual volumes. This capability is referred to as the speech reception threshold (SRT), and is a standard measured value in most hearing tests.

Loss of hearing is sufficiently slow in onset and painless, so as to go unnoticed by many AN patients, who might for example, unconsciously stop using the phone with their affected ear. The time period over which hearing worsens is highly variable and most likely correlates with the rate of tumor growth. However, sudden hearing loss is also a well-recognized phenomenon with these tumors. Furthermore, ringing in the ear (tinnitus) frequently accompanies loss of hearing, and for many patients can be very bothersome. Rarely, tinnitus occurs in the absence of hearing loss, and when this occurs in a single ear, it is sufficient justification for a patient to undergo an evaluation for acoustic neuroma.

Imbalance and vertigo.

Because AN most commonly arise from the vestibular or balance nerve, it should not be surprising that affected patients frequently suffer from some dysequilibrium (loss of balance). When the visual and other sensory systems are working normally, patients can usually compensate for the faulty input that comes from the affected balance nerve. However, patients have a greater tendency to lose their balance than normal individuals especially when walking and turning quickly.

Vertigo is much less frequently associated with acoustic neuromas than lesser disturbances of the balance apparatus. However, when this symptom occurs, it can be extremely disabling. The vertiginous patient experiences profound dizziness, even when at rest, which can be severe enough to mimic seasickness. Interestingly, vertigo tends to occur most often with smaller acoustic neuromas.

Facial weakness

As acoustic neuromas enlarge, they eventually injure the CN VII, or the facial nerve, which controls facial movement on one side of the face. Facial nerve palsies range from hardly noticeable to profoundly disfiguring. The most troubling problems for patients with facial nerve palsies are an inability to completely close the eye as well as difficulty keeping liquids from running out the corner of their mouth when drinking or eating. Typically, however, the facial nerve is able to adjust to even very large acoustic neuromas without causing noticeable facial weakness. When facial weakness accompanies smaller CPA tumors, it raises the suspicion that the tumor is arising from the facial nerve, and is a facial nerve schwannoma, instead of the much more common vestibular schwannoma.

Facial numbness

The largest acoustic neuromas produce facial numbness or ironically severe pain (trigeminal neuralgia) in some patients. Pain or numbness can involve any portion of the face or inside the mouth from the jaw up to the scalp on the affected side. The characteristic pain consists of brief lightening-like jabs that is characteristic of trigeminal neuralgia. The combination of facial weakness and numbness can be very disabling if it involves the region around the eye. Patients with this combination of problems have difficulty protecting the cornea (outer surface of the eye) from abrasion.

Diagnostic Procedures

Imaging

At present, the diagnosis of acoustic neuroma is most commonly established by imaging the brain and skull base with an MRI (magnetic resonance imaging) scan. Although the procedure can be a somewhat claustrophobic experience for some patients, the test is not painful and generally takes less than 30 minutes to complete.

Therefore, whenever symptoms suggest a diagnosis of acoustic neuroma, an MRI with intravenous contrast (dye with special magnetic properties) is indicated. Alternatively, the most modern thin section computerized tomographic scanning (CAT scanning) will also demonstrate the vast majority of acoustic neuromas, provided dye is used. However, MRI almost invariably will reveal even the smallest AN, and is therefore preferred for initial diagnosis.

The classic appearance on MRI is a contrast enhancing (appears white on an MRI image) lesion involving the porus acusticus, which is that portion of the skull bone through which the cochlear (for hearing), vestibular and facial (innervates the facial muscles) nerves pass when exiting the skull. The MRI appearance for nearly all acoustic neuromas is so typical that further tests to confirm the diagnosis are almost never warranted. However, in nearly all patients with acoustic neuromas, a hearing test (or audiogram), should be done to measure the damage inflicted by the tumor on the cochlear nerve.

Hearing Tests

Approximately 70% of patients with acoustic neuromas suffer measurable hearing loss when tested with pure tone audiometry. This test is designed to measure at different frequencies the minimal volume of sound that a patient can reliably detect. The most frequently encountered finding is decreased high frequency hearing loss on the same side as the tumor. AN can also distort the character of sound that reaches a patient’s auditory brain centers. This problem can manifest itself through difficulty understanding telephone conversations in the affected ear. To measure this phenomenon, a standard part of an audiogram includes a measure of speech discrimination or speech reception threshold (SRT). If an abnormal audiogram leads to a preliminary suspicion of acoustic neuroma, contrast MRI is warranted to establish a definitive diagnosis.

Treatment Options

Therapeutic options for managing acoustic neuromas have changed dramatically over the past 10-20 years. However, these options for managing acoustic neuromas continue to evolve, and in parallel, the results of longer-term clinical studies are being reported in the medical literature. Despite such progress (or ironically as a result of such new found knowledge), there is no universal consensus among the different surgical specialists that are involved in treating AN, as to the single best treatment modality.

Each approach has merits and risks. Nevertheless, given both the primary goal of this website and the interests of the typical visitor, we will herein emphasize radiosurgical options, and especially the unique approach made possible by the CyberKnife. However, it is important that any patient with an acoustic neuroma, who is confronted with making the difficult decision about the best treatment for their specific circumstances, thoroughly review and understand all the different therapeutic options. The following information is not intended to be all inclusive, but rather should represent a reasonable starting point for a motivated patient or interested family member to learn more about the treatment of AN.

Treatment Option: Surgical Resection

The standard treatment for acoustic neuromas for more than 50 years has consisted of open surgical resection. Huge refinements in operative technique, especially the introduction of microsurgical (microscope-assisted open surgery) methods, have greatly improved outcomes with this disorder. However, facial weakness is not an uncommon complication of open surgery. In addition, it is only possible to save some hearing in approximately half of the most favorable cases (small tumors) even when operated on by the most skilled microsurgeons.

Different surgical approaches (translabyrinthine, middle fossa, suboccipital, etc.) have been devised to minimize the risk of specific complications, but such a discussion is beyond the scope of this account. With any standard open surgical resection, there is also a risk of further complications, such as spinal fluid leak or infection. Finally, stroke and peri-operative death are uncommon but very real possible events.

For nearly every patient with a very large acoustic neuroma (defined as greater than 4-5 cm in diameter) that is severely pressing on and compromising brain function, it is widely believed (although not universally so) that open microsurgical resection is the treatment of choice. Oftentimes such surgery cannot be delayed and is truly lifesaving. For the vast majority of patients with smaller AN, stereotactic radiosurgical ablation remains an important treatment option.

Treatment Option: Stereotactic Radiosurgery

Radiosurgical Overview

Because of the above risk profile for open surgical resection, therapeutic alternatives are highly desirable in the eyes of very many patients with acoustic neuromas and their physicians. Stereotactic radiosurgery, a novel minimally-invasive procedure for ablating (destroying) brain tumors has been used now to treat acoustic neuromas for almost 20 years.

Although this procedure has evolved considerably over this period of time, several studies using this technique have already reported very favorable long-term clinical results. As a result, radiosurgery is rapidly becoming an effective treatment alternative for most patients with acoustic neuromas. In particular, the extensive experience has enabled various single fraction radiosurgical dose parameters to be refined, thereby minimizing the risk profile from this treatment. Meanwhile, the introduction of the CyberKnife® concept now permits the use hypofractionated (staged) radiosurgical ablation for acoustic neuroma. This new approach has allowed the risk profile from radiosurgical ablation to be further improved.

Single Session Radiosurgery

Approximately 20 years ago the first acoustic neuroma patients were treated inside the Gamma Knife with frame-based radiosurgery. Of necessity, such treatment was initially administered during a single large session. This use of one “fraction” stemmed in large part from the need to target with a stereotactic frame, a device that attached to a patient’s skull by means of four screws. The frame was and continues to be necessary for the precise targeting of the Gamma Knife’s 201 beams of cobalt radiation towards the acoustic neuroma.

Since the frame is relatively uncomfortable, it was only really practical to perform radiosurgery in one day during a single session. Despite this apparent limitation, the results with single session radiosurgery have ushered in a revolution in the management of AN, and in many ways and for many years, have defined the gold standard for radiosurgical ablation of AN.

The earliest radiosurgical techniques have evolved considerably and include: 1) better imaging for targeting, 2) more complex and conformal shaping of the radiation, and 3) improved understanding of the optimal radiation dosing. As a result of these developments, the current outlook for a patient undergoing single session radiosurgery is a better than 98% chance of tumor control. Furthermore, the likelihood of facial nerve injury is only 1%, and in more than half of all patients, useful hearing can be saved. Since some patients may experience delayed deterioration in hearing many years after single stage radiosurgery, the exact hearing preservation rate is a little uncertain.

Ultimately, in those AN patients who have already suffered a significant loss of hearing, single session radiosurgery continues to represent an important treatment alternative to surgical resection.

Staged Radiosurgical Ablation

Although single session radiosurgery is a very powerful tool for managing AN, there has been growing interest and experience in treating these tumors with fractionated radiation, sometimes referred to as FSR. The objective behind this refinement is to further decrease the risk of injuring either the hearing or facial nerves. Spreading a dose of radiation over several (typically 3-5) treatments, known as staged radiosurgery, allows normal tissues, like the cochlear and facial nerves more time to recover from an injurious, but non-lethal, dose of radiation.

Using typical radiation doses and staged radiosurgery, there is almost no risk of facial nerve injury. Meanwhile, the rate of hearing preservation is approximately 85% at 2 years; the remaining patients experience some deterioration in their hearing, but most often this is not severe. To date there appears to be almost no risk of this procedure causing complete deafness. It will take many years of further follow-up to be sure that this level of hearing preservation is long-lived, but so far clinical experience and theory suggest that staged radiosurgery should be the safest of all treatments for AN.

Summary

The modern surgical treatment of acoustic neuromas continues to deliver ever better patient outcomes. Standard microsurgical resection (open surgery using a microscope) is most advisable for very young patients or those with large tumors. Meanwhile, stereotactic radiosurgery is an important treatment alternative for almost all other patients with acoustic neuromas. The non-invasive approach embodied by CyberKnife radiosurgery holds the promise of even better rates of hearing and facial nerve preservation. Ongoing studies are in the process of substantiating such clinical results.

References

Flickinger JC, Kondziolka D, Niranjan A, Lunsford LD: Results of acoustic neuroma radiosurgery: an analysis of 5 years’ experience using current methods. Journal of Neurosurgery. 2001 Jan: 94(1):1-6.
Poen JC, Golby AJ, Forster KM, Martin DP, Chinn DM, Hancock SL, Adler, JR Jr.: Fractionated stereotactic radiosurgery and preservation of hearing in patients with vestibular schwannoma: a preliminary report. Neurosurgery. 1999 Dec: 45(6): 1299-305; discussion 1305-7.