What Is Bilateral Acoustic Neuroma?

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About Bilateral Acoustic Neuroma

A vestibular schwannoma (also known as acoustic neuroma, acoustic neurinoma, or acoustic neurilemoma) is a benign, usually slow-growing tumor that develops from the balance and hearing nerves supplying the inner ear. The tumor comes from an overproduction of Schwann cells the cells that normally wrap around nerve fibers like onion skin to help support and insulate nerves.

As the vestibular schwannoma grows, it affects the hearing and balance nerves, usually causing unilateral (one-sided) or asymmetric hearing loss, tinnitus (ringing in the ear), and dizziness/loss of balance. As the tumor grows, it can interfere with the face sensation nerve (the trigeminal nerve), causing facial numbness.

Vestibular schwannomas can also affect the facial nerve (for the muscles of the face) causing facial weakness or paralysis on the side of the tumor. If the tumor becomes large, it will eventually press against nearby brain structures (such as the brainstem and the cerebellum), becoming life-threatening.

A vestibular schwannoma (VS) is a benign primary intracranial tumor of the myelin-forming cells of the vestibulocochlear nerve (8th cranial nerve). A type of schwannoma, this tumor arises from the Schwann cells responsible for the myelin sheath that helps keep peripheral nerves insulated.

Although it is also called an acoustic neuroma, this a misnomer for two reasons. First, the tumor usually arises from the vestibular division of the vestibulocochlear nerve, rather than the cochlear division. Second, it is derived from the Schwann cells of the associated nerve, rather than the actual neurons (neuromas).

Approximately 2,000 to 3,000 cases are diagnosed each year in the United States (6 to 9 per million persons).

Comprehensive studies from Denmark published in 2012 showed an annual incidence of 19-23 per million from 2002 to 2008, over the last 30 years the reported incidence have been increasing, until the last decade in which an approximation of the true incidence may have been found.Most recent publications suggest that the incidence of vestibular schwannomas have been rising because of advances in MRI scanning.

Most cases are diagnosed in people between the ages of 30 and 60, and men and women appear to be affected equally.

Most vestibular schwannomas occur spontaneously in those without a family history. One confirmed risk factor is a rare genetic mutation called NF2.

The primary symptoms of vestibular schwannoma are unexplained progressive unilateral hearing loss and tinnitus, and vestibular (disequilibrium) symptoms. Treatment of the condition is by surgery or radiation, and often results in substantial or complete hearing loss in the affected ear. Observation (non-treatment) over time also usually results in hearing loss in the affected ear.

Acoustic neuroma, also known as vestibular schwannoma, is a noncancerous and usually slow-growing tumor that develops on the main (vestibular) nerve leading from your inner ear to your brain. Branches of this nerve directly influence your balance and hearing, and pressure from an acoustic neuroma can cause hearing loss, ringing in your ear and unsteadiness.

Acoustic neuroma usually arises from the Schwann cells covering this nerve and grows slowly or not at all. Rarely, it may grow rapidly and become large enough to press against the brain and interfere with vital functions.

Treatments for acoustic neuroma include regular monitoring, radiation and surgical removal.

Diagnosed

Unilateral/asymmetric hearing loss and/or tinnitus and loss of balance/dizziness are early signs of a vestibular schwannoma. Unfortunately, early detection of the tumor is sometimes difficult because the symptoms may be subtle and may not appear in the beginning stages of growth. Also, hearing loss, dizziness, and tinnitus are common symptoms of many middle and inner ear problems (the important point here is that unilateral or asymmetric symptoms are the worrisome ones).

Once the symptoms appear, a thorough ear examination and hearing and balance testing (audiogram, electronystagmography, auditory brainstem responses) are essential for proper diagnosis. Magnetic resonance imaging (MRI) scans are critical in the early detection of a vestibular schwannoma and are helpful in determining the location and size of a tumor and in planning its microsurgical removal.

Symptoms

Early symptoms are easily overlooked, sometimes mistaken for the normal changes of aging or attributed to noise exposure earlier in life, often delaying diagnosis. The most prevalent symptoms in patients suffering from vestibular schwannoma is hearing loss (94 %), tinnitus (83 %) and vertigo (49 %).

Signs and symptoms of acoustic neuroma are often subtle and may take many years to develop. They usually arise from the tumor’s effects on the hearing and balance nerves. Pressure from the tumor on adjacent nerves controlling facial muscles and sensation (facial and trigeminal nerves), nearby blood vessels, or brain structures may also cause problems.

As the tumor grows, it may be more likely to cause more noticeable or severe signs and symptoms.

Common signs and symptoms of acoustic neuroma include:

  • Hearing loss, usually gradual — although in some cases sudden — and occurring on only one side or more pronounced on one side
  • Ringing (tinnitus) in the affected ear
  • Unsteadiness, loss of balance
  • Dizziness (vertigo)
  • Facial numbness and very rarely, weakness or loss of muscle movement

In rare cases, an acoustic neuroma may grow large enough to compress the brainstem and become life-threatening.

Cause

The cause of acoustic neuromas is usually unknown; however there is a growing body of evidence that sporadic defects in tumor suppressor genes may give rise to these tumors in some individuals. In particular, loss or mutation of a tumor suppressor gene on the long arm of chromosome 22 is strongly associated with vestibular schwannomas.[medical citation needed] Other studies have hinted at exposure to loud noise on a consistent basis.

One study has shown a relationship between acoustic neuromas and prior exposure to head and neck radiation, and a concomitant history of having had a parathyroid adenoma (tumor found in proximity to the thyroid gland controlling calcium metabolism).[medical citation needed] There are even controversies on hand held cellular phones.

Whether or not the radiofrequency radiation has anything to do with acoustic neuroma formation, remains to be seen. To date, no environmental factor (such as cell phones or diet) has been scientifically proven to cause these tumors. The Acoustic Neuroma Association (ANA) does recommend that frequent cellular phone users use a hands free device to enable separation of the device from the head.

Although there is an inheritable condition called Neurofibromatosis Type 2 (NF2) which can lead to acoustic neuroma formation in some people, most acoustic neuromas occur spontaneously without any evidence of family history (95%).

NF2 occurs with a frequency of 1 in 30,000 to 1 in 50,000 births. The hallmark of this disorder is bilateral acoustic neuromas (an acoustic neuroma on both sides) usually developing in late childhood or early adulthood, frequently associated with other brain and spinal chord tumors.

The cause of acoustic neuromas appears to be a malfunctioning gene on chromosome 22. Normally, this gene produces a tumor suppressor protein that helps control the growth of Schwann cells covering the nerves.

What makes this gene malfunction isn’t clear, and in most cases of acoustic neuroma, there is no identifiable cause. This faulty gene is also inherited in neurofibromatosis type 2, a rare disorder that usually involves the growth of tumors on balance nerves on both sides of your head (bilateral vestibular schwannomas).

Diagnosis

The Gold Standard for diagnosis of vestibular schwannoma is without doubt Gadolinium enhanced magnetic resonance imaging (MRI) yet several examinations may arise suspicion of vestibular schwannomas.

Routine auditory tests may reveal a loss of hearing and speech discrimination (the patient may hear sounds in that ear, but cannot comprehend what is being said). Pure tone audiometry should be performed to effectively evaluate hearing in both ears. In some clinics the clinical criteria for follow up testing for AN is a 15 dB differential in thresholds between ears for three consecutive frequencies.

An auditory brainstem response test (a.k.a. ABR) is a much more cost effective screening alternative to MRI for those at low risk of AN. This test provides information on the passage of an electrical impulse along the circuit from the inner ear to the brainstem pathways. An acoustic neuroma can interfere with the passage of this electrical impulse through the hearing nerve at the site of tumor growth in the internal auditory canal, even when hearing is still essentially normal.

This implies the possible diagnosis of an acoustic neuroma when the test result is abnormal. An abnormal auditory brainstem response test should be followed by an MRI. The sensitivity of this test is proportional to the tumor size – the smaller the tumor, the more likely is a false negative result; small tumors within the auditory canal will often be missed. However, since these tumors would usually be watched rather than treated, the clinical significance of overlooking them may be negligible.

Advances in scanning and testing have made possible the identification of small acoustic neuromas (those still confined to the internal auditory canal). MRI using Gadolinium as an enhancing contrast material is the preferred diagnostic test for identifying acoustic neuromas. The image formed clearly defines an acoustic neuroma if it is present and this technique can identify tumors measuring down to 5 millimeters in diameter (the scan spacing).[citation needed]

When an MRI is not available or cannot be performed, a computerized tomography scan (CT scan) with contrast is suggested for patients in whom an acoustic neuroma is suspected. The combination of CT scan and audiogram approach the reliability of MRI in making the diagnosis of acoustic neuroma.

Treatment

Early diagnosis of a vestibular schwannoma is key to preventing its serious consequences.

There are three options for managing a vestibular schwannoma:

(1) surgical removal,

(2) radiation, and

(3) observation.

Sometimes, the tumor is surgically removed (excised). The exact type of operation done depends on the size of the tumor and the level of hearing in the affected ear. If the tumor is small, hearing may be saved and accompanying symptoms may improve by removing it to prevent its eventual effect on the hearing nerve.

As the tumor grows larger, surgical removal is more complicated because the tumor may have damaged the nerves that control facial movement, hearing, and balance and may also have affected other nerves and structures of the brain.

The removal of tumors affecting the hearing, balance, or facial nerves can sometimes make the patient’s symptoms worse because these nerves may be injured during tumor removal.

As an alternative to conventional surgical techniques, radiosurgery (that is, radiation therapy—the “gamma knife” or LINAC) may be used to reduce the size or limit the growth of the tumor. Radiation therapy is sometimes the preferred option for elderly patients, patients in poor medical health, patients with bilateral vestibular schwannoma (tumor affecting both ears), or patients whose tumor is affecting their only hearing ear.

When the tumor is small and not growing, it may be reasonable to “watch” the tumor for growth. MRI scans are used to carefully monitor the tumor for any growth.

There are three treatment options available to a patient. These options are observation, microsurgical removal and radiation (radiosurgery or radiotherapy). Determining which treatment to choose involves consideration of many factors including the size of the tumor, its location, the patient’s age, physical health and current symptoms.

About 25% of all acoustic neuromas are treated with medical management consisting of a periodic monitoring of the patient’s neurological status, serial imaging studies, and the use of hearing aids when appropriate. One of the last great obstacles in the management of acoustic neuromas is hearing preservation and/or rehabilitation after hearing loss. Hearing loss is both a symptom and concommitant risk, regardless of the treatment option chosen. Treatment does not restore hearing already lost, though there are a few rare cases of hearing recovery reported.

A diagnosis of NF2 related bilateral acoustic neuromas creates the possibility of complete deafness if the tumors are left to grow unchecked. Preventing or treating the complete deafness that may befall individuals with NF2 requires complex decision making. The trend at most academic U.S. medical centers is to recommend treatment for the smallest tumor which has the best chance of preserving hearing.

If this goal is successful, then treatment can also be offered for the remaining tumor. If hearing is not preserved at the initial treatment, then usually the second tumor, in the only-hearing ear, is just observed. If it shows continued growth and becomes life-threatening, or if the hearing is lost over time as the tumor grows, then treatment is undertaken. This strategy has the highest chance of preserving hearing for the longest time possible.[13]

There is variability in the rate of growth of these tumours, and as such, the decision to treat requires consideration of patient’s age and co-morbidities.

The options include 6 :

  • observation and follow-up
  • stereotactic radiosurgery
  • microsurgery: number of approaches are possible, including 8:
  • retrosigmoid (trans-meatal) (suboccipital)
  • able to preserve hearing
  • can be used for large tumours
  • limited view of the internal auditory canal
  • has a greater chance of residual tumour (in the lateral aspect of internal auditory canal)
  • middle cranial fossa
  • best for small intracanalicular tumours
  • able to preserve hearing
  • translabyrinthine
  • careful skeletonisation of the facial nerve required (i.e. intraoperative facial nerve monitoring by needle electromyography with continuous stimulation)
  • loss of hearing guaranteed

Overall tumour recurrence is low, ranging between 1 and 9% 8.

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