Vestibular Schwannoma

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.

Signs and 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 %).

Hearing loss
The first symptom in 90% of those with an acoustic neuroma is unexplained unilateral sensorineural hearing loss, meaning there is damage to the inner ear (cochlea) or nerve pathways from the inner ear to the brain. It involves a reduction in sound level, speech understanding and hearing clarity. In about 70 percent of cases there is a high frequency pattern of loss. The loss of hearing is usually subtle and worsens slowly, although occasionally a sudden loss of hearing may occur(i.e. sudden deafness). Hearing loss can vary from mild hearing loss to complete deafness.

Tinnitus
Unilateral tinnitus (ringing or hissing in the ears) is also a hallmark symptom of acoustic neuroma. Not all patients with tinnitus have acoustic neuroma and not all AN patients have tinnitus. Most of them do however, both before and after treatment.

Balance
Since the balance portion of the eighth nerve is where the tumor arises, unsteadiness and balance problems or even vertigo (the feeling like the world is spinning), may occur during the growth of the tumor. The remainder of the balance system sometimes compensates for this loss, and, in some cases, no imbalance will be noticed. Balance or vertigo is the third most common symptom in patients with acoustic neuromas (50% incidence). The onset of these may be subtle, like disorientation in dark hallways, and be dismissed as age related decline. These symptoms tend to occur later in the development of the tumor.

Pressure in the ear
Vestibular schwannoma patients sometimes complain of a feeling that their ear is plugged or “full”.

Facial weakness or paralysis
Larger tumors can press on the trigeminal nerve (CN V), causing facial numbness and tingling – constantly or intermittently. The facial nerve (CN VII) is rarely affected in the same way; however, due to its proximity to some structures of the inner and middle ear, it can be damaged during radiological treatment or surgical removal of the tumor, particularly in the case of large growths.

At the time some people learn they have an acoustic neuroma, they are also told that this tumor may involve the nerve that controls facial movement. However, it is much more common for treatment, rather than the tumor itself, to damage this nerve, leading to weakness or paralysis of the face. Taste, a sensation that reflects accurately sweet, sour, bitter and bland, is also a function of the facial nerve. Should any of the cranial nerves be damaged or need to be cut during surgery, it is sometimes possible for a neurosurgeon to microsuture the ends together; however, this is a new and very delicate specialist procedure, where long recovery times, incomplete healing and some permanent loss of function are to be expected.

Headache
Recurring headaches are an uncommon symptom, also tending to occur only in cases of larger tumors.

Advanced symptoms
Large tumors may cause disabling and life-threatening symptoms.

Large tumors that compress the adjacent brainstem may affect other local cranial nerves. The glossopharyngeal and vagus nerves are uncommonly involved, but their involvement may lead to altered gag or swallowing reflexes.

Larger tumors may lead to increased intracranial pressure, with its associated symptoms such as headache, vomiting, clumsy gait and mental confusion. This can be a life-threatening complication requiring urgent treatment.

Cause

3D rendering of acoustic neuroma.
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.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).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.

Treatment
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.

Observation
Since acoustic neuromas tend to be slow-growing and are benign tumors, careful observation over a period of time may be appropriate for some patients. When a small tumor is discovered in an older patient, observation to determine the growth rate of the tumor may be indicated if serious symptoms are not present. There is now good evidence from large observational studies that suggest many small tumors in older individuals do not grow, thus allowing tumors with no growth to be observed successfully. If the tumor grows, treatment may become necessary. Another example of a group of patients for whom observation may be indicated includes patients with a tumor in their only hearing or better hearing ear, particularly when the tumor is of a size that hearing preservation with treatment would be unlikely. In this group of patients, MRI is used to follow the growth pattern. Treatment is recommended if either the hearing is lost or the tumor size becomes life-threatening, thus allowing the patient to retain hearing for as long as possible.

Current studies suggest surgeons should observe small acoustic neuromas (those 1.5 cm or less).

Over a period of 10 years of observation with no treatment, 45% of patients with small tumors (and therefore minimal symptoms) lose functional hearing on the affected side; this percentage is considerably higher than that for patients actively treated with hearing-preserving microsurgery or radiosurgery.

Surgery
The goals of surgery are to control the tumor, and preserve function of the involved nerves (i.e. those involved in facial musculature and hearing). Preservation of hearing is an important goal for patients who present with functional hearing.Surgery cannot restore hearing already lost.

Microsurgical tumor removal can be done at one of three levels: subtotal removal, near total removal or total tumor removal. Many tumors can be entirely removed by surgery. Microsurgical techniques and instruments, along with the operating microscope, have greatly reduced the surgical risks of total tumor removal. Subtotal removal is indicated when anything further risks life or neurological function. In these cases the residual tumor should be followed for risk of growth (approximately 35%). If the residual grows further, treatment will likely be required. Periodic MRI studies are important to follow the potential growth rate of any tumor. Near total tumor removal is used when small areas of the tumor are so adherent to the facial nerve that total removal would result in facial weakness. The piece left is generally less than 1% of the original and poses a risk of regrowth of approximately 3%.

There are three main surgical approaches for the removal of an acoustic neuroma: translabyrinthine, retrosigmoid/sub-occipital and middle fossa. The approach used for each individual person is based on several factors such as tumor size, location, skill and experience of the surgeon, and whether hearing preservation is a goal. Each of the surgical approaches has advantages and disadvantages in terms of ease of tumor removal, likelihood of preservation of facial nerve function and hearing, and post-operative complications.

During surgery, intraoperative neurophysiological monitoring of the facial, acoustic and lower cranial nerves can reduce the risk of injury. In particular, following the 1991 NIH National institutes of Health Acoustic Neuroma Consensus Panel, the use of facial nerve monitoring has become a standard practice in the United States to reduce the risk of facial paralysis.

With massive tumors that compress the brainstem and cerebellum, staged surgical approaches or subtotal surgical resection followed by stereotactic radiosurgery may reduce the risks to life, brain and cranial nerves

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