Acoustic Neuroma

Types of Acoustic Neuromas (Vestibular Schwannomas)
Causes
Symptoms
Diagnosis
Treatment

An acoustic neuroma is a type of non-cancerous (benign) brain tumor. It’s also known as a vestibular schwannoma. A benign brain tumor is a growth in the brain that usually grows slowly over many years and does not spread to other parts of the body.

An acoustic neuroma, also known as a vestibular schwannoma, is a rare benign (non-cancerous) growth that develops on the eighth cranial nerve. This nerve runs from the inner ear to the brain and is responsible for hearing and balance (equilibrium). Although there is no standard or typical pattern of symptom development, hearing loss in one ear (unilateral) is the initial symptom in approximately 90 percent of affected individuals. Additional common findings include ringing in the ears (tinnitus) and dizziness or imbalance. The symptoms of an acoustic neuroma occur from the tumor pressing against the eighth cranial nerve and disrupting its ability to transmit nerve signals to the brain. An acoustic neuroma is not cancerous (malignant); it does not spread to other parts of the body. The reason an acoustic neuroma form is unknown.

Types of Acoustic Neuromas (Vestibular Schwannomas)

There are two types of acoustic neuromas:

Sporadic, unilateral acoustic neuromas. These tumors only grow on one side of the body in 95% of patients. They occur from sporadic (sudden), nonhereditary mutations. These unilateral acoustic neuromas may develop at any age, but most commonly occur in people between the ages of 30 and 60.
Genetic, bilateral acoustic neuromas. Acoustic neuromas on both sides of the body only occur in people who have the genetic disorder neurofibromatosis type 2, a mutation in chromosome 22 that affects the gene responsible for the production of Schwann cells. These patients often have other schwannoma-like tumors throughout the body, and treatments for these tumors are often different from the treatment for unilateral tumors.

Causes

The exact cause of an acoustic neuroma is unknown. Most cases seem to arise for no apparent reason (spontaneously). No specific risk factors for the development of these tumors have been identified.

A variety of potential risk factors for acoustic neuroma have been studied including prior exposure to radiation to the head and neck area (as is done to treat certain cancers) or prolonged or sustained exposure to loud noises (as in an occupational setting). Research is underway to determine the specific cause and risk factors associated with an acoustic neuroma.

In a small subset of cases, acoustic neuromas occur as part of a rare disorder known as neurofibromatosis type II. This rare genetic disorder is usually associated with acoustic neuromas affecting both ears at once (bilateral). (For more information on this disorder, choose “neurofibromatosis” as your search term in NORD’s Rare Disease Database.)

An acoustic neuroma arises from a type of cell known as the Schwann cell. These cells form an insulating layer over all nerves of the peripheral nervous system (i.e., nerves outside of the central nervous system) including the eighth cranial nerve. The eighth cranial nerve is separated into two branches the cochlear branch, which transmits sound to the brain, and the vestibular branch, which transmits balance information to the brain. Most acoustic neuromas occur on the vestibular portion of the eighth cranial nerve. Because these tumors are made up of Schwann cells and usually occur on the vestibular portion of the eighth cranial nerve, many physicians prefer the use of the term vestibular schwannoma. However, the term acoustic neuroma is still used more often in the medical literature.

Related Disorders

Symptoms of the following disorders can be similar to those of acoustic neuromas. Comparisons may be useful for a differential diagnosis.

Meniere’s disease is a rare disorder affecting the inner ear that is characterized by periodic episodes of rotary vertigo or dizziness; progressive, fluctuating, low-frequency (low-pitch) hearing loss; ringing in the ears (tinnitus), and a feeling of fullness or pressure in the ear. Symptoms may occur daily or only a couple of times a year. Symptoms may develop suddenly. Over time hearing loss and tinnitus may become permanent. Vertigo can be severe and cause nausea, vomiting, and sweating. The exact cause of Meniere’s disease is unknown (idiopathic). (For more information on this disorder, choose “Meniere” as your search term in the Rare Disease Database.)

Bell’s palsy is a non-progressive neurological disorder of one of the facial nerves (7th cranial nerve). This disorder is characterized by the sudden onset of facial paralysis that may be preceded by a slight fever, pain behind the ear on the affected side, a stiff neck, and weakness and/or stiffness on one side of the face. Paralysis results from decreased blood supply (ischemia) and/or compression of the 7th cranial nerve. The exact cause of Bell’s palsy is not known. Viral (e.g., herpes zoster virus) and immune disorders are frequently implicated as a cause of this disorder. There may also be an inherited tendency toward developing Bell’s palsy. (For more information on this disorder, choose “Bell’s palsy” as your search term in the Rare Disease Database.)

Meningioma is a rare and typically benign (non-cancerous) tumor that can mimic an acoustic neuroma. While they can occur in any area of the lining of the brain and spinal cord (dura), meningiomas can form adjacent to the hearing and balance nerves and cause symptoms of unilateral (one-sided) hearing loss, dizziness, or imbalance, facial numbness, and/or swallowing difficulty. Magnetic resonance imaging (MRI) can often distinguish between an acoustic neuroma and a meningioma but there are times when even a skilled radiologist is unable to definitively differentiate between the two. If surgery is performed, a pathologist will make the final diagnosis based on the microscopic appearance (histology).

Symptoms

Some individuals, especially those with small tumors, may not have any associated symptoms (asymptomatic). However, even small tumors, depending upon their location, can cause significant symptoms or physical findings.

Acoustic neuromas are slow-growing tumors that can eventually cause a variety of symptoms by pressing against the eighth cranial nerve. Hearing loss in one ear (the ear affected by the tumor) is the initial symptom in approximately 90 percent of patients. Hearing loss is usually gradual, although in some rare cases it can be sudden. In some cases, hearing loss can also fluctuate (worsen and then improve). Hearing loss may be accompanied by ringing in the ears, a condition is known as tinnitus, or by a feeling of fullness in the affected ear. In some cases, affected individuals may have difficulty understanding speech that is disproportional to the amount of hearing loss.

Acoustic neuromas can also cause dizziness and problems with balance such as unsteadiness. In rare cases, dizziness or balance problems may occur before noticeable hearing loss. Because these tumors usually grow very slowly, the body can often compensate for these balance problems.

Although slow-growing, acoustic neuromas can eventually become large enough to press against neighboring cranial nerves. While rare, symptoms resulting from the involvement of other cranial nerves include facial weakness or paralysis, facial numbness or tingling, and swallowing difficulties. Facial numbness or tingling can be constant or it may come and go (intermittent).

In some patients, acoustic neuromas may grow large enough to press against the brainstem, preventing the normal flow of cerebrospinal fluid between the brain and spinal cord. This fluid can accumulate in the skull, leading to a phenomenon called hydrocephalus, which causes pressure on the tissues of the brain and results in a variety of symptoms including headaches, and an impaired ability to coordinate voluntary movements (ataxia), and mental confusion. Headaches may also occur in the absence of hydrocephalus and some rare cases may be the first sign of an acoustic neuroma. In very rare cases, an untreated acoustic neuroma that presses on the brain can cause life-threatening complications.

One-sided Hearing Loss

Over 90% of people with acoustic neuromas develop some degree of one-sided (unilateral) hearing loss. People with this type of hearing loss may have difficulty hearing in noisy settings and locating where a sound is coming from. If a person tends to hold a phone to a certain ear or has a hard time following conversation in a crowded room, these may be signs of hearing loss.

The hearing loss usually gets worse over the years and may lead to total deafness in one ear. In some people (about 5% of patients), hearing loss may develop suddenly. The loss may be partial or total, and spontaneous recovery is possible. Sudden hearing loss may be the first event that leads to a diagnosis, or it may occur months or years before the tumor is discovered.

Hearing loss can occur from compression or infiltration of the tumor on the hearing nerve or secretion of substances toxic to hearing. It can affect the range of sounds heard, as well as the clarity of sound. Hearing loss can occur even if the tumor is not growing.

A small portion of patients with acoustic neuroma may not yet have hearing loss from the tumor. Mild hearing loss due to an acoustic neuroma might not even be noticeable, which can result in a delayed diagnosis.

Ear Fullness

People with an acoustic neuroma might have a sensation of fullness in the ear as if water is in it. This sensation is typically caused by hearing loss from the tumor.

Noise in the Ear (Tinnitus)

Tinnitus is a very common symptom of acoustic neuromas and many other inner ear conditions. People with acoustic neuromas may experience a high-pitched tone in the ear affected by the tumor. In other cases, the tinnitus can sound like hissing, buzzing, or roaring — like when putting a seashell to the ear.

While most patients with acoustic neuromas have both tinnitus and hearing loss in one ear, some may experience tinnitus without losing hearing. Tinnitus can come and go or be constant — with single or multiple tones — and can sound quiet or overwhelmingly loud.

Balance Problems and Vertigo

Because acoustic neuromas arise from the vestibular nerve responsible for balance, unsteadiness or balance problems may be early symptoms of acoustic neuroma. Nearly half of people with acoustic neuromas notice these symptoms, which tend to worsen if the tumor grows. Large acoustic neuromas may compress parts of the cerebellum, which may lead to falls. Patients tend to fall toward the side of the tumor.

The balance system can compensate for the loss of balance, so it may stabilize.

True vertigo (the sensation of spinning or tilting) is not commonly associated with acoustic neuromas, but it can sometimes occur due to tumor growth or bleeding.

Other Signs of Acoustic Neuroma

Acoustic neuromas can also put pressure on other important cranial nerves that are adjacent to where these tumors grow.

Numbness in the face can result from a tumor pressing on the trigeminal nerve. There may be ongoing or periodic numbness and facial tingling on the side of the acoustic neuroma. Tingling (paresthesia) may be near the corner of the mouth or on the cheek. There may also be eye irritation or redness due to numbness in the eye that prevents appropriate blink reflexes.
Facial twitching or weakness can result from the tumor pressing on the facial nerve. This can cause twitching (tics or spasms) of the eye, eyebrow, forehead, or mouth muscles. Less often, you might notice weakness in the face. Facial weakness often does not occur until acoustic tumors grow quite large, and it is less common at the time of diagnosis.
Swallowing problems can occur from the tumor pressing on the vagal and hypoglossal nerves. These nerves control several aspects important to swallowing, including sensation in the throat and movement of vocal cords and the tongue.
Change in taste and tear production is a less common symptom, but they should be evaluated by a doctor. The facial nerve helps control taste and tear formation. Pressure on the nerve can cause dry eyes or even unexpected tears, as well as changes in taste perception.
Headache and pressure: As the acoustic neuroma grows, it can press on the lining of the inside of the skull (the dura). The dura has sensory fibers that can transmit the sensation of pressure. The headache that results from the acoustic neuroma can be dull or aching, and it is usually on one side of the head. The pain may radiate to the neck or the top or front of the head.

These symptoms can be caused by many other, more common health issues such as cholesteatoma, labyrinthitis and vestibular neuritis, and Meniere’s disease. If you have more than a few of these symptoms (especially if they don’t go away or become worse), your doctor can help you decide whether more testing is necessary.

Any symptoms tend to develop gradually and often include:

hearing loss that usually only affects 1 ear
hearing sounds that come from inside the body (tinnitus)
the sensation that you’re moving or spinning (vertigo)
persistent headaches
temporary blurred or double vision
numbness, pain, or weakness on 1 side of the face
problems with limb co-ordination (ataxia) on 1 side of the body
a hoarse voice or difficulty swallowing

Diagnosis

A diagnosis of an acoustic neuroma is made based upon a thorough clinical evaluation, a detailed patient history, identification of characteristic findings, and a variety of specialized tests. Such tests include hearing exams, x-ray scans such as magnetic resonance imaging (MRI) or computed tomography (CT), a specialized test that evaluates balance (electronystagmography), and brainstem auditory evoked response (BAER).

An MRI uses a magnetic field and radio waves to produce cross-sectional images of particular organs and bodily tissues. During CT scanning, a computer and x-rays are used to create a film showing cross-sectional images of certain tissue structures. MRIs are the most sensitive study to confirm the presence of an acoustic neuroma.

An electronystagmography test evaluates balance by detecting abnormal, involuntary eye movements, a condition known as nystagmus. Nystagmus may occur as a result of inner ear complications such as an acoustic neuroma.

A BAER exam checks hearing and neurological function and interaction by recording the brain’s response to certain sounds. Since an acoustic neuroma can disrupt the nerve pathway that relays sound from the ear to the brain, a positive result of a BAER exam could be caused by these tumors.

Acoustic neuroma diagnosis includes:

A hearing test (audiometry). This is a test of hearing function that measures how well you hear sounds and speech. It is usually the first test performed to diagnose acoustic neuroma. A doctor asks you to listen to sounds and speech while you are wearing earphones attached to a machine that records responses and measures hearing function. If you have an acoustic neuroma, your audiogram may show the following.
Increased pure tone average (PTA). This metric evaluates how loud a sound frequency needs to be before you hear it.
Increased speech reception threshold (SRT). This metric evaluates how loud speech needs to be before you hear it. Similar to the pure-tone average, the higher the score, the worse the hearing.
Decreased speech discrimination (SD). This metric evaluates how many words you can detect, one ear at a time. The lower the score, the worse the hearing.
Imaging scans of the head. If other tests point to a possibility of acoustic neuroma, MRI can confirm the diagnosis. MRI with a contrast dye can help pinpoint the tumor. If an acoustic neuroma is present, it will soak up more dye than normal brain tissue and appear clearly on the scan. MRI commonly shows a densely “enhancing” (bright) tumor in the internal auditory canal.

Treatment

The treatment of an acoustic neuroma may involve observation (if the tumor is small and does not cause symptoms), surgical removal (microsurgery or excision) of the tumor, or the use of radiation to stop the tumor from growing (radiation therapy or radiosurgery).

Observation
This option may be preferred in affected individuals where no associated symptoms are present or where a small tumor is not growing or growing at a slow rate. This period of observation may be called “watch and wait”. In elderly individuals who do not have symptoms, watching and waiting may be appropriate because an acoustic neuroma may not require treatment during an individual’s normal life expectancy and the inherent risks and complications of removal can be avoided.

Watchful waiting is also appropriate if an individual with hearing in only one ear is found with an acoustic neuroma in that ear. The patient may choose to live with the acoustic neuroma as long as it is not a life-threatening condition rather than risk further hearing loss that can potentially occur from therapy.

If an acoustic neuroma eventually causes symptoms, then radiation therapy or microsurgery may be necessary. There is not a single, “best” therapy for all affected individuals. The specific location and size of an acoustic neuroma as well as an affected individual’s overall level of hearing and general health are all considered when determining which treatment method is used.

Microsurgery
Surgery performed with specialized instruments under a microscope (microsurgery) may be necessary for some individuals with an acoustic neuroma. Microsurgery allows physicians to perform surgery on very small body parts.

During microsurgery, a physician may remove all or part of an acoustic neuroma. Partial tumor removal is undertaken to reduce the risk of unwanted surgical complications. In other words, it may be easier and safer to take out part of the growth rather than the whole tumor. If the tumor is very large or if the person is older, partial removal may be more appropriate. Further surgery may be necessary for the future if partial tumor removal is performed.

When total tumor removal is indicated, the objective of the procedure is to protect the facial nerve and avoid facial paralysis. In addition, the surgeon tries to preserve hearing as much as possible in the affected ear.

Three different surgical approaches are commonly used for individuals with an acoustic neuroma: retrosigmoid (suboccipital), middle fossa, and translabyrinthine. The size and location of the tumor as well as additional factors are all weighed when determining which approach is used.

Radiation Therapy (Radiosurgery or Radiotherapy)
Three-dimensional focusing of radiation has become more accurate in recent years so that affected individuals may be treated at one session on an outpatient basis or smaller doses may be delivered over several sessions. The objective is to aim so accurately that the tumor cells are affected and damage to surrounding cells is minimized. Radiation therapy can stop the growth of a tumor. Radiation therapy provides a non-invasive treatment option for individuals with an acoustic neuroma, but in some patients, it may take weeks, months, or even a couple of years to see significant effects from this treatment. Tumors treated with radiation therapy can start to grow again at some point later on.
Surgery to remove the tumor. This is a highly effective treatment for acoustic neuromas. Hearing loss that has already occurred from the tumor cannot be reversed, but the remaining hearing can be preserved in some cases. Surgical tumor removal can often address balance problems, facial numbness, and other symptoms.
Stereotactic radiosurgery. This form of radiation therapy delivers precisely targeted radiation to the tumor while avoiding the surrounding healthy tissue. Radiation does not cause the tumor to go away. Rather, the goal is to stop or slow the growth. Typically, radiation therapy is not recommended for young patients and those with larger tumors.
Suboccipital or retrosigmoid craniotomy. During this procedure, the surgeon accesses the acoustic neuroma from the back of the head. It offers the best view of the brainstem, particularly for the nerves involved in swallowing, which can be affected if a tumor is large. This approach is often recommended for patients with larger tumors. It also might preserve hearing for patients with smaller tumors, because the inner ear structures are preserved.
Translabyrinthine craniotomy. During this procedure, the surgeon removes the bone behind the ear to access the tumor through the inner ear. This provides the best view of the entire length of the facial nerve and can require less retraction of the brain. However, it requires going through the structures of the inner ear and does not allow the preservation of hearing. This approach is generally considered for patients who have no functional hearing.
Middle fossa craniotomy. This is an option for patients with smaller acoustic neuromas and intact hearing. It is generally considered to provide the greatest chance of hearing preservation, but there is a slightly greater risk of facial nerve weakness after the surgery, and it cannot be performed for medium or large-size tumors.
Proton beam therapy. This type of radiation therapy uses high-energy beams of positively charged particles called protons. Protons are delivered to the affected area in targeted doses to treat tumors and minimize radiation exposure to the surrounding area.

Investigational Therapies

There are many ongoing research investigations to better understand why acoustic neuromas form and continue to enlarge. For example, growth rates of acoustic neuroma tumors are related to the expression levels of the common inflammatory chemical cyclooxygenase 2. Recent research has shown that the use of aspirin (a commonly used cyclooxygenase 2 inhibitor) among patients with an acoustic neuroma is correlated with less tumor growth in a retrospective analysis. This was further supported by in vitro research showing that aspirin may lead to decreased rate of acoustic neuroma growth tumor culture.

Several investigations are underway looking into the potential use of other drugs for the treatment of acoustic neuromas primarily in patients with neurofibromatosis type II (NF2) with promising results. These include drugs that inhibit several cellular mechanisms including vascular endothelial growth factor (bevacizumab and PTC299), phosphoinositide-dependent kinase-1 (OSU-03012), and ERBB2 receptor (Trastuzumab), epidermal growth factor (Erlotinib and Lapatinib), and p-21 activated kinases (IPA-3).

While continued work needs to be done, research into these and other drugs may lead to effective treatments for those with familial and sporadic acoustic neuromas in the future.

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government website.

Complications

Post-treatment problems (from either surgery or radiation therapy) may include cranial nerve deficits such as facial weakness or numbness, hearing loss, and dizziness. Headache, obstruction of fluid that surrounds the brain and spinal cord (cerebrospinal fluid), and/or decreased mental alertness due to blood clots or obstruction of the flow of cerebrospinal fluid can also occur. Cerebrospinal fluid leakage or an infection that produces meningitis are rare complications of surgical therapy.

The facial nerve may be damaged by the acoustic neuroma or as a result of surgery. In some affected individuals, it may be necessary for the surgeon to remove portions of the facial nerve, resulting in temporary or permanent facial paralysis. The regrowth of the nerve (regeneration) and restoration of function to the muscles of the face may take up to a year. If the facial paralysis persists, a second surgery may be performed to connect the healthy portion of the facial nerve to another nerve such as the hypoglossal nerve (the nerve that controls the tongue) in the neck or the nerve to masseter (the nerve that helps with chewing) in the face. This may bring some improvement in the function of the muscles several other surgical procedures that can aid in reanimating the face that can improve the function and appearance of the weakened side of the face.

Eye problems may develop in some individuals following surgical removal of an acoustic neuroma. Facial weakness can bring about incomplete eyelid closure on the affected side which may lead to irritation of the cornea. In rare instances, this has the potential to lead to blindness of the affected eye. The eye must be kept moist with frequent use of artificial tears, and a barrier applied during sleep, such as a moisture chamber, or taped closed. The use of an eye patch is discouraged as it may contribute to corneal damage.
Double vision (diplopia) may occur if there is pressure on the 6th cranial nerve, and there may be impairment of the muscles of the eyelids. Artificial tears or eye lubricants may be needed.

Additionally, if prolonged facial paralysis is not treated, then the food may “get lost” in the mouth on the affected side, which could contribute to dental problems.

References