NORD gratefully acknowledges Howard W. Francis, MD, Associate Professor, and Yuri Agrawal, MD, Assistant Professor, Division of Otology and Neurotology, Department of Otolaryngology-Head and Neck Surgery, Johns Hopkins University, for assistance in the preparation of this report.
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). There is no standard or typical pattern of symptom development, although hearing loss in one ear (unilateral) is the initial symptom in approximately 90 percent of cases. Additional findings include ringing in the ears (tinnitus) and dizziness. 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 forms is unknown.
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 cases. 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 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. Although rare, complications 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 cases, 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 brain and results in a variety of symptoms including headaches, an impaired ability to coordinate voluntary movements (ataxia), and mental confusion. Headaches may also occur in the absence of hydrocephalus and in 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.
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 under way 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.
Acoustic neuromas affect women more often than men. Most cases of acoustic neuroma develop in individuals between the ages of 30 and 60. Although quite rare, they can develop in children. Acoustic neuromas are estimated to affect 1 in 100,000 people in the general population. Approximately 2,500 new cases are diagnosed each year. The incidence has risen in the last several years, which some researchers attribute to the greater frequency in which small tumors are recognized. However, many individuals with small acoustic neuromas may remain undiagnosed, making it difficult to determine its true frequency in the general population.
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 a 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.
The treatment of an acoustic neuroma may involve observation (if the tumor 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).
This option may be preferred in cases 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, watch and wait 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.
Surgery performed with specialized instruments under a microscope (microsurgery) may be necessary in 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 in 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, 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, alternatively, 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 stops the growth of a tumor. Radiation therapy provides a noninvasive treatment option for individuals with an acoustic neuroma, but in some cases it may take weeks, months or even a couple years to see significant effects from this treatment. Tumors treated with radiation therapy can start to grow again at some point later on.
Post-treatment problems (from either surgery or radiation therapy) may include: cranial nerve deficits such 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 flow of cerebrospinal fluid can also occur. Cerebrospinal fluid leakage or an infection that produces meningitis are rare complications of surgical therapy.
Damage to the facial nerve and eye problems are two potential complications of treatment for an acoustic neuroma. The facial nerve may be damaged by the acoustic neuroma or as a result of surgery. In some cases, 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 the hypoglossal nerve (nerve that controls the tongue) in the neck. This may bring some improvement in function to the muscles of the face. There are also other surgical procedures to aid in reanimating the sagging face.
Eye problems may develop in some individuals following surgical removal of an acoustic neuroma. Facial weakness in particular leaves the affected eye vulnerable to damage of the cornea potentially leading to blindness. The eye must be kept moist with frequent use of artificial tears, and a barrier applied especially 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 it is possible that food may "get lost" in the mouth on the affected side, which could contribute to dental problems.
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