Diagnosing Acoustic Neuroma

Acoustic neuroma, also called vestibular schwannoma, is a slow-growing, benign tumor that develops on the eighth cranial nerve—the nerve that carries signals from the inner ear to the brain. These signals help the brain process sounds as well as information that allows you to keep your balance. Acoustic neuromas arise from Schwann cells, which provide insulation—called myelin—that helps nerve cells with transmission of electrical signals.

Because these tumors grow from and press on the eighth cranial nerve, the first symptom people may experience is hearing loss in one ear. Tinnitus, or ringing in the ear, is also common. Balance problems may also make people with acoustic neuromas feel dizzy or lightheaded, causing instability and imbalance.

As acoustic neuromas grow, they can press on an important nearby nerve called the seventh cranial nerve, causing facial weakness. The seventh and eighth cranial nerves run from the inner ear area through a bony structure of the skull called the internal auditory canal to the brain stem, the area at the base of the brain.

Pain or numbness may also occur when the tumor gets larger and if it presses on the fifth cranial nerve, also called the trigeminal. The fifth cranial nerve runs from the brain stem to various muscles in the face.

Most acoustic neuromas develop spontaneously, meaning for no apparent reason. However, people with a genetic condition called neurofibromatosis type 2 have an increased risk of developing this type of tumor, almost always on the eighth cranial nerves on both sides of the head, or bilaterally.

To diagnose an acoustic neuroma, NYU Langone doctors perform a physical exam, ask about your medical history and symptoms, and request hearing testing and brain imaging.

Audiogram

If you are having problems with your hearing, your doctor may order a hearing test called an audiogram. During this test, you sit in a soundproof booth and raise your hand each time you hear certain sounds in the headphones you’re wearing. Your responses are used to create a graph that shows an audiologist at what frequencies and volume you can perceive sound. During an audiogram, the audiologist also tests how well you understand speech, including how well you hear words in noise.

These results can tell your doctor how a tumor might be affecting your hearing. Asymmetric unilateral hearing loss, meaning the loss occurs in or is greater in one ear, may be a sign of acoustic neuroma; it usually prompts your doctor to order other tests.

Otoacoustic Emissions Test

When the cochlea, the part of the inner ear that processes sound, is stimulated, it produces vibrations called otoacoustic emissions. These vibrations typically echo into the middle ear and can be measured with a small probe that your doctor places in the ear canal. People with hearing loss because of damage to the cochlea have unusual vibrations that can be detected with this test.

Auditory Brain Stem Response Test

An auditory brain stem response test provides doctors with information about how well the sound signal is traveling from the inner ear, along the eighth cranial nerve, to the brain stem and to its upper auditory areas. Delay in this signal transmission may prompt doctors to conduct further tests.

During an auditory brain stem response test, a doctor attaches small metal discs called electrodes to your head. The electrodes measure brain wave activity in response to sound that is delivered through a set of headphones or ear inserts. Unusual test results are a sign of acoustic neuroma.

Vestibular Response Test

To find out if the vestibular, or balance part of the eighth cranial nerve, is working properly, a doctor may order a vestibular response test called an electronystagmography. During the test, doctors place electrodes around your eyes to record eye movements in response to warm and cold air that is gently blown into the outer ear. Unusual and atypical eye movements during the test may indicate that the vestibular nerves are involved. This can help doctors predict on which particular vestibular nerve the tumor is growing.

MRI Scan

NYU Langone physicians use an MRI scan to confirm if an acoustic neuroma is present. This scan uses a magnetic field to create computerized, three-dimensional pictures of the soft tissues in and around the brain. To improve the quality of the image, doctors inject a contrast dye, called gadolinium, into a vein before the scan. With this dye, an acoustic neuroma appears brighter on the scan than surrounding healthy brain tissue.

CT Scan

MRI scans cannot be performed if a person has a pacemaker or other metals in the body. Instead, the doctor may use a CT scan. For this test, a series of X-ray images is sent to a computer to create three-dimensional, cross-sectional pictures of the head. Before the test, you may receive an injection of contrast dye into a vein to enhance the CT image.

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