Diagnosis of Acoustic Neuromas

Diagnostic Evaluation of Acoustic Neuromas

Thanks to advances in imaging techniques, health care providers can often locate small acoustic neuromas when they are still confined to the internal auditory canal. Tumors are typically classified as small (up to .05 cm), medium (.05 cm to 2 cm), or large (2 cm to 4 cm or larger).

The diagnostic evaluation of the patient suspected of having an acoustic neuroma typically consists of three parts:

  • Detailed patient history and physical examination
  • Neurological examination
  • Diagnostic testing

Patient History and Physical Examination

While taking the patient's medical history, the health care professional will ask for detailed information regarding the patient's symptoms, such as when they began, when they occur, and how long they last. The physical examination is an important way that the clinician can rule out other conditions that may be responsible for the symptoms.

Neurological Evaluation

Since individual symptoms of acoustic neuromas can also be associated with many other conditions, the physician will need to observe the clinical presentation of the patient and will look for various behaviors, including:

  • Facial drooping towards one side
  • Signs of vertigo
  • Unsteady gait
  • Drooling
  • Unilateral dilation of the pupil of the eye

Diagnostic Testing

There are two types of tests that are used to diagnose acoustic neuromas:

  • Auditory (hearing) studies
  • Radiographic (imaging) studies
Auditory Studies

An audiogram is a simple auditory test that is used to determine if hearing loss is present in each ear. The person is seated in a sound proof chamber while wearing earphones. Sounds of varying loudness and frequencies are presented separately to each ear and the patient indicates when they hear a sound (such as by raising a hand). Hearing is measured in decibels (units of loudness). Normal hearing ranges from 0 to 20 dB at all frequencies. The person's response (or lack of response) to varying frequencies indicates impairment in speech discrimination (when a person can hear sound but does not understand what is being said). If an audiogram is abnormal, an MRI (magnetic resonance imaging) is usually ordered to confirm the diagnosis.

Unilateral hearing loss is so common with acoustic neuromas that many clinicians will assume the diagnosis of acoustic neuroma unless proven otherwise. It is estimated that approximately 65% of patients with acoustic neuromas at the time of diagnosis have a high-frequency hearing loss, up to 10% may have a low frequency hearing loss, and up to 10% may have no hearing at all on the affected side. Reports vary regarding the percentage of patients that present with reduced speech discrimination with estimates varying between 40% and 70%. Approximately 12% of patients with acoustic neuromas may have normal audiograms and normal speech discrimination scores.

An auditory brainstem response test (ABR, BAER, or BSER) provides information about the integrity of the passage of an electrical impulse along the pathway from the ear to the brainstem. Any disruption of the signal indicates poor functioning of the auditory nerve. Electrodes are placed on the ears and head while a click stimulus is presented through soft, foam earplugs. When an acoustic neuroma is present in the internal auditory canal, it typically disrupts the passage of the electrical impulse along the nerve. The sensitivity of this test (how well it detects acoustic neuroma) is considered to be 100% for tumors larger than 2 cm. and approximately 90% for tumors 1.1 to 2 cm. Its reliability is more limited for very small tumors. The ABR is considered to be the most reliable audiometric test for the diagnosis of acoustic neuroma.

Radiographic Studies

Magnetic resonance imaging (MRI) is the most commonly used imaging scan to identify and diagnose acoustic neuromas. In this test, harmless magnetic pulses and radiofrequency waves are intermittently passed through the area of the body that is being tested. An enhancing material called gadolinium is used to detect if a tumor is present. MRI offers excellent visualization of soft tissue and the gadolinium enhances the brightness of an acoustic neuroma. Magnetic resonance imaging with the use of gadolinium is considered the "gold standard" of imaging for the diagnosis of acoustic neuromas.

A computerized tomography scan (CT scan) may be ordered in cases where an MRI cannot be performed (such as the presence of a pacemaker). Since an ordinary CT scan will not reveal small tumors still confined to the internal auditory canal, contrast materials are used to enhance the image of acoustic neuromas. In general, CT offers limited differentiation and resolution of soft tissue and is, therefore, not precise enough to delineate all acoustic neuromas.

While a CT scan and an audiogram can provide valuable information to aid diagnosis, an MRI and ABR test are generally considered to be the most accurate diagnostic tests for identifying the presence of an acoustic neuroma.

The caloric stimulation test with electronystagmography (ENG) is less commonly used for examination of nystagmus (involuntary, rapid horizontal, vertical, or rotary movement of the eyeballs) which can be an indicator of vestibular dysfunction. This test appears to lack specificity in identifying other inner ear problems and may appear as normal in people with acoustic neuromas.

Researchers from Vanderbilt University in Nashville recently described three distinct risk groups of patients presenting to the clinician with unilateral hearing loss who may fit the profile for a diagnosis of acoustic neuroma. The three risk groups are:

  • Low risk group - patients in this group present with limited symptoms and the hearing loss (unilateral or bilateral) can be attributable to a specific cause. The risk of acoustic neuroma is less than 5%. An ABR test is performed to rule out acoustic neuroma.
  • Intermediate risk group - patients in this group experience a sudden sensorineural hearing loss in one ear or a persistent, unexplained, unilateral tinnitus. The risk of acoustic neuroma is between 5-30% and an MRI with gadolinium should be done to rule out acoustic neuroma.
  • High risk group - patients in this group experience a unilateral sensorineural hearing loss, tinnitus, and reduced speech discrimination. The risk of acoustic neuroma is greater than 30%. Patients should undergo an MRI with gadolinium and periodic follow-up with ABR.