Identifying the Acoustic Neuroma
Advances in medicine have made possible the identification of small acoustic neuromas (those still confined to the internal auditory canal). Routine auditory tests may reveal a loss of hearing and speech discrimination (the patient may hear sounds in that ear, but cannot comprehend what is being said). An audiogram should be performed to effectively evaluate hearing in both ears. A loss in one ear should prompt an MRI.
Magnetic resonance imaging (MRI) is the preferred diagnostic test for identifying acoustic neuromas. Gadolinium, an enhancing contrast material, is used during the scan to reveal the tumor. The image formed clearly defines an acoustic neuroma if it is present. Currently MRI with contrast is the “gold standard” by which the diagnosis is confirmed. This technique can identify tumors measuring only a few millimeters in diameter.
An auditory brainstem response test (a.k.a. ABR, BAER, or BSER) may be done in some cases. This test provides information on the passage of an electrical impulse along the circuit from the inner ear to the brainstem pathways. An acoustic neuroma can interfere with the passage of this electrical impulse through the hearing nerve at the site of tumor growth in the internal auditory canal, even when the hearing is still essentially normal. This implies the possible diagnosis of an acoustic neuroma when the test result is abnormal. An abnormal auditory brainstem response test should be followed by an MRI.
When an MRI is not available or cannot be performed, a computerized tomography scan (CT scan) with contrast is suggested for patients in whom an acoustic neuroma is suspected. The combination of CT scan and audiogram approach the reliability of MRI in making the diagnosis of acoustic neuroma.