How are acoustic neuromas (vestibular schwannomas) diagnosed?
The diagnosis of an acoustic neuroma is often triggered by a patient’s symptoms. The most common presenting feature of acoustic neuromas, occurring in 90% of patients, is unilateral hearing loss. When "pure tone audiometry" is used, the most common finding is high frequency hearing loss. The hearing loss is progressive in most patients, but in approximately 12% of patients the hearing loss may occur suddenly. Other symptoms of the acoustic neuroma include asymmetric tinnitus (ringing in the ear, occurring in 80% of patients), dizziness, and disequilibrium (difficulty with balance). These symptoms will often prompt a doctor to order a hearing test, called an audiogram. If the hearing test confirms that one ear has more hearing loss than the other, then a magnetic resonance imaging (MRI) scan of the head is typically ordered. Early diagnosis offers the best opportunity for successful treatment.
- Hearing test (audiometry): A test of hearing function, which measures how well the patient hears sounds and speech, is usually the first test performed to diagnose acoustic neuroma. The patient listens to sounds and speech while wearing earphones attached to a machine that records responses and measures hearing function. The audiogram may show increased "pure tone average" (PTA), increased "speech reception threshold" (SRT) and decreased "speech discrimination" (SD).
- Brainstem auditory evoked response (BAER): Historically, this test was performed in some patients to provide information on brain wave activity as a response to clicks or tones. The patient listens to these sounds while wearing electrodes on the scalp and earlobes and earphones. The electrodes pick up and record the brain's response to these sounds, but these tests have been shown to be less accurate in diagnosing acoustic neuromas than MRI.
- Scans of the head: If other tests show that the patient may have acoustic neuroma, magnetic resonance imaging (MRI) is used to confirm the diagnosis. MRI uses magnetic fields and radio waves, rather than x-rays, and computers to create detailed pictures of the brain. It shows visual “slices” of the brain that can be combined to create a three-dimensional picture of the tumor. A contrast dye is injected into the patient. If an acoustic neuroma is present, the tumor will soak up more dye than normal brain tissue and appear clearly on the scan. The MRI commonly shows a densely "enhancing" (bright) tumor in the internal auditory canal. If the patient is unable to get an MRI, a computed tomography (CT) scan may be ordered instead.
- Because MRI and CT scans of the head are often ordered for many different conditions, in some cases, acoustic neuromas are discovered incidentally.