In over 90% of the patients with AN, the first symptom is a reduction in hearing on one ear due to involvement of the VIIIth nerve. The hearing loss may be noticed as a diminished ability to understand spoken words, particularly in difficult listening conditions as with telephone use. Although hearing loss with diminished speech understanding is relatively common in older individuals with both ears affected, the hearing loss and reduced clarity that is more pronounced in one ear should prompt an evaluation by your doctor. With acoustic neuroma, hearing loss is often accompanied by ringing in on ear-- "tinnitus". The hearing loss is usually subtle and worsens very slowly over a period of time. In about 5% of cases, there may be a sudden loss of hearing. Some patients may experience a sense of fullness in the affected ear. Unfortunately, since hearing loss is often mild and there is no pain, there may be a delay in seeking medical attention.
The audiogram is a very sensitive test for hearing. The audiogram measures the "pure tone average," (PTA) the "speech reception threshold" (SRT) and the "speech discrimination" (SD) scores.
- Pure tone average (PTA) measures the hearing of individual frequencies of sound. The patient is asked to listen to tones of different frequencies (bass up to treble; low up to high pitch). When the patient is first able to hear the tone in the earphone, that signifies the loudness (in "decibels" the unit of loudness) when the sound is heard. This is repeated for each of approximately eight frequencies. The PTA is the "average" of the level in decibels for the tested frequencies. The higher the PTA the worse the hearing.
- The speech reception threshold (SRT) measures the lowest loudness (in decibels) for detection of speech. Often it is similar in magnitude to the pure tone average. The higher the SRT the worse the hearing.
- The speech discrimination score (SD) is the percentage of spoken words that can be identified when the words are presented in the earphone (one side at a time). The SD has a "hearing level" above threshold at which the words are read. The lower the SD percentage, the worse the hearing.
The mechanism of hearing loss is compression and/or infiltration of auditory nerve fibers. Pressure applied to the nerve trunk may desynchronize or stagger impulses within individual fibers within the nerve, thereby disrupting the clarity of sound. Impairment of the blood supply to the nerve or inner ear may also play a role. In the majority of patients, hearing loss progresses gradually over many years, eventually leading to total deafness in the ear on the side of the tumor. The loss is usually unilateral, or asymmetric, and mainly involves the higher frequencies of sound. Characteristically, speech discrimination is affected more than the pure tone hearing loss (pure tone average).
Less than 2% of patients presenting with sudden hearing loss will ultimately prove to have an AN. Other forms of hearing loss are relatively frequent. In a study of a large group of AN patients, only two-thirds had a high frequency biased hearing loss and approximately 20% had loss at the lower frequencies of sound. A sudden decrease in hearing, often associated with viral infection or vascular occlusion, occurs in about one in eight AN patients. Presumably, sudden hearing loss results from tumor compression. In some patients it may be triggered by head trauma or vigorous physical exercise. The loss may be partial or total, and spontaneous recovery is possible. Sudden loss may be the first event that leads to a diagnosis, or may occur months or years prior to the tumor's discovery.
A small proportion of AN patients have either normal hearing or symmetric hearing loss. In a recent series, 15 % of AN patients had subjectively normal hearing, of whom only 4 % had normal audiograms SRT (speech reception threshold) < 25 dB, SDS (speech discrimination score) >85 %). In addition, 7% of patients had audiometrically symmetrical hearing.
Definition: Tinnitus (pronounced tin’-it-is) is the perception of ringing, hissing, or other sound in the ears or head when no external sound is present. Tinnitus can be intermittent or constant—with single or multiple tones—and its perceived volume can range from subtle to overwhelming. In the United States, an estimated 12 million people have tinnitus.
Tinnitus is very frequent in AN as it is many inner ear diseases. In most cases, the tinnitus is high pitched and localized to the tumor ear. However, the symptom may be non-localizing and may be of unusual pitches. A few patients with AN present with unilateral tinnitus in the absence of subjective hearing loss. Thus, unilateral tinnitus occurring without explanation is an indication for an evaluation for AN.
Vestibular nerve: Since the tumor usually arises from the vestibular nerve which is responsible for balance, unsteadiness or balance problems may be one of the earlier symptoms in the growth of the tumor. Since the remainder of the balance system compensates for this loss, balance problems may stabilize.
Redundancy: There is bilateral innervation of the brainstem by each vestibular nerve. This means that the left vestibular nerve innervates the left and right brainstem, and the right vestibular nerve does the same. This "redundancy" means that it is very difficult for a tumor to completely remove all balance. Similarly, if one of the vestibular nerves is resected as a tumor is removed, the other side appears able to restore a level of activity that enables the restoration of balance.
True vertigo is not commonly associated with AN. In a recent series, only one in five of patients had vertigo, most of whom had small tumors. Vertigo is very infrequent with larger tumors at the time of diagnosis. Thus, it appears that vertigo is caused early in AN growth, perhaps by destruction of the vestibular nerve or through interruption of the blood supply to the labyrinth. With growth, the central nervous system appears capable of adapting the loss of balance information from the affected ear, and thus the sense of vertigo subsides.
Dysequilibrium is much more prevalent than vertigo. Dysequilibrium occurs in nearly half of AN patients. In contrast to vertigo, which decreases in incidence with increasing tumor size, dysequilbrium becomes more frequent with larger tumor sizes. The most likely mechanisms involved in causing dysequilibrium are uncompensated unilateral vestibular deafferentation or persistent perverse input from the diseased vestibular nerve.
Cerebellar dysfunction is intention tremor and gait ataxia (unsteady walking). Large ANs indent the lateral cerebellar lobe and peduncles and may compress part of the cerebellum. Outright cerebellar dysfunction is uncommon in AN, and is limited to large tumors. Although little information is available on its exact incidence, truncal (trunk, abdomen, chest) ataxia appears to be more common than limb ataxia. Patients tend to fall towards the side of the tumor.
As the AN grows, the capsule of the tumor presses upon the lining of the inside of the skull (the dura). The dura has sensory fibers that can transmit the sensation of the pressure. The headache that results from the acoustic neuroma can be dull or aching in quality and is usually unilateral. The headache may "radiate" to the neck, top of the head or front of the head.
Muscle innervation: For the acoustic neuromas, pressure upon or infiltration of the tumor into the facial nerve may result in facial weakness. The muscles supplied by the facial nerve include the muscles of facial expression and spontaneous emotion. The muscles for eye closure, raising the eyebrows and wrinkling the forehead are innervated by the facial nerve. Facial weakness is rare at the time of diagnosis, though spasms of the facial nerve caused by tumor compression are not uncommon.
Tears and taste: The facial nerve also has functions of "tear" formation and taste. Pressure on the nerve can cause dry eye or even unexpected "tear" formation. The branch of the facial nerve that controls tearing is the greater superficial petrosal nerve. The branch of the facial nerve that provides the sense of taste to the anterior two-thirds of the tongue is the "chorda tympani."
Facial numbness: As the tumor grows larger in size and starts pressing on other nerves, mainly the trigeminal nerve, facial sensation may be affected. The patients will then experience numbness and facial tingling, constantly or intermittently. There may be facial tingling on the side of the AN.
The tingling or "paresthesia" may be near the corner of the mouth or on the cheek. Facial numbness is not as common as the facial weakness. Patients may also have facial tics or spasms. As the tumor grows larger or presses on the brainstem, the patient will experience headaches, facial weakness, vertigo and unsteady gait due to raised intracranial pressure.