Diagnostic Principals in Neuro-otology: The Auditory System:
Part II

Classification of tinnitus (cont.) RETURN TO OUTLINE

Tinnitus may be classified as mild, moderate, or severe. Mild tinnitus is usually noticed only in quiet environments or at bedtime. It is usually not very disturbing, and the patient can easily be distracted from the tinnitus by other stimuli. Moderate tinnitus is more intense and is constantly present; the patient is conscious of the tinnitus when attempting to concentrate or when trying to sleep. Severe tinnitus may disable individuals to the extent that they are is unable to concentrate on little other than the tinnitus itself.

EXAMINATION OF AUDITORY FUNCTION RETURN TO OUTLINE

Basic Office Examination of Hearing

Whether or not the patient's complaint is one of hearing loss, a basic assessment of auditory function should be part of the neurological examination. The external ear should be inspected with an otoscope to determine the patency of the external ear canal and the integrity of the tympanic membrane. If the external canal is occluded by cerumen, simple tests of hearing may be invalidated. The cerumen should be removed, if possible, with warm water lavage using a syringe with a 5 to 8 cm piece of rubber tubing affixed to the end to avoid injury to the ear. If water lavage has not removed impacted cerumen, a neurologist should refer the patient to an otolaryngologist for removal.

Assuming there is no cerumen in the external ear canal, the tympanic membrane should be inspected. The neurologist should be able to recognize an inflamed, bulging, or scarred drum, and should note whether there is perforation of the tympanic membrane; blood behind the eardrum; or a pulsating blue mass, which may be indicative of a glomus jugulare tumor. Excellent descriptions of tympanic membrane findings may be found in modern texts of otology. At times it may be helpful to inspect the mobility of the eardrum by increasing pressure within the external canal, using a hand-held pneumatic bulb, attached by tubing to an outlet in the otoscope. Little or no mobility of the tympanic membrane suggests fluid or a mass behind the drum, or a fixed ossicular chain.

The office examination of hearing loss may include tuning fork tests of air and bone conduction. Tuning forks at a frequency of 256 or 128 Hz should not be used due to the vibrations they produce by bone conduction, which the patient may mistake for sound; the 512 Hz is the lowest useful frequency. Two standard tuning fork tests are the Weber and Rinne tests.

Weber test RETURN TO OUTLINE
The Weber test is based on the principle that the signal, when transmitted by bone conduction, will be localized to the better hearing ear or the ear with the greatest conductive deficit. The test can determine the type of hearing impairment when the two ears are affected to different degrees. The stem of a vibrating tuning fork is placed on the skull in the midline, and the patient is asked to indicate in which ear the sound is heard. The usual location described is for placement on the forehead; but better locations are the nasal bones or teeth when a stronger bone conduction stimulus is required. In unilateral hearing losses, lateralization to the poorer-hearing ear indicates an element of conductive impairment in that ear. Lateralization to the better-hearing ear suggests that the problem in the opposite ear is sensorineural.

Rinne test RETURN TO OUTLINE
The Rinne test is probably the most commonly used tuning fork test, but the name is usually mispronounced: It is German, not French, and is accentuated on the first syllable (Rin'neh). The Rinne test is a comparison of the patient's hearing sensitivity by bone conduction versus air conduction. A normal individual will perceive the air conducted sound as louder or the same as bone conducted sound. Proper placement of the tuning fork in each situation is important. When testing by bone conduction, the stem fork should be placed firmly on the mastoid, as near to the posterosuperior edge of the ear canal as possible. The stem should not touch the auricle of the external canal, which should be held to the side by the examiner's fingers. Touching the external ear itself could give false results due to vibration of the auricle. When testing by air conduction, the fork is held about 2.5 cm lateral to the tragus. In the Rinne test, when the conduction mechanism is normal in an ear (that is, in individuals with normal hearing and in those with sensorineural hearing impairment), air conduction will be heard better than bone conduction as it is a more efficient means of sound transmission. This finding is termed a positive Rinne. Bone conduction will be heard better than air conduction when there is a deficit in the conduction mechanism and is referred to as a negative Rinne. A conductive deficit of more than 15 db reverses the tuning fork responses (that is, bone conduction is better than air conduction) at 512 Hz. When testing by bone conduction, the examiner should not forget to have the patient remove his or her eyeglasses: the earpiece can interfere with proper placement of the stem of the tuning fork or give inappropriate conduction or vibratory information. Although tuning fork tests allow the examiner to identify a conductive versus a sensorineural loss, and in some cases lateralize the symptomatic ear, it does not evaluate the degree of impairment or the effects of that impairment on speech understanding.
Laboratory Evaluation of Auditory Function RETURN TO OUTLINE

Pure tone audiometry RETURN TO OUTLINE
An audiologic assessment is comprised of pure tone air and bone conduction testing, speech threshold and word discrimination measures. Threshold is defined as the lowest intensity (measured in decibels) an individual can detect a pure tone or speech signal more than fifty percent of the time. Pure tone air and bone thresholds are established for frequencies from 250 Hz to 8,000 Hz. This frequency range is important to the detection and understanding of the speech signal. Hearing is considered normal when threshold sensitivity is between 0 and 25 dB for frequencies of 250 Hz to 8000 Hz (Figure 6). Responses greater than 25 dB are classified by degree as mild, moderate, severe, moderately severe, and profound (Figure 6). Responses at 500 Hz, 1000 Hz, and 2000 Hz are averaged together to compute the pure tone average (PTA).

In the measurement of bone conduction thresholds, pure tones are transmitted via a bone oscillator, usually placed on the mastoid. This signal directly stimulates the cochlea, bypassing the external and middle ear. The presence of decreased air conduction thresholds and normal sensitivity by bone conduction suggests abnormality in the external ear or middle ear system and is termed a conductive hearing loss.

Speech reception threshold RETURN TO OUTLINE
A speech reception threshold is the lowest intensity and equally weighted two syllable word is understood approximately fifty per cent of the time. The pure tone average and speech reception threshold should be within 7 dB of each other. Comparison of the speech reception threshold and the pure tone average serves as a check on the validity of the pure tone thresholds. Discrepancies between these measures may suggest a functional or non-organic hearing loss. Speech discrimination RETURN TO OUTLINE
Speech discrimination is a tool used to assess an individual's ability to understand a speech signal at normal or above normal conversational levels. Most commonly, a phonetically balanced word list of fifty one-syllable words is presented to the patient at a supra-threshold level. The patient's score is represented as a percentage of the number of words correct. Generally, discrimination ability decreases proportionately with an increase of hearing impairment. However, there is an exception in conductive hearing loss where discrimination ability remains relatively good because the inner ear system is normal. Poor discrimination ability in the presence of relatively good hearing sensitivity may suggest retrocochlear pathology such as acoustic neuroma and should be aggressively pursued by the clinician. Immittance test battery RETURN TO OUTLINE
Tympanometry, static acoustic immittance and acoustic reflex threshold measures comprise the acoustic immittance test battery. Tympanometry is a measure of middle ear mobility when air pressure in the external canal is varied. Results are graphically represented with a pressure along the X axis and compliance along the Y axis. Normal tympanograms have a pressure peak point of "50 mm H2O.

Static compliance refers to the ease of flow of acoustic energy through the middle ear. Immittance measures are obtained at +200 mm H2O (first point of compliance, or C1) and again at the point the tympanic membrane is most compliant (second point of compliance, C2). The point at which the tympanic membrane is most compliant allows maximum transmission of energy through the middle ear cavity. Compliance of the tympanic membrane is derived by subtracting C1 from C2. Values less than 0.25 cm3 of equivalent volume indicate a stiff or non-compliant middle ear system. Values greater than 2.5 cm3 suggest an overly compliant system. Abnormalities associated with reduced mobility of the tympanic membrane in associated middle ear structures include otitis media, otosclerosis and large cholesteatomas. Ossicular chain discontinuity is the most common cause of excessive tympanic membrane mobility. Examples are shown in Figure 7. Extremely high equivalent middle ear volume and low static compliance suggests tympanic membrane perforation.

The acoustic reflex threshold is the lowest intensity needed to elicit a contraction of the stapedius and tensor tympani muscles using a pure tone stimulus. The introduction of an intense sound into the ear canal results in a temporary increase in middle ear impedance. This phenomenon occurs bilaterally, however, it is typically measured in one ear at a time. Contralateral reflexes are measured by stimulating one ear and measuring the reflex from the contralateral ear. Ipsilateral reflexes are measured by stimulating and recording from the same ear. Reflexes occur between 70 and 100 dB SPL (Sound Pressure Level) in normal ears. Middle ear abnormalities or significant sensorineural hearing losses may elevate or obliterate the acoustic reflexes. Retrocochlear pathology and facial nerve disorders may also affect contralateral and ipsilateral acoustic reflexes.

Brain stem auditory evoked potentials RETURN TO OUTLINE
Brainstem auditory evoked potentials (BAEPs) are also known as brainstem auditory evoked responses (BAERs) or auditory brainstem responses (ABRs). These physiological measures can be used to evaluate the auditory pathways from the ear to the upper brainstem (Picton, 1990). In addition, ABR threshold testing may be used to determine behavioral threshold sensitivity in infants or uncooperative patients. The most consistent and reproducible potentials are a series of five submicrovolt waves that are seen within 10 msec of an auditory stimulus. These potentials are recorded by averaging 1,000 to 2,000 responses from click stimuli by use of a computer system and amplifying the response ( Figure 8). The anatomical correlates of the five reliable potentials have only been roughly approximated. Wave I of the BAEP is a manifestation of the action potentials of the VIII nerve and is generated in the distal portion of the nerve adjacent to the cochlea. Wave II may be generated by the VIII nerve or cochlear nuclei. Wave III is thought to be generated at the level of the superior olive, and waves IV and V are generated in the rostral pons or in the midbrain near the inferior colliculus. The complex anatomy of the central auditory pathway (Benjamin and Troost, 1988), with multiple crossing of fibers from the level of the cochlear nuclei to the inferior colliculus, makes interpretation of central disturbances in the evoked responses difficult. Excellent reviews of the generation of the potential, and interpretation of abnormality, are found in recent contributions.

The brainstem auditory evoked potential (BAEP) is a sensitive, noninvasive diagnostic test for the diagnosis of cerebellopontine angle tumors (Picton, 1990). This test is used to differentiate cochlear from VIII nerve hearing defects and, on some occasions, demonstrates an auditory abnormality when behavioral audiometric testing is still normal. The majority of patients with acoustic tumors had abnormal responses (Baloh and Honrubia, 1990).

The absence of waves III and V has been seen in some patients with vestibular schwannoma and in cerebellopontine angle meningiomas. Such patients often have marked hearing deficits with poor discrimination on behavioral testing, suggesting retrocochlear disease. The absence of all waves should not occur unless a severe hearing loss exists. The most specific evoked potential abnormality is the presence of an increase in interwave intervals. Abnormal interwave latencies (I-III or I-V) are the most specific and sensitive abnormalities seen with cerebellopontine angle tumors. The abnormal prolongation or absence of wave V at increased click rates is also characteristic of retrocochlear pathology. Increased absolute latencies of all waves, when compared to responses from the other ear or to clinical normative data may signify a conductive deficit.

Electrocochleography RETURN TO OUTLINE
Electrocochleography (ECochG) is a method of recording the stimulus-related electrical potentials associated with the inner ear and auditory nerve, including the cochlear microphonic, summating potential (SP) and the compound action potential (AP) of the auditory nerve. This measure is beneficial in the differential diagnosis of certain types of sensory disorders, such as Ménière's disease or cochlear hydrops. The amplitude of the SP and AP is measured and is of primary interest when evaluating an ear for increased endolymphatic pressure.

A summary of tests used for distinguishing central versus peripheral auditory disorders is presented (Table). It should be pointed out that most neurologists consider "Central" as just being the brain, brainstem and spinal cord, but otolaryngologists consider "central" as being those location but also the eigth nerve proximal to the cochlea. Thus in the neuro-ototlogic definition discussions and the Table include eigth nerve tumors and other conditions of the cerebellopontine angle as central.

 

Table 1. Tests for Central Versus Peripheral Auditory Disorders**

 

 

 

Peripheral

Central

Pure Tone

+

-

Electrocochleography

+

-

Speech Reception Threshold (SRT)

+

-

Speech Discrimination

+

+

Tympanometry

+

-

Tone Decay

-

+

Bekesy Audiometry

+

+

Acoustic Reflexes*

+

+

Recruitment

-

+

** Some tests not discussed in text. Please refer to standard references, for example Jackler and Brackman, 1994.

+ Useful.

- Not useful.

+ May be helpful, depending upon result.

* Positive findings depend on site of lesion.

THERAPY FOR AUDITORY DISORDERS RETURN TO OUTLINE

Therapy for auditory disorders is largely the province of the otolaryngologist and the audiologist. The neurologist interested in neuro-otology however, should have some knowledge of therapy to promote appropriate referrals. The nature of the treatment program depends, of course, on the exact diagnosis, that is the type of hearing loss and the age of the patient. Both medical and surgical therapies are appropriate depending upon the nature of the disorder. Medical or surgical therapy is used in conductive losses due to otitis media. Surgery is the primary therapy for hearing loss caused by otosclerosis, usually manifest as a conductive type of hearing loss, as described earlier. However, almost every type of non-conductive hearing loss may be helped by a variety of amplification devices and/or counseling. Amplification RETURN TO OUTLINE
Contrary to a commonly held misconception, sensorineural hearing loss may be helped by the use of a hearing aid. It should be recognized, however, that hearing aids only compensate for loss of sensitivity, but the matter in which increased loudness is achieved may reduce distortion and significantly increase discrimination in certain situations. Modern hearing aids use the latest microcircuitry and signal-processing techniques, such as digital filtering, to improve significantly the effectiveness of amplification.

In addition to hearing aids, devices such as telephone amplifiers, television/radio access systems, personal listening systems, and alerting devices are designed to improve communication in difficult listening situations. There are many assistive devices on the market and new systems and modifications are appearing at an accelerating rate.

We note that the hearing aid is the most important rehabilitative tool available for the management of sensorineural hearing loss, however, counseling should represent a central focus of any management strategy for the hearing-impaired adult. In addition, the hearing-impaired should receive counseling both before and after the provision of amplification.

Cochlear implantation RETURN TO OUTLINE
Lastly, cochlear implants have proven to be extremely beneficial for those individuals with severe to profound hearing loss who receive minimal benefit from amplification (Waller and Roland, In press; Balkany et al, 1994). MANAGEMENT OF TINNITUS RETURN TO OUTLINE
The complete evaluation of the tinnitus patient should be approached from a dual perspective. The patient with tinnitus, regardless of location, type, or severity, must first have a thorough otologic and audiologic examination. If there are accompanying symptoms, a complete neurological examination may be appropriate. The patient with an isolated symptom of a persistent, yet unexplained, tinnitus should receive follow-up examinations at definite intervals when initial medical, otologic, and neurological studies reveal no evidence of disease. Tinnitus may be the first symptom of disorder, appearing long before any other symptom, including hearing loss. When medical and otologic examination fail to disclose a remediable cause for the tinnitus, or when a diagnosis is ascertained for which no known medical therapy is presently available at present, the tinnitus patient should undergo further evaluation to determine the most appropriate nonmedical avenue for rehabilitation.

When a specific otologic cause for the tinnitus is identified, otologic management is indicated. When a lesion or disease process is not identifiable, however, then tinnitus management is more difficult. Given no underlying otologic disease, there is at present no effective surgery or medical therapy for the treatment of tinnitus.

Research on the effectiveness of pharmacological therapy for tinnitus, although certainly encouraging, involve medications, such as carbamazepine, lidocaine, and intravenous barbiturates, whose potentially serious side effects limit their usefulness. There is some suggestion that relatively low doses may prove effective in tinnitus management.

Masking RETURN TO OUTLINE
The use of masking as a management tool in the treatment of the tinnitus patient has met with mixed success over the years. The audiologist should remain cognizant of factors such as the patient's perception of the pitch and loudness and the overall spectral intensity of the masking signal. The referring neurologist should be aware of these issues as well.

Tinnitus maskers are designed to provide relief to the tinnitus sufferer by introducing an external masking sound into the effected ear or ears, thereby minimizing or eliminating the perception of the tinnitus. Although the use of tinnitus maskers has not proved universally successful, masking is still a feasible technique that cannot be ignored. The actual efficacy of tinnitus maskers in the average tinnitus patient is probably less than 30%. The use of a hearing aid may be more beneficial by addressing the primary hearing problem.

Biofeedback RETURN TO OUTLINE
Experience with tinnitus patients reveals that many have relatively high levels of anxiety, tension, or other symptoms of chronic stress. There is a significant correlation between tinnitus and tension. Biofeedback as a treatment in the management of tinnitus was first reported in the literature in the mid-1970s. These early studies reported the use of biofeedback as effective in the relief of tinnitus or the associated annoyance produced by it. Biofeedback is quite effective for enhancing relaxation, as are traditional relaxation procedures. When used together, muscle tension and general life stresses are reported to be reduced. Counseling RETURN TO OUTLINE
The need for effective counseling is one important aspect of tinnitus management regardless of the management approaches taken with a given patient. Many patients are frightened by the presence of tinnitus and need a careful and clear explanation of the disorder, coupled with firm reassurance from both the neurologist and the audiologist. In light of the various effects tinnitus may have on a given patient counseling must be directed toward all of the patient's difficulties, not this specific problem in isolation.

BIBLIOGRAPHY RETURN TO OUTLINE

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