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

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

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.

Tests of Auditory Function

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 14):

Responses greater than 25 dB are classified by degree as mild, moderate, severe, moderately severe, and profound (Figure 14). 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 (SRT)

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

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

Tympanometry, static acoustic immittance and acoustic reflex threshold measures comprise the acoustic immittance test battery.

Tympanometry

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.(Figure 15):

Static compliance

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 15. Extremely high equivalent middle ear volume and low static compliance suggests tympanic membrane perforation.

Acoustic reflex threshold

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.