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 
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.

Pure tone audiometry  
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 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 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.

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.