A patient’s ability to hear should be roughly estimated in a general office setting. In the Weber test, a vibrating 512-Hz tuning fork is placed on the midline of the forehead. Patients with normal hearing perceive the vibratory sound as equally loud in each ear. Patients with conductive hearing loss perceive the vibratory sound as louder on the affected side. In the Rinne test, the stem of a vibrating tuning fork is placed on the mastoid tip (with firm pressure) and then the tines are held just in front of the external auditory canal. Normally, the vibratory sound is perceived as being louder at the external auditory canal (ie, air conduction is greater than bone conduction). The reverse indicates a significant conductive hearing loss.
A whisper test, using a Barany box to mask the contralateral ear, can crudely estimate auditory thresholds, but most clinicians find it too inaccurate to be useful. An audioscope combines both otoscopic visualization and the capacity to check hearing at different frequencies and volumes. Any complaint of hearing loss or any abnormality on office screening tests should be followed up with a complete audiogram.
The complete audiogram (see FIG. 103-3) entails a battery of tests for evaluating pure tone and speech reception thresholds and tympanometry. Thresholds for pure tones at octave or half-octave intervals are obtained for frequencies from 250 to 8000 Hz. Testing is done by both air conduction (using earphones) and bone conduction (placing a vibrotac-tile transducer directly on the mastoid).
The speech reception threshold is defined as the intensity at which the patient can correctly identify 50% of a series of spondees (two-syllable words equally accented, eg, cowboy). The speech reception threshold should be within 10 dB of the pure tone average (average threshold in decibels, at 500, 1000, and 2000 Hz). The speech discrimination score is determined by presenting a list of monosyllables at levels above the speech reception threshold and having the patient repeat them; the score is the percentage correctly identified.
A specialized test of speech discrimination, the performance intensity function for phonetically balanced words, differentiates cochlear from retrocochlear hearing loss. As phonetically balanced words are presented with increased intensity, the speech discrimination score rises and then stabilizes in patients with normal hearing or with a cochlear hearing loss. Patients with retrocochlear hearing loss have an initial rise in the score, followed by a dip (a rollover).
Immittance studies, also called impedance audiometry, consist of tympanometry and acoustic reflex measurements.
Tympanometry (see FlG. 103^4) measures the relative change in acoustic immittance at the plane of the tympanic membrane when air pressure changes are introduced in the external ear canal across a range from high positive to high negative pressure. Normally, the immittance is maximal at atmospheric pressure, where air pressure is about equal on each side of the tympanic membrane. The peak in the tympanogram may shift toward a negative air pressure direction if eustachian tube function is compromised, or the peak may disappear when fluid collects in the middle ear.
The acoustic reflex threshold is the lowest sound intensity (between 500 and 4000 Hz) that will produce a reflex contraction of the stapedius muscle. Reflex decay is an abnormal finding involving a decrease in the original reflex amplitude > 50% over a 10-sec test period. Stapes (acoustic) reflex decay suggests a retrocochlear hearing disorder.
If auditory thresholds are ambiguous, or if the possibility of a ret-rocochlear disorder exists, appropriate testing includes the auditory brain stem response. In this test, recording electrodes are attached to the patient’s earlobes and vertex and are connected to a computer that averages responses. Auditory system activity in response to a sound stimulus (a click) is analyzed by the computer and generally results in the delineation of five sequential waves (see FIG. 103-5). Waves I and II are thought to originate from the peripheral and central auditory nerve, wave III from the cochlear nuclei, and wave IV from activity in the superior olivary nucleus. The lateral lemniscus is thought to give rise to wave V. The intensity of the click needed to elicit the characteristic waveforms indicates the auditory threshold, which is determined for each ear individually. Also important is the time at which each wave appears after the sound stimulus (latency). A delay in onset of the entire sequence of waveforms suggests a conductive hearing loss. In the absence of a conductive hearing loss, a delay in onset of wave V strongly suggests a retrocochlear hearing loss.
Results of special tests to determine central auditory function may be invalidated by a concurrent peripheral hearing loss. In general, these tests evaluate a patient’s ability to synthesize distorted or degraded speech signals divided between the two ears, or to detect a specific message in one ear while a competing message is presented in the other ear. In tests using distorted or degraded messages, a cortical (temporal lobe) lesion may cause poor performance in the ear contralateral to the lesion (the contralateral ear effect). Inability to fuse binaural information into a meaningful message may indicate a brain stem lesion.