Loudness recruitment. This phenomenon, mentioned previously, is thought to depend on the selective destruction of low-intensity elements subserved by the external hair cells of the organ of Corti. The high-intensity elements are preserved, so that loudness is appreciated only at high intensities. In testing for loudness recruitment, the difference in hearing between the two ears is estimated and the loudness of the pure-tone stimulus of a given frequency delivered to each ear is then increased by regular increments. In nonrecruiting deafness (characteristic of a nerve lesion), the original difference in hearing persists in all comparisons of loudness, since both highand low-intensity fibers are affected. In recruiting deafness (which occurs with a lesion in the organ of Corti—e.g., Ménière disease, the more affected ear gains in loudness and may finally be equal to the better one. In bilateral disease, recruitment is assessed by the intensity of the stimulus that causes discomfort, about 100 dB (decibels) in normal persons.
2.
Speech discrimination. This consists of presenting the patient with a list of 50 phonetically balanced monosyllabic words (e.g., thin, sin) at suprathreshold levels. The speech-discrimination score is the percentage of the 50 words correctly repeated by the patient. Marked reduction (less than 30 percent) in the speech-discrimination scores is characteristic of eighth nerve (retrocochlear) lesions.
3.
Audiometry. Continuous and interrupted tones are presented at various frequencies. Tracings are made, measuring the increments by which the patient must increase the volume in order to continue to hear the continuous and interrupted tones just above threshold. Clinically, analysis has shown that there are four basic configurations, referred to as types I to IV Békésy audiograms. Type III or IV usually indicate the presence of a retrocochlear lesion, the type II response points to a lesion of the cochlea itself, and type I is considered normal. Related tests, such as threshold tone decay and the short increment sensitivity index, were formerly used to a greater extent than they are currently; therefore, we have not described them here.
4.
Brainstem auditory evoked potentials, or response (BAEP, or BAER) (see Chap. 2). This method provides very refined information as to the integrity of primary and secondary auditory pathways from the cochlea to the superior colliculus. It has the advantage of being accurate in uncooperative and even comatose patients as well as infants who cannot cooperate with audiometry. It is of some value in detecting small acoustic and vestibular schwannomas; in localizing brainstem lesions such as those caused by demyelination; in corroborating the state of brain death, in which all waves, except occasionally the eighth nerve (wave I), responses are abolished; and in assessing sensorineural damage in neonates who have had meningitis or been exposed to ototoxic medications.
5.
The acoustic-stapedial reflex can be used as a measure of conduction in the auditory (and the facial) nerve. This reflex normally protects the cochleas from excessively loud sound. When sound of intensity greater than 70 to 90 dB above threshold hearing reaches the inner ear, the stapedius muscles on both sides contract reflexively, relaxing the tympanum and offering impedance to further sound. It may be tested by insufflating the external auditory canal with pressured air and measuring the change in pressure that follows immediately after a loud sound. The response is muted in patients with conductive hearing loss because of the mechanical restriction of ossicular movement, but otherwise the test is sensitive to cochlear and acoustic nerve lesions.