The acoustic reflex is a contraction of the stapedius muscle of the middle ear in response to loud sound. The pathways for this reflex ascend from the peripheral auditory system to the brainstem and then descend both ipsilaterally and contralaterally, so presentation of a loud sound in one ear results in bilateral contraction of the stapedius muscles. This contraction stiffens the middle ear system, causing a reduction in the transfer of low-frequency energy.
The clinical procedure for assessing the acoustic reflex threshold involves presenting a low-frequency probe tone (i.e., 226 Hz) to one ear, presenting high-intensity signals to the same or the other (contralateral) ear, and monitoring a decrease in the acoustic admittance of the probe tone in response to the presentation of the high-level signal. The minimum stimulus level that results in an observable decrease in acoustic admittance is defined as the acoustic reflex threshold. Acoustic reflex thresholds are usually measured from 500-2000 Hz, in both ipsilateral and contralateral modes, for each ear.
In listeners with normal hearing, the acoustic reflex threshold is elicited at levels approximating 85 dB HL (+/− 10 dB). The acoustic reflex is absent if the signal doesn’t reach the cochlea with sufficient intensity, if there is damage affecting any of the structures along the acoustic reflex pathway, or if there is a stiff middle ear system in the probe ear. Examples of conditions in which the acoustic reflex is absent include conductive hearing loss of 25 dB HL or greater in the stimulus ear, conductive hearing loss of 10 dB HL or greater in the probe ear, sensorineural hearing loss exceeding 75 dB HL in the stimulus ear, a lesion of the facial nerve in the probe ear, and a lesion in the auditory brainstem affecting the crossing pathway of the acoustic reflex arc. The acoustic reflex may also be absent with a lesion of the vestibulocochlear nerve in the stimulus ear, depending on the extent of the lesion. The acoustic reflex is expected to be present in cases of mild, moderate, or moderately severe sensorineural hearing loss associated with a cochlear lesion. The acoustic reflex threshold generally increases as a function of pure-tone threshold in these cases (Silman and Gelfand, 1981).
Otoacoustic emissions (OAEs) are noninvasive, objective measures of cochlear functioning. The cochlea contains two distinct types of sensory hair cells, outer and inner hair cells. When the inner hair cell senses vibration (sound), it will send the information by releasing neurotransmitters at its base to initiate activity in the primary auditory nerve. The outer hair