D3 Otoacoustic Emissions (OAE): OHC Function

Topic

In addition to amplifying and compressing the movement of the basilar membrane in response to sound arriving from the middle ear, outer hair cells also generate their own mechanical energy: when an outer hair cell is activated, it rhythmically contracts and elongates. This active movement not only pushes the basilar membrane forward—in the same direction as the incoming sound—but also directs some of that energy backward, opposite to the direction of the sound that triggered the activation.

The mechanical energy pushed backward by the outer hair cells travels in reverse along the same path an incoming sound would take: it passes through the cochlear fluid toward the oval window, moves the ossicular chain in the direction opposite to normal, causes the tympanic membrane to vibrate from the inside out, and finally exits through the external auditory canal as an actual—albeit very faint—sound known as an acoustic otoemission. Thus, an acoustic otoemission is not an echo of the incoming sound, but rather a new sound generated by the outer hair cells themselves, exiting the ear through the same gateway used by any ambient sound.

Because the acoustic otoemission exiting the external auditory canal is too faint to be heard by the unaided ear, a probe equipped with a highly sensitive microphone is required to seal the canal and capture the emission before it dissipates into the environment. This probe can only record the acoustic otoemission if the outer hair cells generating it remain functional; if these cells are damaged or absent, there is no active movement to push energy backward, and the probe detects no acoustic otoemission in the external auditory canal. The presence of an otoacoustic emission confirms that cochlear function is intact, at least up to a hearing loss level of approximately forty decibels; beyond that threshold, the outer hair cells are too damaged to generate a detectable otoacoustic emission, even if the individual retains some hearing thanks to surviving inner hair cells. Conversely, below that same threshold, the presence of an otoacoustic emission indicates that the outer hair cells are continuing to generate active movement normally—even before the individual has reported any symptoms.

In clinical practice, the two most common methods for generating and recording otoacoustic emissions differ in the type of stimulus presented to the ear to activate the outer hair cells. Distortion-product otoacoustic emissions involve presenting two simultaneous pure tones of different but closely spaced frequencies to the ear; this method exploits the fact that the outer hair cells' response to two simultaneous tones is not merely the sum of the two, but includes a distortion inherent to their behavior—a distortion that the outer hair cells emit back as an otoacoustic emission at a third frequency, which is mathematically related to the original two but distinct from both. In contrast, transient-evoked otoacoustic emissions involve presenting a single brief click containing multiple frequencies to the ear and recording the otoacoustic emission—spanning a wide frequency range—that the outer hair cells produce in response to that single click.

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