Otoacoustic Emissions

Otoacoustic emissions are low-level sounds generated within the cochlea and recorded in the ear canal with a sensitive microphone. Because they arise from the active, energy-producing behaviour of the outer hair cells, their presence provides an objective, non-invasive indicator that the cochlear amplifier is functioning, without requiring any response from the listener.

Definition

Otoacoustic emissions are acoustic signals produced by the cochlea, recordable in the external ear canal, that reflect the activity of the outer hair cells and serve as an objective measure of cochlear (pre-neural) function.

Scope

This entry covers the origin of emissions in outer hair cell function, the main clinical types (spontaneous, transient-evoked, and distortion-product emissions), how they are recorded and judged present, and their role in objective screening. It is a reference description of the method, not clinical interpretation for individuals.

Core questions

• Are outer hair cells generating measurable emissions, indicating cochlear function?
• How do spontaneous, transient-evoked, and distortion-product emissions differ?
• How is an emission distinguished from background noise to judge it present or absent?
• Why are emissions well suited to objective newborn hearing screening?

Key concepts

• Outer hair cells and the cochlear amplifier
• Spontaneous otoacoustic emissions
• Transient evoked otoacoustic emissions (TEOAE)
• Distortion product otoacoustic emissions (DPOAE)
• Signal-to-noise ratio and reproducibility criteria
• Pre-neural (cochlear) measure
• Objective hearing screening

Mechanisms

Outer hair cells contract and elongate in response to sound, amplifying the motion of the basilar membrane; a by-product of this active process is energy that travels back out through the middle ear and is detectable as sound in the ear canal (Kemp 1978). Spontaneous emissions occur without any external stimulus in some normal ears. Transient evoked emissions are elicited by brief clicks and analyzed for a response that reproduces across repetitions above the noise floor. Distortion-product emissions are elicited by two simultaneous tones and appear at predictable additional frequencies created by the nonlinearity of the healthy cochlea, allowing function to be probed frequency by frequency (Probst et al. 1991). Because outer hair cells are vulnerable to noise, ototoxic agents, and many cochlear pathologies, reduced or absent emissions point to cochlear involvement; however, emissions test the cochlea up to the outer hair cells and not the auditory nerve, so they can be present when there is neural dysfunction. A clear middle-ear pathway is required for emissions to be recorded.

Clinical relevance

Otoacoustic emissions give an objective, rapid window onto outer hair cell function that needs no behavioural response, which makes them central to newborn hearing screening and useful for monitoring cochlear status. Because they are pre-neural, they are interpreted alongside neural measures such as the auditory brainstem response. This entry describes how emissions are recorded and what they reflect; it is not a basis for individual diagnosis or treatment.

Epidemiology

Otoacoustic emissions are one of the two physiological measures underpinning universal newborn hearing screening worldwide, alongside the automated auditory brainstem response, and are widely used in programmes for the early detection of hearing loss (JCIH 2007).

History

David Kemp's 1978 demonstration that the human ear emits sound in response to stimulation established the existence of evoked otoacoustic emissions and implicated an active cochlear process. A comprehensive review consolidated the spontaneous, transient-evoked, and distortion-product categories and their measurement (Probst et al. 1991), and emissions were subsequently adopted as a core tool of early hearing detection programmes (JCIH 2007).

Key figures

• David Kemp
• Rudolf Probst
• Brenda Lonsbury-Martin
• Glen Martin

Seminal works

• kemp-1978
• probst-1991

Frequently asked questions

Where do otoacoustic emissions come from?

They arise from the active motion of the cochlea's outer hair cells, whose energy travels back out through the middle ear and is detectable as faint sound in the ear canal.

Why are otoacoustic emissions used in newborn hearing screening?

They are objective and fast, require no behavioural response, and indicate whether the cochlea's outer hair cells are working, which makes them well suited to testing newborns.

Can emissions be normal when there is still a hearing problem?

Yes; emissions reflect cochlear outer hair cell function up to but not including the auditory nerve, so they can be present when the problem lies in neural transmission.