Otoacoustic emission

Otoacoustic Emissions (OAEs) are sounds that are generated from within the inner ear. Having been first described by David Kemp in 1978, OAEs have since become a fundamental tool in the field of audiology for the assessment of cochlear function. This phenomenon reflects the activity of the Outer Hair Cells (OHCs) of the cochlea, and its presence or absence can provide valuable information about the health of these cells, which are crucial for normal hearing.

Types of Otoacoustic Emissions

There are several types of OAEs, each with its own method of elicitation and clinical significance. The most commonly encountered types are:

• Spontaneous Otoacoustic Emissions (SOAEs): These are sounds emitted without an external stimulus. Not all individuals have SOAEs, and their presence is often associated with normal hearing sensitivity.

• Transient Evoked Otoacoustic Emissions (TEOAEs): TEOAEs are elicited using a brief acoustic stimulus (click or tone burst). The response is a series of sounds that occur within milliseconds after the stimulus. TEOAEs are widely used in newborn hearing screening programs.

• Distortion Product Otoacoustic Emissions (DPOAEs): DPOAEs are generated by presenting two continuous tones of different frequencies. The cochlea nonlinearly processes these tones, producing emissions at frequencies that are mathematically related to the original tones. DPOAE testing is useful for frequency-specific assessment of cochlear function.

Clinical Applications

OAE testing plays a critical role in the early detection of hearing loss, especially in newborns and infants. It is a non-invasive, quick, and objective method that does not require the patient's active participation, making it ideal for use in individuals who are difficult to test using conventional audiometry. The main applications of OAEs include:

• Newborn Hearing Screening: To detect congenital hearing loss early, allowing for timely intervention and management.
• Diagnostic Audiology: To differentiate between sensory (cochlear) and neural (auditory nerve) components of hearing loss.
• Ototoxicity Monitoring: To detect early signs of damage to the cochlea from ototoxic medications, allowing for adjustments in therapy.
• Noise-Induced Hearing Loss Monitoring: To identify early damage to the cochlea from exposure to excessive noise.

Limitations

While OAEs are a valuable tool in assessing cochlear function, they have limitations. The presence of middle ear pathology, such as fluid or infection, can prevent OAEs from being detected, even if cochlear function is normal. Additionally, OAEs cannot assess auditory pathways beyond the cochlea, so normal OAE results do not rule out all types of hearing impairment.

Conclusion

Otoacoustic emissions are a cornerstone in the field of audiology, providing a non-invasive and objective measure of cochlear health. Their application in newborn hearing screening has significantly improved the early detection and management of hearing loss, leading to better outcomes for those affected. Despite their limitations, OAEs remain an essential part of the audiologist's toolkit for assessing auditory function.