Otoacoustic emission (OAE) tests measure sounds produced by the inner ear, specifically the cochlea. These low-intensity sounds, often described as echoes, are generated by the active movement of tiny sensory hair cells within the cochlea. The presence of these emissions indicates these hair cells are functioning as expected. OAE testing provides a non-invasive and objective way to assess inner ear health.
The Ear’s Own Sounds
Otoacoustic emissions are produced by the inner ear, particularly the cochlea’s outer hair cells. When sound waves enter the ear, they travel through the outer and middle ear, causing vibrations that reach the fluid-filled cochlea. This creates a traveling wave along the basilar membrane.
The outer hair cells, located on the basilar membrane, amplify these vibrations. These specialized cells possess electromotility, rapidly changing their length in response to electrical stimulation. This active expansion and contraction enhances the basilar membrane’s movement, amplifying incoming sound.
This active amplification by outer hair cells generates a tiny sound that travels backward through the middle ear and into the ear canal. This sound can then be detected by a sensitive microphone placed in the ear canal. The presence of these emissions confirms the cochlea’s active mechanical properties, which contribute to the ear’s acute sensitivity and frequency selectivity.
Detecting Hearing Issues
Otoacoustic emission testing is a valuable tool in audiology, particularly for screening hearing in various populations. Its objective nature means it does not require a behavioral response, making it suitable for newborns, infants, and young children who cannot participate in traditional hearing tests. It also extends its utility to individuals with developmental disabilities or those who are sleeping or comatose.
Newborn hearing screenings widely use OAE tests to identify potential hearing issues shortly after birth. Early detection of hearing loss is important for prompt intervention, which can improve language and communication development in affected children. The test is non-invasive and generally takes only a few minutes to complete, involving the placement of a small, soft probe in the ear canal.
Beyond newborns, OAE testing also monitors hearing health, especially for individuals at risk of noise-induced hearing damage or those taking ototoxic medications. It can detect changes in outer hair cell function even before hearing loss becomes noticeable through conventional pure-tone audiometry. OAEs are a useful part of a comprehensive audiological evaluation.
Different Kinds of Otoacoustic Emissions
Otoacoustic emissions are categorized by how they are elicited, providing distinct information about cochlear function. The two primary types used in clinical testing are Transient Evoked Otoacoustic Emissions (TEOAEs) and Distortion Product Otoacoustic Emissions (DPOAEs). Both are evoked OAEs, meaning they are produced in response to an auditory stimulus.
TEOAEs are generated by brief, broadband sounds, such as clicks or tone bursts, presented to the ear. These emissions provide a general overview of cochlear function across a broad range of frequencies. TEOAEs are commonly used in newborn hearing screening programs due to their quick and efficient nature.
DPOAEs are elicited by presenting two simultaneous pure tones of different frequencies to the ear. The healthy cochlea generates additional sounds, known as distortion products, at specific frequencies mathematically related to the two input tones. DPOAEs offer more frequency-specific information about outer hair cell function and are useful for assessing hearing at various frequencies.
A third type, Spontaneous Otoacoustic Emissions (SOAEs), occurs without any external auditory stimulation. These continuous, low-level sounds are present in approximately 30-50% of individuals with normal hearing. While their presence generally indicates a healthy cochlea, their absence does not necessarily signify a hearing impairment, limiting their diagnostic value in routine clinical practice.
Interpreting OAE Test Results
OAE test results are presented as either a “pass” or a “refer.” A “pass” result indicates that otoacoustic emissions were detected, suggesting normal outer hair cell function and a low probability of significant hearing loss. This outcome implies the cochlea is responding appropriately to sound.
Conversely, a “refer” result means that emissions were not detected or were too faint, indicating a potential issue with outer hair cell function or sound transmission through the outer or middle ear. This outcome does not definitively diagnose hearing loss but signals a need for further, more comprehensive audiological evaluation by an audiologist. Conditions like fluid in the middle ear or ear canal debris can prevent OAEs from being recorded, leading to a “refer” result even if inner ear function is normal.
It is important to understand the limitations of OAE testing. A “pass” result does not guarantee perfect hearing, as OAEs primarily assess the function of the outer hair cells in the cochlea. They do not evaluate the entire auditory pathway, such as the inner hair cells, the auditory nerve, or the brain’s processing of sound. Therefore, a person could pass an OAE test but still have a form of hearing loss that affects other parts of the auditory system.