The ability to hear is essential for social interactions, cognition, and understanding emotions. Thus, infants and young children need to have their hearing checked as soon as possible to determine if they have any type of hearing loss. Even a mild hearing loss can lead to the child’s inability to learn how to speak clearly or to understand language. If caught early, hearing loss can be treated. Thus, hearing should be evaluated before a newborn leaves the hospital.
Causes of hearing loss include but are not limited to premature birth, complications at birth, frequent ear infections, and being exposed to very loud sounds even if it is for a brief time. Hearing loss comes in two different forms: (1) conductive hearing loss, which is caused by interference in the transmission of sound to the inner ear, and (2) sensorineural hearing loss, which is caused by malformation, dysfunction, or damage to the inner ear. Thus, tests have been developed to determine the function of hearing. These physiologic tests include auditory brainstem response, auditory steady state response, tympanometry, middle ear muscle reflex, and otoacoustic emissions.
Auditory brainstem response predicts how well a baby’s inner ear and lower part of the auditory system (brainstem) is functioning. Auditory steady state response is performed under sedation and a computer is used to pick up the brain’s response to a sound and establish hearing level. Tympanometry is actually a procedure to show how well the eardrum moves when a soft sound and air pressure are introduced into the ear canal. Middle ear muscle reflex tests how well the ear responds to loud sounds, which in a healthy ear causes the middle ear to contract. Finally, otoacoustic emissions test how healthy the outer hair cells are and if they are functioning properly. Otoacoustic emissions tests are of clinical importance because they are a simple, noninvasive test for hearing defects in newborns as well as in children who are too young to cooperate with conventional hearing tests.
Otoacoustic emissions are sounds originating from the cochlea in the ear. A microphone that is fitted into the ear canal records these sounds. When sound stimulates the cochlea, the outer hair cells vibrate, producing a nearly inaudible sound that echoes back to the middle ear. A person with normal hearing can produce otoacoustic emissions, but if an individual has hearing loss that is greater than 25 to 30 decibels (dB), he or she will not produce otoacoustic emissions. In the United States, an otoacoustic emissions test is typically performed as part of the newborn hearing screening program and can detect blockages of the outer ear canal and the presence of middle ear fluid or damage to the outer hair cells.
Types of Otoacoustic Emissions
There are two types of otoacoustic emissions: spontaneous and evoked. Spontaneous otoacoustic emissions are emitted from the ear without any sort of stimulation and can be measured with a microphone fitted into the ear canal. About 35 to 50 percent of the population can detect at least one spontaneous otoacoustic emission. Most people are completely unaware of spontaneous otoacoustic emissions and about 10 percent of people perceive them as a very irritating tinnitus, or the hearing of a sound when one is not actually present.
Evoked otoacoustic emissions are currently evoked with three different processes: stimulus frequency, transient-evoked, and distortion product. Stimulus frequency—evoked otoacoustic emissions are detected by the difference between a stimulus wavelength and the recorded wavelength the individual hears. Transient-evoked otoacoustic emissions are evoked via a clicking or tone burst (pure tone) stimulus. Response from the clicking can cover a frequency ranging up to around 4 kilohertz (kHz), but a tone burst will only produce a response that has the same frequency as the pure tone. Lastly, distortion product otoacoustic emissions are evoked with a pair of primary tones with particular intensities and ratios. These tones are presented at the same time to the cochlea; the response determines the intermodulation distortion and the other determines the reflection by the cochlea.
See also: Age-Related Hearing Loss; Auditory Hallucinations; Auditory Processing Disorder; Brainstem Auditory Evoked Potentials; Cochlea; Deafness; Tinnitus
Akron Children’s Hospital. (2012). Hearing evaluation in children. Reviewed by Thierry Morlet. Retrieved from https://www.akronchildrens.org/cms/procedures_tests/Otoacoustic_Emissions_Test/
Kemp, David T. (2002). Otoacoustic emissions, their origin in cochlear function, and use. British Medical Bulletin, 63(1), 223—241. Retrieved from http://bmb.oxfordjournals.org/content/63/1/223.full
Shahid, Ramzan, Michaela Vigilante, Heidi Deyro, Irma Reyes, Beverly Gonzalez, & Stephanie Kliethermes. (2015). Risk factors for failed newborn otoacoustic emissions hearing screen. Clinical Pediatrics (Philadelphia). http://dx.doi.org/10.1177/0009922815615826