Transitory evoked otoacoustic emissions, or TEOAEs, are acoustic responses from the inner ear to a brief and broadband acoustic stimulus. Thus, brief acoustic stimuli elicit a mechanical response from the inner ear that is transmitted externally as sound.
What is transitory evoked otoacoustic emissions?
Transitory evoked otoacoustic emissions, or TEOAEs, are acoustic responses from the inner ear to a brief and broadband acoustic stimulus. The measurement of transitory evoked otoacoustic emissions is a very important examination in hearing diagnostics to clarify hearing impairments, especially hearing loss. It supports selective testing of the inner ear as well as the outer hair cells, which allows clarification of possible causes of hearing impairment. These acoustic-based transmissions enter the ear canal via the ossicles and eardrum, in the opposite direction to the perception of sound. Approximately 97 percent of people have been shown to have such transitory evoked otoacoustic emissions. They are among the most common clinical uses and are also known as Kemp echoes as classic otoacoustic emissions. The cause of such transitory evoked otoacoustic emissions is the outer hair cells located in the cochlea. The cochlea partially amplifies sound waves with a so-called cochlear amplification mechanism, allowing hearing at low sound pressure levels. However, the hair cells pass on these oscillations with a slight delay, reverberate and are measured in the auditory canal after a short sound event with a longer response signal. The measurement as well as the triggering of the transitory evoked otoacoustic emissions is an area of audiometry and allows to diagnose possible diseases of the inner ear. The signal, if the inner ear is damaged, becomes weaker or may fade altogether.
Function, effect, and goals
Transitory evoked otoacoustic emissions are elicited by clicks and then transmitted to the ear as click sounds at moderate volume using a probe in the external auditory canal. The reflected noise in the pauses is measured up to the emitted noise and filtered by means of a special computer program and visualized in a graphical representation. This procedure provides an individual image of each ear, which is both unique and individual, similar to a fingerprint. In order to exclude random hits, the measurements are divided and compared in their partial results. If there is a high level of agreement between the two measurements, this is considered evidence of transitory evoked otoacoustic emissions. Transitory evoked otoacoustic emissions, TEOAE, are important for the clarification of a possible hearing loss. Because transitory evoked otoacoustic emissions up to 35 decibels can reveal a possible hearing loss, they are a valuable tool for screening tests. Transient evoked otoacoustic emissions are already used for newborns, but they are considered less frequency specific due to their broad-based stimulus effect. With the help of transitory evoked otoacoustic emissions, hearing disorders originating from the inner ear can be detected in the newborn hours after birth. In addition, the measurements allow exclusion of a sound perception disorder above 20 to 30 decibels. In most cases, the emissions have a very reduced sound pressure level at up to less than 10 decibels, measurements of very quiet sound signals that would be almost impossible without electronic post-processing. In combination with various measures, they provide important results if the measurement environment contains as little background noise as possible. In this case, the probe is optimally positioned in the auditory canal, sealed with foam or silicone, and the quietest possible situation in the environment is ensured. The measurement itself only takes a maximum of one tenth of a second and is repeated over a period of several minutes. Approximately 300 measurements are taken in the process. Due to the relatively constant reactions of the ear, messages are generated which are helpful values for signal reconstruction.The responses of the outer hair cells and their arrangement provide information about the transitory evoked otoacoustic emissions. Thus, for signal components with a click stimulus, the outer hair cells close to the oval window respond at a high frequency. For signal components with lower frequencies, the outer hair cells respond in the direction of the basilar membrane. With different travel times of the so-called retrograde traveling waves, i.e. against the direction of sound, analyses are possible and thus also statements about the actual hearing in certain frequency ranges. Measurement of transitory evoked otoacoustic emissions is also used to monitor reduced inner ear functions in the event of a hearing loss or noise-induced hearing loss. Inner ear damage favored by medications such as chemotherapeutic agents, dehydrating agents, or even certain antibiotics can be detected early.
Risks, side effects, and hazards
Meaningful measurement of transitory evoked otoacoustic emissions is not possible, for example, if the ambient noise is too loud, the probe channels are obstructed, or there is a conductive disorder such as in middle ear disease. However, the reverse is not true for inner ear disease in the absence of evidence of transitory evoked otoacoustic emissions. Certain forms of hearing loss that lie quasi behind the outer hair cells, due to central causes or neural, auditory nerve-related hearing disorders, cannot be detected by measurement. However, the measurement is ideal because this examination does not require the active assistance of the patient. Thus, problem-free measurement in a newborn is also possible. However, the transitory evoked otoacoustic emissions are only useful up to a certain degree of hearing loss. They diminish with advanced age, and sometimes they are absent altogether. Nevertheless, subjective hearing as well as the audiogram may be normal. To reliably determine hearing ability, it is advisable to combine several test procedures to obtain an optimal overall result.
