Transient evoked otoacoustic emissions

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Transient evoked otoacoustic emissions (abbreviation TEOAE , from Greek otos = ear) are the acoustic response of the inner ear to a short, broadband acoustic stimulus.

TEOAE are a phenomenon that can be observed in the hearing of humans and many animals. Brief sound stimuli, so-called clicks, lead to a mechanical reaction in the inner ear. This reaction is transmitted to the outside again as sound. According to current research, the cause of TEOAE is the outer hair cells in the cochlea , which can partially amplify sound waves (so-called cochlear amplification mechanism), which is helpful for hearing low sound pressure levels. Since the hair cells their vibrations give slightly delayed and even after short events reverberate, a slightly longer response signal can be measured in the ear canal after a brief sound event. Since this signal is weaker in damage to the outer hair cells of the inner ear, or even disappear, the measurement of the teoae can for diagnosis of diseases can be used in the inner ear.

The triggering and measurement of TEOAE is one area of audiometry .

Triggering and measurement

In the case of emissions caused by clicks, a probe in the external auditory canal emits click noises of medium volume into the ear. In the short pauses between the clicks, the reflected noise and the noise emitted by the ear are measured by a microphone in the probe, and the noise emitted by the ear, the otoacoustic emissions, is filtered out and graphically displayed with the help of suitable computer programs. Each ear gives off a highly individual image, which is uniquely similar to that of the fingerprint. To ensure that the noise is not random, the measurement computer divides the measurements into two parts and compares the two partial results. If they match to a high percentage, otoacoustic emissions are proven.

The emissions have a very low sound pressure level, some of which is -10 dB SPL . The measurement of such quiet sound signals is actually not possible without electronic post-processing. Therefore, several measures are used to obtain a usable result. On the one hand, the measurement environment must contain as little background noise as possible. This is achieved through the position of the probe in the ear canal, its sealing with foam or silicone adapters and relative calmness. On the other hand, the measurement, which itself only lasts a tenth of a second or less, is continuously repeated over several minutes (in practice approx. 300 measurements). Since the response of one ear to the same stimulus is practically always the same, the measuring computer can use the many measured responses for a signal reconstruction by averaging. Assuming a mean-free noise, which is independent of the OAE, the signal-to-noise ratio increases by 3 dB when the measurement time is doubled .

Statement of the measurement results

The presence of TEOAE proves the activity of the outer hair cells with great certainty. The arrangement of the outer hair cells represents a frequency-location coding ( tonotopy ). Signal components of the click stimulus with high frequencies stimulate the outer hair cells near the oval window to vibrate, while signal components with low frequencies stimulate the outer hair cells in the direction of the apical end of the basilar membrane . Due to the different transit times of the retrograde traveling waves, the emitted noises can be analyzed separately. Thus, specific statements about the hearing ability in individual frequency ranges are possible. Proof of the existence of TEOAEs confirms the functionality of the outer hair cells. Ambient noise, clogged probe channels and sound conduction disturbances such as B. Middle ear diseases can prevent TEOAE from being measured even though they are produced by the inner ear. Thus, the reverse conclusion from missing TEOAE to a disease of the inner ear cannot be made solely by TEOAE measurements. Types of hearing loss that lie "behind" the outer hair cells in the course of the auditory pathway, e.g. B. neural or central causes are not recorded by the OAE measurements.

application

The measurements of the TEOAE are u. a. Used for newborn hearing screening, which can detect hearing impairments caused by the inner ear just a few hours after birth. This allows you to assess the inner ear hearing ability in the range of 1–5 kHz. Sound sensation disturbances above 20–30  dB HL can be excluded by measuring the TEOAE. However, it does not record neural (auditory nerve-related) hearing disorders. In clinical studies, measurement systems for TEOAE usually show a sensitivity of 99% and more, but only a specificity of 90% to 95%. This means that with a frequency of inner ear hearing loss of 0.1% in newborns, every 10th to 20th child shows no TEOAE even though there is no hearing damage. Such numbers, however, are inherent in the nature of the screenings and are accepted in order to be able to reliably identify all hearing damage as possible. The main advantage of this measurement, however, is that the patient does not have to actively participate in the measurement. Only then is it possible to use it in newborns.

See also

literature