Otoacoustic emission

Otoacoustic emissions (in short: OAE, from Greek ous, genitive otos = ear) are active, acoustic emissions of the ear, the retrograde, that is opposite to the direction in the perception of sound, get over the way ossicles and tympanic membrane in the ear canal and can be recorded there with the help of highly sensitive measuring microphones. You are detectable in about 97 % of people occur in all terrestrial vertebrates and even in the hearing organs of insects.

History of OAE

In the 1940s, the future Nobel Prize winner Georg von Békésy postulated its traveling wave theory, however, some concepts of the cochlear processing concepts could not explain ( among other things because he experimented with "dead" cochleae ). The physicist Thomas Gold suspected an active feedback mechanism and turned to Békésy with the request to repeat his experiments also on living inner ears. However, this took a different approach and gold turned away from the acoustic research because it its postulated energy radiations were not detectable with the then possible measurement methods in the ear canal. It was not until 30 years later succeeded in a British physicist, David Kemp, prove Gold's hypothesis by evoked OAE could measure.

Emergence of the OAE

The OAEs are generated in the inner ear of the outer hair cells. These have various mechanisms to shorten, so that the basilar membrane can be active and frequency-specific biased. This feature leads to an enhancement of the mechanical sound stimulus, so that even with little sound energy that are excited for the transmission of impulses to the auditory nerve competent inner hair cells. The activity of the outer hair cells is affected by various influences from the brain and stations of the auditory pathway. The active strain of the basilar membrane generates the acoustic energy can be measured as OAE. Therefore OAE are only detectable in healthy ears, the OAE remain from when changing or loss of hair cells.

Types of OAE

There are two different types of OAE:

• Spontaneous OAE
• Evoked ( caused by acoustic stimuli ) OAE

Spontaneous OAE ( SOAE ) occur in 35-50 % of healthy ears and are for the producer himself can not be heard (as defined for tinnitus). Your formation mechanism is unclear, they are diminished by ear harmful or noxious noise pollution. You have no significant clinical importance.

Evoked OAE ( EOAE ) occur during or shortly after an acoustic stimulation of the ear. Depending on the shape of the acoustic stimulus different subgroups of evoked OAEs are distinguished:

Measurement of OAE

Since the level of OAE are very low, very sensitive measurement microphones must be used. They are housed together with a transducer which generates the stimulus in the ear canal probe. This probe is sealed to the acoustic attenuation of ambient noise with a resilient material against the ear canal wall. In order to clarify the results and for reducing Störgeräuscheinflüssen the stimulus measurement phases are repeated several times and the results subjected to a mathematical averaging method. According to a Fourier analysis and frequency level graphs can be displayed. The measurement of OAE is only possible with nearly normal middle ear conditions.

Clinical Use

The measurement of OAE closes a gap of so-called objective Diagnosis of hearing (without activity of the subjects) between the middle ear diagnostics based on tympanogram and the Hörnervendiagnostik by BERA. With the OAE can be specifically tested the function of the cochlea. The TEOAE be like, used for their detection up to a hearing loss of <35 dB ( HL) as a screening test, for example the newborn, but are due to the broad-based stimulus frequency hardly specific. Also known as topodiagnosis to estimate the damage location at hearing they are used. The DPOAE are often issued as an "independent hearing test " because they can sense the cochlear frequency- specific, but the cut-off of failure is not as sharply defined as in the TEOAE. So DPOAE are still detectable at hearing losses up to 50 dB ( HL). By a level- dependent saturation behavior of the OAE, however, the hearing loss can be extrapolated by measuring the growth function.