Otoacoustic emissions

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Otoacoustic emissions (short: OAE, from Greek ous , genitive otos = ear ) are active, acoustic emissions from the inner ear that are retrograde, i.e. H. contrary to the direction of sound perception, the ossicles and eardrum enter the auditory canal and can be recorded there with the help of highly sensitive measurement microphones. They are detectable in approx. 97% of people, occur in all terrestrial vertebrates and even in the hearing organs of insects .

Story of discovery

In the 1940s, the later Nobel Prize winner Georg von Békésy postulated his traveling wave theory , which, however , could not explain some aspects of the cochlear processing concepts (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 on living inner ears as well. However, the latter pursued a different approach and Gold turned away from inner ear research, as his postulated energy emissions into the ear canal could not be detected with the measurement methods possible at the time. It was only 30 years later, in July 1978, that the British physicist David Thomas Kemp (born February 24, 1945) of the Royal National Throat, Nose and Ear Hospital in London, succeeded in proving Gold's hypothesis by measuring evoked OAE.

Creation of the OAE

The OAE are generated in the inner ear by the stereocilia of the outer hair cells , which in addition to their function as mechanoreceptors also have the function of motors ( motility ). This motor function of the hair bundles is used to mechanically amplify the recorded sound signals and thus to improve the frequency coordination and frequency resolution of the hearing organ. In mammals, there is also a second motor function by changing the length of the cell bodies of the outer hair cells, synchronized with the acoustic frequencies ( cochlear amplifier ). The motor activity of the hair cells generates additional acoustic energy, which reaches the ear canal via the fluids in the inner ear and the middle ear and can be measured there as OAE. OAE are only detectable in ears that do not show major hearing loss. If the hair cells are damaged or fail, the OAE do not occur. The activity of the hair cells is influenced by descending ( efferent ) nerve connections from the brain.

Section through the cochlea : structure of the organ of Corti

Types of OAE

There are two main types of OAE:

  • spontaneous OAE
  • evoked OAE (produced by acoustic stimuli)

Spontaneous OAE (SOAE) are narrow-band (tonal) acoustic signals continuously emitted by the hearing without external stimulation, which can be measured in the external auditory canal with a microphone probe. At least one SOAE can be proven in approx. 35–50% of all persons. The frequencies are mostly between 500 and 4500 Hz and are very stable, but the levels fluctuate significantly between -30 and +10 dB SPL.

It is typical of SOAE that they are influenced by neighboring external tones. They behave like van der Pol oscillators when they are influenced by external forces.

SOAE are usually not heard by the people concerned themselves. A proportion of around 1–9%, however, hears SOAE as a disruptive tinnitus . Their mechanism of origin is unclear. They are reduced by ear-damaging noxa or noise pollution. They are of no major clinical importance.

Evoked OAE (EOAE) arise during or shortly after acoustic stimulation of the ear. Depending on the form of the acoustic stimulus, different sub-types of evoked OAE are distinguished:

  1. Transient evoked otoacoustic emissions (TEOAE) can be detected after a brief acoustic stimulus (click or tone burst). As the "classic" OAE, they are also known as Kemp echoes. Most commonly used clinically.
  2. Stimulus frequency emissions (SFOAE) are evoked by a sine tone as continuous OAE. Clinically of no major importance.
  3. Distorsively produced otoacoustic emissions (DPOAE) are generated by two simultaneous sine tones (f1 and f2). In the nonlinear system of the cochlea it causes distortion (English distortion ) that stand as an amplitude increase in the measured spectrum. The frequency and amplitudes of the distortion products depend on the ratios of the stimulation frequencies and amplitudes, which can be described mathematically. In humans, a ratio of has proven to be particularly meaningful. Clinically also very important because of the possibility of frequency specificity.

Measurement of the OAE

Since the levels of the OAE are very low, very sensitive measurement microphones must be used. These are placed in an ear canal probe together with a sound transducer that generates the stimuli. This probe is sealed against the wall of the ear canal with an elastic material for acoustic dampening of the ambient noise. To make the result more precise and to reduce the effects of background noise, the stimulus measurement phases are repeated several times and the results are subjected to a mathematical averaging process. Frequency level diagrams can be displayed after a Fourier analysis . The measurement of the OAE is only possible with approximately normal middle ear conditions.

Clinical use

The measurement of the OAE closes a gap in the so-called objective hearing diagnostics (without activity of the test person) between the middle ear diagnostics using a tympanogram and the auditory nerve diagnostics using BERA . The function of the cochlea can be specifically tested with the OAE . The TEOAE are often used as a screening test because of their detection up to a hearing loss of <35 dB (HL), e.g. B. used in newborn hearing screening, but are hardly frequency-specific because of the broad-based stimulus. They are also used as topodiagnostics to assess the damage location in the case of hearing loss. The DPOAE are often given as an "independent hearing test" because they can scan the cochlea in a frequency-specific manner, but the cut-off of failure is not as sharply defined as with the TEOAE. DPOAE are still detectable in hearing losses of up to 50 dB (HL). Due to the level-dependent saturation behavior of the OAE, however, the hearing loss can be extrapolated by measuring the growth function.

literature

  • Rolf Hauser: Application of otoacoustic emissions: a compendium for clinics and practices . Enke, Stuttgart 1995, ISBN 3-432-26491-7 .
  • Sebastian Hoth, Katrin Neumann: The OAE manual . Thieme, Stuttgart 2006, ISBN 3-13-142561-X .

Individual evidence

  1. ↑ DT Kemp: Stimulated acoustic emissions from within the human auditory system. In: J Acoust Soc Am. 64, 1978, pp. 1386-1391.
  2. ^ G. Long: Perceptual consequences of the interactions between spontaneous otoacoustic emissions and external tones. I. Monaural diplacusis and aftertones In: Hearing Research. Volume 119, 1998, pp. 49-60.
  3. ^ MJ Penner: An estimate of the prevalence of tinnitus caused by spontaneous otoacoustic emissions. In: Arch Otolaryngol Head Neck Surg. Volume 116, Number 4, April 1990, pp. 418-423.
  4. ^ T. Janssen, HP Niedermeyer, W. Arnold: Diagnostics of the cochlear amplifier by means of distortion product otoacoustic emissions . In: ORL: Journal for Oto-Rhino-Laryngology and Its Related Specialties . tape 68 , 2006, ISSN  0301-1569 , p. 334-339 , doi : 10.1159 / 000095275 .