OtoAcoustic Emissions (OAE s)

Transcription

1 OtoAcoustic Emissions (OAE s) Phenomenon and applications in audiological diagnostics Measurement procedures TEOAE and DPOAE Physiological backgound, functional models Acknowledgment: several illustrations and diagrams are taken from a tutorial presentation of Prof. Sebastian Hoth, Heidelberg Otoacoustic emissions (Dillier: lecture Medical Acoustics)

2 Differentiation and quantification of hearing disorders

3 Advantages of OAE measurement in audiology Objective response Response specific for cochlea Sensitive test Short test procedure Only passive cooperation required

4 Clinical applications Detection of hearing disorder (newborn screening, babies, children, adults with suspected aggravation, functional hearing loss) Monitoring of cochlear function (ototoxic drugs, noise, degenerative processes, intraoperative monitoring) Audiological differential diagnostics: specific for cochlear lesions

5 Types of otoacoustic emissions Spontaneous otoacoustic emissions (SOAE) Transitory evoked otoacoustic emissions (TEOAE) Distortion product emissions (DPOAE) Stimulus frequency emissions (SFOAE)

6 Otoacoustic emissions (OAE) Spontaneous otoacoustic emissions (SOAE) Evoked otoacoustic emissions (EOAE) Poststimulatory OAE Perstimulatory OAE Transitory evoked OAE (TEOAE) Stimulus frequency otoacoustic emissions (SFOAE) Otoacoustic distortion products (DPOAE)

7 Otoacoustic emissions

8 Measurement equipment for OAE recordings Th. Janssen, in Praxis der Audiometrie (Lehnhardt & Laszig, 2000)

9 Principle of evoked otoacoustic emission measurements (EOAE) Stimulus Response Amplitude Amplitude Time TEOAE Delayed OAE Transitory Evoked OtoAcoustic Emissions DPOAE Distortion products Distortion Product OtoAcoustic Emissions Frequency

10 TEOAE: Transitory (short stimulus, e.g. click) Evoked OtoAcoustic Emissions

11 OAE: verification/optimization of stimulus/measurement conditions 1. Probe Cerumen Patency? Orientation Leakage Stability 2. Stimulus Level Ear canal response 3. Environment Ambient noise Sound attenuation Room acoustics Ear plugs Remote from tympanic membrane 4. Patient Respiration Movements Cables

12 Probe related sources of error Th. Janssen, in Praxis der Audiometrie (Lehnhardt & Laszig, 2000)

13 Improvement of signal/noise relation by 3-fold summation: Emission Noise 3-fold test signal amplitude 1,7-fold noise signal amplitude

14 SNR improvement is only dependent on number of averages (n): SNR improvement (Gain G): S N( n) AS ( n) AN ( n) ( cs n) ( cn n) n G = 20 log = 20 log = 20 log = 20 log = 10 log n S N(1) A (1) A (1) c cn n S Otoacoustic N emissions (Dillier: lecture Medical S Acoustics)

15 TEOAE: documentation of measurement and result

16 Properties of true OAE? Typical response properties: Good measurement conditions Stimulus STABILITY A - B DIFF reproducible response REPRO > 60% SNR > 6 db Emission amplitude between 0 and 25 db SPL Emission duration > 6 ms Initially fast, later slower oscillations Amplitude decreases with increasing delay Amplitude decreases for Otoacoustic higher emissions (Dillier: frequencies lecture Medical Acoustics)

17 Polarity averaging and Binomial statistics: What is the percentage of a positive amplitude at a fixed temporal interval of a random signal? Polarity average Binomial distribution: Meas. signal with Polarity average Meas. signal n>5: Gaussian distribution with and Polarity average Time If the polarity average reaches a value of α n, the signal is probably not a random process (sig. level 1-α)

18 Nonlinearity and distortion Output (linear) Amplitude Time Input (linear) Original signal Linear Amplification Intensity Nonlinear amplification Additional frequencies Frequency Schematic diagram

19 Effect of cubic terms in the transfer function Input u 1 (t) Nonlinear system Output u 2 (t) ( sinω t + t) 1( t) = c 1 sinω u u2 ( t) = a1u 1( t) + a2u1 ( t) + a3u1 ( t) +... Quadratic distortions: 2 u ( t) =... cos( ω ω ) t + sin 2ω t + cos( ω + ω ) t + sin ω t Cubic distortions: 3 u t) =... sin(2ω ω ) t + sin(2ω ) t 1 ( ω2 + terms with 3ω 1 2ω 1 + ω2 2ω + ω 2 1 3ω 2 Distortion product

20 Equipment to measure distortion products (DPOAE s) Th. Janssen, in Praxis der Audiometrie (Lehnhardt & Laszig, 2000)

21 Stimulation using two sinewaves: Superposition of the primary travelling waves and origin of distortion products stimulus f 1 L 1 = 70 db SPL stimulus f 2 = 1.2 * f 1 L 2 = 70 db SPL DP 2f 1 -f 2 Base f 2 Apex

22 Stimulation using two sinewaves: Superposition of travelling waves f1 f2 Base Apex f2 f1 Base Apex Overlap: maximum very close to f2

23 Spectrum of DPOAEsignal Th. Janssen, in Praxis der Audiometrie (Lehnhardt & Laszig, 2000)

24 Audiological interpretation of OAE: OAE s are results of the active cochlear amplification The more effective this amplification is the more sensitive the hearing the larger the emissions The OAE amplitude is inversely proportional to the hearing threshold

25 TEOAE or DPOAE? Advantages of TEOAE Advantages of DPOAE Smaller probe Up to 30 db hearing loss Technical artefacts relatively ease to avoid Less measurement noise Up to 50 db hearing loss Automatic interpretation Suitable for screening Systematic inner ear diagnostics If possible both TEOAE and DPOAE!

26 TEOAE-Measurement at day 2 or 3 pass refer repeat refer pass ABR etc. Ok HI or CI

27 DPOAE Screening device (BioLogic)

28 OAE-Echoscreen (Mack GmbH)

29 OAE Screener Echocheck/Echosensor (Otodynamics Ltd)

30 GSI 60 DPOAE (Grason Stadler)

31 What does an alerting screening result mean? Hearing loss prevalence (language development)...: 3 : 1000 Sensitivity... : 100 % Specifity... : 92 % What is the probability that a child with positive screening result at the first OAE test session has indeed a hearing loss? Only in 3 out of 1000 children is a hearing disorder to be expected Out of 1000 tested normal hearing children 80 will show an alerting screening result 3 / 83 = 0,0361 Positive predictive value... : 3,61 % According to T. Steffens, Regensburg

32 Nonlinear cochlear mechanics: Sensitivity and frequency selectivity at low stimulation levels Johnstone BM, Patuzzi R, Yates GK (1986) Basilar membrane measurements and the travelling wave. Hear Res 22:

33 Structure and function of outer hair cells Tectorial membrane Cuticular plate K + Fast motility: transversal force on the hair bundle. Active amplification? Slow motility: longitudinal force under efferent control. Adaptation at high levels? Electrical tuning Deiters cells efferent afferent After Kim 1986

34 Cochlear Micromechanics Amplification and sharpening of basilar membrane vibrations through nonlinearity electromotility of outer hair cells amplification feedback nonlinearity Preyer 1996

35 Cochlear sound processing and generation of OAE Threshold 0 db Threshold ~ 50 db

36 Active filter mechanisms