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1. OTOACOUSTIC EMMISIONS OAE Dr Meshwa P. Oza Resident ENT

2. HOW DO WE HEAR?

3. Otoacoustic emissions The sound emitted by the biological activity of the normal cochlea which can be picked up, recorded and measured by placing microphone in the deep external auditory meatus is called otoacoustic emissions(OAE). OAEs are low-intensity sounds/acoustic signals that are generated by the cochlea and propagate through the middle ear into the ear canal where they are recorded .

4. OAEs are probably not essential to hearing, but rather are the by- product of active processing by the outer-hair cell system. They are most probably generated by active mechanical contraction of the outer hair cells , spontaneously or in response to sound. OAEs reveal, with considerable sensitivity, the integrity of outer-hair cell function.

5. MECHANISM OF PRODUCTION OAEs are considered to be related to the amplification function of the cochlea. The supersensitive cochlear amplifier which is responsible for the very precise hearing sensitivity and very sharp tuning is actually a bio-mechanical device that involves mechanical activity within the cochlea. This mechanical activity within the cochlea generates an acoustic signal i.e., sound.

6. Stimulus delivered to the ear invokes movement of the basilar membrane, which in turn causes the OHCs to move, or be deflected. When the OHCs move, their stereocilia bend in one direction or the other. Ions rush in and rush out, changing the membrane potential within the hair cell. The changes in voltage across the plasma membrane lead to OHC length changes (shortening and lengthening), which are called electromotility . MECHANISM OF PRODUCTION

7. The electromotility of the OHCs has a feedback effect on the basilar membrane, causing it to vibrate. Therefore, the electromotility of the OHCs is thought to be the mechanism which underlies OAEs. When the basilar membrane moves, the hair cells are set into motion and an electromechanical response is elicited. While an afferent signal is transmitted and an efferent signal is emitted. The efferent signal is transmitted back through the auditory pathway, and the signal is measured in the outer ear canal. MECHANISM OF PRODUCTION

8. TYPES SPONTANEOUS EVOKED Transient-evoked OAEs (TEOAEs) Distortion-product OAEs (DPOAEs) Stimulus frequency OAEs (SFOAE)

9. TYPES OF OAE Spontaneous otoacoustic emissions (SOAEs) - Sounds emitted without acoustic stimulus (ie, spontaneously). Transient otoacoustic emissions (TOAEs) or transient evoked otoacousticemissions (TEOAEs) – Sounds emitted in response to an acoustic stimuli of very short duration; usually clicks but can be tone-bursts.

10. Distortion product otoacoustic emissions (DPOAEs) – Sounds emitted in response to 2 simultaneous tones of different frequencies. Sustained-frequency otoacoustic emissions (SFOAEs) – Sounds emitted in response to a continuous tone.

11. PREREQUISITES Unobstructed outer ear canal Seal of the ear canal with the probe Optimal positioning of the probe Absence of middle ear pathology Functioning cochlear outer hair cells A quiescent patient: Excessive movement or vocalization may preclude recording. Relatively quiet recording environment: A sound booth is not required, but a noisy environment may preclude accurate recording.

12. PREREQUISITES Visual inspection, and preferably tympanometric measurement prior to OAEs recordings, will help determine if middle ear and external ear abnormalities might reduce or block acoustic transmission of OAEs from the cochlea to the microphone. Reduction of ambient noise picked up by the microphone is achieved by a tight fit of the probe into the ear canal. Patient -generated sounds can be minimized by instructing the patient to be still and not to talk during testing.

13. SPONTANEOUS OAE Spontaneous OAEs (SOAEs) are narrowband signals that occur in the ear canal without the introduction of an eliciting signal. Spontaneous emissions are present in over half of all normal-hearing ears and absent in all ears at frequencies where sensorineural hearing loss exceeds approximately 30 dB. SOAEs originate from outer-hair cells corresponding to that portion of the basilar membrane tuned to their frequency.

14. SOAEs are low intensity, continuous, very narrow band or pure tone sounds that cannot be heard by the person in whose ear it is being generated. But it can be picked up by placing a very sensitive miniature microphone in the external auditory meatus just lateral to drum.

15. If the SOAEs can be detected in an individual, it indicates that the individual has normal hearing at the frequency at which the SOAEs are being generated. No idea can be had whether the person has normal hearing at other frequencies.

16. It is not known why SOAEs are detectable in only 50%(with improved recording techniques 70%) of the normal hearing individuals. Many explanations have been offered but the one that is generally accepted is that some normal abberations in physical structure of the organ of corti, like a fourth row of outer hair cells in the cochlea is the probable cause of generation of SOAE.

17. Normal range SOAEs in adults have been found to have an average intensity of -3dB to 2.6dB . In infants the intensity is significantly higher and is about 8.5dB of average. In certain cases it can be upto 20dB. The frequency of spontaneous otoacoustic emission is usually between 1000 to 2000 Hz in adults and between 3000 to 4000 Hz in infants.

18. The intensity and frequency of SOAEs change over short periods of time and also with age. In old age -above 60 years, not only intensity , but also the prevalence of SOAEs decreases even if hearing is normal. Present more frequently in females than males and are detectable more in right than left ear. The cause is not known. In some patients, multiple SOAEs are present all together in the same ear instead of single sound as present in most ears.

19. Clinical implications Detection of SOAE in an ear indicates that hearing threshold is within normal limits in and around the frequency at which it is generated. The average hearing threshold for all frequencies combined together in an ear in which SOAEs are detectable is definitely within 30dB, i.e., better than 30dB. So if in an ear there is an SOAE detectable whose frequency is 1500 Hz, then average threshold of all frequencies combined together is better than 30dB.

20. If there is mild deafness in one or two frequencies but SOAEs are detectable in an ear, then the hearing threshold at the frequency of SOAE is better than 20dB and the hearing loss in those one or two frequencies is mild so that, average hearing threshold is within 30dB for that ear.

21. EVOKED OTOACOUSTIC EMISSIONS(EOAE) It does not occur spontaneously and has to be evoked by presenting a sound stimulus to the ear. It is hence termed as Evoked OAE. It is sometimes also referred to as Transient OAEs, since they are not continuous like SOAEs. A special type of Transient OAE which is now very popular is the Distortional Product OAE(DPOAE).

22. The character, i.e., the frequency, latency, duration, etc. of the evoked otoacoustic emission is dependent upon the character of evoking stimulus. The otoacoustic emission in response to a sound stimulus of a particular frequency occurs from that part of cochlea which is tuned for that particular frequency. Eg ., if a low frequency sound is presented to cochlea, the EOAE will be generated from the apical region of cochlea, since apex is tuned for low frequency sounds- these will have longer latency period and have to travel from the farthest end of cochlea to middle ear and to EAC.

23. The emissions from cochlea are true sounds and not electrical impulses. Hence, they are recorded using microphones and not electrodes. EOAEs reduce very rapidly as deafness increases and are undetectable when deafness is above 30 to 35 dB .

24. The EOAEs are usually elicited by presenting clicks at intervals of 20 milliseconds and the response is obtained as a series of waves or osscilations having latency between 5 to 10 milliseconds after the onset of stimulus. This temporal separation helps in visual identification and separation of the transient -evoked emissions from the stimulus that evoked them, that is also recorded.

25. Since the OAE are very very faint sounds of 0 to 15 dB, they can be easily masked by extraneous noise. They are generated from 2 sources : 1) the ambient environmental noise 2) the biological noise of blood flow, respiration, swallowing, etc. generated internally within the subject’s body. These noises are generated at random whereas OAE noise is generated at a particular time after the presentation of the sound stimulus. NOISE

26. If a large number of sound stimuli are presented for evoking the TOAE/DPOAE and then summated and averaged, the random and unsynchronized extraneous noise will be cancelled. The extraneous noise comprising of biological sounds and ambient environmental sounds are recorded in the OAE recording as “noise floor”. OAE is measured and documented as OAE generated above the noise floor.

27. SOLUTION As these noises mask the OAE, it is always necessary to 1) carry out the test in a very quiet environment 2)Sedate the child before the test is done so that child is relaxed and internally generated sounds are lesser. To get a good OAE recording, the noise floor has to be minimized.

28. TYPES OF OAE MACHINES 2 types of OAE machines are available:- 1) clinical machine in which detailed waveform of OAE, noise floor and stimulus is graphically depicted 2) OAE screener in which either a light glows or figure appears indicating presence of OAE. For clinical establishment, a screener which costs much lesser, suffices, but clinical model is better as OAE waveform can also be recorded.

29. If a measurable EOAE is recorded, it indicates that the subject has a normal middle ear and cochlear mechanism. If it is absent, it indicates that there is some problem in middle ear and cochlea which needs to be investigated. Impedance audiometry, Electro cochleography and BERA can then be done.

30. EOAE is very useful for objective screening of hearing in neonates since it is: Sensitive Non invasive Requires very little time Very simple to perform Does not require qualified audiologist or medical person

31. Transient OAE Measurable in essentially all normal hearing persons. Irrespective of age, a very reliable response can be objectively obtained right from newborns to very old debilitated and even unconscious patients if the person has normal middle ear and inner ear function. Can be elicited or evoked by click stimuli or tone bursts.

32. TOAE in response to a click or broadband sound:- broadband sound with more energy in the mid frequency region. TOAE generated by tone burst will have same frequency as that of evoking sound. TOAE elicited from an individual with normal hearing will be unique for that individual as regards the frequency spectrum, latency of the different frequency components and amplitude of the response.

33. Usually elicited by sounds of 80 to 85dB . Using sounds of higher decibels do not have any added advantages. TEOAE response saturates after about 70 to 80dB stimulus level. At low intensities of upto 50 to 60dB, increasing the intensity of evoking stimulus increases the amplitude of TEOAE response in a linear fashion . The average loudness(amplitude/intensity) of TEOAE response recorded with a sound stimulus of 80 to 85dB is about 20dB SPL in neonates and just less than 10dB SPL in adults.

34. IMPLICATIONS TEOAE is most commonly employed for neonatal hearing screening. In neonatal hearing screening, an OAE test is carried soon after birth to screen whether the newly born child has normal hearing or not.

35. WHEN TO DO?? Different studies have been carried out to find out the optimum time after birth when the test is best done. Within 36 hours:-75% At 108 hrs :- 95% General experience- within 24 hours of birth- about 93% of ears if 2 to 3 attempts are made and if ear is cleaned of any slimy substance before the test is done.

36. TEOAE test can be done with both click/broadband sounds as well as puretone sounds- click stimulus is better suited for eliciting the response, the click stimulus is better suited for the test. Click sounds elicit a more robust response. Stimulus artifacts are more with tone burst TEOAE.

37. TEOAE response that is elicited from an ear after presenting a click sound is suppressed if a sound is simultaneously presented to the other ear. This is true not only for TEOAEs but also for SOAEs. Not clear why this happens.

38. There must be some inhibitory effect of the CNS on the cochlear hair calls as the TEOAE and SOAE responses are known to be generated from biological activity of outer hair cells. This unique feature of OAE being suppressed by another sound has been used as a test for confirming a newly discovered disease entity called auditory neuropathy .

39. Distortional Product Otoacoustic Emission When 2 different frequencies were presented simultaneously to the ear, the evoked otoacoustic emission was neither a mixture of the 2 frequencies, nor the 2 discrete frequencies present in the stimulus. It was a sound of another frequency which was totally different from the frequencies of the two sound stimuli. This is termed as distortional product OAE. Just like TEOAE, they can be detected in nearly all normally hearing ears.

40. By convention, the 2 sounds used foreliciting the DPOAE are termed as:- f1 primary -frequency of puretone sound- lower frequency f2 primary- frequency of puretone sound- higher frequency than f1 L1 - intensity level of the f1 primary L2 - intensity level of the f2 primary

41. The most robust DPOAE response was obtained when the f2 to f1 ratio is 1.22:1 F2 should be f1*1.22 or the frequency of f2 should be 1/3 octave higher than f1 When f2 to f1 ratio is 1.22, then the frequency of the evoked DPOAE is 2f1-f2.  

42. NORMAL RANGE The average intensity of DPOAE response is 0 to 10 dB or may be 15dB SPL in some cases. The DPOAE response is about 50 to 75 dB lesser than intensity of the primaries.

43. Usually the f1 primary is presented at 65dB and f2 primary is presented at 50 or 55 dB but small changes are permissible. 2f1-f2 represents the cochlear function of the f2 frequency region of cochlea. By changing the f1 and f2 frequencies, it can be ascertained whether hearing threshold is normal at different frequencies and different zones of cochlea can be tested for normal function. L1 and L2 intensity level of less than 70dB must be used.

44. USES DPOAEs are absent in patients with sensorineural hearing loss greater than 40 dB. They are useful in screening infants for hearing loss. Detection of early cochlear damage due to ototoxicity and noise trauma. In early ototoxicity or noise trauma, even when there is no evidence of deafness on pure tone audiometry, absence of DPOAE in higher frequencies, at 4000 Hz is a sign of early cochlear damage.

45. DISADVANTAGES It can identify hearing loss 10% perfectly if hearing loss is more than 40 dB, but if it is 20-40 dB, it fails to identify in 10% cases. Identification of hearing loss is less accurate in low frequencies. It cannot predict hearing threshold and its clinical use till now is limited only to screening for hearing impairment. TEOAE is marginally more sensitive for low frequency hearing loss whereas DPOAE is slightly better in identifying high frequency hearing loss.

46. OAEs are not of much help in differential diagnosis of deafness. Does not identify the extent of hearing impairment of exact site of lesion. Absence of OAE just indicates probably a cochlear lesion but does not tell us the condition of the higher auditory pathways from auditory nerve to auditory cortex.

47. Auditory neuropathy Desynchronisation or degeneration in auditory nerve, but the cochlear function is totally normal. Since the OAE is generated from cochlea, OAE is present in auditory neuropathy even though patient has hearing problems and abnormal PTA and BERA findings. These patients have very poor understanding of speech. OAE is the most vital tool to identify auditory neuropathy. Diagnostic hallmark is presence of OAE with abnormality in BERA test.

48. RESULTS The OAE test gives a PASS or FAIL result. FAIL suggests there is possibly hearing impairment. Hence most OAE instruments do not give FAIL as a result but instead of it gives REFER as the result. In these cases patient is to be referred for other tests like BERA for threshold estimation to confirm presence/absence of deafness and also to quantify deafness if any.

49. Non pathologic problems that can cause absence of OAEs : Poor probe tip placement or poor seal: Most current equipment alerts clinicians to these problems. Cerumen occluding the canal or blocking a probe port. Debris and foreign objects in the outer ear canal. Vernix caseosa in neonates: This is common immediately after birth. Uncooperative patient: Usually, recordings simply are not obtained .

50. Pathologic problems that can cause absence of OAEs : Outer ear : Stenosis Otitis Externa Polyp Tympanic membrane : Perforation of the eardrum

51. Middle ear : Otosclerosis Middle ear disarticulation Cholesteatoma Bilateral otitis media

52. Bilateral otitis media Even in the presence of normal cochlear function, OAEs generally are absent in the presence of otitis media. OAE testing is best conducted after the otitis media has cleared. If the patient cannot be tested later, when the otitis has cleared, no harm exists in attempting to record OAEs. If OAEs are present (as in a very small percentage of patients with otitis media), that information could be useful. If they are absent (as in most patients with otitis media), no conclusions about cochlear function can be drawn.

53. Cochlea : Exposure to ototoxic medication or noise exposure (including music): OAE changes may precede threshold changes in the conventional frequency range. Any other cochlear pathology.

54. Conditions that do not affect OAEs : CN VIII pathology: If CN VIII pathology also affects the cochlea ( eg , vestibular schwannoma that decreases cochlear vascular supply), OAEs are affected. Central auditory disorder

55. Conditions that elicit abnormal OAEs and normal behavioral thresholds : Tinnitus : OAEs may be abnormal in the frequency region of the tinnitus. Excessive noise exposure (may cause increase or decrease in amplitude): No clear correlation to noise-induced threshold changes is noted. Ototoxicity Vestibular pathology

56. Conditions that elicit normal OAEs and abnormal behavioral thresholds : Functional hearing loss Attention deficits Autism Possibly, inner hair cell damage but normal outer hair cells (reported for animals but no human reports yet) Auditory neuropathy: This includes central auditory nervous system dysfunction and CN VIII auditory dysfunction.

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