OAE and BERA ( otoacoustic emissions and brainstem evoked response audiometry)

Otoacoustic emissions and brainstem evoked response potentials
Objective hearing evaluation

1. OAE and BERA

2. Otoacoustic emissions Otoacoustic Emissions are the sound that result from energy generated in cochlea Propagated through middle ear Transmitted through External Auditory cannal ; Captured by a microphone in External Auditory cannal

3. Hypothesis of OAE generation A healthy, living cochlea demonstrates nonlinear behavior and refined frequency specificity at low stimulus levels, similar to the characteristics demonstrated by individual hair cells and auditory nerve fibers Cochlear amplifier are biological mechanisms that enhances vibration of basellar at peak of travelling wave particularly at low stimulus levels Outer haircells main contributers R educed auditory sensitivity, broader tuning, and abnormal response growth when OHCs are damaged or missing

4. OAE and Outer Hair Cells Preneural phenomenon Present even if 8 th CN severed OAE evoked during low stimulus are vulnerable to agents like Acoustic trauma Hypoxia Ototoxic medication

5. Measurement of OAE Sensitive miniature microphone fits in Ear cannal Microphone housed in a small probe that coupled to ear a foam or rubber tip Output to be amplified Noise has to be reduced – Procedure in sound treated room Fan or sound producing instrument switched off Child test done during asleep, Adult avoid movements and talk Probe has to be secured tightly

6. OAE divided into Spontaneous and evoked OAE Evoked OAE divided into Stimulus frequency OAE Transcient OAE Distortion OAE

7. Spontaneous OAE SOAEs are measured in the absence of external stimulation SOAEs appear as puretone -like signals coming from the ear.

8. + ve in 50% of healthy indivudals Considered as a screening test rather than a diagnostic test If SNHL present more than 30dB SOAE not positive More in females

9. Stimulus frequency OAE Occur at same frequency and at same time when a pure tone is introduced into ear Microphone records Puretone used for evoking and SFOAE SFOAE has to be filtered Common filtering method – Evoking frequency with SFOAE measured. Suppressive tone given – Vector subtraction of tone alone and tone presented with suppressor tone

10. Not used routinely in clinical practice SFOAE equally efficient in finding hearing loss in 1k and 2k and superior to other EOAE at .5k

11. Transient evoked OAE As their name suggests, TEOAEs are measured following the presentation of a transient or brief stimulus . Also called COAE or Click Evoked OAE A click or toneburst is presented to the ear, and the response occurs following a brief time delay. TEOAE checked in different frequencies Best to detect hearing loss in 2k and 4k

12. A click or toneburst is presented to the ear, and the response occurs following a brief time delay . Measurement of TEOAEs is accomplished using time synchronous averaging Even with averaging Noise : TEOAE ratio is high The energy from the stimulus may also persist in the ear canal long enough to obscure the onset of the TEOAE response

13. BASIC CHARACTERISTICS OF TEOAES AND THE EFFECTS OF STIMULUS AND RECORDING PARAMETERS Frequency dispersion – higher frequency appears first then lower frequency Or higher frequency have lesser latency period

14. Distortion-Product Otoacoustic Emissions Distortion-product OAEs (DPOAEs) are measured simultaneously with the presentation of two puretone stimuli, called “primaries”, to the ear . The frequencies of the primaries are conventionally designated as “ f 1” and “ f 2” ( f 1 < f 2) and the corresponding levels of the primaries as “ L 1” and “ L 2 .” OAE output are mathematically related to frequencies f 2– f 1, 2 f 1– f 2, 3 f 1–2 f 2, 2 f 2– f1

15. Characterstics of DPOAE The frequency separation of the two primaries, generally described as the f 2/ f 1 ratio, influences the DPOAE level that will be measured

16. BRAIN STEM EVOKED RESPONSE audiometry

17. Auditory pathway

18.   BERA is an objective way of eliciting brain stem potentials in response to audiological click stimuli. These waves are recorded by electrodes placed over the scalp .   This   investigation was first described by Jewett and Williston in 1971 . Never a substitute for other audiological tests

19.  The stimulus either in the form of click or tone pip is transmitted to the ear via a transducer placed in the insert ear phone or head phone . The wave froms of impulses generated at the level of brain stem are recorded by the placement of electrodes over the scalp.

20.  The standard electrode configuration for BERA involves placing a electrode over the vertex of the head, and ear lobe or mastoid prominence. One more earthing electrode is placed over the forehead. This earthing electrode is important for proper functioning of preamplifier.  

21. Wave I appeared to be generated by the more distal aspect of cochlear nerve and Wave II by the more proximal aspect of the auditory nerve Wave III in humans is a CN-generated response Wave IV is likely generated primarily by the superior olivary complex Wave V of the ABR is likely generated for the most part by fibers of the lateral lemniscus as they enter into the area of the inferior colliculus

22. Interpretation Wave I : small amplitude, delayed or absent may indicate cochlear lesion Wave V : small amplitude, delayed or absent may indicate upper brainstem lesion I – III inter-peak latency: prolongation may indicate lower brainstem lesion. III – V inter-peak latency: prolongation may indicate upper brainstem lesion. I – V inter-peak latency: prolongation may indicate whole brainstem lesion. Shortening of wave the interval with normal latency of wave V indicate cochlear involvement.

23. PATTERNS OF ABR FINDINGS IN VESTIBULAR SCHWANNOMA Absolute Latency Delay An absolute latency delay for Wave V is likely vestibular schwannoma to be considered Absolute latency delay is also seen in cochlear, retocochlear or conductive hearing loss

24. other measures of the ABR as well as patient history can become important and helpful . When Wave V latency measurements are greater than 6.1 ms in response to a click stimulus presented at a moderately high intensity level, the possibility of an eighth nerve tumor should be considered

25. Interwave Latency Delay Because of the fact that most eighth nerve tumors grow between the generator sites of Waves I and III, the I–III interwave interval (IWI) is likely to be extended in these cases In many clinics and laboratories a I–III IWI extending beyond 2.4 ms is considered abnormal

26. Many times 1 and 3 waves wont be present If present 85% IWI delay present The I–V IWI may also be used in the detection of vestibular schwannoma Absence of wave 2 causes suspicion. Suspected case of acoustic neuromas where 2 nd were is patent rule out involvement of cochlear nerve

27. Interear Latency Comparisons An interear latency comparison, or interaural latency difference (ILD ), using Wave V measurements obtained from the left and right ears can be diagnostically relevant, especially when used alongside interwave latency measurements . Wave V is the only wave of the ABR that is present in each ear conform. So wave 5 is taken for measurement

28. Should be aware that a asymmetrical hearing loss produce abnormal ABR. Correction factors like 0.1ms for 10Db used at 4KHz greater than 50dB Change in 0.3 to 0.4 ms is diagnostic

29. Amplitude Comparison of Waves Wave 5: wave 1 amplitude ratio is considered as a reliable source The test should be repeated atleast once with no difference in amplitude more than 20% 5:1 ratio is less than .75 is indicative of vestibular schwanommas Incase of wave 4 and 5 complex the peak of complex is considered

30. Waveform Morphology and Absence of Waves Absence of wave is another indicator of VS Wave 3 is most commonly absent wave Presence of wave to rule out cochlear involvement of VS Wave 1 present others absent conforms a retro cochlear involvement Loss of early waves is suspecious of VS not a reliable indicator

31. Repetition Rate Shifts Controversial This measure involves the comparison of the latency of Wave V at low and high rates of presentation such as 11 clicks per second to 81 clicks per second . If there is a greater than expected latency increase or a disappearance of Wave V during the high rep rate recording, then the interpretation is for possible eighth nerve pathology.

32. Laterality Small tumours will produce ipsilateral ABR affected When tumour is large contralatral ABR wave 3 to 5 affected due to compression of tumour

33. ABR AND BRAINSTEM INVOLVEMENT Absolute Latency Delay Wave 1 or 2 not useful Delay in wave 3 or 4 or 5 without a delay in 1 or 2 indicates a brainstem involvement in adjacent areas

34. Interwave Latency Delay 3-5 waves or 1-5 waves delay can be due to brainstem involvement 1-3 delay may be due to cochlear nerve involvement or caudal brain stem If the I–III IWI is extended but Wave II present indicates brainstem involvement only 3-5 indicates brainstem involvement 1-5 latency delay is present subcomponents has to be checked

35. Interear Latency Comparisons Not reliable as majority of brainstem lesions are bilateral

36. Amplitude Comparison of Waves Neurologists have generally supported the use of the V–I amplitude ratio as a diagnostic criteria for brainstem involvement When these conditions are met then the V–I amplitude ratio, though not highly sensitive, is highly specific if a ratio of less than 0.75 is used

37. Waveform Morphology and Absence of Waves Brainstem involvement often results in a highly dependable index for brainstem abnormality when Waves I and III are present and the IV–V complex or Wave V is absent When Wave III is absent and Wave V is present, one should look at the I–V IWI. If this interval is normal, then it is likely that the response is normal . The absence of all waves can also occur in individuals with brainstem involvement with severe periphral hearing loss

38. Contralateral Effects Large mass lesions on one side, such as an acoustic neuroma, can compress and cause ABR brainstem finding on the opposite side when stimulating that ear The ear opposite the large lesion, if the hearing is reasonably good, will show normal Waves I and III and an abnormal III–V interval or compromised V or IV–V complex This kind of information can be clinically useful when there may be no ABR response or no hearing on the involved side

39. Repetition Rate Shifts Some reports indicate that when rep rate functions are abnormal other ABR indices are also abnormal, Not much reliable in brainstem pathology