Otoacoustic Emissions (OAEs) in Children and Adults

Recommended Procedure: Clinical Application of Otoacoustic Emissions (OAEs) in Children and Adults

Date: February 2023
Review date: February 2027

General Foreword

The British Society of Audiology (BSA) provides Practice Guidance based on the best available evidence and consensus on good practice at the time of publication. The BSA does not guarantee the interpretation and application of this guidance and is not liable for any loss or damage arising from its use. Comments on the document are welcomed.

Authors

Produced by: The Electrophysiology Special Interest Group (EPSIG) and the Professional Guidance Group.

Key Authors:

• Dr. Ghada Al-Malky (University College London)
• Dr. Paul Bacon (Sheffield Teaching Hospitals)
• Dr. Peter Bray (Diagnostic Group, Demant)
• Dr. Siobhán Brennan (University of Manchester; Sheffield Teaching Hospitals)
• Dr. John E FitzGerald (Norfolk & Norwich University Hospitals NHS Trust)
• Dr. Michelle Foster (Leeds Teaching Hospitals NHS Trust)
• Dr. Guy Lightfoot (ERA Training & Consultancy Ltd.)
• Professor James W Hall III (Osborne College of Audiology, Salus University, Pennsylvania, USA & Department of Communication Sciences and Disorders, University of Hawaii, Hawaii, USA)

Declarations of interests:

• ERA Training & Consultancy Ltd offer training courses in ERA testing, training and accreditation in ABR peer review and offer clinical support for centres performing ABR testing.
• Diagnostic Group, Demont design and sell OAE equipment through Interacoustics, Maico and GSI.

Shared Decision-Making:

The service user should be involved in shared decision-making when undertaking audiological intervention, receiving subsequent information and understanding how it will impact on the personalisation of care. Individual preferences should be taken into account and the role of the clinician is to enable a person to make a meaningful and informed choice.

Contents

1. Abbreviations
2. Introduction
   • Development of the recommended procedure
   • Background and aims
   • Scope
3. Types and classifications of OAEs
   • Stimulus-based classification
   • Source-based classification
4. Equipment selection
   • Standards
   • Types of available equipment
5. Preparation
   • Equipment preparation
     • Stage B calibration
     • New Probe calibration
     • Stage A checks
     • Coupler tubes
     • OAE probe tips and precautions against cross infection
   • Test environment and recording conditions
     • Noise
   • Patient / Carer instructions
   • Probe fitting
6. TEOAE measurement
   • Stimulus parameters
     • Stimulation level
     • Click stimulus waveform
     • Recording window
   • TEOAE analysis and interpretation
7. DPOAE measurement
   • Stimulus and recording parameters
   • DPOAE analysis and interpretation
8. Clinical applications
   • Hearing screening
   • Monitoring cochlear function
     • Monitoring for ototoxicity
     • Monitoring for noise- or music-induced hearing loss
   • Diagnostic assessment of cochlear function
     • Auditory neuropathy spectrum disorder (ANSD)
     • Non-organic hearing loss (NOHL / pseudohypacusis)
     • Non-cooperative (non-compliant) subjects
   • Relationship between pure tone audiometry and OAE findings
9. References
   Appendix A. Examples of DPOAE and TEOAE displays for the Biologic, Interacoustics Titan and Otodynamics instruments.
   Appendix B. Defining what constitutes a significant change in DPOAEs
   Appendix C. Troubleshooting when interpreting the results
   Appendix D. Summary UK: Pilot New-born Hearing screening

1. Abbreviations

• AABR: Automated Auditory brainstem response (screening test)
• ABR: Auditory brainstem response (full assessment/ diagnostic test)
• ANSD: Auditory Neuropathy Spectrum Disorder
• AN: Acoustic Neuroma
• BM: Basilar Membrane
• CR: Clear response
• DPOAE: Distortion-product Otoacoustic Emissions
• ME: Middle ear
• MEE: Middle ear effusion
• MRSA: Methicillin-resistant Staphylococcus aureus (bacteriological infection)
• NCR: No clear response
• NHSP: Newborn Hearing Screening Programme (England)
• NICU/SCBU: Neonatal intensive care unit / Special care baby unit (terms used interchangeably). The NHSP NICU/SCBU screen protocol applies to those babies on the unit for ≥48 hours.
• NIHL: Noise-induced hearing loss
• NOHL: Non-organic hearing loss
• OAE: Otoacoustic emissions
• OHC: Outer hair cell
• PCHI: Permanent Childhood Hearing Impairment - defined here as ≥40dBHL averaged over 0.5, 1, 2 & 4 kHz pure tone audiometry thresholds. It includes both sensorineural and permanent conductive impairments.
• SFOAE: Stimulus-frequency Otoacoustic Emissions
• SNHL: Sensorineural hearing loss
• SNR: Signal-to-noise ratio
• SOAE: Spontaneous Otoacoustic Emissions
• TEOAE: Transient Evoked Otoacoustic Emissions
• TW: Travelling wave
• VRA: Visual Reinforcement Audiometry

2. Introduction

2.1. Development of the recommended procedure

This procedure was developed by members of the Electrophysiology Special Interest Group (EPSIG) in accordance with BSA Procedure for Processing Documents (2003).

2.2. Background and aims

• Prof. David Kemp first reported OAEs in 1978 (Kemp, 1978).
• OAEs are sounds of cochlear origin recorded through a microphone in the external ear canal.
• They are produced by the motion of the cochlea's sensory outer hair cells (OHCs) in response to auditory stimulation (Kemp, 2002).
• Successful stimulation and detection of OAEs indicates functioning of both the middle and inner ear when recording from a patent ear canal.
• OAEs are used for newborn hearing screening worldwide, including in the UK (Davis et al, 1997; Norton et al., 2000; Chapchap & Serge, 2001; Uloziene & Grandori, 2003; Stevens et al., 2014; Rissmann et al., 2018).
• Research supports other clinical applications in children and adults, such as:
  • Confirmation of hearing status as part of a test battery
  • Diagnosis of spurious and false hearing loss (HL)
  • Identification and diagnosis of auditory neuropathy spectrum disorder (ANSD)
  • Evaluation of central control mechanisms
  • Longitudinal monitoring and assessment of the effect of ototoxic drugs.

Misconceptions related to the clinical application of OAEs:

1. OAEs are only useful for Newborn Hearing Screening: OAEs also have a significant role in the diagnosis and management of many pathologies, affecting all of the age groups seen in audiology.
2. Diagnostic OAEs can be analyzed only as 'CR' or 'NCR': Detailed diagnostic analysis can provide much more information that can be clinically useful e.g. as part of a diagnostic test battery or when monitoring different changes in frequency responses to indicate progressive cochlear damage.
3. TEOAEs and DPOAEs provide the same information: Each present a different aspect of OHC function as they are generated through different mechanisms. They can therefore sometimes complement each other.
4. OAEs provide the same information provided by pure-tone audiometry (PTA): OAEs and pure tone audiometry are very different measures of auditory function. Although normal OAEs are often associated with normal hearing sensitivity and abnormal OAEs with hearing loss, abnormal OAEs may be recorded in persons with normal pure tone thresholds and, in contrast, normal OAEs may be recorded in persons with hearing loss. OAEs and pure tone audiometry provide complimentary information. OAEs are not a true measure of “hearing”. OAEs along with other objective auditory measures offer a cross-check for diagnostic behavioural audiological testing.

This recommended procedure aims to clarify the measurement, analysis, and interpretation of OAE findings in different clinical settings and populations and to provide guidance for common clinical applications of OAEs, including:

1. Hearing screening of children or adults
2. Hearing monitoring to assess cochlear damage caused by ototoxic agents or noise (including hearing conservation programmes)
3. Diagnostic assessment or differential diagnosis of patient populations at risk for cochlear dysfunction, such as:
   • Sensory versus neural auditory dysfunction in auditory neuropathy spectrum disorder (ANSD), auditory processing disorder (APD), acoustic neuroma (AN)
   • Non-organic (false) hearing loss
   • Non-cooperative and difficult-to-test patients

A standardized method should be used for each of these cases in order to allow for clear, comparable and accurate interpretation of outcomes across services and to establish the use of this tool within the audiological battery of tests.

2.3. Scope

This procedure offers guidelines for selected screening and diagnostic applications of OAEs in pediatric and adult populations.

The development and operation of newborn hearing screening programs, including universal newborn hearing screening, is outside the scope of this document. However, test parameters for newborn hearing screening with OAEs are addressed briefly under Section 6 to 8 of this document.

These guidelines do not cover all evidence-based clinical applications of OAEs or OAE applications in all disorders or pathologies. Readers interested in exploring this further are advised to visit text books such as Otoacoustic Emissions: Principles, Procedures and Protocols (Dhar & Hall, 2018) or Otoacoustic Emissions: Clinical applications (Robinette & Glattke, 2007).

These guidelines focus on the technical procedure of carrying out and reporting results of transient-evoked OAEs (TEOAEs) and distortion-product OAEs (DPOAEs) testing, as these are the most clinically applicable OAE test types at the present time.

The following measurements and interpretation are outside the scope of this document:

• spontaneous OAEs and stimulus-frequency OAEs
• contralateral suppression of OAEs to assess inhibitory efferent auditory pathways.

Also note that the use of OAEs in newborn hearing screening programs is not specifically addressed here however, some of this document will be valuable in gaining a better understanding of OAE use in screening.

3. Types and classifications of OAEs

3.1. Stimulus-based classification

The original and conventional classification of OAEs was based on whether they required a stimulus to evoke an OAE response or whether they were present spontaneously.

1. Spontaneous OAEs (SOAEs): OAEs recorded in the external ear canal without presentation of an external stimulus.
2. Evoked OAEs:
   • Transient-evoked OAEs (TEOAEs): OAEs evoked by the presentation of a broadband click/ Chirp or less commonly a tone burst stimulus,
   • Distortion-Product OAEs (DPOAEs): OAEs evoked by the presentation of two closely linked simultaneously presented pure-tones (f1 and f2),
   • Stimulus-frequency OAEs (SFOAEs): OAEs evoked by a pure-tone stimulus and are detected by the vectorial difference between the stimulus waveform and the recorded waveform using methods such as the interleaved suppression technique.

This simple classification is widely used, however it implies that all OAEs provide the same information in relation to cochlear function and only differ by the type of evoking stimulus (Probst, Lonsbury-Martin & Martin, 1991), but this is not the case.

3.2. Source-based classification

Evoked otoacoustic emissions arise from a mix of two fundamentally different mechanisms/sources namely, linear coherent reflection and non-linear distortion mechanisms (Shera and Guinan, 1999):

• Reflection source: This is the main generation model for TEOAEs and SFOAEs at low levels, where emissions are generated by the reflection of the traveling wave from the normal yet imperfectly alignment of OHCs.
• Distortion source: This is the main generation model for DPOAEs. This is where the areas of the basilar membrane stimulated by two tones (the lower frequency 'f1' with stimulus level L1 and the higher frequency ‘f2’ with stimulus level L2) overlap, multiple 'intermodulation' distortions are generated. The largest and most commonly recorded being 2f1-f2 (figure 1).

TEOAEs are related to the strength of the travelling wave which depends on electromotility (expansion or contraction of the OHCs). DPOAEs demonstrate the equally important non-linear aspect of OHC physiology (e.g. transduction) (Dhar & Hall, 2018). Clinically, this means that each of these types of OAEs provide slightly different information and so when tested together may give more detailed information regarding the integrity of the cochlear function.

4. Equipment selection

4.1. Standards

The relevant British Standards relating to the technical construction and characteristics of OAE equipment are published in the BS EN 60645-6:2010 document. This is identical to the International Electro technical Commission (IEC) 60645-6:2009 document. Instruments are divided into two categories, for screening and diagnostic purposes in these standards. As some national screening programmes pre-date these standards, users should refer to the technical requirements for screening equipment as specified by their own programme. Currently, there are no American National Standards Institute (ANSI) standards for OAE equipment.

4.2. Types of available equipment

Commercially available equipment for both TEOAEs and DPOAEs can be classified as either 'screening' or 'diagnostic' and often one instrument can perform all functions (both screening and diagnostic TEOAEs and DPOAEs).

With screening OAE equipment, minimal control is required from the operator with 'automatic decision making' regarding the stimulus waveform and the response waveform. The operator often cannot view the stimulus or the response waveform. The equipment reports either a 'Pass' or 'Refer' when minimal pre-set stop criteria are reached. Screening OAE equipment is designed so recording is fast (e.g. DPOAE screening equipment may only collect OAE data for 2 frequencies per octave).

With diagnostic OAE equipment, the operator has more control over the settings of the equipment. Also, the operator can view the stimulus and response waveform, and other parameters such as noise levels, the number of artefacts, and the artefact rejection limit. The operator can then decide when to start and finish recording or to extend test time to reach the desired recording quality encapsulated in the signal-to-noise level ratio (SNR) parameter. Testing for diagnostic purposes typically takes longer than screening tests as 5 or 6 half octave bands maybe assessed in TEOAE diagnostic testing and in DPOAE diagnostic testing > 16 points per octave may be analyzed. Higher SNRs are also desirable for the most accurate recording of an OAE level.

5. Preparation

5.1. Equipment preparation

5.1.1. Stage B calibration

Equipment should have a documented (Stage B) calibration record on a timely basis as per manufacturer recommendation (e.g. annually). Regular safety and electrical testing is also required in accordance with local protocols.

5.1.2. New Probe calibration

New probes should be set up and checked as per the manufacturer instructions. Before using an OAE probe for the first time it is recommended to perform a probe calibration check to keep as a reference of the probe’s original performance for comparison over time. From then on, it is recommended that regular probe calibration checks are performed frequently. Refer to manufacturers guidelines to identify the tolerances for accepting a correctly functioning probe.

5.1.3. Stage A checks

Probes are vulnerable to blockage by wax and debris as well as mechanical damage to the speaker and microphone. Frequent Stage A checks, defined below are required, preferably prior to each clinical test session, after cleaning or servicing of the probe and whenever a fault is suspected or unexpected results are obtained, to ensure that the equipment is producing consistent outcomes. Ideally the stage A checks should be carried out in a acoustically quiet surroundings.

The Stage A check should include:

• a visual inspection for any obvious signs of damage of the device or probe or probe blockage.
• a probe test to check probe performance (i.e. a measure of the loudspeaker outputs and microphone sensitivity within its usual test cavity and comparison against the initial measures made at delivery with accepted tolerances as per manufacturer’s recommendation).
• an occlusion test, where possible, to ensure no artefactual 'false OAE' is being generated in the recording system and low levels of probe “noise” floor.
• a test recording in a test cavity to ensure there is no 'false OAE' present in either the recording or stimulating systems
• a real ear biological check with a known response to confirm adequate function.

5.1.4. Coupler tubes

Probe performance can normally be confirmed using the manufacturers probe test facility e.g., probe cavity test. Performance can degrade if the coupler tubes are blocked with debris or wax.

5.1.5. OAE probe tips and precautions against cross infection

The probe tips provided by the manufacturer must be used with the appropriate instrument. Use of alternative tips affects the acoustics, stimulus settings, recording output, and increases chances of ear discomfort, dislodging or impaction of that tip in the ear canal. Disposable probe tips should be discarded after a patient has been tested, in order to avoid cross infection and maintain hygiene requirements for health and safety. However, if the manufacturer states that repeated use is acceptable, appropriate cleaning procedures between patients must be applied to meet local infection control guidelines.

If a patient with a Patient Safety Alert for an infection with a biological hazard such as MRSA needs to be tested, OAE recording can be performed, but the equipment needs to be appropriately covered according to the local health and safety protocols and after recording is complete the equipment needs to be cleaned according to local health and safety protocols and replacement of the coupler tubes is recommended where possible.

5.2. Test environment and recording conditions

5.2.1. Noise

Sources of noise can be generated acoustically in the environment or produced physiologically by patients. Although OAEs probes are not sensitive to electrical noise, high electrical and radio fields could induce noise into the sensitive OAE detection circuits. Proximity to powerful electric installations should be avoided.

OAEs probes can be recorded effectively in a quiet room and do not necessarily need to be performed in a sound-treated room (Gorga et al., 2000, Cone-Wesson et al., 2000). However, efforts must consistently be made to minimize sources of ambient acoustic noise, e.g. closing the door to the test room, requesting that persons in the test room refrain from talking, turning off the power for any unnecessary noisy equipment/ lights/fans, and locating the patient away from any noise sources. Continuous noise, such as air conditioning, ventilation, and road traffic noise may be more problematic than occasional short-lived noise that is more likely to be rejected by the artefact reject system and reduced through additional signal averaging (Kemp, 2002). Noise usually impacts more on measurement of OAEs for lower frequency stimuli (< 1500 Hz). A deep well-fitted probe is important to achieve the target stimulus level within the ear canal and to minimize adverse effects of external noise whilst enhancing the OAE signal. The noise rejection level should be set appropriately. Setting high rejection levels in the presence of background noise can be counter-productive and should be avoided.

It is important to take steps to minimize patient generated noise. Adult and older pediatric patients should be asked to remain still and quiet and to avoid talking or chewing. Movement of the probe wire over clothing can also cause noise. Optimal OAE recordings are made from infants and young children who are not chewing, sucking, or crying. However, it is often possible to record clinically useful OAEs under less-than-ideal test conditions.

5.3. Patient / Carer instructions

OAE recording does not require the patient to be awake, conscious, or provide behavioral responses to the stimuli. All that is required is to be able to fit the probe in the patient’s ear canal securely and for the patient to remain still and quiet for the duration of testing. Babies are best tested while sleeping or, if awake, in a very settled state.

Prior to initiating OAE recording, the operator should provide the patient with a brief explanation of the procedure and what is expected of the patient:

1. A small probe with a soft tip will be placed into the external ear canal.
2. There is no need for the patient to listen to the sounds or to tell the operator if the patient hears the sounds. The machine will automatically record sounds produced by the ears.
3. The patient only needs to sit quietly and to relax while the test is underway.
4. The patient is reminded to as much as possible refrain from moving, speaking or chewing during the procedure.

5.4. Probe fitting

A good probe fit is an essential requirement for obtaining accurate recordings of OAEs. A good probe fit with deep probe insertion reduces the ambient noise, seals the stimulus in the ear canal, minimizes stimulus ringing, avoids the need to hold the probe and increases the probability of measuring an OAE with low background noise.

The main factors that can affect probe fitting include:

• Choosing the correct size for the probe tip by closely inspecting the size of the ear canal opening. Manufacturers of OAE equipment provide a range of probe tip sizes suitable for neonatal, pediatric and adult ear canals.
• Accurate consideration of the shape and angle of the ear canal either by visual inspection and/or otoscopy.
• Noting and removing as indicated debris, foreign objects, and excessive cerumen in the ear canal.
• Adequate technical skill and clinical experience of the person performing the OAE test.

6. TEOAE measurement

6.1. Stimulus parameters

Table 1 shows typical stimulus and recording parameters for measurement of TEOAEs. Selected parameters may vary with devices from different manufacturers.

6.1.1. Stimulation level

The recommended stimulation level for click-evoked TEOAE measurement is between 81 and 87 dB peak equivalent sound pressure level (dB peSPL) with an average target level of 84 dB peSPL. These levels typically evoke a robust TEOAE if hearing thresholds are 20 dB HL or better (Kemp, 1978; Norton et al., 2000; Glattke & Robinette, 2002).

Infants have significantly smaller ear canals compared to older children and adults. As a result, sound pressure level of the stimulus will be higher if the instrument calibration does not adjust the stimulus levels according to the ear canal size of the ear being tested. The optimal selection of the ‘neonate setting’ and use of the ‘auto-adjust’ feature will allow for the ear canal size to be accounted for. However, caution needs to be taken in trying to use the ‘auto-adjust’ feature to readjust the stimulus level automatically rather than securing a deep probe fit. This feature aims to compensate for the difference in ear canal size but does not compensate for inappropriate probe fitting and will not ensure achievement of a true repeatable stimulus level.

6.1.2. Click stimulus waveform

Ideally, a clear positive and negative deflection over a maximum period of 1ms, followed by a straight line to indicate the absence of oscillations (or ringing) of the waveform, is required. Strong resonances (sharp peaks) increase the risk of artefactual responses and should be avoided.

‘Stimulus Stability’ demonstrates any change in probe fit from the start of the test. The closer this stimulus stability is to 100% the more confident the tester is that the stimulus remained stable over the duration of the recording. Users should obtain guidance from device manufacturers regarding the acceptable ranges of stimulus stability for their specific TEOAE device (usually ≥ 85% is acceptable).

6.1.3. Recording window

To prevent stimulus artifacts in the analysis TEOAE window, the start of data collection with clinical devices is delayed for 2.5 - 4 ms following the click stimulus. This will inevitably lead to the limitation in ability to record the higher frequency components of the response. The length of the data collection time will vary depending on the stimulus rate used. The end of data collection and analysis window is typically 12 ms to 20 ms. The longer window length allows more low frequency (<=1kHz) OAEs to be collected due to their longer latency, but the test time is longer. The shorter window can be used to minimize testing time especially where there is low frequency noise (e.g., infant screening) or where only higher frequencies are of clinical interest (e.g., ototoxic monitoring).

6.2 TEOAE analysis and interpretation

Responses to sets of clicks are sub-averaged and alternately sent to two different buffers (A & B). After the required sub-averages have been collected in each buffer, the test is complete and two TEOAE waveforms are overlapped and displayed on the screen (Response waveform window). The extent to which the two waveforms are correlated is often expressed as a “Reproducibility” percentage (provided by correlation of A & B). The term repeatability could also be used to describe the correlation or agreement between TEOAE waveforms.

In healthy ears a TEOAE is generally considered to be “Clear Response” (CR) when it has:

• An amplitude that may vary from ~ -10 dB SPL to ~ +30 dB SPL and
• A signal-to-noise ratio (SNR) of ≥ 6 dB SPL

This conclusion is made for each frequency band tested. It is most common to interpret TEOAEs from their frequency analysis. Typically, a 1/2 octave analysis is provided. Adequate reproducibility is assessed by the signal to noise ratio in each band. Depending on whether this is considered an overall acceptable response is based on the clinical application of TEOAEs. For example, in newborn hearing screening, achieving these outcomes in two or more half octave bands may be considered a ‘pass’. It is appropriate to conclude:

• “No Clear Response (NCR)” outcome when adequately low noise levels(≤ -5 dB SPL) are achieved and the required TEOAE amplitude and SNR scores are not achieved.
• “Inconclusive” outcome when recording conditions are not adequate to allow for low noise levels to be reached e.g. due to noisy or incomplete recordings or a poor probe fit. In such conditions it cannot be determined whether OAEs are NCR or CR but obscured by noise.

In general, TEOAEs will not be detected for patients with a cochlear hearing loss involving outer hair cell dysfunction greater than 35 dB HL, although this is dependent upon the hearing loss configuration (e.g. an OAE can be obtained with good low frequency hearing in the presence of a high frequency hearing loss).

TEOAEs are present in 99% of cases when all audiometric hearing thresholds are better than 20 dB HL (Robinette, Cevette, & Probst (2007). However, TEOAEs may be NCR for persons with subtle cochlear dysfunction who have hearing thresholds within normal limits. Similarly, TEOAEs are NCR for cochlear hearing loss involving outer hair cells when hearing thresholds are greater than 40 dB HL.

7. DPOAE measurement

7.1. Stimulus and recording parameters

DPOAE stimulation requires the simultaneous presentation of two pure-tone frequencies (f1 is the lower frequency primary tone at level L1 and f2 is the higher frequency primary tone at level L2). These are closely spaced and typically set at a frequency ratio of 1.22. DP amplitudes, the corresponding noise levels around the same frequencies and the signal-to-noise (SNR) ratio, calculated by subtracting the noise from the DPOAE, are used to determine the confidence of the result.

Stimulus levels of L1 = 65 and L2= 55 or 50 dB are typically used in clinical DPOAE recordings (Petersen et al., 2017).

DPOAEs can be recorded for f2 stimulus frequencies within the range of 500 Hz to over 10,000 Hz. The DPOAE level is plotted against the f2 frequency in the 'DPgram' as this frequency best represents the originating place in the cochlea.

7.2. DPOAE analysis and interpretation

DPOAE analysis for all test frequencies is performed for amplitudes, noise floors, and DP-noise floor differences relative to normative data. Manufacturer values for normative data are dependent on algorithms used for signal processing and DP detection. The following criteria and categories are offered as a guide for initial analysis of DPOAEs recorded with clinical devices.

DPOAE Clear Response Present and Normal

• DPOAE amplitude within an appropriate normal region (usually > 0 dB SPL)
• SNR (i.e. DP – noise floor) ≥ 6 dB
• Low noise levels (ideally < -10 dB SPL)

DPOAE Clear Response Present but Abnormal

• DPOAE amplitude below normal limits (e.g., < 5%ile of normal and usually < 0 dB SPL)
• SNR (DP – noise floor) ≥ 6dB
• Low noise levels (ideally < -10 dB SPL)

DPOAE No Clear Response

• SNR (DP – noise floor) < 6 dB SPL
• Low noise levels (ideally < -10 dB SPL)

In general, DPOAEs at test frequencies are expected to be CR present and normal (amplitudes within an appropriate normal region) if pure tone hearing thresholds are better than 15 dB HL, CR present but abnormal for cochlear (outer hair cell) hearing loss (pure tone hearing thresholds) within the range of 15 to 40 or 50 dB HL, and NCR for cochlear (outer hair cell) hearing loss greater than 40 or 50 dB HL.

8. Clinical applications

There are three general clinical applications of OAEs: The first application is screening to detect cochlear dysfunction in apparently normal populations (e.g. newborn infants, preschool children, young school age children) or adult populations at risk for hearing loss (e.g. industrial workers, musicians, military personnel, persons with recreational noise exposure, or adults with learning disabilities).

The second application is to monitor OAE levels for changes with time, also in patients at risk for developing cochlear dysfunction (e.g., ototoxicity monitoring or industrial monitoring). The third application is to include OAEs in a test battery for diagnosis of auditory dysfunction and hearing loss, specifically to provide information about the type, degree, configuration or site of auditory dysfunction

8.1. Hearing screening

Automated OAEs (TEOAEs and DPOAEs) are used throughout the world for hearing screening. TEOAEs are the technique of choice within the UK Newborn Hearing Screening programs. Additional information can be found with the following links: https://www.gov.uk/topic/population and http://www.thebsa.org.uk/resources/.

The Pass/Refer criteria used in the UK newborn hearing screening program are intended to identify those persons with bilateral moderate or worse permanent childhood hearing impairment (PCHI). Typical pass criteria for an automated OAE is a SNR of 6dB with a minimum signal level of -5dB SPL per half octave band in at least 2 frequency bands and a minimum overall signal of > 0dBSPL.

OAE hearing screening may also be combined with automated auditory brainstem response (ABR) hearing screening to minimize false-negative and false-positive screening errors (Hall, Smith & Popelka, 2004; Joint Committee on Infant Hearing, 2019).

8.2. Monitoring cochlear function

8.2.1. Monitoring for ototoxicity

DPOAEs are included in clinical practice guidelines for ototoxicity monitoring (American Academy of Audiology, 2009). Cochlear damage caused by ototoxic drugs initially affect the OHCs at the high frequency basal turn of the cochlea before extending towards the apical end. This selective damage potentially makes DPOAE testing a very effective monitoring tool, as it is capable of assessing the early high frequency OHC damage, before speech frequencies are affected and preferably before the appearance of audiometric hearing loss (American Academy of Audiology, 2009).

The specific rationale for use of DPOAEs rather than TEOAEs is the ability to monitor outer hair cell function for frequencies above about 4000 Hz where ototoxic effects first occur.

Whenever possible, comprehensive audiological assessment including DPOAE measurement should be performed before exposure to the ototoxic drug or within the first 48 hrs of the first dose of cisplatin or 72 hrs of the first dose of aminoglycosides.

Changes in DPOAE amplitude (not SNR) that exceed the acceptable test-retest range of variability from baseline, particularly for the highest test frequencies, are considered evidence of ototoxicity-induced cochlear damage. It is not necessary for DPOAE amplitudes to decrease below normal limits before ototoxicity is suspected.

8.2.2 Monitoring for noise- or music-induced hearing loss

Chronic exposure to high levels of sound or even short duration exposure to transient high impact sound initially produces outer hair cell dysfunction that is detected with OAE monitoring.

Noise-induced hearing loss commonly affects the 3000 to 6000 Hz frequency region of the cochlea as evidence by the classical ‘audiometric noise notch’. DPOAEs are a more suitable tool for monitoring for sound-induced hearing loss as they yield information on outer hair cell function throughout the frequency range of interest.

Monitoring of noise-exposed workers should include baseline OAE measurement, ideally before a subject is initially exposed to loud sounds and then annual monitoring with OAEs for the first two years of employment and then at three-yearly intervals, to compare shifts in OAE amplitude thresholds at the different test frequencies and assess if they have changed significantly beyond the accepted test-retest variability limit.

In general, a reliable decrease in DPOAE amplitude (not SNR), of 6dB from baseline at frequencies ranging between 1000 to 6000 Hz should be considered as evidence of cochlear dysfunction and would warrant follow-up and management.