The Healthy Hearing Program Diagnostic Assessment Protocols for Audiological Practice

Transcription

1 The Healthy Hearing Program Diagnostic Assessment Protocols for Audiological Practice Edited by Tegan Keogh Co-Edited by Nuala Beahan 1

2 FOREWORD The following diagnostic protocols represent information supported by relevant and most recent scientific literature. The broader aim is to establish a gold standard of practice amongst paediatric diagnostic audiologists which facilitates evidence based clinical practice. Document Revision History: Version Date Prepared By Comments 1 1/12/2009 Tegan Keogh Review in June 2010, and 6 monthly thereafter. and Nuala Beahan Approvals: 1 Version Name Title/Responsibility Date Signature Anticipated Reviews: Who When Audiology Working Group June

3 ACKNOWLEDGEMENTS The editor of this document wishes to acknowledge that the following sections are heavily drawn, with permission, from the United Kingdom s NHSP s Neonatal Hearing Assessment Protocols. Sections one to eight (i-viii) are replications of the NHSP protocol, with some amendment for application, to the Queensland State-wide Healthy Hearing Program. The following 1. Stevens, J (ed). (2008). Auditory Brainstem Response Testing in Babies using Clicks: A recommended test protocol (version 2.0), 2. Mason, S (ed). (2002). Auditory Brainstem Response Testing in Babies using Tone-Pip Stimulation: A recommended test protocol, 3. Stevens, J (ed). (2002). Bone Conduction Auditory Brainstem Response Testing in Babies: A recommended test protocol, 4. Stevens, J (ed). (2009). Provisional guidelines for using Auditory Steady State Responses (ASSR) in babies: Suggested parameter values for recording frequency-specific ASSR using Natus Biologic MASTER / GSI Audera / ICS Chartr EP 200/ Interacoustics Eclipse systems, 5. Stevens, J (ed). (2002). Transient Evoked Oto-Acoustic Emission (TEOAE) Testing in Babies: A recommended test protocol, 6. Baldwin, M (ed). (2008). Tympanometry in Babies Under 6 Months: A recommended test protocol (version 2.0), 7. Williamson, T (ed). (2002). Behavioural Observation Audiometry: A recommended test protocol, 8. Day, J (ed). (2008). Visual Reinforcement Audiometry Testing of Infants: A recommended test protocol (version 2.0), In addition, significant acknowledgement is due to Ms Kelly Nichols and Ms Lisa Shannon for supplying a clinical protocol framework from which to commence. And, Ms Nuala Beahan for her efforts in co-editing several of the test protocols. 3

4 The Audiology Working Group, and in particular, Mr Ron Linning is acknowledged for the development of the Minimum Standards. Appreciation extends to Dr Joseph Kei, Dr Carlie Driscoll, and Dr Wayne Wilson for their valuable input and advice. Also to the members of the Audiology Working Group; Ms Jackie Moon, Ms Kelly Nichols, Ms Janice Wu, Mrs Sreedevi Aithal, Mr Ron Linning, Ms Suma Ajith, Ms Tania Sargent, and Ms Trang Pham. Lastly, appreciation is also extended to colleagues of the Healthy Hearing Team who have contributed in some way to the development of this document, including Ms Shirley Glennon, Ms Leanne White, Mr Gavin Bott, Ms Hayley Harpham, & Ms Nuala Beahan. 4

5 CONTENTS FOREWORD... 2 ACKNOWLEDGEMENTS... 3 CONTENTS... 5 THE HEALTHY HEARING PROGRAM AIMS AND OBJECTIVES... 8 PROGRAM AIM... 8 PROGRAM OBJECTIVES... 8 AIMS OF THE HEALTHY HEARING PROGRAM DIAGNOSTIC ASSESSMENT PROTOCOLS FOR AUDIOLOGICAL PRACTICE...10 MINIMUM STANDARDS...11 Minimum Standards Audiological Assessment of Neonates and Infants BACKGROUND INFORMATION...17 DIAGNOSTIC REFERRALS...17 SURVEILLANCE REFERRALS...17 FUTURE PROTOCOLS TO BE DEVELOPED...19 (i) Click-Evoked ABR Test Protocol SCOPE...21 PATIENT PREPARATION...21 STIMULUS...23 DATA COLLECTION AND ANALYSIS...25 CALIBRATION...29 ARTEFACTS...29 HEALTHY HEARING CLICK-EVOKED ABR QUICK SUMMARY PROCEDURE...30 QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR CLICK ABR:...31 NORMATIVE DATA...33 GLOSSARY...35 REFERENCES...36 (ii) Tone-Burst ABR Test Protocol INTRODUCTION...38 SCOPE...38 PATIENT PREPARATION...38 STIMULUS...40 DATA COLLECTION AND ANALYSIS...41 CALIBRATION...43 HEALTHY HEARING TONE BURST ABR QUICK SUMMARY PROCEDURE...45 QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR TONE BURST ABR...46 REFERENCES...47 (iii) Bone Conduction ABR Test Protocol INTRODUCTION...49 SCOPE...49 PATIENT PREPARATION...49 STIMULUS...50 DATA COLLECTION AND ANALYSIS...52 CALIBRATION...55 QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR CLICK BONE ABR:...55 USE OF TONE BURST STIMULI...56 QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR TONE BURST BONE ABR:...57 REFERENCES...57 (iv) Auditory Steady State Response Test Protocol INTRODUCTION...60 EQUIPMENT...60 STIMULUS PARAMETERS

6 CALIBRATION...63 HEADPHONES...63 HEALTHY HEARING AUDITORY STEADY STATE QUICK SUMMARY PROCEDURE...64 QUICK GUIDE: RECOMMENDED RECORDING PARAMETERS FOR ASSR...64 REFERENCES...66 (v) Transient Evoked Otoacoustic Emissions Test Protocol INTRODUCTION...68 SCOPE...68 TYPES OF OAE AND RECORDING TECHNOLOGY TEOAEs and DPOAEs...68 PRACTICAL ASPECTS...69 CRITERIA FOR DIAGNOSTIC TESTING...69 RECORDING DETAILS...71 HEALTHY HEARING TRANSIENT EVOKED OTOACOUSTIC EMISSIONS QUICK SUMMARY PROCEDURE...72 QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR TEOAEs:...73 FOOTNOTES...74 REFERENCES...74 (vi) High Frequency Tympanometry Protocol (1000 Hz) INTRODUCTION...76 SCOPE AND AIMS...76 MEASUREMENT...77 EQUIPMENT...80 TYMPANOGRAM (TRACE) INTERPRETATION...81 QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR TYMPANOMETRY...83 REFERENCES...84 TYMPANOMETRY APPENDIX...88 (viii) Behavioural Observation Audiometry INTRODUCTION...91 Aims of BOA...91 SUGGESTED PROCEDURE:...92 Test environment: Noise-maker suggestions...93 REFERENCES...96 (viii) Visual reinforcement audiometry testing of infants INTRODUCTION...99 SCOPE...99 FACILITIES & PREPARATION STIMULI TEST PROCEDURE INTERPRETATION OF RESULTS REPORTING OF RESULTS STAFF TRAINING AND EXPERTISE QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR VRA GLOSSARY REFERENCES CASE HISTORY SAMPLE QUESTIONS SUMMARY CRITERIA FOR DISHARGE FROM THE HEALTHY HEARING PROGRAM REPORTING GUIDE TO DIAGNOSING A SIGNIFICANT HEARING IMPAIRMENT (PERMANENT / CHRONIC HEARING LOSS) RECOMMENDED PRACTICES ROLE OF THE MULTI-DISCIPLINARY TEAM ROLE OF THE AUDIOLOGIST ROLE OF THE QUEENSLAND HEARING LOSS FAMILY SUPPORT FACILITATOR: HANDLING OF PATIENT FAILURE TO ATTEND AND DECLINE OF SERVICE

7 SURVEILLANCE REFERRALS- MINIMUM TEST BATTERY FOR SURVEILLANCE Monitoring the Risk of Progressive Hearing Loss: Healthy Hearing Report of PCHI not identified by the newborn hearing screen Appendix A Check list for daily and monthly function check of auditory brainstem response systems

8 THE HEALTHY HEARING PROGRAM AIMS AND OBJECTIVES PROGRAM AIM Early detection of permanent childhood hearing loss, supported by early intervention is the primary aim of the Healthy Hearing Program. Ultimately, the goal is to maximize linguistic competence and literacy development for children who are deaf or hard of hearing and this should be measured by the proportion of these children achieving linguistic skills equivalent to their non-verbal performance IQ at 3 years of age. PROGRAM OBJECTIVES The following table contains overall Healthy Hearing Program objectives, performance and outcome indicators. The objectives relevant to diagnostic audiology are highlighted. OBJECTIVE Performance Indicator Outcome Indicator 1. Optimise early detection of permanent childhood hearing impairment in neonates by providing newborn hearing screening before 3 months of age; 2. Provide diagnostic audiology assessment to relevant neonates by 6 months of age; * At least 95% of all children born in Queensland will be offered the hearing screen at birth. Rate of neonates screened per year (minimum should be no less than 95%, aim is 100% follow-up rate) Age of neonate at screening target is 3 months corrected age. Referral rate for formal audiological testing after screening per year target no greater than 4%. Ideally around 2-3%. * Minimise false positives - target <4% and false negatives - target is zero.. * Begin diagnostic audiological assessment by 2 weeks following refer for bilateral refers and 6 weeks unilateral. 1-2 children per thousand with a bilateral sensorineural hearing loss >40dBl detected before 3 months. * Rate of neonates offered the hearing screen. Currently - to end of >98% * Rate of neonates who are screened. Currently - to end % *Proportion of all infants screened by 3 months. Proportion screened by 6 months. Currently - average age at screening - 40 wks * Rate of neonates referred to audiology directly from screening and from targeted surveillance. Currently - to end Direct Refers % and Targeted Surveillance % *Total no & percentage of neonates tested false-positive and/or false-negative. *Total number and percentage of neonates beginning assessment within timeframes. * Rate of neonates diagnosed by 3 months. 8

9 3. Ensure equitable access to the Healthy Hearing Program for all neonates irrespective of social, economic, cultural or geographic circumstances; 4. Ensure the Healthy Hearing Program is standardised and provided using multidisciplinary, evidence based screening, diagnostic, treatment and habilitation protocols; 5. Provide maintenance care for all children who have a cochlear implant at the time of the commencement of the Healthy Hearing Program; 6. Provide an outreach hearing diagnostic and treatment service in the Cape York and Torres Strait region for children from an Aboriginal or Torres * 95% of neonates referred from the Healthy Hearing Program will be diagnosed and referred to Australian Hearing and/or Cochlear Implant Program by 6 months or 12 months Corrected Age, respectively. * Percentage of neonates diagnosed and referred to Australian Hearing and/or Cochlear Implant Program by 6 months or 12 months Corrected Age, respectively. * See above * See above * Appropriate resources for Indigenous and * Proportion of infants from Indigenous non-english speaking families are and non-english speaking background available. offered and participating in screening is * Support for rural and remote families is comparable to representation in available and offered. population. * Benchmarking - National and * All screening indicators comparable International Standards - eg: SCREENING: with national and international best First Refer rates, Second Refer rates, practice. Capture rate, Targeted Surveillance rates. * Audit and quality processes in place for *AUDIOLOGY: see above + 1-2/1000 all program components. SNHL detected * Children with hearing loss appropriately * Development Diagnostic Assessment identified. Protocols for Audiological Practice * Diagnostic Assessment for Audiological Practice developed and * TREATMENT & HABILITATION: implemented. Literature Review on Early Intervention for * Literature Review finalised and made children with a Hearing Loss to be available to all program and early undertaken. intervention services. * All children with CI who are managed through the Mater or RCH CI program have access to maintenance services. *Average age at intervention - Cochlear * Audit of CI children s access to Implant target is 12 months corrected age maintenance services. (or clinically appropriate). * Age at implantation. * Managed by Royal Childrens Hospital * Managed by Royal Childrens Hospital and Cairns Hospital and Cairns Hospital 9

10 Strait Island background. AIMS OF THE HEALTHY HEARING PROGRAM DIAGNOSTIC ASSESSMENT PROTOCOLS FOR AUDIOLOGICAL PRACTICE 1. To ensure effective, efficient and accurate diagnosis of Permanent Childhood Hearing Loss (PCHI) for children referred from the Healthy Hearing Program, either at birth or for surveillance at later ages. 2. To ensure consistency and equity of access to high quality diagnostic services throughout the state. 3. To ensure diagnostic procedures are evidence-based and reflect best practice for diagnostic audiological assessment for neonates and infants. 4. To provide the basis for: Consistent and accurate information gathering for program reporting and evaluation purposes; Service Level Agreements with districts; Contracting with private paediatric diagnostic audiology services. Training and credentialing of all Paediatric Diagnostic Audiologists and New Graduates. At the time of this document s preparation there were no schemes in place to train Audiologists in the area of Paediatric Diagnostic Audiology. It was, however, recognised that a new graduate or inexperienced Audiologist was unable to practice competently in the area without significant supervision. For this reason, Audiologist s wishing to practice in the area of Paediatric Diagnostic Audiology under the auspices of the Healthy Hearing Program must have demonstrated a significant period (2 years) of paediatric diagnostic audiology experience. Should an Audiologist not meet this period of practice then they will be required to undergo a training and mentorship program with an experienced Paediatric Audiologist. All Audiologists will be required to complete the Healthy Hearing Diagnostic Assessment Protocols for Audiological Practice (DAPAP) training to be able to practice paediatric diagnostic audiology for the Healthy Hearing Program. As the DAPAP will be a constantly evolving document refresher courses will be required every two years. A specific program of credentialing will be developed to accompany these training requirements. 10

11 MINIMUM STANDARDS Background At the outset of the Healthy Hearing program a set of minimum audiology practice standards were developed. The minimum standards were devised according to the level of care provided at that facility (level 1, level 2 and level 3 or tertiary). The minimum standards initially served as a guideline for Audiological Practice in the assessment of neonates and babies. However, as the Healthy Hearing Program progressed it was recognised that these standards did not provide the significant detail required for a quality program in Audiology practice and assessment. Consequently, the minimum standards have served as a basis for development of this document. They are included in this document for background purposes only and are no longer recognised as the minimum standard of assessment. This document replaces the minimum standards and all facilities are now required to follow the same assessment protocol. It is acknowledged that new or developing sites (eg. in remote or rural areas, or new hospitals) may not meet the protocol due to equipment or resource limitations. This will be rare, negotiated with the State-wide Director of Healthy Hearing and always precede referral to a site that can complete the audiological assessment according to protocol. 11

12 Minimum Standards Audiological Assessment of Neonates and Infants Entry at any level as clinically appropriate (i.e. Level 1, 2, 3 not necessarily sequential) Referral between levels as appropriate eg may be diagnosed with AN in 3 and then return to level 2 for monitoring Facility Role/Requirements Comment Capacity Skills/Resources Equipment Appropriate Patient (Example) Level I Minimum requirements for Level 1assessment: (Replaces or validates screening procedure) Minimum PASS requirements: A. Click evoked ABR - Pass = Wave V present down to 30 dbnhl bilaterally, and Waves I, III and V identifiable at suprathreshold levels B. OAE - Pass = S/N ratio 6dB or more in 3 frequency bands across the 1 4 K khz range To evaluate if the baby has functionally normal hearing bilaterally. To report to Healthy Hearing Program using appropriate format. To appropriately refer on as clinically indicated. Proforma for standardised reporting to be drafted. Including contact details of Healthy Hearing, place for parents to sign for release of information, place for parents to sign to say they have been told and understand results and next steps required Administer, interpret and report on results obtained in audiological assessment of neonates and infants with ABR, high frequency tympanometry and otoacoustic emissions. To appropriately report results to parent/carers. Limited access to facilities to establish threshold across the speech spectrum Some experience is providing parents regarding diagnosis of hearing loss in neonates Auditory Brainstem Audiometry (ABR) air and bone conduction (click) Diagnostic Otoacoustic emissions Access to some basic behavioural measures of responsiveness to sound i.e. noise makers First audiological assessment after a double refer from screening. Baby back transferred home prior to screening were local screening in not available or not appropriate. 12

13 Facility Role/Requirements Comment Capacity Skills/Resources Equipment Appropriate Patient (Example) Level II If a baby does not PASS: In addition to level 1 skills: In addition to level 1 equipment: Minimum requirements for Level II assessment (minimum standard) Essential: A. Click ABR Air conduction bilaterally to threshold levels. B. Click ABR Bone conduction. C. Frequency specific measure ASSR or tone burst at 1 KHz showing threshold for sensorineural losses. Desirable: A. Waves I, III and V seen at suprathreshold levels B. High frequency Tympanometry. C. Ipsilateral reflex D. Additional frequency specific information obtained E. Frequency specific measure ASSR or tone burst at 1 To obtain site of lesion information. To establish threshold estimates across the major speech frequencies bilaterally. To appropriately support families through the diagnosis of a hearing loss. To link families to services according to Healthy Hearing clinical pathways. To appropriately report to Healthy Hearing Program. A list of information to be routinely covered in informational counselling is required. This will assist to standardise information and to support less experiences audiologists. More discussion required regarding management of each possible outcome of this level i.e. pass, conductive, sensorineural hearing loss More definition and standardisation is required regarding noise makers and use of these + Administer, interpret and report on results obtained in the audiological assessment of neonates and infants with: behavioural observation audiometry, frequency specific electrophysiological measures of hearing sensitivity, cochlear microphonic. + To provide appropriate information to families in a timely, appropriate and unbiased manner + Demonstrated experience in providing informational counselling to parents regarding diagnosis of significant hearing loss - Does not require access to tertiary level audiological or medical facilities such as sedation, cochlear implant programs High frequency tympanometry Frequency specific electrophysiologica l equipment e.g. auditory steady state response, tone burst ABR Facilities for appropriate behaviour assessments including VROA and noise makers Optional: Noise makers and sound level meter or a standardised generator (more definition required) Any baby where the initial audiological results were not WNL. Confirmation of hearing loss. Monitoring of conductive hearing loss. Surveillance of healthy hearing babies Initial assessment for baby with high likelihood of hearing loss would be recommended to occur in this setting eg syndrome with known risk of hearing loss, strong family history. 13

14 Facility Role/Requirements Comment Capacity Skills/Resources Equipment Appropriate Patient (Example) KHz showing threshold for conductive losses. Level III In addition to level 1 and 2 level skills: In addition to level 1 and 2 level equipment: Audiological minimum standards as per level I for PASS and level II for further assessment Tertiary level (i.e. hospital setting) Experience in management of medically and developmentally complex paediatric population Capacity for procedures such as sedation, cochlear implants etc Access to multidisciplinary team and tertiary level medical management eg ENT Baby with complex audiological, social or medical presentation. Profound SNHL Auditory neuropathy. All permanent SNHL for access to hearing impairment clinics. 14

15 Facility Role/Requirements Comment Capacity Skills/Resources Equipment Appropriate Patient (Example) Surveillance Centres Minimum requirements for Surveillance assessment: Separate information across the speech frequencies obtained by VROA with insert headphones or OAEs. Pass = VROA thresholds db SPL 25 at 0.5 khz 20 at 1, 2 and 4 khz AND To provide monitoring of babies identified as at risk of progressive hearing loss To provide follow-up for babies referred back into the system eg parent concern Aim is to provide separate ear information across the major speech frequencies Reporting - Will the template allow for reporting of this population? Referral pathways if loss identified One adequately trained and experienced audiologist Available support personnel as required. Administer, interpret and report on results obtained in the audiological assessment. Essential: VROA facilities (Optional with B/C and separate ear capabilities) Tympanometry Otoacoustic emissions Parent concern GP concern Identified by Healthy Hearing as requiring surveillance Identified by audiologist as requiring surveillance OAE Present with S/N ratio 6 db or greater across three frequencies across 1 4 khz range And 15

16 Facility Role/Requirements Comment Capacity Skills/Resources Equipment Appropriate Patient (Example) Middle Ear Analysis 16

17 THE HEALTHY HEARING PROGRAM AND AUDIOLOGY BACKGROUND INFORMATION The aim of Audiology within the Healthy Hearing Program is to identify hearing loss which may impact on a child's speech and language development. There are two streams to the Program Diagnostic and Surveillance; DIAGNOSTIC REFERRALS Children referred to the Audiology Department from the Healthy Hearing Program who have had their hearing screened in the nursery using AABR and referred twice (either unilaterally or bilaterally), or have been referred directly due to factors that make them unsuitable for screening (eg. atresia, craniofacial abnormalities) All bilateral refers from the screening AABR are aimed to be seen for Audiological assessment within 2 weeks from the date of the referral, provided the infant is medically stable. All unilateral refers from the screening AABR are aimed to be seen for Audiological assessment within 6 weeks from the date of referral, provided the infant is medically stable. SURVEILLANCE REFERRALS Children referred from the Healthy Hearing Program who have passed screening bilaterally, but have had High Risk Indicators identified in their medical history, placing them at risk for progressive or late onset hearing loss. See the Surveillance referral pathway to determine audiological assessment timeframes. 17

18 (1) DIAGNOSTIC REFERRALS- The Minimum Test Battery The minimum requirements, for discharge with normal hearing, when assessing a baby referred from screening include the following assessments on BOTH ears; (a) Click-evoked ABR (assesses broad spectrum, but is most sensitive in the 2-4 khz Regions). (b) Assessment of hearing at 1 and 4 khz must include one of the following; Option 1: Tone burst ABR. Option 2: ASSR. Option 3: TEOAE testing (1-1.5 and 4 khz). (c) High Frequency Tympanometry (1000 Hz probe tone). A baby is required to have normal test results in all of the above assessments prior to discharge. A baby suspected of a having a hearing impairment is required to repeat the click-evoked ABR, in addition, the following will also be performed; (d) Bone conduction ABR. (e) Toneburst ABR across the frequency spectrum, or ASSR testing across the frequency spectrum. Frequency specific information is required for referral to Australian Hearing and reasonable access to rehabilitation. The following sections contain test protocols for all assessments that may be conducted throughout the course of Audiological assessment appointments; (i) Click-evoked Auditory Brainstem Response Test (Click ABR) (ii) Tone-burst Auditory Brainstem Response Test (Tone-burst ABR) (iii) Bone Conduction Auditory Brainstem Response Test (BC ABR) (iv) Auditory Steady State Evoked Potentials (ASSRs) (also known as Steady State Evoked Potentials, SSEPs) (v) Transient Evoked Otoacoustic Emissions (TEOAEs) (vi) High Frequency (1000Hz Probe Tone) Tympanometry (vii) Behavioural Observation Audiometry (BOA) (viii) Visual Reinforcement Audiometry (VRA) 18

19 All measures should be undertaken to adhere to the test protocols. In extraordinary circumstances, deviation may occur. In this instance, the clinician is required to use appropriate clinical decision making before doing so, document the circumstances, and keep records regarding the deviation. The above tests do not exclude the ability to use additional tests, such as; (a) Distortion Product Otoacoustic Emissions (b) Acoustic Reflex Testing The use of these tests is highly encouraged, to provide additional clinical information. They may be performed at the clinician s discretion but are not mandatory. FUTURE PROTOCOLS TO BE DEVELOPED It is envisaged that a protocol regarding Auditory Neuropathy Spectrum Disorder will be inserted into this document in the near future. Protocols for DPOAE testing and Acoustic reflex testing will also be developed in future versions of the protocol. 19

20 Click-Evoked ABR (i) Click-Evoked ABR Test Protocol Air-conduction Auditory Brainstem Response testing in babies using clicks- A Recommended Test Protocol 20

21 Click-Evoked ABR (i) Click-Evoked ABR Test Protocol Air-conduction Auditory Brainstem Response testing in babies using clicks- A Recommended Test Protocol SCOPE The ABR Protocol aims to provide guidelines for testing babies in the first few months of life by ABR using click stimuli for the purposes of assessing hearing sensitivity. Tone burst ABR and bone conduction ABR are covered in separate documents. Clicks are currently employed for newborn hearing screening. For diagnostic assessment of hearing impairment, further frequencyspecific information is required, obtained by the use of tone burst ABR or auditory steady state responses. The document covers the technical procedure of carrying out an ABR test and the reporting of the results. It does not cover practical aspects such as handling of the baby or criteria to be used to decide whether a baby has passed or failed an ABR screen. Equipment that carries out automatic ABR is available and is used in newborn hearing screening only. The test protocol in this type of equipment is usually fixed. This document refers to diagnostic ABR (i.e. non automated) ABR. Any changes in this protocol should be performed with careful consideration and with appropriate clinical justification. PATIENT PREPARATION Precautions against cross infection Local procedures should be adhered to, including the handling of infections such as MRSA. Suggested minimum practice is that all equipment other than the electrodes, preamplifier and earphone should remain outside the baby s cot or, when the baby is not in a cot, should be placed at a distance from the baby. Any item in the cot that is likely to come into contact with the baby should be cleaned, using a suitable method, before being used on another baby. Electrodes are dealt with under the next section. The earphones and electrode leads should be cleaned with soap and water or swabbed down with alcohol gel. Testers should wash their hands thoroughly before and after handling the baby and make use of alcohol gel as prescribed by local infection control procedures. It is recommended that local advice is sought regarding infection control methods. 21

22 Click-Evoked ABR Choice of electrodes and application Disposable electrode tabs should be used at all times, this minimises contact of the electrode leads with the baby. It is recommended that disposable electrodes are used to facilitate sterility. It is important to devise a sterile procedure and the following is recommended if disposable electrode tabs are not used. The skin should be carefully abraded by using a suitable sterile abrasive electrode paste and clean gauze. If non disposable electrodes are being used, the electrodes, after cleaning, should be placed in a solution such as chlorhexidine solution to disinfect them. However this does not sterilise the electrodes and local advice should be sought on effective sterile procedures. The International Organisation of Societies for Electrophysiological Technology (OSET) has published useful guidelines on the prevention of infection in electrophysiological procedures 2. The amplitude of any artefact resulting from induced electrical interference is proportional to the difference in the electrode impedances. A low value for the difference in impedances is most easily obtained by ensuring all electrodes have low impedances. The impedance, as measured between each electrode pair, should be in the range 1000 to 5000 ohms. Although in good recording conditions and in a screened room higher electrode impedances can be tolerated, it has been found that on a Special Care Baby Unit, values below 5000 ohms are normally necessary to reduce mains (50Hz) interference to an acceptable level. At high stimulus levels a high electrode impedance would also give an unacceptably large stimulus artefact which would obscure the initial part of the response and may activate the data rejection facility so that no data are averaged. Electrode location One channel or two channel recording can be used. In single channel recording, electrodes are located as follows: Negative electrode: ipsilateral mastoid or nape of neck. If using mastoid, use a low mastoid position unless wishing to record the post-auricular myogenic response. Positive electrode: high forehead as near as possible to Cz a and midline. The fontanelle should be avoided but the electrode should be placed as close as possible to this otherwise the ABR response will be reduced in size. 22

23 Click-Evoked ABR Common electrode: contralateral mastoid when using the ipsilateral mastoid electrode for the negative electrode. If the nape of the neck is used for the negative electrode, the common electrode can be placed on the forehead (at least 4cm from the positive electrode) or either mastoid, whichever is the more practical in the individual case. The above electrode configuration should result in wave V being plotted upwards on the display. If this is not the case then the positive and negative electrode connections should be reversed. There is some evidence that the nape of the neck rather than the mastoid gives a larger wave V response although the signal to noise ratio may not be any higher. However if wave I is required (for neuro-diagnostic purposes) the mastoid should be used. There is a slight increase in the amplitude of the response if the positive electrode is placed nearer to the vertex position (Cz) a. In Two channel recording, electrodes are located as follows: Negative electrode: ipsi-lateral mastoid and contralateral mastoid. Positive electrode: high forehead as near to Cz as possible, avoiding the fontanelle. Common electrode: forehead. Contralateral electrodes will be used in the two channel recording. Note that if bone conduction ABR is also to be carried out this may influence the choice of the electrode placement for air conduction ABR. If the option of two channel BC ABR recording is being considered then the common electrode can be placed on the forehead as the ipsilateral and Cz a is the standard position used in adults. It is defined in the electrode system for electroencephalography. For the purpose here it can be taken as the point along the midline of scalp half way between the bridge of the nose (nasion) and the start of the skull at the rear of the head (inion). STIMULUS Stimulus and stimulus rate An electrical click of 100μs at a stimulus rate of around 22.1 per second is recommended, with alternating polarity to minimise stimulus artefact. A figure such as 22.1/s, which does not have a common factor with mains frequency, should be chosen to minimise any mains artefact. A check should be made that the stimulus rate allows the data collection to be completed prior to the next 23

24 Click-Evoked ABR stimulus being given. This can be checked by timing the collection of 1000 sweeps and confirming that the time taken is = (1000 stimulus rate). The level of stimulus output should be monitored by listening to the earphone at critical points during the test. Stimulus levels should be recorded in dbnhl. The nhl can be taken to imply the use of either ISO (2007)3, NHSP-recommended calibration values 4, or other calibration datum that is referenced to the average psycho-acoustic threshold of a group of normally hearing young adults. For testing carried out under the Healthy Hearing Program, the advice in the NHSP assessment guidelines should be adhered to. The relevant text is: Note that the polarity of the stimulus can affect the response waveform and in cases of ANSD it may be useful to record responses to both rarefaction and condensation clicks. The majority view, based on current practice, was that for newborn hearing screening and threshold measurement alternating polarity should be used to minimise the stimulus artefact. The stimulus presentation should occur at 70, 50, and 30 dbnhl as a minimum for both ears. Two traces at each level are required. Should a hearing impairment be suspected, then clinical threshold seeking is required, with a minimum of two waveform required at threshold. Earphone This should be of sufficient quality to deliver a stimulus up to 140 dbspl peak (about 107 dbnhl) without distortion. TDH39/49 headphones are recommended. However, insert earphones (e.g. type EAR-3A) may be used. Calibration may be more difficult with insert earphones due to greater variation in enclosed volume. However, insert earphones reduce the need for masking. If insert earphones are used, care should be exercised that wax is not compacted by the probe, so blocking the sound pathway. Care should also be taken to ensure that the method of insertion is standardised (eg 3 mm marked on the insert tubing). Earphones should be centred over the ear canal ensuring that the ear canal is not collapsed due to excess pressure. 24

25 Click-Evoked ABR Warning insert earphones If insert phones are used in babies, the recommendation is that they should not be used above 85 dbnhl unless they include a microphone to automatically adjust the stimulus level for ear canal volume. The reason for this is that a baby has a much smaller ear canal which can give up to about a 20 db rise in the stimulus level compared to the same insert earphone used in an adult (Voss & Herrman, 2005; Rance & Tomlin, 2006; Sininger et al, 1997). This figure is thought to diminish over the early months of life as the ear canal grows. In view of the evidence suggesting that insert earphones giver higher stimulus levels the following provisional values are proposed by the NHSP. These values MUST be added to the stimulus level in dbnhl to correct for the effect of the smaller ear canal volume in babies less than 3 months of age in comparison to adults. Hence, if performing a click-evoked ABR and a threshold of 20 dbnhl is obtained, that threshold is to be corrected to 30 dbnhl (as per the table below). Table 1. Provisional correction to insert earphone stimulus level for the effect of the smaller baby ear canal volume. Click 500Hz 1000Hz 2000Hz 4000Hz The application of the insert headphone correction factors will be updated as appropriate in future versions of this protocol pending developments of research in this area, and the findings of the Healthy Hearing Audiology working group. DATA COLLECTION AND ANALYSIS Amplifier and artefact rejection level The sensitivity/gain should be set such that any electrical activity greater than about ± 10μV is rejected. Some users have found that lower rejection levels down to ± 5μV are better. A value of between ± 5μV and ± 10μV is therefore recommended. The equipment manuals should be checked to determine what value of amplifier sensitivity is required to achieve these rejection levels. 25

26 Click-Evoked ABR The use of higher rejection levels is not recommended when recording conditions are difficult (e.g. high muscle activity). The result is likely to be a poorer signal to noise ratio in the averaged signal. Babies need to be sleeping or in a very settled state when testing. Filters Low frequency (high pass): A value between 20 to 100 Hz is recommended. A value between 20 to 30 Hz has been found to give the best signal to noise ratio of wave V near threshold. High frequency (low pass): A value around 1500 Hz is recommended. There is little response energy above this frequency. A higher value generally adds more electronic noise from the amplifier. Notch filter Under normal recording conditions this will not be required as 50 Hz mains artefact should be absent or minimal with good electrode practice using current equipment. If mains artefact levels are high it is better to identify and remove the source of the problem rather than use a notch filter which can introduce distortion. Window length and Averaging The longest response acquisition window consistent with the chosen stimulus rate should be used. This will normally be less than 15 ms (see section on stimulus rate above). The number of sweeps per replication accepted by the averager should be varied depending on both the size of the response and the level of background activity. It will normally vary between about 1200 and 2000 sweeps, although higher numbers will be required when the responses are small or the background noise is high. Typically a figure of 2000 sweeps is recommended with a minimum of Exceptionally there may be such a large ABR response or low background activity that fewer sweeps can be used. The number of rejected sweeps will depend on the level of background activity and the rejection level that has been chosen. Whenever the electrical activity from the baby exceeds the rejection levels for long periods of time data collection must be halted until the baby has settled for the reason given below. Recordings made where there is a high number of rejected sweeps must be treated with caution. Do not widen the reject levels in an attempt to complete testing on an unsettled baby. 26

27 Click-Evoked ABR Data rejection facilities on current equipment are effective in rejecting most unwanted sweeps. However, when there is a long period of high background activity and the data collection is not interrupted there will be a greater proportion of sweeps containing higher levels of noise added to the averaged response. Display The convention of plotting wave V upwards is proposed as it is the most common in current practice. It is recommended that the display be always set at a fixed number of μv/division. The amplitude (y) and time (x) scales should be such to ensure that small waveforms near threshold are visible. It is recommended that 0.1μV on the response amplitude (y) axis is the same size on the display as 1 2 ms on the time (x) axis. It is also recommended that the same display scales are used for all tests. An automatic display gain must not be used as an inappropriate gain for assessment of the response may be set. Criteria for accepting the presence of a response and establishing the ABR threshold For most current ERA recording systems acceptance of the presence of a response depends on visual interpretation. Replication of responses (as defined below) is therefore essential if a correct visual interpretation is to be made. The only exception to this should be where the result is not used in the determination of threshold. For example, if the first stimulus level is 50 dbnhl and a flat trace is obtained then the best use of time may be not to replicate until a response is observed at higher stimulus levels and it is clear which levels need to be replicated to determine threshold. For an infant suspected of having hearing impairment, there should be a minimum of a replicated response at threshold, at 10 db or 5 db below and at 10 db or 5 db above threshold. The exception to this is where a clear response is obtained at the recommended Healthy Hearing discharge level of 30 db nhl. In this situation a replicated response at 30 db nhl is sufficient. For each stimulus level the result should be marked in one of three ways. (a) response present, (b) good recording conditions but no response was recordable or, (c) recording conditions too poor to establish a result. 27

28 Click-Evoked ABR For each stimulus level the result should be marked in one of three ways. (a) response present, (b) good recording conditions but no response was recordable or (c) recording conditions too poor to establish a result. For (a) there must be a high degree of correlation between the replications and a characteristic waveform. For (b) the replications must be low in amplitude (peak to peak below about 0.05μV) with no significant correlation and for (c) the replications will have amplitudes above about 0.05μV peak to peak without a high degree of correlation. Where condition (a) is not met and there is some evidence of a response further replication may be able to determine whether a response is present or not. For example, two further replications could be carried out and the replications could be added in pairs to produce two summed traces, each the average of a higher number of sweeps. In any decision, all recorded traces at each stimulus level should be considered and only those rejected where there is a good technical reason or where the noise levels (e.g. electro-myogenic activity) were high. Results should clearly be marked as to whether they are threshold or the lowest level tested. It should also be made clear in the results when threshold measurement was not possible due to the maximum stimulus limits of the equipment. A useful convention (used on esp) is to use = for threshold, for lowest level tested (where the threshold may be lower) and > for threshold above maximum level tested. So for example 35dBnHL would mean a clear response (condition a) at 35dBnHL but not tested below this level and >80dBnHL would mean no response (condition b) at 80dBnHL but not tested above this level. At the completion of the ABR appointment, all traces MUST be reviewed by a peer and cosigned regarding the agreement of interpretation. The co-signer should have a minimum o two years of paediatric ABR experience. This process MUST occur regardless of the institutional setting and the degree of experience of the Audiologist. Should an Audiologist work as a sole practitioner, then the Healthy Hearing Program will ensure that that Audiologist is linked to a peer reviewer to verify all ABR traces. Traces MUST be peer-reviewed and co-signed prior to the completion/distribution of the diagnostic report. Definition of ABR threshold for Healthy Hearing The ABR threshold has been defined in the NHSP assessment guidelines 1 as the lowest level at which a clear response is present, with the absence of a recordable response at a level 5 or 10dB 28

29 Click-Evoked ABR below the threshold, obtained under good recording conditions. This definition will be adopted for the Healthy Hearing Protocol. The ABR wave latencies will be compared to the normative data established by Jacobsen & Morehouse (1984). Wave latencies that fall outside the normative data should be reported as abnormal. Furthermore, the interaural latency difference should be no greater than 0.3 ms. CALIBRATION A Stage A check should be carried out on the ERA equipment prior to use. Details of this are available in Appendix A. ISO (2007) provides reference equivalent threshold sound pressure levels (RETSPL) for click and tone burst stimuli used for AC ABR for certain types of transducer. It also provides a standard for reference equivalent threshold force levels (RETFL) for use for click BC ABR for the B71 type bone vibrator. However there are no RETFLs for tone burst BC ABR. A procedure for calibration is given in IEC (2007)5. In July 2005, a provisional set of reference levels for AC and BC ABR was agreed for the NHSP: and the latest version of these figures is published on the NHSP website 4, with the ISO389-6 values used where appropriate. These values should be used to calibrate equipment used for hearing assessment. It is important to note that the stimulus rate will affect the psycho-acoustic threshold but not the ABR threshold 6. When performing a subjective (stage A) listening check therefore, the use of the recommended stimulus rate near 50 per second will make the clicks sound about 3 db too loud. Bear this in mind or perform the listening check at 20 per second. It is important that the physical intensity of the stimulus does not change as the stimulus repetition rate is changed since this can introduce errors. This is difficult to detect subjectively and NHSP can advise which instruments require users to apply a correction for this problem. ARTEFACTS Recording system checks The equipment should be checked at regular intervals (monthly is recommended) for system artefacts. This applies not only to click ABR but to all the protocols for auditory evoked potentials. Using the normal protocol for testing, connect the electrodes together. Run the normal protocol twice and check that a flat trace is obtained with a minimal level of residual system noise (peakto-peak below 0.05μV) and no correlation between repetitions. 29

30 Click-Evoked ABR At regular intervals (monthly is recommended), when testing a baby, carry out an additional control recording to check that there are no artefacts in the recording. Do this on a baby where a clear ABR response has been obtained. Block the sound from reaching the ear and repeat the test with the stimulus still on. This can be achieved by covering the earphone or by sealing the insert earphone tube (ensure that the sound is not audible). A flat trace should be obtained with a minimal level of residual EEG (peak-to-peak below 0.05μV for a quiet baby) and no correlation between repetitions. Control recordings when testing babies It is also recommended that a control recording is carried out with the stimulus acoustically blocked whenever the ABR response is marginal and/or is of the form that could be an artefactual response: e.g. in tone burst ABR it is often the case that only wave V and SN 10 are observed. Mains artefact could produce a similar response. If the response is artefactual and is not an electro-physiological response to the sound stimulus it will still be present when the stimulus is acoustically blocked. Note that marginal responses, that do not take the form of a potential artefact, may not need to be checked if well supported by clear repeatable responses at 5 or 10dB higher stimulus levels. Action required If artefactual responses are observed, then it is essential to determine their source and remove them from the recording process. Advice should be sought and if necessary manufacturers contacted, so that the source of the artefacts can be eliminated. REPORTING NOTE: Please be aware that Australian Hearing may apply correction factors to convert ABR thresholds to estimated hearing thresholds (db ehl) suitable to be used for hearing aid fitting. Australian Hearing conversions are based upon ABR thresholds being reported in db nhl. Healthy Hearing diagnostic audiology reports should state which decibel scale is being reported, and any correction or calibration factors applied to the thresholds in addition to those covered in this document. HEALTHY HEARING CLICK-EVOKED ABR QUICK SUMMARY PROCEDURE 30

31 Click-Evoked ABR - Both left and right ears must be tested, regardless of whether the infant has a bilateral or unilateral refer. - Testing is routinely carried out at 70, 50 and 30 dbnhl, and at least 2 traces, at each test level, are required. Clinical judgement and threshold seeking is required if waves are not identifiable at these levels. - A child is deemed to have functionally normal hearing if a repeatable wave V is seen down to 30 dbnhl. - Major wave components (I, III and V) should be present at suprathreshold levels (70dB nhl or higher as necessary). - A latency intensity function for the absolute latency of wave V is to be plotted and compared to age appropriate norms 7. - The central transmission time at supra-threshold level (i.e. 70dB nhl) should be compared between ears for symmetry (significant asymmetry = interaural latency difference > 0.3 msec). QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR CLICK ABR: Electrode Location: One Channel: Positive: High forehead/vertex Negative: (i) Ipsilateral mastoid or (ii) nape Common: (i) Contralateral mastoid or (ii) forehead/mastoid Two Channel: Negative electrode: ipsi-lateral mastoid and contralateral mastoid. Positive electrode: high forehead as near to Cz as possible, avoiding the fontanelle. Common electrode: forehead. Contralateral electrodes will be used in the two channel recording. Stimulus: Alternating polarity click*: Rate 22.1 to 33.1/s + 31

32 Click-Evoked ABR Stimulus presentation level: 2 traces required at 70, 50 and 30 dbnhl as a minimum for normal discharge. Clinical threshold seeking required if loss is suspected. Earphone: TDH39/49 preferably (testing down to 30 db) EAR-3A or insert earphones with correction factors (testing down to 20 db to include correction factor) Coupler value for 0dBnHL: Amplifier reject levels: Filters: Refer to NHSP calibration data 4 ±5 to ± 10 uv Low frequency: Hz High frequency: 1500 Hz Window length: Number sweeps averaged per replication : Display: 15 ms maximum Minimum 1200 Typically μv on amplitude axis = 1 2ms on time axis * refer to section on stimulus and stimulus rate. +And deviations from the recommended rate should have a documented reason for deviation. 32

33 Click-Evoked ABR NORMATIVE DATA Paediatric norms below represent upper limits of normal for wave V in each population (i.e. mean latency + 2SD) 12 Jacobsen Pre-Term Data 10 Jacobsen Post-Term Data 11 9 Milliseconds Milliseconds dbnhl dbnhl wks wks wks wks wks 2 mths 6 mths 12 mths Adult ABR Norms + 2 SDs Jacobson & Morehouse (1984) Weeks CA Mean + 2SDs dbhl I III V I-V Weeks CA Mean + 2SDs dbhl I III V I-V

34 Click-Evoked ABR Weeks CA Mean + 2SDs dbhl I III V I-V Weeks CA Mean + 2SDs dbhl I III V I-V months Mean + 2SDs dbhl I III V I-V months Mean + 2SDs dbhl I III V I-V months Mean + 2SDs dbhl I III V I-V Adult Mean + 2SDs dbhl I III V I-V

35 Click-Evoked ABR ADULT NORMS CLICK dbnhl WAVE V: CLICK RATE = 11.1pps MALES AND FEMALES COMBINED 90dB 80dB 70dB 60dB 50dB - 2 SD MEAN SD WAVE I V: - 2 SD MEAN SD GLOSSARY ABR Auditory brainstem response AC Air conduction AN/AD Auditory neuropathy /auditory dys-synchrony BC Bone conduction dbnhl decibels Hearing Level (the n is a leftover from days before an international calibration reference was available and the scale used was derived from nominal or normal studies) ERA Electric Response Audiometry esp e-screener Plus (Electronic record system for NHSP) NHSP Newborn Hearing Screening Programme (England) RETFL Reference Equivalent Threshold Force Level RETSPL Reference Equivalent Threshold Sound Pressure Level 35

36 Click-Evoked ABR REFERENCES 1/ NHSP Guidelines for early assessment and management of babies referred from the Newborn Hearing Screening Programme. Newborn Hearing Screening Programme (England) 2/ International Organisation of Societies for Electrophysiological Technology. (1999). Guidelines for infection control in the Clinical Neurophysiology department. 3/ ISO (2007). Acoustics - Reference zero for the calibration of audiometric equipment - Part 6: Reference threshold of hearing for test signals of short duration. International Organisation for Standardisation, Geneva. 4/ NHSP ABR calibration specification and ABR calibration data. Newborn Hearing Screening Programme (England). 5/ IEC (2007). Audiometers: Part 3: Auditory test signals of short duration for audiometric and neuro-otological purposes. International Electrotechnical Commission, Geneva 6/ Lightfoot G, Sininger Y, Burkard R, Lodwig A. (2007). Stimulus repetition rate and the reference levels for clicks and short tone bursts: a warning to audiologists, researchers, calibration laboratories and equipment manufacturers. Am J Audiol, 16: / Jacobsen, & Morehouse. (1984). A comparison of auditory brainstem response and behavioural screening in high risk and normal infants. Ear Hear, 5:

37 Tone-Burst ABR (ii) Tone-Burst ABR Test Protocol Auditory Brainstem Response Testing in Babies using Tone Burst Stimulus- A Recommended Test Protocol 37

38 Tone-Burst ABR (ii) Tone-Burst ABR Test Protocol Auditory Brainstem Response Testing in Babies using Tone Burst Stimulus- A Recommended Test Protocol INTRODUCTION Many aspects of the methodology for the tone burst ABR are similar to those already published in the click ABR protocol 2 and will be referenced accordingly. This document will therefore address in more detail the issues which are specifically relevant to the use of the tone burst. There were many points of debate and in some areas it has been necessary to give a range for the final values for parameters as a single optimum value could not be agreed. In most cases these differences are not critical to the clinical use of the test. The optimum values may change as the results of more research are published. Please note that this protocol applies to the tone burst ABR when used as a measure of hearing threshold. In this document the term tone burst is considered equivalent to the term tone pip. SCOPE The document sets out to provide guidelines for testing babies in the first few months of life using the ABR evoked by tone burst stimuli for the purposes of assessing hearing sensitivity. Implementation and application of the tone burst ABR requires a greater level of knowledge and interpretation skills when compared to the click ABR. It is therefore recommended that testers should become conversant and confident with the click ABR before using the tone burst stimulus. This document covers the technical procedure of carrying out a tone burst ABR test and its use primarily as an audiological threshold tool at the follow-up stage following referral from the neonatal hearing screen. In some clinical situations, the click ABR and transient evoked oto-acoustic emissions should be employed as oto-neurological tools to assist with the differential diagnosis of cochlear loss, auditory neuropathy, and retrocochlear pathology. These investigations are outside the scope of this document. PATIENT PREPARATION The general preparation of the patient for the ABR test is the same for both tone burst and click stimuli. The same precautions against cross infection and choice and application of electrodes should therefore be adopted as described in the click ABR protocol 2. 38

39 Tone-Burst ABR Test Environment Ideally the threshold ABR test should be performed in a sound proofed room or environment which meets the same standards as used in pure tone audiometry. In addition, levels of electrical interference (eg 50 Hz mains) should be sufficiently low in the test environment such that the signal baseline is not adversely affected. Clinics should not be sited close to potential sources of interference such as high powered mains equipment, transformers, or plant equipment. In some circumstances ABR testing is performed outside the designated clinic area for example on the ward or in the operating room. In this situation, levels of acoustical and electrical interference must be sufficiently low so as not to influence the results of the test. Careful selection of the local test area or room may be necessary in order to achieve satisfactory environmental conditions. Electrode location One channel or two channel recording can be used. In single channel recording, electrodes are located as follows: Negative electrode: ipsilateral mastoid or nape of neck. If using mastoid, use a low mastoid position unless wishing to record the post-auricular myogenic response. Positive electrode: high forehead as near as possible to Cz a and midline. The fontanelle should be avoided but the electrode should be placed as close as possible to this otherwise the ABR response will be reduced in size. Common electrode: contralateral mastoid when using the ipsilateral mastoid electrode for the negative electrode. If the nape of the neck is used for the negative electrode, the common electrode can be placed on the forehead (at least 4cm from the positive electrode) or either mastoid, whichever is the more practical in the individual case. The above electrode configuration should result in wave V being plotted upwards on the display. If this is not the case then the positive and negative electrode connections should be reversed. There is some evidence that the nape of the neck rather than the mastoid gives a larger wave V response although the signal to noise ratio may not be any higher. However if wave I is required 39

40 Tone-Burst ABR (for neuro-diagnostic purposes) the mastoid should be used. There is a slight increase in the amplitude of the response if the positive electrode is placed nearer to the vertex position (Cz). In Two channel recording, electrodes are located as follows: Negative electrode: ipsi-lateral mastoid and contralateral mastoid. Positive electrode: high forehead as near to Cz as possible, avoiding the fontanelle. Common electrode: forehead. Contralateral electrodes will be used in the two channel recording. STIMULUS Type of stimulus A brief tonal stimulus with a rise and fall period in the range of 1 to 2 cycles each and a plateau of 1 to 2 cycles is recommended. A tone burst with 2 cycle rise and fall times and 1cycle plateau is widely used, often referred to as a cycle tone burst 3. The ramp for the rise and fall of the stimulus can be either linear or of Blackmann type. The tonal frequency is typically in the range of 500 Hz to 4 khz. There is some evidence that ipsilateral masking noise (high pass or notched noise) can improve the frequency specificity of the test. The application of this masking technique is outside the scope of this document. It is advised that expert guidance is sought. The polarity of the tone burst stimulus has minimal effect on the response waveform. It is recommended therefore that an alternately inverted stimulus polarity is used for all testing. This also has the advantage of minimising the effects of stimulus artefact in the final averaged waveform. The level of stimulus output should be monitored by listening to the earphone at critical points during the test. Stimulus level for each tone burst frequency should normally be recorded in dbnhl i.e. as referenced to the average psycho-acoustic threshold of a group of normally hearing young adults. Presentation rate A stimulus presentation rate of 37 per second is recommended, with alternating polarity to minimise stimulus artefact. A value such as 37/s, which does not have a common factor with mains frequency, should be chosen to minimise any time-locking of mains interference. A check should be made to ensure that this stimulus rate allows data collection of each individual sweep to be completed prior to presentation of the next stimulus. This can be performed by timing the collection of 1000 sweeps and checking that the time taken is 1000 divided by the stimulus rate. A stimulus rate of 37/s is compatible with the recommended sweep time of 20 to 25 ms for collection of tone burst ABR data. 40

41 Tone-Burst ABR Earphone The requirements of the earphone for tone burst stimuli are similar to those for the click. This should be of sufficient quality to deliver a stimulus of up to around 135 db SPL peak (about 110 dbnhl ) without distortion. A TDH39/49 or insert earphone e.g. type EAR-3A are suitable. Calibration may be more difficult with insert earphones due to greater variation in enclosed volume. However insert earphones reduce the need for contralateral masking. If insert earphones are used then care should be taken to ensure that wax is not compacted by the probe so blocking the sound pathway. Earphones should be centred over the ear canal ensuring that the ear canal is not occluded by any excess pressure. DATA COLLECTION AND ANALYSIS The tone burst ABR waveform is generally smaller, broader and longer latency than the click ABR, particularly with mid to low tonal frequencies. It often consists of a slow wave V component followed by a negative trough, sometimes called the SN10. The data collection parameters are adjusted in order to capture this type of response. Amplifier artefact rejection An artefact rejection limit in the range of +/-10 to +/-15 μv is recommended in order to reject a high proportion of noisy signals. This will improve the signal to noise ratio and assist the detection of small responses close-to-threshold. Levels above +/-25μV should not be used. The level of stimulus artefact rejection is often controlled by the sensitivity/gain selection on the signal amplifier. Filters The recommended filter bandwidth on the signal amplifier is a low frequency cut-off (high pass) of 20 Hz or 30 Hz, and a high frequency cut-off (low pass) of 1000Hz to 3000Hz. These settings are the same as for the click ABR and will enable the slower waves of the ABR to be recorded (wave V and SN10). Notch filter A 50 Hz mains notch filter should not be used as this may distort or attenuate the slower components of the response (wave V and SN10). 41

42 Tone-Burst ABR Window length and Averaging The recommended window length is in the range 20 to 25ms in order to be sure of acquiring the complete SN10 component of the ABR (see section on stimulus rate above). The number of sweeps per replication accepted by the averager should be varied depending on both the size of the response and the level of background activity. Since the tone burst ABR is generally less well-defined than the click ABR, a minimum of 2000 sweeps is recommended. Increasing this number to 3000 or 4000 may be advisable for identification of small responses close to threshold or if the signal baseline is noisy. The number of rejected sweeps will depend on the level of background activity and the reject level that has been chosen. Whenever the electrical activity from the baby exceeds the reject levels for long periods of time it is recommended that data collection is halted until the baby has settled for the reason given below. Recordings made where there is a high number of rejected sweeps should be treated with caution. Data rejection facilities on current equipment are effective in rejecting most unwanted sweeps. However, when there is a long period of high background activity and the data collection is not interrupted there will be a greater proportion of sweeps containing noise which might distort the averaged response. Replication of the waveform is an excellent technique for differentiating between genuine response and noise. Display The convention of plotting wave V upwards (SN10 down) is proposed as it is the most common approach in current audiological practice. It is recommended that the display be always set at a fixed number of μv/division. The amplitude (y) and time (x) scales should be such to ensure that small waveforms near threshold are visible. It is recommended that 0.05μV on the response amplitude (y) axis (or 0.1μV for larger responses) has the same size on the display as 1 to 2 ms on the time (x) axis. An automatic display gain is not recommended as an inappropriate gain for assessment of the response may be set. Criteria for accepting the presence of a response At least two replications of a response are required at the stimulus level which will be recorded as the lowest level at which a response was obtained. Single recordings at one stimulus level are only acceptable at high suprathreshold levels and providing that they form part of an intensity series of recordings. Since the tone burst ABR is generally less well-defined than the click ABR it 42

43 Tone-Burst ABR is essential to have a good recording conditions with a quiet signal baseline, even then it may necessary to use 3 or even 4 replications at one stimulus level in order to reach a definitive and reliable interpretation, particularly with stimulus levels close to threshold of the response. For each stimulus level the result should be marked in one of three ways. (a) response present, (b) good recording conditions but no response was recordable or (c) recording conditions too poor to establish a result. For (a) there must be a high degree of correlation between the replications and a characteristic waveform. For (b) the replications must be low in amplitude (peak to peak below about 0.05μV) with no significant correlation and for (c) the replications will have amplitudes above about 0.05μV peak to peak without a high degree of correlation. Threshold of the response must be determined from an intensity series of traces, using stimulus levels both above and below the estimated threshold. This will enable a reliability and robust interpretation of response threshold. It is recommended that increments of 10 db are used with at least one stimulus level 10dB below threshold, one at threshold and one at 10 db above. Results should be clearly marked as to whether they indicate a threshold or the lowest level tested. It should also be made clear in the results when a threshold measurement was not possible because of reaching the maximum limits of stimulus intensity available on the equipment. Independent auditing of the results should not give thresholds that are more than 10 db different from those originally recorded. The method of measuring threshold should be stated i.e. whether it is the lowest clear response or an interpolated value. CALIBRATION There are no international standards on the RETSPL values for tone burst used for ABR. A procedure for calibration is given in IEC (1994) 4. It is important to note that the stimulus rate will affect the sound level of the psycho-acoustic threshold of the tone burst to a much greater extent than the threshold of the ABR. In the IEC (1994) standard a stimulus rate of 20/s is recommended or the rate that is used when testing clinically. These recommendations are open to question due to the effect of stimulus rate noted above. If a local biological calibration is carried out, it is recommended that this is performed using a relatively slow stimulus rate of 10 per second. 43

44 Tone-Burst ABR Samples of tone burst data from four centres are presented here which describe the peak-to peak equivalent sound pressure level (dbppespl) for 0 dbnhl (RETSPL). The exact measurement parameters for each centre differ slightly with respect to stimulus rate, coupler and tone burst characteristics but serve as a useful guide to the approximate values expected. Site Reference Earphone 500Hz dbppespl 1kHz dbppespl 2kHz dbppespl 4kHz dbppespl Vancouver Stapells & TDH Oates 3 Nottingham Mason 5 TDH Los Angeles Sininger Insert et al 6 (1.5 khz) Not recorded Sheffield Stevens 7 TDH ppe = peak to peak equivalent It is important to be aware that calibration values will be different for insert type earphones when compared to the TDH39/49. Also existing calibration values on commercial equipment need to be checked as there is a likelihood that these may relate to a long duration burst of pure tone rather than the shorter tone burst. For a more detailed explanation please consult the appropriate reference. You are strongly advised to seek expert help if you are not familiar with the measurement of peak to peak equivalent SPL. It is informative to compare these values of the ppespl for the tone burst with the recommended value for the click (33 dbppespl 2 for a TDH39 earphone). Warning insert earphones If insert phones are used in babies, the recommendation is that they should not be used above 85 dbnhl unless they include a microphone to automatically adjust the stimulus level for ear canal volume. The reason for this is that a baby has a much smaller ear canal which can give up to about a 20 db rise in the stimulus level compared to the same insert earphone used in an adult (Voss & Herrman, 2005; Rance & Tomlin, 2006; Sininger et al, 1997). This figure is thought to diminish over the early months of life as the ear canal grows. 44

45 Tone-Burst ABR In view of the evidence suggesting that insert earphones giver higher stimulus levels the following provisional values are proposed by the NHSP. These values MUST be added to the stimulus level in dbnhl to correct for the effect of the smaller ear canal volume in babies less than 3 months of age in comparison to adults. Hence, if performing a 500 Hz TBABR and a threshold of 25 dbnhl is obtained, that threshold is to be corrected to 30 dbnhl (as per the table below). Provisional correction to insert earphone stimulus level for the effect of the smaller baby ear canal volume. Click 500Hz 1000Hz 2000Hz 4000Hz The application of the insert headphone correction factors will be updated as appropriate in future versions of this protocol pending developments of research in this area, and the findings of the Healthy Hearing Audiology working group. REPORTING NOTE: Please be aware that Australian Hearing may apply correction factors to convert ABR thresholds to estimated hearing thresholds (db ehl) suitable to be used for hearing aid fitting. Australian Hearing conversions are based upon ABR thresholds being reported in db nhl. Healthy Hearing diagnostic audiology reports should state which decibel scale is being reported, and any correction or calibration factors applied to the thresholds in addition to those covered in this document. HEALTHY HEARING TONE BURST ABR QUICK SUMMARY PROCEDURE - Both ears must be assessed. - Testing is routinely carried out at 60, 40 and 30 dbnhl. - At least 2 traces at each test level are required. - The presence of wave V at 30 dbnhl (or with appropriate insert corrections) is considered functionally normal at any given frequency, and can exclude all but a mild hearing loss. - Latency and morphology of responses must be considered in determining presence versus absence of a response. 45

46 Tone-Burst ABR QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR TONE BURST ABR Electrode Location: One Channel: Positive: High forehead/vertex Negative: (i) Ipsilateral mastoid Common: (i) Contralateral mastoid or (ii) forehead/mastoid Stimulus: Tone burst with alternating polarity: Stimulus presentation level: 2 traces required at least at threshold. Stimulus timing: Rise-plateau-fall typically Blackman cycles Stimulus rate: Typically 37 /s Earphone: TDH39/49 preferably EAR-3A (insert earphones) with correction factors. Coupler value for 0dBnHL: Depends on exact stimulus and measurement parameters (see above Table) Amplifier reject levels: Filters: ±10 to ± 25 uv Low frequency: Hz High frequency: Hz Window length: Number sweeps averaged per replication : Display: ms maximum Minimum μv or 0.1 μv on amplitude axis 1ms or 2 ms on time axis 46

47 Tone-Burst ABR * refer to section on stimulus and stimulus rate. + This is only an example of a suitable rate (refer to section on stimulus and stimulus rate). REFERENCES 1/ Davis A, Bamford J M, Wilson I, et al. (1997). A critical review of the role of neonatal screening in the detection of congenital hearing impairment. Health Technology Assessment 1, 10. 2/ Stevens JC et al. (1999) Click auditory brainstem responses testing in babies- a recommended test protocol. British Society of Audiology News bulletin, December Issue 3/ Stapells D R, Oates P (1997) Estimation of the pure-tone audiogram by the auditory brainstem response: A review. Audiol Neurootol 2: / IEC (1994). Audiometers: Part 3: Auditory test signals of short duration for audiometric and neuro-otological purposes. (Equivalent to BS EN : 1995) 5/ Mason S M. (1999). Calibration of tone pip stimuli. (personal communication) 6/ Sininger Y S, Abdala C, Cone-Wesson B. (1997). Auditory threshold sensitivity of the human neonate as measured by the auditory brainstem response. Hearing Research 104: / Stevens J C. (2001). Calibration of tone pip stimuli (personal communication) 47

48 Bone Conduction ABR (iii) Bone Conduction ABR Test Protocol Auditory Brainstem Response Testing in Babies using Bone Conduction- A Recommended Test Protocol 48

49 Bone Conduction ABR (iii) Bone Conduction ABR Test Protocol Auditory Brainstem Response Testing in Babies using Bone Conduction- A Recommended Test Protocol INTRODUCTION With the publication of the systematic review on Neonatal Hearing Screening 1 a national group has had meetings to discuss some of the issues of implementation of Universal Neonatal Hearing Screening. Several workshops on technical issues have been held at which it was felt that it would be useful to produce consensus documents on test methodology where this was possible. Click ABR (air conduction) was the first of these documents 2. This document extends this protocol to cover the use of click bone conduction stimuli. The use of tone bursts in bone conduction ABR is dealt with in a section at the end of the protocol and should be used in conjunction with the protocol for tone burst ABR which has been produced separately. There were many points of debate and in some areas it has been necessary to give a range for the final values for parameters as a single optimum value could not be agreed. In most cases these differences are not critical to the clinical use of the test. The optimum values may change as the results of more research are published. SCOPE The document sets out to provide guidelines for testing babies in the first few months of life by bone conduction (BC) ABR using click stimuli for the purposes of assessing hearing sensitivity. Clicks are currently employed for neonatal screening and most diagnostic assessment in the UK. However it should be noted that this may change as other electrophysiological methods such as tone burst ABR become more widely used. The document covers the technical procedure of carrying out an ABR test and the reporting of the results. It does not cover practical aspects such as handling of the baby or criteria to be used to decide whether a baby has passed or failed an ABR screen. PATIENT PREPARATION The procedure for patient preparation is similar to that used for click air conduction (AC) ABR and reference should be made to the relevant sections in that document. Only supplementary information is given here. 49

50 Bone Conduction ABR Precautions against cross infection The bone conductor should be cleaned in the same way as the earphone for AC ABR. Choice of electrodes and application Bone conductors can generate large artefacts, particularly at high stimulus levels. Low electrode impedances are of particular importance for BC ABR. Electrode location A one or two channel recording is recommended with electrodes located as given below. The purpose of two channel recording is to record the contralateral response to aid in the determination of which cochlea is being stimulated. One channel recording: (electrodes as used for air conduction click ABR) Negative electrode: ipsi-lateral mastoid or nape of neck. Positive electrode: high forehead as near to Cz as possible, avoiding the fontanelle. Common electrode: contralateral mastoid when using the ipsi-lateral mastoid electrode for the negative electrode. If the nape of the neck is used for the negative electrode, the common electrode can be placed on the forehead (at least 4 cm from the positive electrode) or either mastoid, whichever is the more practical in the individual case. Two channel recording: Negative electrode: ipsi-lateral mastoid and contralateral mastoid Positive electrode: high forehead as near to Cz as possible, avoiding the fontanelle. Common electrode: forehead The above electrode configurations should result in wave V being plotted upwards on the display. If this is not the case then the positive and negative electrode connections should be reversed. The mastoid location has been recommended rather than the nape for the negative electrode for the two channel recording as this allows for the recording of wave I, if present. STIMULUS Stimulus and stimulus rate For a single channel recording where it is not required to measure wave I an electrical click of about 100μS at a stimulus rate of around per second is recommended, with 50

51 Bone Conduction ABR alternating polarity to minimise stimulus artefact. A figure such as 33.1/s, which does not have a common factor with mains frequency, should be chosen to minimise any mains artefact. A check should be made that the stimulus rate allows the data collection to be completed prior to the next stimulus being given. This can be checked by timing the collection of 1000 sweeps and confirming that the time taken is 1000/stimulus rate. Higher rates would reduce the time taken to record wave V. However as the rate is increased above 50 per second the maximum recording window must be reduced below 20 ms. There will be a loss of the latter part of the SN10 waveform which may offset the advantage achieved by faster recording of the wave V waveform. For one or two channel recording where an attempt is made to record wave I, the rate should be lower. A provisional rate of 11.1 /s is recommended. The level of stimulus output should be monitored by listening to the earphone at critical points during the test. Stimulus levels should normally be recorded in dbnhl i.e. as referenced to the average psycho-acoustic threshold of a group of normally hearing young adults. A correction needs to be applied for age. This is given below. Note that the polarity of the stimulus can affect the response waveform and in cases of auditory neuropathy it may be useful to record responses to both rarefaction and condensation clicks. The majority view, based on current practice, was that for neonatal screening and threshold measurement alternating polarity should be used to minimise the stimulus artefact. Bone conductor This should be of sufficient quality to deliver a stimulus up to 50 dbnhl without distortion. A suitable bone conductor is the Radioear type B-71. The B70 may offer an alternative. A check should be made that the impedance of the bone conductor is correct for the equipment being used. The bone conductor should be frequently checked to make sure that the sound output is as expected. A listening test near threshold, and at maximum output level should be carried out. The sound output becomes distorted at high sound levels. The output of the bone conductor measured on an artificial ear should be checked at high levels for waveform distortion. The level of the output should also be checked to see that it follows the value of the attenuator. Location of bone conductor. The bone conductor should be placed on the mastoid, avoiding the mastoid electrode. The bone conductor lead should be kept away from the electrode and electrode lead. This site gives a higher 51

52 Bone Conduction ABR stimulus level when compared to a forehead placement. There is also evidence of an inter-aural attenuation of approximately 20 db in babies, which is taken advantage of by a mastoid location. Pressure to apply A moderate force should be used to apply the bone conductor. Tests on an artificial mastoid have demonstrated an error of no more than 2 db over a wide range of applied forces. Effect of age on stimulus There is a change in the effectiveness of the stimulus in the neonatal period and a correction should be applied to the value obtained from the psycho-acoustic threshold value found from adults. Table 1 give the proposed correction values for age 5 Gestational age 36 weeks 40weeks 46 weeks 52 weeks Correction (db) The values should be added to the stimulus setting. For example a 30dBnHL stimulus as biologically calibrated in adults would be ( ) = 40 dbnhl for a 40 weeks gestational age baby. DATA COLLECTION AND ANALYSIS Amplifier and artefact rejection level The values used should be the same as for AC ABR. The sensitivity/gain should be set such that any electrical activity greater than about +/- 25uV is rejected. Some users have found that lower rejection levels down to +/- 10uV are better. A value between +/-10 and +/-25uV is therefore recommended. The equipment manuals should be checked to determine what value of amplifier sensitivity is required to achieve these rejection levels. The use of higher rejection levels is not recommended when recording conditions are difficult (e.g. high muscle activity). The result is likely to be a poorer signal to noise ratio in the averaged signal. Filters The filters should be the same as those used for AC ABR. Low frequency (high pass): A value between 20 to 100 Hz is recommended. A value between 20 to 30 Hz has been found to 52

53 Bone Conduction ABR give the best signal to noise ratio of wave V near threshold. High frequency (low pass): A value of around 1500 Hz is recommended. There is little response energy above this frequency. A higher value generally adds more electronic noise from the amplifier. The order of the filters in different machines varies. Check the manual to determine what the order of the filter is. A higher order filter has a greater effect but may introduce more distortion of the waveform. If the order is greater than two or the manual is unclear, refer to expert help to see if filter values should be altered. Notch filter Under normal recording conditions this will not be required as 50 Hz mains artefact should be absent or minimal with good electrode practice using current equipment. If mains artefact levels are high it is better to identify and remove the source of the problem rather than use a notch filter which can introduce distortion. Window length and Averaging The values for the parameters are the same as for AC ABR. The longest response acquisition window consistent with the chosen stimulus rate should be used. This will normally be between 18 and 20 ms, although could be longer when a lower stimulus rate is used to record wave I (see section on stimulus rate above). The number of sweeps per replication accepted by the averager should be varied depending on both the size of the response and the level of background activity. It will normally vary between about 1500 and 2500 sweeps, although higher numbers will be required when the responses are small or the background noise is high. Typically 2000 sweeps is recommended with a minimum of Exceptionally there may be such a large ABR response or low background activity that fewer sweeps can be used. The number of rejected sweeps will depend on the level of background activity and the rejection level that has been chosen. Whenever the electrical activity from the baby exceeds the rejection levels for long periods of time it is recommended that data collection is halted until the baby has settled for the reason given below. Recordings made where there is a high number of rejected sweeps should be treated with caution. Data rejection facilities on current equipment are effective in rejecting most unwanted sweeps. However, when there is a long period of high background activity and the data collection is not interrupted there will be a greater proportion of sweeps containing higher levels of noise added to the averaged response. 53

54 Bone Conduction ABR Display The convention of plotting wave V upwards is proposed as it is the most common in current practice. It is recommended that the display be always set at a fixed number of μv/division. The amplitude (y) and time (x) scales should be such to ensure that small waveforms near threshold are visible. It is recommended that 0.1 μv on the response amplitude (y) axis has the same amplitude on the display as 1 2 ms on the time (x) axis. An automatic display gain is not recommended as an inappropriate gain for assessment of the response may be set. Criteria for accepting the presence of a response At least two replications of a response are required at the level which will be recorded as the lowest level at which a response was obtained. A third replication should be carried out if there is any uncertainty in the result or if tests are only being carried out at one stimulus level. If the results at this level are not absolutely unambiguous, a further pair of recordings should be obtained at a level 10 db higher to limit any possible error to 10dB. For each stimulus level the result should be marked in one of three ways. (a) response present, (b) good recording conditions but no response was recordable or (c) recording conditions too poor to establish a result. For (a) there must be a high degree of correlation between the replications and a characteristic waveform. For (b) the replications must below in amplitude (peak to peak below about 0.05μV) with no significant correlation and for (c) the replications will have amplitudes above about 0.05μV peak to peak without a high degree of correlation. The above protocol should be followed for screening. For threshold measurement it is recommended that increments of 10dB are used with a minimum of one stimulus level 10dB below threshold, one at threshold and one 10dB above. Results should clearly be marked as to whether they are threshold or the lowest level tested. It should also be made clear in the results when threshold measurement was not possible due to the maximum stimulus limits of the equipment. Independent auditing of the results should not give thresholds that are more than 10dB different from those originally recorded. The method of measuring threshold should be stated i.e. whether it is the lowest clear response or an interpolated value. 54

55 Bone Conduction ABR CALIBRATION There are no international standards on the RETFL values for clicks used for BC ABR. A procedure for calibration is given in IEC (1994) 3. It is important to note that the stimulus rate will affect the sound level at psycho-acoustic threshold. In the IEC (1994) standard a stimulus rate of 20/s is recommended or the rate that is used when testing clinically. These recommendations are open to question due to the effect of stimulus rate noted above. If a local biological calibration is carried out, it is provisionally recommended that this is carried out at a stimulus rate of 10 per second. It is recommended that centres of excellence that have carried out biological calibration are approached for advice and implementation of a satisfactory procedure. REPORTING NOTE: Please be aware that Australian Hearing may apply correction factors to convert ABR thresholds to estimated hearing thresholds (db ehl) suitable to be used for hearing aid fitting. Australian Hearing conversions are based upon ABR thresholds being reported in db nhl. Healthy Hearing diagnostic audiology reports should state which decibel scale is being reported, and any correction or calibration factors applied to the thresholds in addition to those covered in this document. QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR CLICK BONE ABR: Electrode Location: One Channel: Positive: High forehead/vertex Negative: (i) Ipsilateral mastoid or (ii) nape Common: (i) Contralateral mastoid or (ii) forehead/mastoid Two Channel: Negative electrode: ipsi-lateral mastoid and contralateral mastoid. Positive electrode: high forehead as near to Cz as possible, avoiding the fontanelle. 55

56 Bone Conduction ABR Common electrode: forehead. Contralateral electrodes will be used in the two channel recording. Stimulus: Alternating polarity click: Rate /s + Stimulus presentation level: At least two traces at threshold required. Bone Conductor: Radioear B71 or B70 Coupler value for 0dBnHL: Refer to NHSP calibration data 4 Amplifier reject levels: Filters: ±10 to ± 25uV Low frequency: Hz High frequency: 1500Hz Window length: Number sweeps averaged per replication : 20 ms maximum Minimum 1200 Typically 2000 Display: 0.1μV on amplitude axis = 1 2ms on time axis + This is only an example of a suitable rate (refer to section on stimulus and stimulus rate). USE OF TONE BURST STIMULI The procedure is the same as for click BC ABR except that a different stimulus is used leading to some changes in the recording parameters. These are summarised in the table below. The justification for the nature and rate of the stimulus is can be found in the tone burst AC ABR protocol. Stimulus artefact will be a particular problem given the combination of the high levels of electromagnetic field radiated from the bone conductor and the long length of the tone burst stimuli. Low electrode impedances are particularly important. 56

57 Bone Conduction ABR QUICK GUIDE: RECOMMENDED PARAMETERS AND SETTINGS FOR TONE BURST BONE ABR: Electrode Location: As per Click Bone Conduction ABR Stimulus: Alternating tone burst. Rate 39.1/ Stimulus Timing: Bone Conductor: Rise-plateau-fall typically Blackman cycles Radioear B71 or B70 Coupler value for 0dBnHL: Refer to NHSP calibration data 4 Amplifier reject levels: Filters: ±10 to ± 25uV Low frequency: Hz High frequency: 1500 Hz Window length: Number sweeps averaged per replication : Display: ms maximum Minimum 2000 Typically μV on amplitude axis = 1 2ms on time axis * refer to section on stimulus and stimulus rate. + This is only an example of a suitable rate (refer to section on stimulus and stimulus rate). REFERENCES 1/ Davis A, Bamford J M, Wilson I, et al. (1997). A critical review of the role of neonatal screening in the detection of congenital hearing impairment. Health Technology Assessment (1) (10). 57

58 Bone Conduction ABR 2/ Stevens JC et al. (1999). Click auditory brainstem responses testing in babies- a recommended test protocol. British Society of Audiology News bulletin, December Issue. 3/ IEC (1994). Audiometers: Part 3: Auditory test signals of short duration for audiometric and neuro-otological purposes. (Equivalent to BS EN : 1995) 4/ IEC (1990). Mechanical coupler for the measurement of bone vibrators. (to become IEC ). 5/ Webb H D. (1993). Auditory screening in high risk neonates: an evaluation of the bone conduction auditory brainstem evoked response test. M. Med Sci. University of Sheffield, UK 58