Examination of figure 6 reveals that certain portions of the waveforms are overlapping and quite similar, while a large amount of separation exists within other segments of the waveforms.
A good estimate of the noise level may be obtained by finding the place where there is the maximum amount of separation and multiplying the magnitude of that separation by one-third (personal correspondence with Dr. Lightfoot).
When appropriate steps have been taken to reduce noise, EEG noise levels can be averaged to below 25 nV with fewer than 1,000 sweeps. For noisier patients, we may have to average to 2,000 sweeps or beyond (e.g., 4,000-6,000 sweeps).
5. THRICE AS NICE
While there is no unanimous rule stating how much larger the ABR wave V should be relative to the EEG noise, most experts suggest that the response should be 2.5 to 4 times greater.
At Hearts for Hearing, if the response is at least three times greater than the noise, then it is not necessary to continue averaging until the residual noise reaches 25 nV.
6. 100-PERCENT GUARANTEED
Auditory brainstem responses that are recorded at the same presentation level should be highly replicable.
If we are uncertain whether a response exists after collecting two waveforms, then we run a third waveform. If there is a true physiologic response, then the third waveform should be almost identical to the first two. Any differences among the waveforms are noise.
In addition, we may also use digital analysis to add the waveforms together, yielding a collective response that, if a response does exist, should possess a wave V. This result corroborates the presence of the wave V in individual waveforms.
Finally, for threshold-level response, we must be certain that the display scale allows for visualization of low-level responses.
Again turning to the UK guidelines http://hearing.screening.nhs.uk/getdata.php?id=19345, we set the display scale so that 0.1 µV on the vertical axis (i.e., ABR amplitude) corresponds to the same distance as 1 ms on the horizontal axis (e.g., time scale).
If we suspect that a low-level response exists, then we “zoom in” so that.05 µV corresponds to the same distance as 1 ms.
7. CAN YOU HEAR ME NOW?
To minimize the margin for potential error, we suggest using a bracketing approach in auditory brainstem response similar to that used in behavioral audiometry.
We begin the ABR assessment with a 2,000-Hz tone burst presented at 60 dB nHL and then seek to obtain a response at 30 dB nHL in order to rule out a significant hearing loss.
If no response is present at 30 dB nHL—that is, a probable hearing loss exists—or if there is a questionable response, then we increase the presentation level 10 dB above the suspected threshold to confirm the presence of a clear response.
For infants with hearing loss, we collect two waveforms at a presentation level that is 10 dB below the response threshold.
Some experts recommend the completion of two “response-absent” waveforms. However, if the sufficiently averaged EEG response clearly shows no response with an objective residual noise level less than 25 nV, then it is highly unlikely that a response would be identified by an additional waveform.
We do not obtain waveforms 10 dB below the response threshold level if we have collected clear responses at levels ruling out a significant hearing loss greater than 25 dB nHL.
8. NO CROSSING
For ABR testing, there is potential for crossover to the opposite ear when bone-conduction testing is used or when there is a large difference between air-conduction thresholds obtained for the two ears.
To limit the likelihood of crossover, the clinician may routinely present 50-55 dB nHL of broadband noise to the opposite ear, which is unlikely to affect the response obtained on the test ear and should sufficiently mask the opposite ear.
If a more refined approach to determining masking necessity and levels is desired, the reader is referred to Guy Lightfoot's excellent summary called “Masking the ABR https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCoQFjAA&url=http%3A%2F%2Fhearing.screening.nhs.uk%2Fgetdata.php%3Fid%3D21673&ei=Rml2U-rqOY-jqAa_wYGoCQ&usg=AFQjCNElsP08fA9M4sfO_Fi5JlBn2Xu2nw&sig2=wXX7O03M5oeGb2PF8MjWwQ&bvm=bv.66917471,d.b2k.” Dr. Lightfoot's masking calculator spreadsheet is described in the article.
Two-channel recordings should be made during bone-conduction testing so that the latencies of the responses may be compared between the two channels. The latency of wave V should be earlier for the channel that corresponds to the responding ear.
9. WHAT'S NEXT?
Since we set up the parameters within our ABR protocols to be similar to those used by David Stapells and colleagues in generating correction factors for the estimation of behavioral threshold from tone-burst ABR thresholds, we feel comfortable using the same correction factors: -15, -10, -5, and 0 dB at 500, 1,000, 2,000, and 4,000 Hz, respectively.
For children who have no response present at equipment limits, we subtract the frequency-specific correction from the highest level we assessed at each tone-burst frequency.
10. READ ALL ABOUT IT
To read more about auditory brainstem response interpretation, we recommend the following publications:
* Stapells DR. Frequency-specific ABR and ASSR threshold assessment in young infants. In: Seewald RC, Bamford JM, eds. A Sound Foundation Through Early Amplification 2010: Proceedings of the Fifth International Conference . Stafa, Switzerland: Phonak; 2010: 67-106. http://www.phonakpro.com/content/dam/phonak/gc_hq/b2b/en/events/2010/Proceedings/Pho_Chap_04_Stapells_final.pdf
* United Kingdom's Newborn Hearing Screening and Assessment: Guidance for Auditory Brainstem Response Testing in Babies. http://hearing.screening.nhs.uk/getdata.php?id=19345
* British Columbia Early Hearing Program: Diagnostic Audiology Protocol. http://www.courses.audiospeech.ubc.ca/haplab/bcehp_diagnosticaudiologyprotocolsdec2007.pdf?bcsi-ac-0f50f852aecff21e=226972DC00000002MFyPq1D+qsvzwtVz0vcUbkn5RwAnCwAAAgAAADffKgCEAwAAAQAAABcgCAA
* Hall JW III, Swanepoel DW. Objective Assessment of Hearing. San Diego, CA: Plural Publishing, Inc; 2010.
* Hall JW III. New Handbook for Auditory Evoked Responses. Boston, MA: Allyn & Bacon; 2007.
The clinician who is new to infant ABR assessment should seek mentorship from an audiologist seasoned in the art and science of ABR testing in young children. Don't fly solo until you are certain you can navigate turbulent skies.
Even better, establish a local peer review network in which valuable constructive feedback is obtained on your practice.© 2014 by Lippincott Williams & Wilkins, Inc.