Auditorybrain stem response is used clinically to estimate threshold for newbornhearing screening or determining whether a hearing loss may be sensory, neuralor retro-cochlear. The ASSR overcomes some of the limitations of ABRtesting because, the ASSR is evoked by pure tones that areamplitude and/or frequency modulated.
The followingare two studies that compared the threshold estimates from auditorysteady-state response (ASSR) tests with the thresholds of click and toneburst-evoked auditory brainstem responses (ABR): Study 1:Comparison of ASSR With ABR Results:Compared ASSR test results inrelation to click-ABR (c-ABR). Behavioral Threshold Tests, ABR ThresholdTests and ASSR Threshold Tests were conducted in order to obtain the results. Results: Correlationswere determined between ABR threshold (in nHL) with each audiometric threshold (in dB HL) and between ASSR thresholds and audiometric thresholds (both in dB HL). Theresults revealed that both c-ABR and ASSR have highly and statisticallysignificant correlations.Smaller threshold forASSR than ABR was the discrepancy between behavioral and evoked potentialthreshold. Discussion: C-ABR and ASSR threshold estimates may beuseful to predict pure-tone threshold for infants and children who have hearingthresholds in normal to severe-to-profound range as data suggested.
In ASSR, it defines threshold as the lowestlevel at which a statistically significant result was obtained. In ABR, itdefines threshold as the lowest level for which a time-domain waveform wasvisually detected by an observer. small differences were noted between ABR andASSR correlation. Strengths: Ø This study considered the instrumentoutput limits and excluded data with profound hearing loss.Ø Data with period between the ABR,ASSR and behavioral threshold tests was more than two months were excluded fromthe study.Ø ASSR results are obtained using PCwhich avoids bias judgments.
Weakness: Ø Clicks evoke more synchronous neuralresponse than modulated tones, which is a factor that could contribute to thestudy results.Ø Case selection criteria excludedprofound losses at any frequency. Ø Adultscalibration values were used to report thresholds and it has been revealed thatthresholds are different in infants compared to adults.
Ø ABR results are obtained by anobserver which may result in bias judgments. Study 2:Direct Comparison of ASSR and tone burst evoked ABR: ASSR tests used 500 and 4000Hz, and responseswere detected automatically using algorithms. ABR used two-channel electrodemontage and neuro scan system to obtain results, and responses were detectedvisually by the subject. To estimate threshold, Tone burst ABR (tb-ABR) andASSR were used.
Nonetheless, ABR used tone burst stimulus while ASSR usedamplitude modulated + frequency modulated tone.Result: Elevated thresholds for 500Hz relatively tothose for 4000Hz. Additionally, tone burst thresholds were elevated. Discussion: 500Hz tb-ABR visual detection resulted in lowerthreshold estimated in comparison to other measures at 500Hz and at 4000Hz italso resulted in the lowest threshold estimates. and the 500Hz ASSR at 74Hz itresulted with high threshold estimates and at 4000Hz it was not significantlydifferent at 95Hz than the one obtained in tb-ABR.
Strengths: Ø In contrast to the 1st study whichcompared tb-ABR to ASSR at one frequency, this study used more than onefrequency in comparison. Ø Allparticipants had normal pure-tone thresholds.Ø Both tests ABR and ASSR were carriedin the same settings, room that is sound-treated, custom-built with lights off. Weaknesses: Ø During ASSRtesting, most of the participants slept, which affects their thresholds (lowerthresholds obtained)Ø For each participant, only one ear was testedin both ABR and ASSR.
Ø No formal assessment was made of subjectstatus during the experimental procedures. Ø Normal hearing subjects, while tests areusually used to estimate threshold in infants, children and adults with hearingloss.Ø Stimulus type was different in both tests,ABR used frequency specific while ASSR used AM and FM, which causesdifficulties in comparing threshold estimatesØ Normal ASSR thresholds range reported in thestudy varied across researchers.