(PP506) Comparison of Commercially Available Auditory Brainstem Response Stimuli at Threshold Intensity Levels

The auditory brainstem response (ABR) is instrumental to evaluate auditory nerve pathologies and estimate hearing threshold for infants, children, and adults. Although regularly utilized in clinical practice, current stimuli used (i.e., click and tone-burst) do not consistently yield reliable threshold findings. In this study, a comparison of the 100-µs broadband click and the 5-ms iChirp will be used to determine which stimuli provides more accurate results for the estimation of behavioral thresholds in normal hearing adults.

Summary: 

Background

The auditory brainstem response (ABR) is an integral objective estimate of hearing thresholds, especially for difficult-to-test populations. Stimuli used to evoke an ABR have been traditionally either broadband acoustic click or tone-burst signals. These types of stimuli, however, inconsistently produce robust and repeatable waveform amplitudes, specifically at low intensity levels (Elberling et al, 2010; Maloff & Hood, 2014; Speidel & Beck, 2016). It has been suggested that the broadband click causes waveform variability due to the inevitability of the traveling wave, which stimulates the basal region of the cochlea before the apical (Dau, 2000). Therefore, the response is limited to a narrow region of instantaneous stimulation along basilar’s membrane. The chirp, on the other hand, off-sets the timing of the high frequency spectrum relative to its low frequency component. The low frequency component stimulates the cochlea earlier than the high frequency component, which promotes simultaneous activation of the cochlea (Dau, 2000). This synchronous activity is thought to contribute to greater neural synchrony of afferent action potential firing and in turn, result in more reliable latencies and larger waveform amplitudes. The iChirp, is a newly developed stimulus that is commercially available through the Intelligent Hearing System (IHS) SmartEP platform. The iChirp holds promise as a better alternative to the standard click for use in threshold estimation that is comparable to behavioral thresholds. We hypothesize that the iChirp will result in more robust ABR waveforms to threshold when compared to the click stimulus.

Methods

This study is approved by the Institutional Review Board at Northwestern University. Subjects (N=40, age 18-25) will have normal otoscopic, tympanometric, and air-conducted pure tone audiometric (octaves 250-8000 kHz, thresholds <20 dB HL) findings. Electrodes will be placed on the high forehead (Fz, active), right mastoid (M2, reference), and left mastoid (M1, ground), and impedances will be < 5 kΩ (< 3 kΩ interelectrode impedance). Stimuli (iChirp and click) will be presented at a rate of 19.1/s to the right ear with an ER-3A insert earphone. Responses will be filtered (100-3000 Hz), amplified (100,000X), and sampled (50 kHz) for a 12.5-ms time window. A second, repeatable waveform will be obtained for threshold reliability (2048 sweeps). Stimuli will be first presented at 70 dB nHL and decreased until threshold is determined by a repeatable wave V (peak-to-trough amplitude). Paired t-tests will be used to compare amplitude differences at threshold and correlation analysis will be performed with behavior thresholds for both stimuli.

Conclusion

It is important to establish a reliable stimulus that will produce ABR threshold findings in good agreement with behavioral thresholds for routine clinical utility. This is fundamental to properly diagnose and treat individuals with hearing disorders where the audiogram alone cannot determine hearing status (i.e., difficult-to-test populations and infants). For our study, we predict the iChirp stimulus may provide more accurate results for clinical estimates of behavioral thresholds in normal hearing adults. We believe our findings may be instrumental for future studies investigating its use in normative protocols for individuals across the lifespan.