(PP503) Neurodiagnostic ABR Methods: Merging Theory with Clinical Application

The auditory brainstem response (ABR) is an established clinical tool for estimating hearing threshold sensitivity and differential neurodiagnostic procedures. Long established theoretical framework suggests ABR waveform components are significantly affected by modifying parameters such as electrode placement, stimulus rate, and stimulus polarity. Here we tested these theoretical assertions by recording ABRs using different electrode montages, click rates, and click polarities in 77 normal-hearing young adults. In doing so, we provide data to support the use of specific recording parameters for optimal ABR recordings in clinical settings.

Summary: 

While the auditory brainstem response (ABR) is an established clinical tool, there are many aspects of ABR acquisition that one must consider for optimal recordings such as the placement of electrodes and stimulus parameters. Theoretical assertions about the ABR along with animal and human studies have guided clinical practices. Here we aim to revisit theoretical assertions regarding electrode montage, click rate, and click polarity and provide data to support optimal recording parameters for clinical use.

Based on theory, horizontal and vertical electrode montages emphasize different components of the ABR because of the relationship between their recording planes and the orientation of dipole spread within the auditory pathway. We predicted a horizontal montage would enhance earlier waves (e.g., I) due to the horizontal orientation of the auditory nerve’s fiber tracks. Conversely, a vertical montage would enhance later occurring waves (e.g., IV/V) due to the vertical orientation of the fiber tracks of upper brainstem structures. 

It is theorized that an increase in click rate leads to a decrease in phase locking based on multiple interrelated mechanisms such as desynchronization of participating nerve fibers and neural adaptation/fatigue. Based on this assumption, we predicted that higher click rates would result in an overall decrease in amplitudes and increase in latencies and that earlier waves would be more vulnerable to amplitude effects and later waves more vulnerable to latency effects.

As for click polarities, theory states rarefaction and condensation oppositely deflect the basilar membrane upon stimulation and therefore produce either immediate or delayed neural responses (respectively). The differences in transduction events produced by these two polarities should result in conflicting response characteristics in early auditory evoked potentials (i.e., ABRs). More specifically, we predicted that rarefaction polarity leads to earlier response times (i.e., absolute wave latencies) than condensation polarity, given the excitatory nature of the rarefaction phase.

Neurodiagnostic ABRs to a broadband 100-μs click were collected on 77 normal-hearing subjects between 21-34 years of age. Evoked responses were recorded using Intelligent Hearing Systems (IHS; Miami, FL) Smart EP platform. Based on the theory of interest, montage, click rate, or click polarity was changed while every other parameter remained within standard ABR settings (i.e., vertical montage, 19.3/s click rate, rarefaction polarity). Wave amplitudes and latencies were compared across the two montages and the two polarities using a Wilcoxon matched pairs signed-rank test. Wave amplitudes and latencies were compared across click rates using a Friedman analysis.

Vertical montage recordings produced larger amplitudes for waves I and V compared to horizontal montage with no change in latencies. ABR measurements between 11.1/s and 19.3/s click rates were negligible, 33.3/s produced variable amplitude and latency effects, and 66.6/s produced decreased amplitudes and increased latencies across all waves. Negligible absolute wave latency differences were found between click polarities, but significant increase in wave amplitudes were observed from the rarefaction polarity. From our review of theory and subsequent findings, we recommend recording neurodiagnostic ABRs in a vertical montage to a rarefaction click at 19.3/s for efficient acquisition of robust waveforms.