Analog and digital filtering of ABR: ipsi- and contralateral derivations

In audioneurological evaluations, peak latency is an important parameter regarding the determination of possible wave delays. Digital filtering entails suppression of less informative low-frequency components without phase distortion, thus accentuating the peak readings. Ipsi- and contralateral recordings have improved the reliability as regards the identification of ABR waves. This applies specifically to the wave IV-V complex. The purpose of this study has been to compare 1) analog and digitally filtered waveforms and 2) ipsi- and contralateral derivations.

Two-channel ABR data were collected from 120 unselected subjects referred for assessment of possible retrocochlear diseases. The analog filter bandwidth was 30 to 3000 Hz. Each response was subsequently digitally filtered with a bandwidth of 300 to 2500 Hz, and a comparison of wave identification between the analog and digitally filtered responses was performed. Wave identification was also compared between the digitally filtered ipsi- and contralateral responses, and the differences of the wave latencies between the two derivations were calculated.

Digital filtering improves ABR wave identification. For the digitally filtered waveforms, a better wave identification is found ipsilaterally for waves I to III, whereas the opposite is found for wave IV. Wave V identification is identical in both derivations. Furthermore, significant ipsi- and contralateral latency difference were found for all waves except for wave IV. This must be taken into consideration if ipsi- and contralateral derivations are to be summed.