Auditory Brainstem Response (ABR) Findings With Click and CE-Chirp Stimulations in Noise-Exposed Participants.

Dear Editor,

We read with great interest the article by Pushpalatha and Konadath[1] entitled “Auditory brainstem responses for click and CE-chirp stimuli in individuals with and without occupational noise exposure.” The effort to study the noise-exposed individuals by means of auditory brainstem response (ABR) recorded with click and CE-chirp stimulations should be commended. In general, the results obtained were in line with the findings from previous studies that investigated the effects of noise on the central auditory nervous system. The authors then concluded that the CE-chirp stimulus was more effective than the click in identifying early pathological changes due to occupational noise exposures. Nevertheless, in regard to the reported study findings, we wish to highlight some issues that might be worthy of consideration.

Upward chirp stimulations have been shown to produce more robust ABR waveforms than the conventional click stimuli.[2345] By utilizing the specific duration and bandwidth of chirps, the travel time differences along the cochlear partition are compensated leading to optimized neural synchrony and enhanced ABR waveforms.[23] At similar sensation levels, many studies have demonstrated that the ABR amplitudes to chirps can be as large as twice the amplitudes of ABR evoked by clicks in healthy participants.[235] Many types of chirp stimuli have been constructed and studied in healthy participants,[2345] as well as in participants with hearing losses.[67] Among them, CE-chirp stimulus (in honor of Claus Elberling, Ph.D.) has been extensively studied and is commercially available for clinical applications.

As highlighted in our previous article,[8] the ABR latencies evoked by CE-chirp should be interpreted with caution. When the CE-chirp stimulus is used for recording ABR with the commercially available auditory-evoked potential devices, the earlier ABR latencies are indeed expected, which are not related to enhanced neural synchrony. The onset and offset times of CE-chirp have been adjusted in such a way that the ABR waveforms would show shorter latencies than the conventional click stimulus. As reported elsewhere, the indicator for the enhanced neural synchrony by the chirp stimulation is the increased ABR amplitude, not the shorter latency.[235] In fact, when ABR is recorded with an unmodified chirp (i.e., the original chirp without the temporal adjustment) and click, bigger ABR amplitudes and more prolonged ABR latencies are produced by the chirp stimulus.[2] Pushpalatha and Konadath[1] performed sufficient statistical analyses, wherein both intra- and intergroup comparisons were made. The intergroup analyses (between normal and noise-exposed participants) on ABR latencies for each stimulus were appropriate. However, comparing the ABR latencies between click and CE-chirp stimulations for each group (intragroup comparison) may not be beneficial in regard to the aforementioned latency issue of chirp.

In addition, the ABR results obtained with CE-chirp at high-intensity levels should be analyzed with care. As repeatedly demonstrated elsewhere, the performances of chirp-evoked ABR are optimum at low stimulation levels.[2345] At high-intensity levels (>60 dB nHL), an upward spread of excitation would occur resulting in the desynchronization of neural excitation, and thus, ABR waveforms with poorer morphology are produced.[9] In fact, at high stimulation levels, the click stimulation would produce more robust ABR waveforms than the chirps.[29] As an effort to overcome this limitation, level-specific CE-chirp stimuli have been constructed and are also available commercially for clinical use.[9] In the study by Pushpalatha and Konadath,[1] for CE-chirp stimulation, the high percentage of absent waves I and III, as well as the high variability of latency data (in both normal and noise-exposed groups), might be related to the use of high stimulation level (80 dB nHL) in the ABR recording.

Nevertheless, the authors found significant wave III abnormalities in the noise-exposed participants for the click stimulation, suggesting that the cells of the cochlear nucleus could be compromised when exposed to noise. This finding is promising and consistent with the previous reports. In line with this, Lesperance et al. [10] found that the average cell area of the rostral anteroventral cochlear nucleus was 22% smaller in the noise-exposed ear than in the normal hearing ear of guinea pigs. This result, however, was only seen when both the outer and inner hair cells were damaged. It is clear that more studies on humans are required before a concrete conclusion can be made in this area of research. This issue is perhaps worth discussing to further educate the readers.

Finally, we agree with the authors that latency is a more reliable measure than amplitude in ABR recording. However, based on the latency issue highlighted earlier, again, the latency aspect of ABR evoked by CE-chirp stimulus should be interpreted with care.

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Conflicts of interest

There are no conflicts of interest.