The Verification of ABR Response by Using the Chirp Stimulus in Moderate Sensorineural Hearing Loss

Med. J. Cairo Univ., Vol. 81, No. 2, September: 21-26, 2013 www.medicaljournalofcairouniversity.net The Verification of ABR Response by Using the Chirp Stimulus in Moderate Sensorineural Hearing Loss SOHA M. HAMADA, M.D.; SALWA M. ABDEL LATIF, M.D. and HODA I. ABUMOUSSA, M.D. The Department of Audiology, Hearing & Speech Institute, Giza, Egypt Abstract Background: The Brainstem Auditory Evoked Potentials (BAEP) are historically recorded using clicks. An alternative to the click have been developed aiming to a simultaneous neuronal activation along the basilar membrane, and, consequently, the recording of responses with larger amplitude, this stimulus, named chirp. Objectives: Assessment of ABR responses in adults subjects with normal hearing sensitivity and in patients with moderate degree of SNHL by using chirp stimuli. Comparing the results of chirp ABR with those obtained from the same individuals using click stimuli. Methodology: Thirty young adult subjects ranging from 20 to 40 years with normal hearing sensitivity (group 1) and thirty subjects of same age group with moderate degree of SNHL (group 2) were evaluated. ABR recorded using low frequency (1000Hz) and high frequency (4000Hz) chirp stimulus and the results were compared with those obtained by click stimulus presented monaurally. Results: In this research we found a statistically significant difference as regards wave V latency between click, low frequency and high frequency chirp in all test conditions in subjects with normal hearing sensitivity. Also there is a statistically significant difference as regards wave V latency between click and high frequency chirp stimulus at both stimuli level used in patients with moderate degree of SNHL with shorter latencies by chirp stimulus. Shorter latency of wave V is found with high frequency chirp than low frequency chirp in group 1 at 10SL and in group 2 at 10 and 20SL. As regards wave V amplitude a statistically significant difference between click and chirp in all conditions in (group 1) and a statistically significant difference was found as regards wave V amplitude between click and chirp in all conditions except between click and low frequency chirp at (group 2). Wave V amplitude is larger with high frequency chirp in both groups. Conclusions: Response to chirp stimulus is characterized by shorter latency when compared to click stimuli and latency of wave V is shorter with high frequency chirp than with low frequency chirp. Wave V amplitude is larger with chirp stimuli than those obtained with click and amplitude of wave V is larger with the high frequency chirp than low frequency chirp. Key Words: Chirp stimulus ABR Frequency specific chirp. Correspondence to: Dr. Soha Mohamed Hamada, E-mail: sohahamada@hotmail.com Introduction THE Brainstem Auditory Evoked Potentials (BAEP) are historically recorded using clicks that, due to their sudden start and broadband composition, supposedly activate synchronically a wide region of the cochlea [2]. However, studies have shown that the response to the click is not totally synchronized. When a click reaches the basilar membrane, the resulting sound wave takes a considerable amount of time to reach the apex of the cochlea, from the base. The response peak occurs milliseconds later in the low frequency regions than in the higher frequency regions. For this reason, the basilar membrane cells are not stimulated at the same time, resulting in an asynchronous depolarization of neurons. More time is needed for the low frequency region to attain the maximum dislocation reached at the base of the cochlea, in such a way that all neurons along the basilar membrane are depolarized simultaneously [2-5]. These observations evidenced negative points of the click stimulus in registering the BAEP, but also suggested points that could be modified in the elaboration of new stimuli. An alternative to the click have been developed aiming at a simultaneous neuronal activation along the basilar membrane, and, consequently, the recording of responses with larger amplitudes [2-5]. This stimulus, named chirp. The concept of the chirp was first applied to auditory electrophysiology in 1985 [5] and has since been studied intensively for its use within the auditory field [4]. Claus Elberling and a group of researchers have been developing several studies with the aim to elaborate a chirp model that compensates the temporal dispersion of the sound wave in the cochlea from equations based on BAEP latencies recorded in humans. Patented as CE-chirp in honor of Claus Elberling, this stimulus was elaborated based on real models of the travel time of the sound wave 21

22 The Verification of ABR Response by Using the Chirp Stimulus in the human cochlea [4,6,7]. It has been shown repeatedly that a chirp is more efficient than a corresponding click in the recording of ABR, and the auditory steady-state response (ASSR). The results found by Elberling et al., (2007) showed that a chirp is a more efficient stimulus than a click for the recording of early auditory evoked responses in normal hearing adults using transient sounds at a high rate of stimulation [4]. Wegner and Dau [8] results demonstrated the importance of cochlear processing for the formation of ABR. Moreover, they found that the broadband chirp elicited a larger wave-v amplitude than the click in the unmasked condition as well as in all masking conditions where high-pass noise was presented in addition to the signal. The results demonstrated that the increased synchrony obtained with the chirp stretches over the entire frequency region. The chirp may be particularly interesting for clinical use in the low-frequency region below about 0.5-1kHz. The drop in response amplitude of the chirp ABR over the click ABR for higher levels of stimulation in normal hearing individuals implies that there might be an upper level of stimulation beyond which the chirp is no longer more effective than the click. From the latency behavior of the chirp ABR, it is suggested that the chirp emphasizes individual characteristics more than the corresponding click [9]. A chirp that evokes the largest broadband ABRs in normal-hearing subjects changes in duration with level. That is, being relatively short at higher levels (50dB nhl) and relatively long at lower levels and near the threshold. However, the changes in amplitude in response to chirps of different durations are not very large, and it is therefore uncertain whether the outcome from using such chirps actually would outweigh the instrumental complexity of implementation. It appears that the largest advantage of using the chirp over the click is found at the lower levels of stimulation [10]. Aim of the work: Assessment of ABR responses in adults subjects with normal hearing sensitivity and adults patients with moderate degree of SNHL by using click and chirp stimuli. Comparing the results of chirp ABR with those obtained from the same individuals using click stimuli. Subjects and Methods The subjects of this study were two groups: Group (1) consists of thirty young adult subjects ranging from 20 to 40 years (mean=28 years) with normal hearing sensitivity, their average pure-tone hearing thresholds <20dBHL and Group (2) consists of thirty patients with moderate degree of SNHL, their average pure-tone hearing thresholds 50dBHL across frequencies from 250 up to 8000Hz of same age group (mean=36 years). They were selected randomly from outpatient clinic of hearing and speech institute after exclusion of patients complicated with element of conductive hearing loss. The effects of any degree of hearing loss will be compensated by presenting the stimulus at 20 and 10dBSL (relative to behavioral threshold). Equipment: Clinical Audiometer Interacoustics model AC40. Immittancemeter Ineracoustics model AZ 26. ERA, Interacoustics model Eclipse using both the click stimulus and chirp stimulus. Ethical aspects: A written consent was signed by all subjects showing their acceptance regarding participation in this study. Each subject was informed about all steps and any possible side effects. The research was approved by the ethical committee of the general organization of teaching hospitals and institutes. All subjects in this research were submitted to the following: Full history taking. Otologic examination. Basic audiological evaluations (Pure tone audiometry, Speech audiometry & Immittacemetry). ABR recorded using low frequency (1000Hz), high frequency (4000 Hz) chirp stimulus and click stimulus presented monaurally using insert earphones EAR TONE. With the subjects in relaxed state, Reference electrodes were placed on the right (A2) and left (Al) mastoids, and the active (Fz) and ground (Fpz) electrodes. The recording was conducted only with impedance below 3kg. Filters of 100Hz to 3kHz were used during the recording. were presented at a rate of 21.1 stimuli per second in rarefaction polarity. Chirp stimulus were presented at a rate of 44.1 stimuli per second using alternate polarity. The Brainstem Auditory Evoked Potential (BAEP) recording started with click stimulus. After the click stimulus, the BAEP recording with chirp stimulus was initiated on the same ear using the intensity of 20 and 10SL. Then the same recording order was conducted on the opposite ear. The amplitude and latency of recordings were determined by visual identification of the wave V. Statistical analysis of the amplitude

Soha M. Hamada, et al. 23 and latency data was carried out using SPSS system (Statistical package for social sciences) (versionl6), IBM Corporation, USA. Results In this research we compared the ABR responses elicited in normal hearing subjects and in patients with moderate degree of SNHL by both click and chirp stimuli. The wave V peak-to peak amplitude was measured from the wave V peak to the next trough (in gv) and wave V latency was determined by the wave V peak (in msec). It was assumed that responses from each ear of a subject could be treated independently, so the results from right and left ears were pooled (n=60 ears for each test). Mean and standard deviation of wave V latency in group 1 and group 2 are shown in (Table 1). Shorter latencies of wave V is noticed with chirp stimuli (Figs. 1,2). Tables (2,3) show the comparison of wave V latency between click, low frequency and high frequency chirp stimulus in group 1 and group 2 respectively. Paired sample t-test shows a statistically significant difference as regards wave V latency between click, low frequency and high frequency chirp in all test conditions in group 1 (Table 2). However paired sample t-test shows a statistically significant difference as regards wave V latency between click and high frequency chirp stimulus only at both stimulus level in group 2 (Table 3). Shorter latency is more evident with high frequency chirp than low frequency chirp in group 1 at 10SL and in group 2 at 10 and 20SL (Tables 2,3). Table (4) shows mean and standard deviation of wave V amplitude in group (1) and group (2). Wave V amplitude is larger with chirp stimuli than those obtained with click in both groups (Figs. 3,4). Tables (5,6) compare between wave V amplitude elicited by click, low frequency and high frequency chirp stimulus in group 1 and group 2 respectively. Paired sample t-test shows a statistically significant difference as regards wave V amplitude between click and chirp in all conditions in group 1 (Table 5). Paired sample t-test shows a statistically significant difference as regards wave V amplitude between click and chirp in all conditions except between click and low frequency chirp at in group 2 (Table 6). Wave V amplitude is larger with high frequency chirp in both groups (Tables 5,6). We noticed that wave V morphology is better in most of tested subjects when using chirp stimulus as compared to click stimulus. Table (1): Mean and standard deviation of wave V latency (msec) in group 1 and group 2. Stimulus Intensity Group 1 Group 2 7.102±0.303 5.740±0.472 6.105±0.274 5.595±0.139 Low frequency 6.450±0.571 5.607±0.523 chirp 5.567±0.750 5.405±0.433 High frequency 6.356±0.663 5.082±0.287 chirp 5.589±0.792 4.482±0.114 8 7 6 5 4 3 2 1 0 o El / Mean ` Standard Deviation 4Y 4Y 60 6 6,6 \P. 4, Ace.. - 0 1 Fig. (1): Mean and standard deviation of wave V latency (msec) in group 1. 6-5 4 3 2-0 Mean Standard Deviation) + 4 Av 44 44 N s 6 0 Fig. (2): Mean and standard deviation of wave V latency (msec) in group 2. Table (2): Comparing between wave V latency with click, in group 1. Stimulus Low frequency chirp Low frequency chirp High frequency chirp High frequency chirp Mean±S.D. 7.102±0.303 6.450±0.571 6.105±0.274 5.567±0.750 7.1.02±0.303 6.356±0.663 6.105±0.274 5.589±0.792 0.029* 0.032* 0.023* 0.034* as regards wave V latency between click, low frequency and high frequency chirp in all test conditions.

24 The Verification of ABR Response by Using the Chirp Stimulus Table (3): Comparison between wave V latency with click, in group 2. Stimulus Mean±S.D. P Low frequency chirp 5.740±0.472 5.607±0.523 0.646 frequency chirp Low High frequency chirp High frequency chirp 5.595±0.139 5.405±0.433 5.740±0.086 5.082±0.287 5.595±0.139 4.482±0.114 0.654 0.024* 0.001 * as regards wave V latency between click and high frequency chirp stimulus at 10 and. Table (4): Mean and standard deviation of wave V amplitude (t.tv) in group 1 and group 2. Table (5): Comparing between wave V amplitude with click, in group 1. Stimulus Low frequency chirp Low frequency chirp High frequency chirp High frequency chirp Mean±S.D. 0.270±0.133 0.487±0.193 0.395±0.217 0.522±0.321 0.270±0.193 0.545±0.273 0.395±0.217 0.586±0.291 P 0.022* 0.034* 0.015* 0.021 * as regards wave V amplitude between click and chirp in all conditions. Stimulus Intensity Group 1 Group 2 Table (6): Comparing between wave V amplitude with click, in group 2. 0.270±0.133 0.364±0.125 0.395±0.217 0.478±0.108 Low frequency chirp 0.487±0.193 0.498±0.150 0.522±0.321 0.570±0.132 High frequency chirp 0.545±0.273 0.606±0.180 0.586±0.291 0.646±0.233 Stimulus Low frequency chirp Low frequency chirp High frequency chirp High frequency chirp Mean±S.D. 0.364±0.125 0.498±0.150 0.478±0.108 0.570±0.132 0.364±0.077 0.606±0.180 0.478±0.108 0.646±0.233 0.043* 0.053 0.011 * 0.034* as regards wave V amplitude between click and chirp in all conditions except between click and low frequency chirp at. 0.7 0.6-0.5-0.4 0.3-0.2 0 44 / 44 v, t4 i r e' r < h6 '. vciks. 4>y Fig. (3): Mean and standard deviation of wave V amplitude (t.tv) in group 1. Mean MI Standard Deviation) - a=9 1 4.6' -0 7 4 117 4 4 6 Fig. (4): Mean and standard deviation of wave V amplitude (t.tv) in group 2. Discussion This study measured ABR responses elicited by chirp stimuli (1000 & 4000Hz) in adults subjects with normal hearing sensitivity and in patients with moderate degree of SNHL. Comparing the results of chirp ABR with those obtained from the same individuals using click stimuli. A recent study that evaluated the auditory brainstem responses (ABRs) evoked by chirps of different durations (sweeping rates) 1111, the study demonstrated that shorter duration of chirp stimuli are most efficient at higher levels of stimulation whereas longer duration of chirp stimuli are most efficient at lower levels. In this research we used chirp stimulus with rates (44.1) stimuli per second. Mechanisms other than the traveling wave delay, in particular, upward spread of excitation and changes in cochlear-neural delay with level, are suggested to be responsible for these findings. As the click ABR threshold has been shown in adults

Soha M. Hamada, et al. 25 and children to correlate most strongly with the best behavioral threshold in 1000 to 4000Hz frequency range [14], we used in our work chirp at 1000 and 4000Hz and compared the results with those obtained with click ABR. The octave-band chirps are constructed by decomposing a broadband chirp, and constitute a subset of the chirp. The delay compensations of the proposed model are similar to those found in the previous experimental study, which thus verifies the results of the proposed model. Another study was done that used band-limited chirp stimuli that compensate for frequency-dependent cochlear delays. They found that wave V amplitude increased and latency decreased as stimulus frequency increased. The decrease in latency is consistent with high-frequency responses arising from basal regions of the cochlea Hi. These results matched with our findings as regards amplitude and latency of wave V in both groups. Petteri found that in normal-hearing subjects, the chirp latencies were either approximately the same or smaller than the click latencies. Based on their results from both normal-hearing and SNHL subjects, Significant differences between corresponding chirp and click were found only in three cases: In amplitudes between the 60dB chirp and click; in latencies between the 40dB chirp and click; and in latencies between the 60dB 4kHz NB chirp and tone burst TB. Shorter latencies have been reported for the chirp stimulus [9]. These results corresponding with our results as we found a statistically significant difference as regards wave V latency between click, low frequency and high frequency chirp in all test conditions in group 1. Also we noticed a statistically significant difference as regards wave V latency between click and high frequency chirp stimulus at both stimuli level in group 2 with shorter latencies by chirp stimulus. As regards wave V amplitude a statistically significant difference was found between click and chirp in all test conditions in group 1. We noticed a statistically significant difference as regards wave V amplitude between click and chirp in all conditions except between click and low frequency chirp at that was found in group 2 with larger amplitude elicited by chirp stimulus. Recent researches for CE-Chirp observed that, there were a significantly larger amplitudes of wave V with chirp than those observed with clicks for all intensities, except at 80dBnHL [13]. These results are expected and similar to previous studies that compared chirp and click stimuli in the BAEP recording [4,6,7]. Some authors observed that, wave V amplitude increases with the CE-Chirp stimulus and decreases with the click stimulus when the intensity level decreases from 80 to 60dB. Studies have suggested that these findings are due to the fact that, with the CE-Chirp stimulus, there is a broader propagation of the sound wave in the cochlea at high intensities, so that regions that would not respond are just stimulated. This results in an overstimulation, which reduces the response amplitude [3,6,8]. Petteri, also found a significant dependency of both wave V amplitude and latency on the stimulus intensity for chirp and click stimuli. Our study revealed same results as the amplitude of wave V decrease with lowering the intensity in case of click, low and high frequency chirp in both groups. When the presence/absence of waves I, III and V was analyzed at high intensities, both waves I and III tended to disappear with the CE-chirp stimulation as we noticed in our research. It is worth mentioning that waves I and III have great diagnostic value in the BAEP recording [14]. Conclusions: Response to chirp stimulus is characterized by shorter wave V latency when compared to click stimulus and latency of wave V is shorter with high frequency chirp than with low frequency chirp. A statistically significant difference of wave V latency was noticed when comparing between click, low frequency and high frequency chirp in all test conditions in normal hearing subjects. While in patients with moderate degree of SNHL a statistically significant difference was found between click and high frequency chirp stimulus. Wave V amplitude is larger with chirp stimuli than those obtained with click and amplitude of wave V is larger with the high frequency chirp than low frequency chirp. A statistically significant difference of wave V amplitude was found between click and chirp in all conditions in normal hearing group and in patients with moderate degree of SNHL except between click and low frequency chirp at 20dB SL. Recommendations: Chirp stimulus could be used for ABR recording in order to obtain better wave morphology. Further study of the frequency specific chirp stimulus for more correlation of ABR threshods and behavioral thresholds.

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