3. Results
In this study, Tone Burst and NB CE-Chirp ABR data of 24 adults (31
ears) were analyzed retrospectively. The tests could not be completed in
both ears, since 17 individuals could not tolerate repeated ABR
measurements for each ears. The mean age of the participants was
detected at 30.4 ± 8.6 (min: 20 max: 48). Test data of 18 ears from 17
men and 13 ears from seven women were analyzed. The research included 18
(58.1%) right ears and 13 (41.9%) left ears. The absolute latencies of
peak V with TB stimuli were significantly longer than latencies obtained
with NB CE Chirp stimuli at 0.5, 1 and 2 kHz at all sound intensity
levels (p<0.001) (Figure 1, 2) . There was no
difference between TB ABR and NB CE-Chirp ABR latencies at 4 kHz-20 dB
nHL (p=0.374) (Table 1) . The amplitudes of peak V detected with
NB-CE Chirp were significantly larger than TB amplitudes in the level of
4 kHz 60 dB nHL (p=0.038). However, there was no significant difference
was detected at other frequencies for the amplitude in both TB and NB
CE-Chirp ABR (p>0.05).
NB CE-Chirp ABR threshold values closer to PTA 0.5 1, 2 kHz thresholds
than the TB ABR thresholds (p<0.001, CI=2.0-5.9 dB)(Table 2) (Figure 3). While the thresholds obtained
with PTA, TB and NB CE-Chirp stimuli were showing the highest
correlation at the 2 kHz, the least correlation between the measurements
was found at 4 kHz (respectively ICC= 0.855 and 0.673), (Table
2). The correction factor, which was calculated from the mean
difference in the paired t-test, between thresholds obtained from NB
CE-Chirp and TB stimuli and BHT determined by PTA is given in Table 3.
In the posthoc power analysis, the power for the difference between the
threshold values at all frequency and all sound intensities was measured
as 90% and above.
There was no significant difference between thresholds obtained from
PTA, TB and NB CE-Chirp for right and left ears at all frequencies
(p>0.05, paired t-test). While the most consistent
(test-retest reliability) threshold value obtained with PTA at 4 kHz
frequency with TB ABR in the right ear (r=0.709, p=0.001), it obtained
at 2 kHz with NB CE-Chirp ABR in the left ear (r= 0.743, p= 0.004). The
estimated thresholds were similarly correlated with PTA in the right and
left ears. The mean test time with NB CE-Chirp ABR was calculated as
23.6 ± 3.9 minutes and the mean test time with Tone Burst ABR was
measured as 28.2 ± 4.5 minutes. NB CE-Chirp ABR test time was
significantly shorter than the TB ABR test time (p=0.011).
4. Discussion
Ferm et al. (16) compared NB Chirp and Tone pip ABR findings at 1 and 4
kHz and 40 dB nHL of 42 ears of 30 infants in the newborn screening
program. As a result of this study, NB CE-Chirp ABR wave V amplitudes
were found to be larger than Tone pip ABR wave V amplitudes (p=0.001).
In 40 children with normal hearing, the findings of the 0.5, 1, 2, 4 kHz
NB CE-Chirp ABR and Tone-Burst ABR were compared at 80, 60, 40, 20 dB
nHL by Rodrigues at al.(12). At all levels except 500 Hz 80 dB nHL, NB
CE-Chirp ABR wave V amplitudes were found to be greater than Tone-Burst
ABR wave V amplitudes (p<0.05).
In our study, only at 4 kHz 60 dB nHL level, NB CE-Chirp ABR wave
amplitudes were found to be greater than TB ABR wave amplitudes
(p=0.038). Similar to our research results, Megha et al. (25) found no
significant difference in any of the frequencies for amplitude parameter
in TB and NB CE Chirp ABR. At high levels of chirp stimulus,
desynchronization develops as a result of overstimulation on the
cochlear basilar membrane. As a result, it is stated that the wave V
amplitudes decrease (25-27). This effect can be observed, especially
when a high stimulus level is sent to subjects with normal hearing (25,
27).
Studies comparing NB CE-Chirp ABR and Tone Burst ABR in the literature
were frequently conducted with infants. For example, Rodrigues et al.
(12) found that NB CE Chirp ABR latencies were shorter than Tone Burst
ABR latencies in infants at 0.5, 1, 2 kHz. In this study; the mean wave
V latency evoked by 500 Hz (60 dB nHL) NB CE-Chirp was found to be 3.57
(SD: 0.70). In this study, longer wave latencies were found compared to
our research. Because the value of the peak V latency varies among
different age person (28). Our study was carried out with adults.
Megha et al. (25) conducted a study that researched the effects of noise
using NB CE Chirp ABR and TB ABR. Similar to our research, in this
study; the mean wave V latency evoked by 500 Hz NB CE-Chirp was found to
be 2.45 (SD: 0.68) in the control group with normal hearing. Especially
at low frequencies, NB CE-Chirp ABR wave V latencies were observed to be
very short. For Tone Burst stimulus, the ABR mean latencies increased
with decrease in frequency. But NB CE-Chirp stimulus, the ABR means
latencies decreased with decrease in frequency. This is explained by
eliminating the delay in the cochlear travelling wave by using different
transmission times for each of the chirp stimulus frequencies. The low
frequencies in the NB CE-Chirp are being presented early than high
frequency octave bands. Thus NB-CE Chirp provides maximum stimulation
into the cochlea (12, 25).
Ferm et al. (16) reported that the NB CE-Chirp ABR thresholds were found
to be lower than the tone pip thresholds. For this reason, it was
emphasized that NB CE-Chirp stimulus can provide closer responses to
behavioral thresholds.
In our study, ABR thresholds with Narrow Band CE-Chirp stimulus at 0.5,
1 and 2 kHz were found significantly closer to the behavioral hearing
thresholds compared to Tone Burst ABR thresholds (p<0.001,
CI=2.0-5.9). Talaat et al. (29) investigated which of the NB CE-Chirp
ABR and Tone Burst ABR thresholds were closer to the frequency specific
behavioral hearing thresholds. As a result, it was stated that NB CE
Chirp-ABR provides higher sensitivity and accuracy than TB ABR in
estimating behavioral hearing thresholds in young children. Van et al.
(30) compared the proximity of NB CE-Chirp ABR and Tone Burst ABR
thresholds to behavioral hearing thresholds in 23 adults with normal
hearing. In this study, it was stated that using NB CE-Chirp is more
reliable than TB ABR in estimating behavioral hearing thresholds.
Similarly, the ASSR thresholds determined by the NB CE-Chirp stimulus
are highly correlated with the behavioral thresholds (0.5, 1, 2, 4 kHz)
defined with PTA according to the TB ABR thresholds (31, 32).
The hearing thresholds specified as dB nHL in ABR is not equal to
behavioral thresholds determined as dB HL. As a result, it is suggested
to use the correction factor when calculating BHT thresholds (dB HL)
from the thresholds (nHL) determined by ABR (25, 28). The correction
factor, which is used to estimate behavioral hearing thresholds with
using of NB CE-Chirp ABR, can be reduced by up to 5 dB (21, 16).
The shortening of the test time with NB CE-Chirp stimulus is another
advantage. ABR amplitudes are not detectable in 30 % of the click
stimulus and need more test time. Click stimulus produces larger and
easily detectable amplitudes. Likewise, the mean test time of NB
CE-Chirp ABR was detected shorter than the mean test time of TB ABR in
our research (p=0.011). Similarly to this study, Zirn et al. (33)
compared the duration of the test conducted with both ABR methods in
their study including 253 children, and as a result, NB CE-Chirp ABR was
found to shorten the recording time. Ferm et al. (16) reported that the
use of NB CE-Chirp stimulation produced larger response amplitudes, and
therefore increase the signal to noise ratio according to TB clicks, so
shortens the test time.