There are two
studies were conducted to compare click-ABR and tone burst-ABR with ASSR in
issues need to be considered with the comparison of ABR and ASSR which includes
frequency specificity, response generators, the effects of hearing loss, and
automatic detection algorithms. The subjective response detection and the lack
of frequency specificity are found to be a limitation in ABR. Steady-state
response is evoked by pure tones that are amplitude and/or frequency modulated.
The frequency of modulation is linked to the neural generators of the ASSR. The
presence of ASSR is determined in large part by the integrity of the cochlea
and eighth nerve.
STUDY1: COMPARISON OF ASSR WITH ABR RESULTS
Cases of near-normal
to profound hearing loss were reviewed after behavioral, c-ABR, and ASSR
threshold tests were completed. c-ABR Threshold
was determined by visual inspection of the waveform. ASSR Threshold was
determined as the lowest stimulus level that met the phase-locked response criterion.
Results: The results show that both c-ABR and ASSR have
strong and statistically significant correlations with the pure-tone audiogram
in infants and children with various degrees of hearing loss and can be used to
predict pure-tone threshold. The correlation of ABR with pure-tone threshold
was higher than for ASSR at 1 and 2 kHz, whereas at 4 kHz, the correlations
were identical. There was a higher correlation between ASSR and behavioral
thresholds at 500 Hz compared to the c-ABR. The discrepancy between behavioral
and ASSR thresholds were smaller than for ABR.
Discussion: Several factors may contribute to these results. First, clicks
evoke a more synchronous neural response and wide pattern of excitation along
the basilar membrane than modulated tones. In contrast, the rise times of the
individual modulation envelopes in a modulated tone are prolonged relative to
the click. The results reinforce the utility of c-ABR for indicating the
presence of hearing impairment in the 1-to 4-kHz range.
COMPARISON OF ASSR AND TONEBURST-EVOKED ABR.
There is one
problem regarding response detection, that is ASSR is based on an automatic
detection algorithm, whereas the tb-ABR is based on observer inspection of
waveforms. A comparison of ASSR and tone-ABR threshold in normal-hearing adults
using the PC algorithm for ASSR detection and the Fsp
algorithm for ABR detection was undertaken.
Results: tone-ABR and the modulated tone-evoked ASSR thresholds were
similar when both were detected with an automatic detection algorithm and that
threshold estimates varied with frequency, stimulus rate, and detection method.
Differences in threshold for the Fsp-ABR thresholds and 41-Hz ASSR thresholds
were not significant.
Discussion: The study showed that we can use both tone-ABR and ASSR to
estimate threshold of hearing, but the stimulus used and detection method affect
the threshold determination. The advantages of algorithm-based response
detection are substantial. Techniques such as Fsp offer a means for making
response detection and threshold estimation from ABR objective. The present
data and some studies suggest that there are no significant differences in
threshold determination between the two techniques. While other studies showed an
advantage for ASSR in the determination of residual hearing for those with
severe-to profound and profound hearing losses.
Strengths: Comparison of the results of current studies with previous
studies. The first study talked about the correlation between ABR, ASSR, and
behavioral thresholds unlike previous studies talked about the correlation of
one electrophysiological test and behavioral test. The findings of the second
study are in good agreement with those of Rance and colleagues (previous
study). The behavioral test results were reliable.
Weaknesses: In the first study The genders of the participants in first study
were not specified. Behavioral threshold measures and evoked potential
thresholds were separated in time by weeks, sometimes months which resulted in
a possibility of progression in hearing loss for some infants. Degrees
of hearing loss were highly variable. In the first study, the
correlations between behavioral test and electrophysiological test were not
investigated on profound cases because they were excluded. In the second study,
only on ear was tested.