4 | DISCUSSION
This study was designed to retrospectively investigate whether ASSR can
be a prognostic indicator of hearing outcome in patients with SSNHL.
Previous studies reported that mean threshold differences between ASSR
testing and PTA results at different frequencies did not exceed 15
dB.16-19 Thus, if the patients had a ≥ 15 dB HL of
worse ASSR predicted threshold at least one of 0.5, 1, 2, and 4 kHz
frequencies compared to pure-tone threshold, they included in the WASSR
group. Also, for a reliable measure of ASSR thresholds, the ASSR
measurements cannot be made for stimulus levels (on average) at and
above 100 dB HL.15,20 For this reason, subjects with a
≥ 90 dB HL of pure-tone averages were excluded in grouping.
The main finding of the present study is that the worse ASSRs on the
completion of high dose steroid treatment for SSNHL might predict
unfavorable hearing outcome. Although the WASSR group and SBASSR group
had similar initial pure-tone thresholds (Table 1), WASSR group had
significantly worse last pure-tone thresholds compared to SBASSR group
(Fig. 2-A). The proportion of patients with a worse ASSR in the hearing
improved group was only 21%, whereas the proportion in non-improved
group was 60% (Table 2). In univariate and multivariate logistic
regression, patients in the WASSR group were significantly less likely
to have more than 15 dB HL of hearing gain or better than 25 dB HL of
final hearing thresholds than patients in SBASSR group (Table 2).
ASSR represents the synchronous discharge of auditory neurons in the
brainstem, phase locked to the modulation frequency of the stimulus. The
energy in the resultant response is at the modulation frequency and its
harmonics, allowing response detection using automatic and objective
analysis protocols.21-23 Clinically, ASSR can be used
to objectively estimate the frequency-specific hearing thresholds in
individuals with normal hearing sensitivity and with various degrees and
configurations of sensorineural hearing loss.17,19Although ASSR has good predictive value for behavioral hearing
thresholds, there were no significant correlations between the
behavioral thresholds and ASSR derived thresholds at the majority of the
frequencies (1000, 2000, and 4000 Hz) in patients with auditory
neuropathy.24,25 ASSR derived thresholds were
substantially worse than behavioral thresholds in such patients, and
these findings seem to support the notion that poor synchronization is
the cause of poor neural responses. While it is unknown whether neural
damage is worse than hair cell damage in relation to recovery, previous
study reported that wave I latency of auditory brainstem response (ABR),
which is the recording waveform of the auditory nerve, was significantly
correlated with hearing outcomes in patients with
SSNHL.14 Taken together, a worse ASSR might reflect
neural damage of auditory nerve and a worse ASSR than PTA result can be
considered as a one of the poor prognostic factors in SSNHL.
A limitation of this study is that patients with profound hearing loss
were excluded from the analysis. The second limitation is that the last
follow-up timing was inconsistent. Although the duration of follow-up
was similar between WASSR group and SBASSR group, 19 patients had last
audiogram within one month (5 patients in the WASSR group and 14
patients in the SBASSR group).