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).