Discussion
ABR is an objective hearing measurement rarely affected by anesthesia,
sleep and environmental noises. Blackman-gated tone-burst ABR had been
reported to predict the PTA thresholds for a wide range of frequencies
while click evoked ABR only focused on 2kHz to 4kHz PTA thresholds.
Compared to click-ABR, Blackman-gated 1kHz tone-pip ABR worked better in
assessing low-to-mid hearing frequencies which are mainly impaired in
CHL patients. The pre- and intra-fs-ABR only prolonged the surgery for
10 to 15 minutes. Instead of insert earplug, loudspeaker was better in
maintaining sound intensity and aseptic principle.
In terms of different stimuli, both S1 and S3 worked well in OR. The
wave V latency of S3 was longer than that of S1 because longer rise
phase activates less nerve fiber compared to shorter rise phase. In S1,
fs-ABRI and PTAI showed better correlation and in S3, the absolute
intra-fs-ABR value was better in predicting the long-term HI.
Background noise of the air conditioner and anesthesia machine, magnetic
and electronic influence of various machines might affect the
monitoring. Therefore, only the anesthesia machine was left open while
testing. We noticed that the mean hearing threshold of fs-ABR of NH in
OR was 6.08dB higher than that in SPC, but it was not clinically
significant, and would pose little impact on the efficiency of
prediction.
Additionally, the mean bias between post-surgery PTA and intra-fs-ABR
was smaller than that of the pre-PTA and pre-fs-ABR, and the difference
between the two biases were 6.024dB and 14.09dB for S1 and S3
respectively. That meant intra-fs-ABR underestimated the post-PTA for
6.024 and 14.09 in average for S1 and S3 respectively which might be
caused by the following factors: firstly, the revised tympanic membrane
and the underneath cartilage would increase the mass of acoustic
transducer system. It usually took 3-6 months for the cartilage to be
absorbed which was unavoidable. Secondly, blood and serum would pile up
in the external canal during monitoring, which would cause artificial
conductive hearing loss. Necessary steps had been taken to minimize the
impact of local errhysis, like using gelatin sponge with hemostatic at
the external ear canal before testing.
ETD was a known etiology for CHL with a prevalence of 1% in adult and
presented in 70% of patients undergoing tympanoplasty caused by chronic
otitis media or cholesteatoma(10). Most otologists agree that better ETF
is critical for the better long-term HI of middle ear surgery(11). We
also observed that ETD affected the HI in a negative way. ETS-7 had been
demonstrated to be valuable as a diagnostic follow-up method(12).
Subjects with ETS≥7 showed better HI outcome and the coefficient factor
between post-surgery PTA and intra-fs-ABR rose to 0.8, higher than that
of whole S1 (r=0.5). We didn’t find any statistically significant
correlation among parameters in Sub1(ETS<7). Therefore, it is
recommended to pay attention to ETF to optimize the HI.
However, there are several conditions should be taken into consideration
in future work. Firstly, more subjects should be enrolled to test the
feasibility of this method among different kinds of ossiculoplasty.
Secondly, the impact of errhysis on the threshold shift should also be
studied in animal models. Thirdly, an automatic threshold determination
method is now under study and we are trying to further minimize the
testing time to optimize patients’ benefit.