Introduction
Patients with conductive hearing loss(CHL) present hearing and
communication hardship which greatly lower their life quality.
Ossiculoplasty has been proved to be the most effective way to improve
hearing and minimize the air-bone gap(ABG). However, post-surgery
hearing improvement(HI) are influenced by several factors, among which
positioning and coupling of ossicular prosthesis are prerequisites of
long-term HI. However, the revision surgery rate remains a risk both for
patients and surgeons ranging from 8%(1) to 13.2%(2). The reason that
long-term HI presents great variety is that it mainly depends on the
surgeon’s subjective judgments.
Various mechanical and acoustical intra-operative measurements of
ossiculoplasty efficiency had been tried: Laser-Doppler Vibrometry(LDV)
(3), electromagnetic field stimulation(4),
electrocochleography(ECochG)(5), auditory brainstem response(ABR)(6),
auditory steady-state responses(ASSR) and Chirp-ASSR(7). Instead of
stimulating the tympanic membrane (TM), mechanical methods directly
tested the ossicular chain(OC) movement which ignored the TM changes
which could lead to the overestimation of the hearing ability(4).
Additionally, nearly all experiments were carried out in cadaver
temporal bones, few of them confirming the usage of these methods in
patients.
ABR is an objective and noninvasive hearing measurement widely used
clinically and is rarely being affected by sleep, anesthesia and
environmental noise. Tone-burst ABR was reported to have a high linear
correlation with PTA threshold and hearing threshold at 1kHz could
estimate low-frequency PTA thresholds accurately(8). Griffith S. Hsu
firstly applied intra-operative ABR in stapedectomy and demonstrated
that ABR monitoring could reduce the revision surgery rate(6). However,
those studies mainly focused on stapes surgery with intact and uninjured
tympanic membrane, few reported eligible follow-up results, and enrolled
limited subjects. Another obstacle is that insert sponge earphones are
not suitable for middle-ear surgeries. On one hand, the sponge will
absorb blood and effusion during surgery, causing the change of stimulus
sound intensity and threshold shift. On the other, CHL often involves
leisured mastoid and TM needed to be repaired, which leaves changed
middle-ear cavity and reconstructed TM after the surgery. The sponge
insert earplug fail to seal the external canal after the reconstruction,
leading to the leakage of the sound intensity and artificial unknown
threshold shift.
In response to these obstacles, we firstly used the loudspeaker to
replace the insert earplug. The Blackman-gated 1kHz tone-pip was
demonstrated efficient and accurate in measuring hearing threshold in
both normal hearing(NH) and CHL patients both in OR and standard
sound-proof chamber(SPC) in our previous work(9). In this paper, we
modified previous fs-ABR system, test its efficiency in predicting the
long-term post-surgery HI intra-operatively and explore the effect of
ETD on post-surgery HI and fs-ABR system monitoring efficiency.