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.