| Objective:To establish an auditory evoked potential monitoring system in ossicular chain reconstruction,to achieve intraoperative hearing threshold assessment,and to explore an intraoperative method to evaluate the effect of ossicular chain reconstruction.Methods:Based on the characteristics of the ossicular chain reconstruction cavity,a local sound field(LSF)that is closely coupled with the sound tube of an insert earphone was developed;a sound level meter was used to perform physical calibration of both a pure tone signal and a Chirp signal of the LSF,and the pure tone and Chirp sound hearing thresholds of normal hearing subjects(20 people,40 ears)were tested.A psychophysical calibration of above signals of the LSF was conducted,and the calibration results from the physical and psychophysical methods were summarized to adjust the output of the device to make a sound field earphone(SFE).The interaural attenuation of the SFE was tested with a unilateral complete deafness patient model(10 people).An auditory evoked potentials analyzer made by Russian Neurosoft Ltd.(Ivanov,Russia)and the SFE were used to measure the Chirp-evoked auditory steady-state responses(Chirp-ASSRs)of patients(7 cases,15 ears)with conductive hearing loss after anesthesia but before surgery to compare the intraoperative ASSR response and preoperative pure tone hearing thresholds of patients with different degrees of hearing loss and to test the correlation between the response threshold under anesthesia and the preoperative hearing threshold;Regression analysis of the relationship of intraoperative ASSR threshold improvement and the postoperative auditory threshold satisfactory.Results:This LSF was a conical earphone with a height of 20 cm and an opening radius of 4 cm;the respective attenuations of various frequencies(0.5,1,2,and 4 kHz)of pure tone from the sound entrance to the sound exit of the LSF were 44.2,32.4,24.0,and 13.9(physical calibration with a sound level meter,dB)and 46.4±6.3,33.0± 4.8,22.7 ± 3.9,and 16.4 ± 7.2(normal hearing subjects with psychophysical calibration,dB).The respective attenuations of various frequencies(1,2,4 kHz)of Chirp signals were 28.4,25.4,and 13.9(physical calibration with sound level meter,dB)and 30.2± 10.6,26.6 ±8.5,and 16.1 ± 6.4(normal hearing subjects with psychophysical calibration,dB).Based on the attenuation of the LSF,the sound energy output was adjusted,and the SFE was made.The respective interaural attenuations(dB)of the frequencies(Hz)of 0.5,1,2,and 4 kHz of the LSF were 37.5 ± 4.2,44.5 ± 5.5,44.0 ± 6.6,and 42.5 ± 4.9,with an average of 42.1 dB,The response threshold of Chirp-ASSR under anesthesia had a strong correlation with the hearing threshold of pure tone;the Pearson coefficients of various frequencies(1k,2k,and 4 kHz)were 0.56(P = 0.03),0.82(P<0.001),and 0.90(P<0.001),respectively,and the intragroup correlation coefficients were 0.70(P = 0.02),0.90(P<0.001),and 0.95(P<0.001),respectively.The average test time was 7.0 ± 0.7 min.Intraoperative thresholds variation guarantee the postoperative auditory improvement.Conclusion:When the SFE,which was obtained using our self-developed LSF and calibrated acoustic equipment,is combined with an intraoperative test of the Chirp-AS SR response threshold,an objective hearing threshold assessment under anesthesia can be performed accurately to achieve an intraoperative evaluation of the results of ossicular chain reconstruction and the intraoperative thresholds advancement can accurately predict the postoperative auditory improvement. |
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