[Measurement of electrical evoked middle latency response in cochlear implantation operation]

To establish the method of conducting electrical evoked middle latency response (EMLR) monitoring in cochlear implantation operation and further to assess the neural response of auditory pathway under electrical stimulation.

Twenty cases of cochlear implantation subjects were investigated in this study. Fourteen cases were pre-lingual deaf and 6 were post-lingual deaf. The surface recording electrodes were placed on the patients under general anesthesia, with language processor connected to the triggering port of the auditory evoked potential device. After the electrode was implanted, the electrode No.3 was selected to conduct. The electrically evoked auditory nerve compound active potentials (ECAP) were firstly tested in all patients, thereafter the EABR mode was selected, and the stimulation parameters were changed to EMLR mode with monopole biphasic, alternation stimulation, pulse width from 50 to 100 µs, the stimulation intensity decreased or increased from 20 CL above the strength of the ECAP threshold to the reaction threshold with a step of 5CL. To evaluate the correlation between the ECAP thresholds and EMLR, another 6 cases of normal hearing healthy subjects were recruited to record their short-sound evoked auditory middle-latency response (AMLR), as the control of morphology and latency of MLR by electrical stimulation.

The typical AMLR waveforms could be recorded by the composition of five waves in the 6 cases of normal hearing healthy subjects, with an average response threshold of (12.5±8.6) dBnHL, close to the behavioral audiometric threshold (10.8±7.3) dBHL. The EMLR waveforms could be recorded in 20 patients, which was similar to the AMLR waveforms. However, the wave latency and wave interval shortened. There were lower volatility and longer latency in pre-lingual deaf than post-lingual deaf. The EMLR threshold (140.55±9.92) CL was significantly lower than the ECAP threshold (160.75±13.34) CL (t=10.467, P<0.01), a positive correlation between the thresholds was detected (r=0.763, P<0.01).

We successfully established the method of EMLR monitoring in cochlear implantation surgery. The EMLR threshold is lower than the ECAP threshold but it is close to the behavioral audiometric threshold; EMLR can provide neural response information closer to the auditory center, and can serve as an effective objective method to evaluate the effect of hearing rehabilitation.