Recording Procedure of ECochG

Bio-logic® Navigator Pro auditory evoked system 7.0.0 (Natus Medical Inc., San Carlos, CA, USA) was used for electrocochleography. The extra-tympanic electrode connecting a bronze foil was self-made. The bronze foil was chosen due to its availability and lower price than the gold one in order to reduce the cost of routine clinical expenses. Besides, bronze has better electric conductivity, which may improve the acquisition of small signals of the ET electrode compared with the tympanic electrode (29). Furthermore, the placement of a TM electrode on the tympanic membrane must be completed under an otoscope, which is a little difficult and time-consuming for audiologists. ET electrode could reduce the discomfort and made this test more acceptable for patients as well. The reference electrode was placed on the mastoid of the non-tested ear, whereas the ground electrode was placed on the forehead. Prior to electrode placement, all electrode sites were treated with an alcohol wipe and a lightly abrasive skin prepping gel using a cotton swab to ensure maximum adhesion and conductivity. Prior to placing the earphone into the ear canal, the ET electrode was covered with the Ten 20 conductive electrode paste and then surrounding the ER-3A insert earphone. The impedance of the ET electrode was kept below 5 kΩ. The other electrode impedances were kept below 1 kΩ. Alternating click stimuli of 100 μs duration at a rate of 11.3/s at 90 dBnHL of hearing level were presented monaurally. Intensity delay of 0.8 ms and the filter setting of 0.1–3 kHz were used. For ensuring the good quality of the responses, the stimuli were presented with 2,000 sweeps. For adequate visualization, the acquired responses were amplified 100,000 times. The SP and AP data were obtained, and each waveform was replicated at least two times for each subject. During the testing, the examinee lay comfortably on the bed in a soundproof room in the Diagnosis and Treatment Center of Hearing Impairment and Vertigo in Xinhua Hospital.