Cochlear function after selective spiral ganglion cells degeneration induced by ouabain

Background Ouabain, a cardiac glycoside that specifically binds to Na/K-ATPase and inhibits its activity, was applied to gerbils to develop a method for studying auditory neuropathy. Methods Ouabain was applied to the round window of the cochlea in each gerbil by using a piece of gelfoam with 3 μl or 24 μl （1 mmol/L） ouabain solution. The changes of the threshold of auditory brainstem response, cochlear function round window electrocochleography, as well as the morphological changes of the spiral ganglion cells of the cochlea were observed after application of ouabain for 24 hours or 96 hours. Results In ouabain treated gerbils, auditory brainstem response and compound action potential thresholds showed either elevation or no response at all. However, the thresholds of cochlear microphonic and distortion product otoacoustic emissions were not affected. Degeneration and necrosis of some spiral ganglion cells in ears with applications of ouabain （24 hours, 3 μl, 1 mmol/L; 96 hours, 24 μl, 1 mmol/L ouabain）. The number of spiral ganglion cells was decreased （24 hours, 3 μl, 1 mmol/L ouabain） or near to a total loss （96 hours, 24 μl, 1 mmol/L ouabain）.Conclusions These results indicate a high degree of independence between the spiral ganglion ceils and the outer hair cell systems in the cochlear transduction mechanism. The method used in this study would provide a valuable tool for studying auditory neuropathy.

Electrocochleoeraphy in cochlear implantation:Development,applications,and future directions

Electrocochleography(ECochG)is an electrophysiological technique that records electrical potentials generated by different components of the inner ear and peripheral cochlear nerve in response to acoustic stimulation.ECochG responses can be analyzed into(1)cochlear microphonics(CM),(2)auditory nerve neurophonics,(3)summating potential,and(4)compound action potential.Over the past few decades,there have been ongoing refinements in technique and updates in the understanding of recorded potentials.Historically,ECochG found its main application in the diagnostic evaluation of Meniere^disease(MD).However,in the last decade,the focus has shifted towards cochlear implantation(Cl).In patients with residual hearing after Cl,combined electric and acoustic stimulation has resulted in improved hearing and speech outcomes.Despite efforts to mitigate trauma during electrode insertion,hearing preservation rates vary after surgery.During implantation,real-time ECochG offers an opportunity to measure frequency specific CMs elicited from a localized region in the cochlea as the surgeon inserts the electrode array.In extracochlear ECochG recordings,the recording electrode can be placed on the promontory,the stapes,or the tympanic membrane.Intracochlear ECochG can be performed by inserting a recording electrode into the cochlea or by using one of the Cl electrodes as the recording electrode.The loss of intraoperative ECochG signal may indicate cochlear trauma from electrode insertion,but the association between intraoperative ECochG changes and cochlear trauma remains controversial.The ability to monitor cochlear trauma during Cl electrode placement holds promise to improve hearing preservation outcomes,modify surgical techniques,and change electrode design.The goal of this review is to provide a comprehensive overview of the electrophysiology and history of ECochG,discuss its recent applications in Cl,and explore the ongoing research in this expanding field.