Intraoperative Monitoring during Surgery in the Cerebellopontine Angle: A New Tympanic Electrode

Abstract

Preservation of hearing has become a goal in surgery of the cerebellopontine angle. Intraoperative monitoring of the brainstem auditory evoked response (ABR) is the standard in identification of operative maneuvers that diminish the integrity of the eighth nerve. Preoperatively, the amplitude of the ABR is often reduced, limiting the evaluation. The electrocochleogram (EchoG) produces potentials nearer to the site of origin; therefore the amplitude is higher in comparison to the ABR. For recording of the EchoG, transtympanic needle electrodes and intrameatal spring electrodes are used. In this study, a new tympanic electrode was designed, which should be noninvasive and easily insertable. Potentials of this tympanic electrode were then compared to the ABR, which was simultaneously recorded. A spiral-spring electrode with a ball at the tip was designed, which was integrated into the earplug of the click-generator. Potentials of this tympanic electrode were then compared to the ABR, where a needle electrode at the mastoid was used. Placing of the electrode onto the tympanic membrane was achieved very easily in five patients. No additional instrument for the placement was needed. The tympanic membrane was never penetrated by the electrode. The amplitude of the compound action potential (CAP) was more than 2.5 times higher than the wave I of the ABR. Fewer responses were needed for averaging; therefore the time for detection of changes in the waveforms was short. The CAP corresponded in a timely manner to the wave I in the ABR. The tympanic electrode is an easy and safe tool in intraoperative monitoring of the eighth nerve. The CAP of the EchoG is generated at the distal end of the cochlear nerve and should display functional changes of the nerve during operation. The wave I of the ABR is often poorly displayed with tumors in the cerebellopontine angle and can persist in modified form even after transection of the eighth nerve. An accurate and secure depiction of the CAP is desirable to increase the prognostic value of the CAP compared with waves III and V of the ABR. The frequent use of the new tympanic electrode will show whether the EchoG can become routine during surgery in the cerebellopontine angle.