Simultaneous Use of CO 2 Laser with Continuous Monitoring of Eighth Cranial Nerve Action Potential during Acoustic Neuroma Surgery

In a 38-year-old patient with bilateral acoustic neuromas a CO₂ laser mounted on an operating microscope was used to remove the tumor on the right side. The technique of tumor evaporation reduced surgical manipulation. The eighth cranial nerve action potential was continuously monitored with the recording electrode fixed to the proximal trunk of the eighth nerve. Clicks were delivered through an ear mold insert at a sound pressure level of 115 dB at 10 clicks/sec. The preoperative auditory brain stem response (ABR) showed no wave I and a delayed wave V and no response to electrocochleography with a promontory electrode. Tumor removal resulted In an improvement In the abnormal action potential latency from 5.1 msec to 4.32 msec. Overuse of the laser above the cochlear nerve caused a transient reduction in wave amplitude. At the end of the procedure the latency was 4.6 msec, and the amplitude was reduced to 2.3 μV. Audiometry at 1 week showed the pure-tone speech reception threshold reduced from 50 dB preoperatively to 60 dB postoperatively and discrimination reduced from 72% to 52%. There are two advantages to direct monitoring of the eighth nerve action potential: surgical trauma to the auditory nerve can be identified rapidly without averaging as many responses as are necessary when using ABR to monitor surgery; and when there is no preoperative response to electrocochleography or ABR, surgical manipulation can still be guided by direct recording from the eighth nerve. To our knowledge this is the first case in which an acoustic neuroma was removed with a CO₂ laser while the eighth nerve action potential was continuously monitored. The combination of these two new techniques may allow the surgeon to preserve auditory function in a greater percentage of cases.