Laser-Doppler measurements and electrocochleography during ischemia of the guinea pig cochlea: implications for hearing preservation in acoustic neuroma surgery.

Interruption of cochlear blood flow has been implicated as one of the causes of the sensorineural hearing loss that may occur during acoustic neuroma surgery. With the guinea pig as an animal model for cerebellopontine angle surgery, laser-Doppler measurements were used to estimate the cochlear blood flow changes caused by compression of the eighth nerve complex. With compression, the laser-Doppler measurements decreased abruptly; somewhat later, the electrocochleographic potentials declined. When compression was released, laser-Doppler measurements usually returned immediately, followed later by return of the electrical potentials. Some of these potentials, including the compound action potential of the auditory nerve, often became transiently larger than their precompression values. Interposing bone between the laser-Doppler probe and the otic capsule, so that the total bone thickness approximated the thickness of the human otic capsule, decreased the laser-Doppler measurement, but changes caused by compression were still apparent. Thus, although the human otic capsule is much thicker than the guinea pig capsule, it may still be possible to make laser-Doppler estimates of human cochlear blood flow. Laser-Doppler monitoring during acoustic neuroma surgery may be beneficial, because it could give earlier warning of ischemia than is currently available from electrocochleographic monitoring, thereby enabling earlier corrective action. Electrocochleography complements laser-Doppler measurements by indicating the physiologic state of the cochlea.