Measuring the cochlear blood flow and distortion-product otoacoustic emissions during reversible cochlear ischemia: a rabbit model

Impairment to the cochlear blood flow likely induces many types of sensorin eural hearing loss. Models using several small laboratory animals have been described in the literature that permit the simultaneous monitoring of the cochlear blood flow with laser-Doppler flowmetry and cochlear function usi ng evoked responses. However, these models have not permitted a direct appl ication of the resulting knowledge to the human condition, primarily due to differences in the translucence of the otic capsule between species. In th e present study, to approximate conditions relevant to the human patient, t he rabbit was utilized to develop a procedure in which laser-Doppler flowme try could be used to measure the cochlear blood flow ill an animal with an opaque otic capsule. At the same time, the cochlear function was monitored non-invasively using. distortion-product otoacoustic emissions. In this man ner. a laser-Doppler probe was positioned in the round window niche and the cochlear function measured using distortion-product otoacoustic emissions during a systematic series of ischemic episodes. Cochlear ischemia was prod uced by deliberately compressing the eighth nerve complex at the porus of t he internal acoustic meatus, for periods lasting from 1-3 min, while cochle ar blood flow and distortion-product otoacoustic emission measures were obt ained simultaneously before, during and following the occlusion. Results de monstrated that the cochlear blood flow sharply decreased within 1 s after compression onset, whereas distortion-product otoacoustic emissions showed obstruction-induced changes after a delay of several seconds, provided that the blood flow decreased, at least, 40%. Similarly, upon release of the co mpression, the cochlear blood now began to recover within 1 s, whereas the recovery of the corresponding distortion-product otoacoustic emissions was slightly delayed. Although not apparent in the distortion-product otoacoust ic emission recovery time course, the cochlear blood flow consistently over shot its initial baseline value during the recovery process. Thus, although cochlear ischemia produced changes in the distortion-product otoacoustic e mission activity that generally followed the resulting alterations in the c ochlear blood flow, the detailed relationship between the two measures was complex. (C) 1999 Elsevier Science B.V. All rights reserved.