Undetectable pressure increase during induction of acute endolymphatic hydrops by microinjection.

DeMott, J.E. and Salt, A.N.

Abstract

Prior studies by have shown that injection of artificial endolymph into the cochlea at rates less than 60 nl/min for periods up to 15 min produced acute endolymphatic hydrops and /or basally- directed flow of endolymph (Salt & DeMott ARO 19: #543, 1996). In the present study, microinjections into the endolymphatic or perilymphatic spaces were performed while hydrostatic pressure was monitored simultaneously in both compartments using micropressure instrumentation. Time averaging was performed to reduce background pressure fluctuations due to respiration. During injections, virtually identical pressure changes were recorded in both the endolymphatic and perilymphatic compartments. Digital subtraction of the traces demonstrated no detectable change in pressure of endolymph with respect to perilymph during injection into endolymph. It was estimated that less than 0.1 mm Hg pressure increase of endolymph with respect to perilymph occurred while acute hydrops was produced. These results demonstrate that the terms endolymphatic hydrops and endolymphatic hypertension are not synonymous. In the normal animal, the boundary membranes of the endolymphatic system appear to be highly compliant so that pressure differences across the membranes are extremely low. These results are in agreement with those of Takeuchi et al. (Ann. Otol. Rhinol. Laryngol. 100, 244, 1991) in which larger volumes of artificial endolymph were injected more rapidly than in the present study. The predominantly basally-directed endolymph flow observed during endolymphatic injections in our prior study indicates that a structure outside the cochlea, probably the saccule or endolymphatic sac, is even more compliant than the structures bounding scala media in the cochlea.

Study supported by NIH/NIDCD DC01368

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