Time course of endolymph volume increase in experimental hydrops measured in vivo with an ionic volume marker

Abstract

A new method has been developed to measure the cross-sectional area (CSA) of scala media in the living cochlea. The method has some advantages over histological methods, in which tissues may shrink or move during processing. In the present study, scala media CSA was measured in the second turn of guinea-pig cochleas in which endolymphatic hydrops was induced surgically. The area measurement method used an iontophoretic injection of a volume marker into scala media, during which the concentration of marker in endolymph was monitored with an ion-selective microelectrode. The measured marker concentration was inversely proportional to the CSA of endolymph.The marker we used was the anion arsenic hexafluoride (AsF − 6), which was almost ideal for the purpose as it was retained well in endolymph. Area was measured in normal animals and in hydropic animals at times from 4 days to 16 weeks after endolymphatic duct obstruction. The results showed that hydrops develops within days of ablation of the endolymphatic duct. The degree of hydrops was compared with electrophysiological measures of function, including the endocochlear potential, action potential thresholds and the amplitudes of the cochlear microphonic, summating potential and action potentials. In the initial stages of hydrops development, electrophysiological changes were small. In contrast, there were marked functional changes between 8 and 16 weeks, when endolymph volume was no longer increasing. These data support earlier studies which have shown that dysfunction in the hydropic cochlea does not correlate directly with the degree of endolymphatic hydrops. If the same is true for dysfunction in the ears of patients with Ménière's Disease, then it may not be possible to restore normal function simply by alleviating the hydrops. Furthermore, if dysfunction depends on factors other than the degree of hydrops, it may be possible to alleviate symptoms in ways other than by directly reducing hydrops.