Evaluation of a Novel Method for Sampling Scala Tympani Perilymph by Collecting Fluid from the Cochlear Apex.

Salt, A.N. and Hale S.A.

Abstract

The collection of cochlear perilymph samples is technically difficult. A widely used method is to aspirate samples through the round window (RW) membrane. This can result in contamination of the sample with high levels of cerebrospinal fluid (CSF) due to the proximity of the sampling site to the cochlear aqueduct. To increase the proportion of perilymph in the sample, it appeared logical to take samples at a location further away from the cochlear aqueduct. We therefore developed a technique for sampling scala tympani (ST) perilymph from the apex of the cochlea. The goal was to perforate the cochlear apex and collect all the fluid that emerged. To achieve this, a “cup” was constructed around the intact apex with silicone adhesive. When the apex was perforated, positive intracochlear pressure caused a fluid bead to form within the silicone cup. The fluid was drawn by capillary action into a calibrated glass pipette. We evaluated the method using the marker ion trimethylphenylammonium (TMPA). TMPA was applied to perilymph either by RW irrigation or by microinjection into the apical turn. The TMPA concentration of a 10 µL sample obtained by apical sampling was compared with that measured in perilymph by a TMPA-selective electrode prior to taking the sample. Data were interpreted with a finite element model of the cochlear fluids that was used to simulate each aspect of the experiment. The results were consistent with the sample being composed of more than 97 % of the TMPA from ST perilymph, diluted with CSF. The sample concentration can be corrected for the CSF dilution based on the known ST volume, which has been estimated to be 4.7 µL. This method of sampling from the apex is technically straightforward and provides a greater proportion of perilymph in the sample than does sampling through the RW.
 

This study was supported by the National Institutes of Health through the National Institute on Deafness and other Communication Disorders, Grant number DC01368

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