Scala tympani perilymph sampling from the cochlear apex.

Salt, A.N., Mynatt, R., Hale, S and Plontke, S.

 

Abstract

Introduction
Local drug delivery to the inner ear is becoming widely used in the clinical treatment of diseases such as Meniere’s disease. There is therefore an increasing need to document the pharmacokinetics of drugs applied to the inner ear. In many published studies, perilymph for analysis was obtained from the basal turn of scala tympani (ST) through the round window membrane (RWM) or through a perforation of the bony capsule. As the cochlear aqueduct enters ST at the base, such samples are highly contaminated with CSF. A recent study estimated that 10 mL samples taken through the RWM contain less than 20% perilymph. In the present study, an alternative method for obtaining ST perilymph samples with less CSF contamination has been evaluated.
Methods
Perilymph samples were obtained from the cochlea apex of guinea pigs. Prior to perforating the otic capsule, the bone was dried, coated with cyanoacrylate and a “cup” was formed from two-part silicone glue. This permitted fluid to be collected into capillary tubes when the apex was perforated without any loss or contamination. Samples were obtained following local application of the marker TMPA by injection or by irrigation of the RWM, or following application of gentamicin to the RWM.
Results
In experiments where a 10 mL volume was collected from the cochlear apex, the concentrations were consistent with the samples containing the entire ST contents (volume 4.7 mL), “rinsed through” with a similar volume of CSF. Simulations based on measurements made with TMPA-selective electrodes in ST correlated well with the measured samples. In experiments where repeated, 1 mL samples were taken, the longitudinal gradients of TMPA and gentamicin along ST were demonstrated.
Conclusions
Sampling perilymph from the cochlear apex is technically straightforward. The perilymph content of large samples from the apex is higher than that of an equivalent volume taken through the RWM. If sequential, small samples are taken, the longitudinal gradient of drug along the perilymphatic space can be demonstrated. This method offers substantial advantages over sampling from the basal turn of ST.
 

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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