Perilymph Kinetics of FITC-Dextran Reveals Homeostasis Dominated by the Cochlear Aqueduct and Cerebrospinal Fluid

Abstract

Understanding how drugs are distributed in perilymph following local applications is important as local drug therapies are increasingly used to treat disorders of the inner ear. The potential contribution of cerebrospinal fluid (CSF) entry to perilymph homeostasis has been controversial for over half a century, largely due to artifactual contamination of collected perilymph samples with CSF. Measures of perilymph flow and of drug distribution following round window niche applications have both suggested a slow, apically directed flow occurs along scala tympani (ST) in the normal, sealed cochlea. In the p r e s e n t s t u d y , w e h a v e u s e d f l u o r e s c e i n isothiocyanate-dextran as a marker to study perilymph kinetics in guinea pigs. Dextran is lost from perilymph more slowly than other substances so far quantified. Dextran solutions were injected from pipettes sealed into the lateral semicircular canal (SCC), the cochlear apex, or the basal turn of ST. After varying delays, sequential perilymph samples were taken from the cochlear apex or lateral SCC, allowing dextran distribution along the perilymphatic spaces to be quantified. Variability was low and findings were consistent with the injection procedure driving volume flow towards the cochlear aqueduct, and with volume flow during perilymph sampling driven by CSF entry at the aqueduct. The decline of dextran with time in the period between injection and sampling was consistent with both a slow volume influx of CSF (∼30 nL/min) entering the basal turn of ST at the cochlear aqueduct and a CSF-perilymph exchange driven by pressure-driven fluid oscillation across the cochlear aqueduct. Sample data also allowed contributions of other processes, such as communications with adjacent compartments, to be quantified. The study demonstrates that drug kinetics in the basal turn of ST is complex and is influenced by a considerable number of interacting processes.

Gene Therapy Restores Balance and Auditory Functions in a Mouse Model of Usher Syndrome

“…It has been shown that drug delivery in rodent cochlea is much more effective when injected through the semicircular canal than through the round window, because rodents have a large cochlear aqueduct, which is located at the base of the cochlea, next to the round window. 37,38 The AAV8-whirlin delivered through the round window may be diluted by cerebrospinal fluid from the cochlear aqueduct, or it may escape through the cochlear aqueduct, which decreases its concentration and limits its ability to infect the entire cochlea. 38 When AAV8-whirlin gene therapy was delivered through the posterior semicircular canal, the average IHC stereocilia lengths in the eight whirler mice that showed hearing recovery were much longer than in our previous study in which AAV8-whirlin was injected through the round window.…”

“…37,38 The AAV8-whirlin delivered through the round window may be diluted by cerebrospinal fluid from the cochlear aqueduct, or it may escape through the cochlear aqueduct, which decreases its concentration and limits its ability to infect the entire cochlea. 38 When AAV8-whirlin gene therapy was delivered through the posterior semicircular canal, the average IHC stereocilia lengths in the eight whirler mice that showed hearing recovery were much longer than in our previous study in which AAV8-whirlin was injected through the round window. 14 This is likely due to the significantly higher number of hair cells with whirlin expression achieved in the present study (the AAV8-whirlin used in this study is the same as in our previous study).…”

Intracochlear Drug Delivery Through the Oval Window in Fresh Cadaveric Human Temporal Bones

“…In the guinea pig, cerebrospinal fluid (CSF) entry into the scala tympani through the cochlear aqueduct has been described to have a diluting effect on substances delivered through the round window. This dilution has been theorized to derive both from entry of CSF into the scala tympani and from loss of drug into the CSF space (24), with CSF entering at a very slow rate in a sealed, normal cochlea (24,25). This likely contributes to the slow, apically-directed flow within the normal guinea pig cochlea, measured to be approximately 1.6 nL/min (25).…”

Drug delivery into the cochlear apex: Improved control to sequentially affect finely spaced regions along the entire length of the cochlear spiral

“…Constant-rate injections, which were not used, would have yielded reduced flow rate as scala tympani cross section widens. Dilution of solution concentration from mixture with cerebrospinal fluid (CSF) that enters through cochlear aqueduct is one of several reasons why the common procedure of round window niche applications cannot treat the entire cochlear length (Salt et al 2015). In contrast to drug delivery at the cochlear base, apical injections avoid dilutions even at the lower range of injection rates (i.e., 25 nl/min) which are close to the measured sustained volume of cerebrospinal fluid entry of ~30 nl/min through the cochlear aqueduct (cf.…”

“…In contrast to drug delivery at the cochlear base, apical injections avoid dilutions even at the lower range of injection rates (i.e., 25 nl/min) which are close to the measured sustained volume of cerebrospinal fluid entry of ~30 nl/min through the cochlear aqueduct (cf. Salt & DeMott 1998, Salt et al 2015). Thus, from the start of the injection thereafter, the entry of cerebrospinal fluid at the base will be suppressed.…”