Perilymph volume flow measured under physiological conditions

This study made use of an ionic tracer (trimethylphenylammonium, TMPA) to mark perilymph in one region of the cochlea. Perilymph flow was assessed by monitoring tracer movement to other regions with ion selective electrodes. Small, nontoxic, quantities of tracer (typically a 50‐nl bolus of 150 nM TMPACI) were required, since concentrations as low as 1 μM were readily detected. Stringent precautions were taken to seal the injection and recording electrodes into the perilymphatic scalae so that no artifactual flow was induced by perilymph leakage. Results were compared with a mathematical model of tracer dispersion by volume flow and passive diffusion. For scala tympani, TMPA dispersion corresponded to a flow rate of 1.6 nl/m in the apical direction. This rate was significant (p < 0.05), although the physiological effects of such a low flow rate are presumed to be negligible. Perforation of the cochlear apex resulted in flow rates of over 1 μ1/m, almost 1000 times the physiological rate. In scala vestibuli, no significant flow could be detected under normal conditions. These results demonstrate that in the sealed state, longitudinal volume flow of perilymph is extremely low. Artifactual volume flow is readily induced by procedures which involve perforation of the otic capsule. [This work supported by NIH.]