Measurement of Endolymph Flow Rate
Why is the rate of endolymph flow important?To establish how a body fluid is regulated, the most fundamental question is whether the fluid is moving or stationary. For many years is was believed that endolymph was "secreted" in the cochlea and was resorbed by the endolymphatic sac. If this was true, then a flow of endolymph would occur, directed from the cochlea toward the endolymphatic sac. This concept has provided the foundation for the explanation of endolymphatic hydrops development in Meniere's disease. According to this concept, a disturbance of the resorption system in the endolymphatic sac would cause a "backing-up" and an accumulation of endolymph, resulting in the development of hydrops. This concept is shown in the figure below.
Traditional view of endolymph homeostasis
It is known from radiotracer studies that the ions in endolymph are not static, but are actively replaced with time. Potassium (the predominant cation) is turned over at a rate such that half is replaced every 55 mins. The question arises whether the rate of longitudinal flow can account for this turnover of potassium. Since cochlear endolymph has a volume of approximately 2 ul, a replacement of half the volume (1 ul) in 55 mins can be calculated to be equivalent to a flow rate of 18 nl/min. A flow rate of 18 nl/min, if it existed, would account for the measured turnover of potassium.
How is the endolymph flow rate measured?The scala media of the cochlea is basically a very small tube approximately 18 mm long and 0.1 mm2 in cross-sectional area. The total volume of cochlea endolymph is less than 2 ul, (equivalent to a container 1 mm x 1 mm x 2 mm). In order to measure the rate of flow in this tube, specialized techniques are necessary. The methods we developed involved using a chemical marker ion. This is equivalent to pouring colored dye into a river so that it is possible to see the rate of flow. Since we cannot directly see the endolymph, without unduly damaging the cochlea, we monitored the movement of the marker with ion-selective microelectrodes which can be inserted into endolymph with minimal damage. We can make electrodes which can detect the marker in extremely low (micromolar level), non-toxic concentrations. The placement of electrodes in the cochlea for flow measurement, called the "3-electrode method", is shown below.
This figure shows a schematic of how the electrodes are placed in the cochlea. The endolymphatic compartment is identified by the dark band, corresponding to the pigmentation in stria vascularis.
Electrode placement to measure endolymph flow
This figure shows the uncoiled cochlea with the ion-selective electrodes placed each side of the electrode from which marker is injected. Marker is injected without disturbing endolymph volume using iontophoresis, which involves passing a small current (typically 50 nA) through the electrode that ejects the charged ions at a very precise rate. If no flow is present in endolymph (lower left panel), the marker would spread symmetrically from the injection site by diffusion. The same amount would spread in apical and basal directions so that a similar concentration would be detected by the two ion-selective electrodes. If there is flow present in endolymph (lower right panel) is would displace the marker basally, giving a higher concentration at the basal electrode and a lower concentration at the apical electrode.
How flow is measured with the 3-electrode method
Simulations (AVI format) showing the high sensitivity of the method.
Click on images to load animation, then click to start movie
The left panel of each simulation shows the concentrations detected by the two electrodes (red and blue) as a function of time. The right panel shows the concentration as a function of distance along the cochlea, with the electrode locations shown as red and blue. We are simulating 15 mins iontophoresis of marker, followed by 15 mins without marker injected.
If endolymph flowed down the cochlea at 20 nl/min, then the concentrations seen at the two sites would be very different, and the curves would quickly stabilize as a function of time during injection.
This provides an accurate method by which endolymph flow rate can be quantified. The diffusion of the tracer which is included here can be calculated very precisely. In addition, experiments have been performed in fine-diameter plastic tubes, to validate the method and to calibrate the computer model.
You can run simulations for any experimental condition by downloading the Washington University Cochlear Fluids Simulator
Results of Flow Measurements in Cochlear Endolymph
Click on image to load animation, then click to start movie
This movie initially shows the time courses of tracer measured in vivo in cochlear endolymph. One factor which makes the analysis more complex is that in the real experiment, (as opposed to the simulations above) the recording electrodes cannot be placed exactly symmetrically with respect to the injection electrode. This is easily overcome by letting the computer model calculate the curves for the actual distances used in each case. In this example one electrode was 0.59 mm basal to the injection site and the other was 1.3 mm apical to the injection site. The marker (TMA) was injected for the first 15 minutes of the 30 minute observation period. The right panel of the simulation shows the distribution of marker (concentration) along the length of the cochlea. The left panel shows the concentration time courses recorded at the two measurement sites. There are two animations presented. The first shows the time courses calculated for a zero flow rate. The difference between the two recorded time courses in this condition is due to the slightly different distances involved. With a zero flow rate, it can be seen that the calculated curve for the basal electrode is too high, and that for the apical electrode is too low. In order to obtain a better fit, it was necessary to include an apically-directed flow at a rate of 2.5 nl/min. Thus in this experiment, the longitudinal movement of endolymph was apically-directed.
Summary of Measured Endolymph Flow Rates
This figure summarizes the flow rates measured in 14 animals using three different ionic tracers, tetramethylammonium (TMA), trimethylphenylammonium (TMPA) and tetraethylammonium (TEA). In these experiments it can be seen that all measured flow rates were extremely low, and scattered around zero. The mean rate of endolymph flow was calculated to be 0.36 nl/min, directed towards the base, but this low rate is not statistically different from zero. The flow rate is well below that required to account for the ionic turnover of endolymph. Ionic turnover is dominated by other processes such as the ongoing radial current flow that is modulated during acoustic transduction.
Conclusions1) In the normal cochlea, longitudinal flow is far too slow to account for the known turnover of the major ions in endolymph.
2) Explanations for Meniere's disease and hydrops which are based on the endolymphatic sac failing to resorb endolymph are incorrect.
Note, however, that some of our recent studies have shown that endolymph flow can be induced under some experimental conditons. Flow in either apical or basal directions can be induced.
Publications on Endolymph Flow in the Normal CochleaSalt, A.N. and Thalmann, R.: Rate of longitudinal flow of cochlear endolymph. In: Ménière's Disease. Ed., J.B. Nadol, Kugler, Amsterdam, pp. 69-73, 1989.
Salt, A.N. and Thalmann, R.: Interpretation of endolymph flow results. Hearing Res. 33:279-281, 1988.
Salt, A.N. and Thalmann, R.: New concepts regarding the volume flow of endolymph and perilymph. Adv. Oto-Rhino-Laryngol. 37:11-17, 1987.
Salt, A.N., Thalmann, R., Marcus, D.C. and Bohne, B.A.: Direct measurement of longitudinal endolymph flow rate in the guinea pig cochlea. Hearing Res. 23:141-151, 1986.
Page generated by: Alec N. Salt, Ph.D.,
Cochlear Fluids Research Laboratory,
Washington University, St. Louis