Endolymphatic Hydrops

Cross-section of a cochlea with endolympatic hydrops

Image produced in collaboration with Dr Robert Kimura, Harvard University.
This figure shows a section through a cochlea with endolymphatic hydrops. Compared to the normal situation, the endolymphatic space is enlarged. Reissner's membrane can be seen bowed out into scala vestibuli.

Meniere's disease in humans is characterized as a disorder of the endolymph, in which endolymphatic hydrops develops. The symptoms usually associated with Meniere's disease (low-frequency hearing loss, tinnitus (ringing in the ears), episodes of rotatory vertigo (dizziness), and a sensation of "fullness" or pressure in the ear) are believed to result from a malfunction of the endolymph volume regulation mechanisms. Endolymph volume increases slowly, disturbing motion of the basilar membrane and thus giving rise to hearing loss. One explanation of the episodic vertigo is that hydrops develops to the point where a boundary membrane breaks, which results in a release of endolymph into the perilymphatic system.
This development of hydrops, and periodic release of endolymph can be illustrated by these movies, available in MPEG or AVI format.
Help with choice of movie players is available

Development of Hydrops : Movie

210K MPEG OR 310K AVI (Video for Windows)

The movie shows how endolymph enlarges over time until one of the boundary membranes fails. This may not necessarily be a rupture, as shown here, but may be in the form of a more diffuse leak. The release of endolymph results in a major disturbance of the normal chemical composition of the perilymph in scala vestibuli. As scala vestibuli has a wide communication with perilymph of the vestibule, the chemical disturbance will spread into the vestibular system. The high-potassium endolymph is highly toxic to cells not specialized to deal with it. Cells become depolarized (activating transmitter release and afferent nerve fiber activity) and swell in a high-potassium environment, resulting in auditory and vestibular dysfunction.

It should be stressed that there are a number of simplifications in this presentation, including the time course of events. In addition there are other explanations of the dysfunction in the literature, which continue to be investigated

In order to treat endolymphatic hydrops in humans, it is necessary to understand how endolymph volume is regulated in the normal ear. At present, the details of how endolymph volume is regulated are unknown. The primary goal of research in our laboratory is to develop such an understanding. One important aspect of our studies has been to develop methods to measure endolymph volume. Obviously, it is impossible to study how endolymph volume is regulated if endolymph volume cannot be measured. Hydrops can be clearly demonstrated histologically but is difficult to quantify by this technique. We have now developed methods which can measure endolymph volume with accuracy in vivo. In addition we have developed techniques to quantify endolymph volume more accurately using 3-D imaging techniques in conjunction with magnetic resonance microscopy.

One structure which appears essential for normal endolymph volume regulation is the endolymphatic sac. If the endolymphatic sac is ablated in some animals then endolymphatic hydrops develops, similar to that seen in patients with Meniere's disease. For years it was believed that endolymph was secreted in the cochlea and flowed out through the ductus reuniens, to be resorbed by the endolymphatic sac. Our research have shown that this is incorrect. In the normal cochlea the rate of longitudinal endolymph flow is extremely low, so that flow does not make a significant contribution to the homeostasis of ions in endolymph. However, recent studies in our lab suggest that when endolymph volume is disturbed, longitudinal flows may be induced which play a part in the recovery of normal volume. These flows may be directed toward the cochlear apex (when endolymph volume is reduced) or towards the cochlear base (when endolymph volume is increased). The conditions under which flows can be induced, and the role they play in volume regulation are presently being studied.

Back to Home Page

Page generated by: Alec N. Salt, Ph.D.,
Cochlear Fluids Research Laboratory,
Washington University, St. Louis