DC potential and K+ activity in experimental endolymphatic hydrops

Functional changes associated with experimentally induced endolymphatic hydrops

A guinea pig model with surgically induced endolymphatic hydrops of the inner ear has been developed and studied over the past thirty years. The aim of such studies is to obtain insight into physiological processes associated with endolymphatic hydrops in man and in particular in Menière's disease where endolymphatic hydrops is systematically encountered at post-mortem examination of the temporal bones. The present review attempts to draw together the data pertaining to functional modifications of inner ear function in the animal model. For simplicity the data are categorised under five main titles: electrochemical modifications, electrophysiological modifications, pressure and hydrops, sensitivity to other insults and vestibular dysfunction. One of the most striking observations that can be made is that the data originating from different authors are very variable. There is, however, some evidence suggesting that the evolution of the auditory dysfunction could be considered as consisting of a series of different phases. This kind of information could serve as a basic framework for future research on the animal model.

Glycerol-induced changes in the cochlear responses of the guinea pig hydropic ear

Pigmented guinea pigs were chronically implanted with bilateral round-window electrodes. Endolymphatic hydrops was induced by obstruction of the endolymphatic duct on one side. At 1-5 months post-surgery, glycerol was orally administered to each animal and the acute effect on cochlear and eighth nerve responses was investigated. Doses from half to four times the clinical dose of glycerol failed to improve the CAP audiogram in operated ears and were often found to raise the thresholds. Glycerol was found to diminish the amplitude of the summating potential as well as that of the compound action potential. Similar effects were observed for the higher doses in the control ears, although no effect was seen when doses used were equal to or less than the clinical dose. Our data suggest that the cochlear conditions in the guinea pig model of hydrops differ notably from those seen in patients with Ménière's disease.

Ionic environment of cochlear hair cells

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.

Saccular and cochlear endolymphatic potentials in experimentally induced endolymphatic hydrops of guinea pigs

Saccular and cochlear endolymphatic potentials (SEP and CEP) were recorded in 33 normal animals. We confirmed that SEP is not produced in the saccule per se, but is a potential leak from the cochlea. CEP was reduced to one-tenth of the original value when it reached the saccule. Endolymphatic hydrops was provoked in 32 animals, and CEP and SEP were successfully recorded in 29 and 20 cases, respectively. The measurements were performed in the 2nd, 4th and 12th postoperative week. Both CEP and SEP decreased in magnitude as time elapsed after the surgery. This reduction is attributed to the strial dysfunction. In advanced hydrops, CEP transmission to the saccule is greatly inhibited. It is concluded that the saccular dysfunction can occur in the hydropic ear.

Changes in EP and inner ear ionic concentrations in experimental endolymphatic hydrops

Unilateral endolymphatic hydrops was created in guinea pigs by endolymphatic duct obstruction. Using microelectrodes, studies of endocochlear potential (EP) and inner ear ionic concentrations were performed. EP was significantly decreased whereas Na+, K+, and Cl- concentrations in both scala media and scala tympani were not significantly altered. The presence of a decreased EP after endolymphatic duct obstruction appears to be a reliable indicator for the presence of hydrops. This decrease in EP without ionic concentration changes suggests that it is the electrogenic portion of EP which is altered in hydrops.

Neurophysiologic assessment of endolymphatic hydrops

An electrophysiological method for assessing endolymphatic hydrops of the cochlea in an animal model of Meniere's disease is described. Eighth nerve gross action potentials (AP) were evoked by tone bursts presented at discrete phases of a simultaneously delivered 50-Hz pure tone. Hydropic cochleas showed significantly less modulation of AP amplitude and latency than normal cochleas. This method may be useful in objectively evaluating Meniere's disease with electrocochleography.

Functional and morphological changes in experimental endolymphatic hydrops

Electrocochleograms (ECochG) in response to tone bursts and electronystagmograms (ENG) during sinusoidal rotational stimulation were monitored in guinea pigs equipped with chronically implanted round-window and periocular electrodes before and for as long as 90 days after uni- or bilateral surgical obliteration of the endolymphatic duct. The presence of cochlear and saccular endolymphatic hydrops was verified during microdissection of the inner ear, and cochlear hair cell counts were performed in some of the animals. The results indicate that in surgically-induced endolymphatic hydrops, cochlear and vestibular changes occur similar to those observed clinically in Menière's disease: fluctuant threshold shifts, threshold elevation at low frequencies, sensorineural loss at all frequencies with recruitment, increase in the ratio of summating potential to compound action potential (SP/CAP), asymmetry and recruitment in the ENG, and increased vestibular excitability, with occasional post-rotational and spontaneous nystagmus. Functional changes could not be clearly correlated with those seen by light microscopy, since hair cell loss was confined mainly to the apical coil. The observations indicate that the guinea pig with surgically-induced endolymphatic hydrops can be a useful model of Menière's disease, not only on morphological grounds, but also on the basis of functional changes, which over the relatively short term of observation represent a dysfunction rather than a loss of sensorineural structures.

[Experimental studies on the effect of labyrinth anesthesia]

Injection of tetracaine into the tympanic cavity inhibits, independently of histologic lesions of the round window membrane, the active electrogenic pump in the stria vascularis cells. The DC potential in the endolymph is reduced with increasing doses of tetracaine, while the K+-Cl- activity is unchanged for 120 min. This drug does not alter the permeability of Reissner's membrane. Near the stria vascularis area, the oxygen pressure in the endolymph increases significantly after the effects of 4% tetracaine.

Animal models of endolymphatic hydrops

Endolymphatic hydrops in the animal can be produced by various methods. The most promising is obliteration of the endolymphatic duct in the guinea pig. The extent of hydrops is similar to that of Ménière's disease specimens, but the animals are asymptomatic. A better model, utilizing a less invasive method and having the ability to elicit episodic vestibular symptoms, is needed. There is a reasonable doubt that one of the causes for Ménière's disease lies in the abnormal endolymphatic duct and sac. A wide ablation of such pathologic tissue may provide additional pathogenetic information.

Evidence for a perilymphatic origin of the endolymph: application to the pathophysiology of Ménière's disease

The origin of the endolymph was elucidated by kinetic studies of the entry of water and electrolytes into endolymph and perilymph after intravenous administration of radioactive tracers in rats. The compartmental analysis of the data and the comparison of this study with the results of Konishi and associates (Acta Otolaryngol (Stockh) 86, 22-34 and 176-184, 1978), using perilymphatic perfusion of tracers, indicate that perilymph rather than plasma may be considered the precursor of endolymph. Since the cochlear epithelium was found to be freely permeable to water, an alteration of electrolyte transportation across the membranous labyrinth may be involved in the pathophysiology of Ménière's disease. Chloride transport across the cochlear epithelium was investigated using acetazolamide, a specific carbonic anhydrase inhibitor.

Functional and morphological findings of endolymphatic sac

After obliteration of endolymphatic sac and duct, no significant alteration in d.c. potential, K+ activity or protein content during the development of endolymphatic hydrops could be observed. The K+ activity of endolymphatic sac was only one-ninth of that in the cochlear part of the endolymph. After injection of Thorotrast into the endolymphatic sac, aggregated particles were found in the cochlear endolymph 2 days later. Ethacrynic acid (60 mg/kg) caused a decrease in K+ activity in the cochlear endolymph, but an increase in the endolymphatic sac. Intercellular edema was observed both in stria vascularis and in endolymphatic sac by light and electronmicroscopy after ethacrynic acid administration. These results suggest the existence of an active transport mechanism in the endolymphatic sac epithelium.

Scanning electron microscopic observations on the distended Reissner's and saccular membranes in the guinea pig

The surface ultrastructures of membranous labyrinths in normal and hydropic guinea pig inner ears were studied. In normal specimens differences between Reissner's and saccular membranes were observed. The endolymphatic epithelial cells of the saccule were smaller in size and their microvilli varied more in size and distribution. Mesothelial cells of the saccule were often spherical in shape and bulged toward the vestibule. In hydropic guinea pig inner ears, distension of Reissner's and saccular membranes occurred by irregular enlargement of epithelial cells. Microvilli were reduced in size and number. On the severely distended Reissner's membrane, outpouchings, infoldings and holes were observed. Transmission electron microscopic examination revealed no significant difference between the tight junctions of epithelial cells in normal or hydropic Reissner's membranes. Mesothelial cells of Reissner's membrane were more severely degenerated than those of the saccule. Cochlear sensory cells were often degenerated in the apical turn where mesothelial cells were lacking, but saccular sensory cells showed very little change in their surface structures.