Morphological and electrophysiological study of experimental endolymphatic hydrops in guinea pig

Endolymphatic hydrops was experimentally induced in guinea pig ears by obliteration of the endolymphatic sac. One, two, four, and eight months after obliteration the animals were sacrificed for histological examination. Immediately before sacrificing the animal, compound action potentials (AP) and summating potentials (SP) were measured from both, the hydropic and the control ear. After one month, a distention of Reissner's membrane (hydrops) was found which reached the bony wall of scala vestibuli in 50% of the ears. After eight months the majority of the guinea pig ears showed extensive hydrops. In addition, we found thin connections between Reissner's membrane (RM) and the bony wall, ruptures of RM, thickening of RM and defects in the mesothelial cells, edema followed by atrophy of the stria vascularis, and loss of spiral ganglion cells and nerve fibers. The outer hair cells, followed by the inner hair cells, showed progressive degeneration starting at the apical end of the cochlea. One month after obliteration the AP threshold for the hydropic ear did not differ from the threshold for the control ear. Two months after obliteration small AP-threshold shifts were found which increased up to 40 dB after eight months. For 2- and 4-kHz stimuli an enhanced negative SP was found after one month when the extracochlear electrode was placed near the apex. After four and eight months the enhanced negative SP turned into a reduced SP. The enhanced SP is ascribed to the hydrops provoking a deviant resting position of the basilar membrane. The decrease in AP and SP amplitude two and more months after obliteration is ascribed to the loss of hair cells and spiral ganglion cells.