Scanning electron microscopy of hair cells, stereocilia and cross-linkage systems in experimentally induced endolymphatic hydrops.

In this study scanning electron microscopy was used to examine the stereocilia and their cross-linkages in guinea pig cochleas 1, 2, 4 and 8 months after endolymphatic sac obliteration. Initial changes were restricted to the stereocilia of the outer hair cells and consisted of a disarrangement and bulging of the stereocilia with an interruption of their cross-linkage systems. Subsequently, the stereocilia became fused and atrophied. Cross-linkages of inner hair cells remained intact. Loss of both outer and inner hair cells started in the apex and progressed towards the base of the cochlea. These findings indicate that early changes in the micro-architecture of the stereocilia may have a mechanical origin, with pressure fluctuations in the scala media possibly playing an important role.

Electrocochleographic changes in relation to cochlear histopathology in experimental endolymphatic hydrops.

The relation between electrocochleographic and histological changes in experimental endolymphatic hydrops was studied 1, 2, 4, or 8 months after obliteration of the endolymphatic sac. An increase in the compound Action Potential (AP) threshold was found 2, 4, and 8 months after obliteration. This increase was strongly correlated with loss of outer hair cells, nerve fibres and spiral ganglion cells. An enhanced negative Summating Potential (SP) and an enhanced SP-AP ratio were found mainly in animals with an endolymphatic hydrops without further cochlear pathology. A normal or decreased SP and SP-AP ratio was regularly recorded in animals with both an endolymphatic hydrops and a variety of other histopathological changes in the inner ear. An increased second peak (N2) in the AP waveform was recorded from 63% (15/24) of the hydropic ears, equally divided over the four groups. There was no obvious correlation between the increased N2 and other electrophysiological or histological findings.

Electrocochleographic study of experimentally induced endolymphatic hydrops.

An endolymphatic hydrops was induced in the left ear of each of 32 guinea pigs by obliteration of the endolymphatic sac. Both the hydropic ear and the control ear were examined by electrocochleography 1, 2, 4 or 8 months after obliteration of the sac using electrodes on the apex and near the round window. One month after obliteration, the threshold of the compound action potential (AP) and the increase of the AP amplitude with sound pressure level (SPL) were the same as the results recorded from the control ears despite the presence of a histologically confirmed hydrops. In the 2-month group, small AP threshold differences (10-20 dB) were recorded, increasing up to 10-40 dB in the 4- and 8-month groups. In the latter animals we also found a more rapid increase of the AP amplitude with SPL, a finding suggestive of recruitment. In the 1-month group we recorded an enhanced negative SP for 2- and 4 kHz stimuli when the electrode was placed near the apex. The same was recorded for 4-kHz stimuli in the 2-month group. In the 4- and 8-month groups there was a tendency towards a decrease in the negative SP. The SP-AP ratio recorded from the apical position showed the same pattern as the SP amplitude, an increase after 1 month and a tendency to decrease in the following months. Near the round window there was no significant effect of hydrops on the SP amplitude or on the SP-AP ratio. A deviation in the AP-SP wave form was recorded in 69% (22/32) of the guinea pigs.(ABSTRACT TRUNCATED AT 250 WORDS)

Histopathological study of experimentally induced endolymphatic hydrops with emphasis on Reissner's membrane.

We investigated the histopathological changes of the guinea pig cochlea in experimentally induced endolymphatic hydrops 1, 2, 4 and 8 months after obliteration of the endolymphatic sac. In this paper special attention is given to the morphological changes created in Reissner's membrane. Reissner's membrane showed various foldings, bony wall connections, local thickenings, duplications, mesothelial defects, ruptures and atrophy. The incidence of these changes increased with survival time. The stria vascularis and hair cells showed degeneration starting apically and proceeding towards the cochlear base. The possible implications of these findings are discussed.