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.