Impediment of basilar membrane motion reduces overload protection but not threshold sensitivity: evidence from clinical and experimental hydrops.

The mechanical function of the basilar membrane (BM) in the mammalian cochlea has been newly debated after the discovery of frequency selectivity of single hair cells. Decisive information on this matter can be expected from hydropic ears, since hydrops presumably alters cochlear mechanics by (1) impeding BM motion and (2) uncoupling outer hair cell (OHC) stereocilia from the tectorial membrane (TM). Therefore hearing in Menière's disease (MD) was examined analysing data on epidemiology of MD types and audiogram types, 2f thresholds, over-recruitment, loudness intolerance, and otoacoustic emissions. Further, hearing in experimental hydrops (XH) was examined analysing data on: morphological changes on TM and OHCs in relation to hydrops duration; morphological and electrophysiological changes upon acoustic overstimulation. The results were unequivocal on two points: (1) co-occurrence of hydrops and normal hearing thresholds can appear both in MD and XH, (2) co-occurrence of non-hearing-loss hydrops and loudness hypersensitivity is typical both in MD and XH. The conclusion is that BM motion apparently is no necessary element in the chain of cochlear sound transmission but obviously is an auxiliary element for overload protection through resonant absorption. The results are further indicative of audiometric methods for an early detection of incipient MD.