The Effects of Experimental Hydrops versus Increased Intracochlear Pressure on Auditory Function in the Guinea Pig

In order to investigate the physiological consequences of hydrops on auditory function an animal model has been developed in which endolymphatic hydrops is surgically induced and electrophysiological responses are monitored preceeding during and following hydrops induction. These studies have revealed that experimental hydrops systematically results in a fluctuant low frequency hearing loss ( below 4 kHz) which is apparent within a few days of surgery. After about two months there is in addition a very high frequency loss (16 kHz and above). A final mid frequency loss (including 8 kHz), after a further lapse of one month, results in a flat audiogram. In order to investigate whether increased endolymphatic pressure is responsible for the electrophysiological changes observed in endolymphatic hydrops we have recently placed a canula in the endolymphatic canal of normal hearing guinea pigs and hence increased the intracochlear pressure to investigate the effects on the audiogram. There is an immediate high frequency loss which includes 8 kHz, followed by a low frequency loss below 2 kHz and finally a mid frequency loss around 4 kHz. These two experimental procedures result in cochlear pathologies which differ in two principal aspects. The immediate effect of experimental hydrops is a deterioration of the low frequencies whilst that of increased pressure is a deterioration of the high frequencies. Experimental hydrops appears to preserve the functioning of the cochlea best around 8 kHz whilst increased pressure applied at the base of the cochlea appears to preserve the 4 kHz region. The data suggest that increased intracochlear pressure is not likely to account for early low frequency fluctuant hearing losses in experimental hydrops and in Menieres's disease but could account for the evolutive nature of the deafness to include the higher frequencies.