Hypersensitivity of hydropic ears, at frequencies with normal thresholds, to temporary threshold shifts.

We have shown that experimentally-induced hydrops in the guinea pig systematically provokes an early low frequency (6.4 kHz and below) fluctuant hearing loss. The present study was aimed at investigating one aspect of the functioning of that part of the audiogram with normal CAP thresholds (mid and high frequencies) in early hydrops. Temporary threshold shifts (TTS) as a function of pure tone stimulation level (8 kHz, 1 min 75-95 dB SPL), were investigated. The TTS was measured as a function of pure-tone exposure level. The different points on the input/output curve were determined on sequential days in order to allow full recuperation from the previous stimulation. The group of hydropic ears (N = 5) proved to be more sensitive by about 6 dB than the group of control ears (N = 10). The data indicate that whilst short-term endolymphatic hydrops can be characterised by a selective low frequency loss other auditory deficits exist throughout the cochlea and become manifest at supraliminal stimulus levels.