Rapidly fluctuating thresholds at the onset of experimentally-induced hydrops in the guinea pig

Pigmented guinea pigs were chronically implanted bilaterally with a platinum electrode on each round window. After recovery the endolymphatic sac was destroyed and the duct blocked on one side only; the other side was employed as a control. The round window response thresholds on both sides were recorded several times per week over a three month period. There were three main results. A sensitivity loss of up to 20 dB was observed for frequencies between 250 Hz and 6.4 kHz within two weeks post-op. At the end of three months the threshold elevation for these frequencies was as much as 50 dB. On the other hand the thresholds for frequencies between 8 and 16 kHz remained within 10 dB of their pre-operative value for at least two months. The thresholds fluctuated with a shift of as much as 25 dB within 24 h. The threshold elevation was associated with a decrease in the latency, at threshold, of the round window AP response which at frequencies between 250 Hz and 6.4 kHz was as short as that for 8 kHz. This observation suggested that it was the base, only, of the cochlea which responded. The present study has indicated that experimentally induced endolymphatic hydrops in the guinea pig mimics well the progressive and fluctuating hearing loss characteristic of Ménière's disease.