Asymmetry of Evoked Rotatory Nystagmus in the Guinea Pig after Experimental Induction of Endolymphatic Hydrops

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Abstract
We have previously shown by chronic implantation of round window electrodes, that after the experimental induction of endolymphatic hydrops by surgical blockade of the endolymphatic canal, in the guinea pig, there is a low/mid-frequency fluctuant hearing loss, followed by a very high frequency loss and after a few months the audiogram is flat. This evolution reproduces exactly that observed in Meniere's patients. The aim of this study was to investigate the evolution of one aspect of vestibular function in parallel with audiogram changes. The nystagmic responses in the dark were tested every month during the 4 months which followed hydrops induction. There was considerable variation in the number of evoked saccades for different control animals and even between different recordings for the same control animal. However, in general, the number of saccades to right and left rotation was symmetrical for control animals, whereas for hydropic animals there was a period, within the first 2 months post-operation, when there was a reduction of saccades to the left (operated side) with sometimes an increase to the right. This asymmetry which occurred in the period of fluctuant hearing losses did not however appear synchronized with audiogram fluctuations, the nystagmic responses tended to become symmetrical over the 4 months post-operation. Oral administration of glycerol to animals whose nystagmus was asymmetric brought the vestibular response towards symmetry in 50% of the cases. Although episodes of vertigo, as known in Meniere's disease, were never observed in operated guinea pigs, these data indicate that they have some functional vestibular disturbance.