Contribution of endolymphatic fluid shift to caloric response in plugged semicircular canals.

The purpose of this study was to clarify the role of endolymphatic fluid shift in caloric response, using frog posterior semicircular canals (PSCs). PSCs were sutured using 10-0 nylon thread and were used as a model of canal plugging. Compound action potentials (CAPs) of the PSC nerve evoked by a cooling stimulus were recorded. The CAPs after suturing the PSCs were found to be greater than those before suturing. This indicates that the fluid shift effect increases after canal suturing. Additionally, we present a clinical case in which caloric nystagmus was observed after lateral canal plugging. In this case MRI revealed the fluid space from the plugged portion toward the ampulla to be intact. There was another case with lateral canal plugging that showed the same findings on MRI. The above findings support the hypothesis that fluid shift is responsible for the caloric response without the convective flow of endolymph in the plugged canal.