Caloric nystagmus in the lateral recumbent position in normal subjects--the possibility of a participation of velocity storage and other non-convection factors

Abstract

To clarify the role of central velocity storage in caloric nystagmus, 20 normal subjects in the lateral recumbent position underwent recording and analysis using ENG and an infra-red video camera in darkness. The caloric stimulus was applied by pouring 10 ml of water at 20 degrees C for 20 seconds into the ear canal. After irrigation, each subject then turned to an ear-up or an ear-down lateral position. Ninety-nine percent of the rapid phases of the horizontal components were directed to the up-side ear with no relation to the irrigated ear. Sixty-six percent of the vertical components showed vertical nystagmus toward the lower eyelid, and the remaining 34% of the vertical components showed no vertical nystagmus. The direction of the torsional components was from the up-side ear to the leg with no relation to the irrigated ear. After the first phase, the nystagmus was more significantly reversed in the ear-up lateral position than in the ear-down lateral position (p < 0.01). The maximum slow-phase velocity of the first phase horizontal component and the nystagmus frequency of the first phase horizontal and vertical components were significantly smaller in the irrigated ear-up lateral position than in the ear-down lateral position (p < 0.01). The maximum slow-phase velocity of the first phase vertical component was also smaller in the ear-up lateral position (p < 0.05). However, the duration of the first phase vertical component revealed no difference between the two positions. These findings indicate that factors other than endolymph convection are involved in the first phase horizontal and vertical components. A role for central velocity storage in the vertical, components was suggested. When caloric nystagmus was analyzed as one response, including not only the horizontal component but also the vertical and torsional components, the possibility of otolithic involvement was suggested as one of the non-convection factors.