Three Dimensional Analysis of Caloric Nystagmus to Determine Flow of the Endolymph in the Rhesus Monkeys

To determine the flow of endolymph during caloric nystagmus, rhesus monkeys with plugged right anterior and left posterior canals (RALP) and with plugged bilateral lateral canals (LLRL) were tested and compared to 6 normal monkeys. Caloric nystagmus induced by 20℃ water was recorded in eight head positions each 45 deg apart in the plane parallel to the lateral canals which were placed perpendicular to the earth. Eye movements were recorded three dimensionally using a search coil system, and expressed with respect to spacial oriented or semicircular canal oriented coordinate axes. Caloric nystagmus in normal monkeys appeared mostly in the lateral canal and earth-horizontal planes, suggesting contribution of the lateral canal and central integrator. The difference of caloric nystagmus after the plugging expressed on canal axes revealed position-dependent modulation of the maximum slow phase velocity in each canal component. The response in LLRL monkey was 1/10 that of normal animals, and lost the position dependency in the lateral canal component, indicating thermal convection in the lateral canal. The RALP monkey responded actively with irregular position dependency. The difference in response in the plugged canal component between normal and the canal-plugged animals in the plugged canal component changed following a sine function according to the head position, indicating convection in each of the semicircular canal. The direction of endo-lymph flow in the optimum position was suggested to be ampullofugal in the lateral and the posterior canals, and ampullopetal in the posterior canal. These results indicate that caloric response is in part a vector summation of the responses caused by thermal convection in each three semicircular canals, which is modified by the central integrator to appear as space horizontal eye movements