Baseline Shift and Gain Asymmetry in the Caloric Test

The caloric test is quantified using two parameters: unilateral weakness (UW) and directional preponderance (DP). The clinical usefulness of UW, also known as canal paresis, is well established but there is considerable debate about the value of DP. Some laboratories choose not to include DP in the interpretation of the caloric test. One reason for the low clinical value of abnormal DP may be the fact that it can be caused by two distinct pathologies. The first is a static asymmetry in the peripheral or central vestibular pathways and the second is a gain asymmetry in the secondary vestibular neurons within the vestibular nuclei. Because the current formula for calculating DP combines both abnormalities into a single parameter, it is possible that important information is being lost. This article reviews the abnormalities that can cause DP and offers computational methods for separating the contribution of each abnormality. Introduction In the standard bithermal caloric test, right warm and left cool irrigations are expected to generate right-beating nystagmus while left warm and right cool irrigations are expected to generate left-beating nystagmus. In a normal individual, the intensities of all four caloric responses are approximately the same and therefore, there is no significant difference between right-beating and left-beating responses. Some patients however, have directional preponderance (DP) in which responses in one direction are significantly greater than the responses in the opposite direction. DP is defined as the normalized (scaled) difference between the peak nystagmus slow-phase velocities (SPVs) from irrigations that are expected to generate rightbeating nystagmus and those from irrigations that are expected to generate left-beating nystagmus. Mathematical formulas for calculating DP and other caloric parameters are provided in the Appendix. Interpretation of DP The normal limits reported for DP from different studies have ranged from as low as 20% to as high as 50%. Currently, most laboratories consider DP of less than 30% to be within normal limits (Sills et al., 1977). There has been a controversy about the interpretation and clinical value of abnormal DP. Initially, abnormal DP was considered a central finding but this conclusion was reached based on caloric responses that were obtained in the presence of fixation (Fitzgerald and Hallpike, 1942). Therefore, what was considered Baseline Shift and Gain Asymmetry in the Caloric Test Kamran Barin, Ph.D. F O R C L I N I C A L A U D I O L O G Y