Validating the Hypothesis of Otolith Asymmetry as a Cause of Space Motion Sickness

Prior experiments in hypoand hypergravity aboard NASA's KC-135 aircraft demonstrated in nine former astronauts a clear relation between ocular torsional instability in novel gravitational states and a history of space motion sickness (SMS).' When tested in the upright position during 0 G and 1.8 G in parabolic flight, five of these astronauts were found to have high scores of torsional asymmetry, a measure of disconjugate spontaneous torsional eye movements. These five persons had suffered SMS during their previous space missions, and their scores of torsional asymmetry on the KC-135 were directly related to the severity of their SMS symptoms in space. The four former astronauts who had low scores of torsional asymmetry on the KC-135 did not have SMS in space. Mean torsional asymmetry scores of the two astronaut groups differed significantly a tp = 0.01 (two-tailed Wilcoxon ranked sums test). In ground-based testing of torsional asymmetry in these same subjects, there were no differences among the nine astronauts. These results supported the asymmetry h y p o t h e ~ i s , ~ ~ ~ which asserts that a slight anatomical or physiological imbalance of the otolith organs may be well compensated in the usual 1-G environment on earth, but when such an asymmetric system is exposed to novel G states, the prior compensatory equilibration may be disturbed. This failure of compensation is believed to result in unaccustomed vestibular responses, giving rise to the sensory conflict proposed to underlie SMS.4 The present investigation focused on three questions suggested by the results of the previous study: