Ocular torsion as a test of the asymmetry hypothesis of space motion sickness

Abstract

Disconjugate eye torsion induced by O G and 1.8 G during parabolic flight was studied in nine former astronauts in 1990 and eight in 1991, four of whom were included in the previous experiment. The astronauts could be divided into two statistically significant groups on the basis of low and high scores of disconjugacy. When their histories of space motion sickness (SMS) were later revealed, all of the low scorers had not been sick on previous space flights; all the high scorers had had SMS. These data give support to the hypothesis that SMS in one-half or two-thirds of astronauts is due to an otolith, probably utricular, asymmetry in those persons. Ocular disconjugacy tended to increase at O G with increasing numbers of parabolas, this being particularly evident in those subjects with prior SMS. One conclusion: 10 to 20 parabolas are necessary to adequately discriminate those who are subject to SMS from those who are not. Tilting the subjects with high disconjugacy values and presumed otolith asymmetry by small amounts in right ear down or left ear down positions for several exposures to hypergravity did not reveal a lessened amount of ocular disconjugacy; there were actually increased amounts of ocular disconjugacy induced in the tipped positions. We suspect the increased disconjugacy caused by multiple parabolas may have masked any “null” point induced at 1.8 G by small head angulations. Space motion sickness (SMS) appears to be a unique form of motion sickness. It occurs within minutes to hours after entering microgravity environment, typically lasts the first 1 to 4 days in space flight, and may occur in abbreviated form on returning to earth. The symptoms are much like other forms of motion sickness except vomiting may occur with little warning. The substrate of SMS appears to be a loss of the constant force of gravity acting on the vestibular otolith system. In certain subjects in this “sensory mismatched” state, motion sickness may be easily triggered by linear and angular acceleration to the head and by visual stimuli. Between one-half and two-thirds of astronauts and cosmonauts have SMS 3 — and the remainder do not. The most likely explanation is the hypothesis advanced by von Baumgarten and Thümler, 1 i.e. in certain individuals there is an asymmetry in the otolith apparatus on the two sides of the head which becomes compensated in a lifetime in a 1 G environment, but this compensation is lost when exposed to the hypogravity of space flight. One thing that makes SMS unique is that there is no correlation with motion sickness in a earth-based environment, whether it be on land, sea or in the air. This is perhaps the main reason it has not been possible, until the present investigations, to predict who would get SMS. We shall review here work relating alterations in ocular torsional movements induced by repeated exposure to transient episodes of O G and 1.8 G during parabolic airplane flight to SMS. Several questions were asked: Could we predict in a blinded analysis which astronaut-subjects had had prior SMS (and which had not)? Based on what appears to be a positive answer, 4 we then asked whether the observed ocular torsional disconjugacy could be modified by an increasing number of parabolas; and whether observed disconjugacy seen in the subject in the upright position could be reduced or “nulled” by parabolic stimuli with the subjects tipped at different angles in the right ear down and left ear down directions.