Clinical characterization and etiology of space motion sickness.

An inflight, clinically-oriented investigation of SMS was begun on STS-4 and revealed the following: compared to motion sickness on Earth, autonomic signs are significantly different in space motion sickness (SMS) vs. motion sickness (MS) in that sweating is not present, pallor or flushing may be present, and vomiting is episodic, sudden, and brief. Nausea may be present but is more often absent. Onset ranges from minutes to hours, plateaus, and rapidly resolves in 8-72 h with 36 h as average. Postflight reactions have been mild unless deliberately provoked in the early period of re-exposure to gravity. Postflight there is a period of resistance to all forms of motion sickness. There is some evidence for individual reduction in sensitivity on repeated flights. Etiology could not be proven objectively; however, the sensitivity to angular motion, often pronounced in pitch, and theoretical considerations make an intravestibular conflict the most likely cause. Electro-oculogram (EOG), audio-evoked potentials, measurement of fluid shifts, and other studies are inconsistent with a transient vestibular hydrops or increased intracranial pressure as a cause.