Subjective Vertical Conflict Theory and Space Motion Sickness.

BACKGROUND: Space motion sickness (SMS) remains a troublesome problem during spaceflight. The subjective vertical (SV) conflict theory postulates that all motion sickness provoking situations are characterized by a condition in which the SV sensed from gravity and visual and idiotropic cues differs from the expected vertical. This theory has been successfully used to predict motion sickness in different vehicles on Earth. METHOD: We have summarized the most outstanding and recent studies on the illusions and characteristics associated with spatial disorientation and SMS during weightlessness, such as cognitive map and mental rotation, the visual reorientation and inversion illusions, and orientation preferences between visual scenes and the internal z-axis of the body. RESULTS: The relationships between the SV and the incidence of and susceptibility to SMS as well as spatial disorientation were addressed. CONCLUSION: A consistent framework was presented to understand and explain SMS characteristics in more detail on the basis of the SV conflict theory, which is expected to be more advantageous in SMS prediction, prevention, and training.

NASA's human system risk management approach and its applicability to commercial spaceflight.

As planning continues for commercial spaceflight, attention is turned to NASA to assess whether its human system risk management approach can be applied to mitigate the risks associated with commercial suborbital and orbital flights. NASA uses a variety of methods to assess the risks to the human system based on their likelihood and consequences. In this article, we review these methods and categorize the risks in the system as "definite," "possible," or "least" concern for commercial spaceflight. As with career astronauts, these risks will be primarily mitigated by screening and environmental control. Despite its focus on long-duration exploration missions, NASA's human system risk management approach can serve as a preliminary knowledge base to help medical planners prepare for commercial spaceflights.

Space motion sickness: incidence, etiology, and countermeasures.

Space motion sickness is experienced by 60% to 80% of space travelers during their first 2 to 3 days in microgravity and by a similar proportion during their first few days after return to Earth. Space motion sickness symptoms are similar to those in other forms of motion sickness; they include: pallor, increased body warmth, cold sweating, malaise, loss of appetite, nausea, fatigue, vomiting, and anorexia. These are important because they may affect the operational performance of astronauts. Two hypotheses have been proposed to explain space motion sickness: the fluid shift hypothesis and the sensory conflict hypothesis. The fluid shift hypothesis suggests that space motion sickness results from the cranial shifting of body fluids resulting from the loss of hydrostatic pressure gradients in the lower body when entering microgravity. The cranial fluid shifts lead to visible puffiness in the face, and are thought to increase the intracranial pressure, the cerebrospinal-fluid pressure or the inner ear fluid pressures, altering the response properties of the vestibular receptors and inducing space motion sickness. The sensory conflict hypothesis suggests that loss of tilt-related otolith signals upon entry into microgravity causes a conflict between actual and anticipated signals from sense organs subserving spatial orientation. Such sensory conflicts are thought to induce motion sickness in other environments. Space motion sickness is usually treated using pharmaceuticals, most of which have undesirable side effects. Further studies elucidating the underlying mechanism for space motion sickness may be required for developing new treatments.

Development of space motion sickness in a ground-based human centrifuge.

Adaptation of the vestibular system, specifically the otolith organs, to a non-terrestrial environment can result in space motion sickness-like symptoms when the human is reintroduced to the normal, 1 Gz, terrestrial environment. This premise was investigated by exposing nine subjects to 90 min of sustained 2 Gz acceleration in a human centrifuge and then observing and evaluating them at 1 Gz. Five of the subjects developed slight SMS symptoms, three developed moderate, and one developed frank sickness. Postural instabilities in two of the most affected subjects were also observed using the Equitest System post exposure. Long duration exposure to a non-terrestrial G(2Gz) appears to be a potential means for developing SMS-like symptoms in a ground-based human centrifuge.

Comparison of treatment strategies for Space Motion Sickness.

Treatment strategies for Space Motion Sickness were compared using the results of postflight oral debriefings. Standardized questionnaires were administered to all crewmembers immediately following Space Shuttle flights by NASA flight surgeons. Cases of Space Motion Sickness were graded as mild, moderate or severe based on published criteria, and medication effectiveness was judged based on subjective reports of symptom relief. Since October 1989, medication effectiveness is reported inflight through Private Medical Conferences with the crew. A symptom matrix was analyzed for 19 crewmembers treated with an oral combination of scopolamine and dextroamphetamine (scopdex) and 15 crewmembers treated with promethazine delivered by intramuscular (IM) or suppository routes. Scopdex has been given preflight as prophaxis for Space Motion Sickness but analysis showed delayed symptom presentation in 9 crewmembers or failed to prevent symptoms in 7. Only three crewmembers who took scopdex had no symptoms inflight. Fourteen out of 15 crewmembers treated with IM promethazine and 6 of 8 treated with promethazine suppositories after symptom development had immediate (within 12 h) symptom relief and required no additional medication. There were no cases of delayed symptom presentation in the crewmembers treated with promethazine. This response is in contrast to untreated crewmembers who typically have slow symptom resolution over 72-96 h. We conclude that promethazine is an effective treatment of Space Motion Sickness symptoms inflight. NASA policy currently recommends treating crewmembers with Space Motion Sickness after symptom development, and no longer recommends prophylaxis with scopdex due to delayed symptom development and apparent variable absorption of oral medications during early flight days.

Space motion sickness.

Space motion sickness, presumably triggered by sudden entry into a weightless environment, occurred with unexpected frequency and severity among astronauts who flew the Skylab missions. Recovery from symptoms was complete within 3-5 days, and as revealed by the Skylab M131 Human Vestibular Function Experiment, all crewmembers were immune to experimentally induced motion sickness after mission day 8. This syndrome has been recognized as a possible threat to the early mission well-being and operational efficiency of at least some individuals who will fly space missions in the future. The causes of space motion sickness are not clearly understood, nor have satisfactory methods been identified to date for its prediction, prevention and treatment. In order to minimize the potential impact of this syndrome on Space Shuttle crew operations the National Aeronautics and Space Administration has organized a broad program of inter-disciplinary research involving a large number of scientists in the United States. Current research on the etiology of space motion sickness is based to a large extent on the so called sensory conflict theory. Investigations of the behavioral and neurophysiological consequences of intralabyrinthine, as well as intermodality sensory conflict are being performed. The work in this area is being influenced by the presumed alterations that occur in otolith behavior in weightlessness. In addition to sensory conflict, the possible relationship between observed cephalad shifts of body fluids in weightlessness and space motion sickness is being investigated. Research to date has failed to support the fluid shift theory. Research underway to identify reliable test methods for the prediction of susceptibility to space motion sickness on an individual basis includes attempts to (a) correlate susceptibility in different provocative environments; (b) correlate susceptibility with vestibular and non-vestibular response parameters, the latter including behavioral, hemodynamic and biochemical factors and (c) correlate susceptibility with rate of acquisition and length of retention of sensory adaptation. Controlled studies are also being performed during parabolic flight as a means of attempting to validate predictive tests for susceptibility to this syndrome. Research to develop new or improved countermeasures for space motion sickness is underway in two primary areas. One of these involves anti-motion sickness drugs. Significant achievements have been realized with regard to the identification of new highly efficacious drug combinations, dose levels and routes of administration. Although pronounced individual variations must be accounted for in selecting the optimum drug and dose level, combinations of promethazine plus ephedrine or scopolamine plus dexidrine are presently the drugs of choice. Work is also underway to identify side effects associated with anti-motion sickness drug use and to identify new drugs which may selectively modify activity in central neural pathways involved in motion sickness. In addition to research on drugs, efforts are being made to develop practical vestibular training methods. Variables which influence rate of acquisition of adaptation, length of retention of adaptation and transfer of protective adaptation to new environments are being evaluated. Also, included in this area is the use of biofeedback and autogenic therapy to train individuals to regulate autonomic responses associated with motion sickness. While valuable new knowledge is expected to evolve from these combined research programs, it is concluded that the final validation of predictive tests and countermeasures will require a series of controlled space flight experiments.