Space motion sickness and motion sickness: symptoms and etiology

The adverse symptoms of space motion sickness (SMS) have remained problematic since the beginning of manned spaceflight. Despite over 50 yr of research SMS remains a problem that affects about half of all space travelers during the first 24-72 h of a spaceflight. SMS has been treated as another form of motion sickness (MS) despite distinct differences in symptomology. In this review SMS and MS differences are examined and documented based on available data. Vestibular biomechanics that occur during weightlessness coupled with theoretical assertions regarding human evolution have led us to propose a two-component model of SMS. The first component involves conflicting sensory signals inherent to the otolith organs that occur during weightlessness. The second component is a bimodal conflict between the otoliths and semicircular canals that can occur during normal head movements in weightlessness. Both components may inadvertently, and mistakenly, signal that a vestibular malfunction has occurred, hence initiating a protective mechanism that may produce symptoms that discourage activity.