Motion sickness is rarely a pathological condition but rather a normal response to conflicting perceptions of motion by the body's sensory receptors (visual, vestibular, and body proprioceptors). It can also be induced when the pattern of motion differs from that previously experienced, in the absence of expected motion, or viewing a very large screen where the viewer is not actually moving. Only those without a functioning vestibular apparatus of the inner ear are truly immune.
The development of symptoms follows an orderly sequence that varies with the intensity of the stimulus and the susceptibility of the individual. The initial symptom is usually vague discomfort around the upper abdomen ("stomach awareness"), which is followed by nausea and increasing malaise. The face or area around the mouth becomes pale and general sweating begins. With rapid worsening of symptoms ("avalanche syndrome") there can be increased salivation, feelings of body warmth, lightheadedness and often depression and apathy. Vomiting typically follows.
Additional symptoms are frequent, but more variable. These include belching and flatulence, hyperventilation, sighing and yawning, headache, tightness around the forehead or a "buzzing" sensation, drowsiness, lethargy and somnolence, panic or confusion. The lethargy, fatigue, and drowsiness can persist after the stimulus stops and nausea lessens.
Over time, there is a tendency to adapt ("to get one's sea legs"). For most individuals this occurs by 2 to 3 days, although about 5% are said not to adapt and remain symptomatic if the stimulus persists. Returning to stable circumstances, as in returning to shore, can trigger an exacerbation, but this is usually shorter because readaptation is quicker. Personal susceptibility tends to be a stable and enduring characteristic, and is predictive of greater susceptibility in the future.
Boat travel is most likely to cause motion sickness, followed by travel by air, car, and train. The incidence of motion sickness may be illustrated by the following examples: 98% of occupants of life rafts in rough seas vomit; 60% of student aircrew suffer from air sickness (and in 15% it is of sufficient severity to interfere with flying training), but less than 0.5% of passengers in civil aircraft are affected. Differences between subjects in tolerance to provocative motion can, in part, be related to their physical and mental constitution. Infants below the age of 2 years are rarely affected, but with maturation, susceptibility increases rapidly to reach a peak between 4 to 10 years. Thereafter, susceptibility falls progressively so that the elderly (60 years) are relatively immune. In any age group, females are more sensitive than males. Certain dimensions of personality, such as neuroticism, introversion and perceptual style have also been shown to be correlated, albeit weakly, with susceptibility. Motion sickness can also be a conditioned response and a manifestation of phobic anxiety.
Some people are more prone to this condition than others, but factors such as turbulence, anxiety and illness can also trigger motion sickness. As an indication, the market for travel sickness remedies in the UK in 1986 was £3.3 million.