Usually, the first sign of seasickness is pallor, although this occasionally may be preceded by a flushed appearance1. This may be followed by yawning, restlessness and a cold sweat, often noticeable on the forehead and upper lip. Malaise, nausea and vomiting may progress to prostration, dehydration and electrolyte and acid-base imbalance, although these latter and more serious manifestations usually appear only in intractable seasickness during long periods at sea. During this progression, there is often a waxing and waning of symptoms, especially before the actual development of vomiting, and vomiting itself often brings temporary relief.
Tolerance develops to a particular motion, and a person may become acclimatized to specific conditions. If there is a change in the intensity or nature of the motion, the individual may again be susceptible. Continuous exposure to constant conditions usually produces tolerance within 2 to 3 days. Tolerance can also develop to repeated shorter exposures. There is a central nervous system habituation to such a degree that after the person disembarks and the motion is stopped, the person feels that he or she is rocking at the frequency of the original ship exposure.
There is considerable variation in susceptibility to seasickness. With increasing age individuals tend to become more resistant, and at least one study suggests that girls and women are more susceptible2. This susceptibility is said to result from a lack of experience with the situations that produce seasickness. Overindulgence in food and alcohol before exposure, and especially the night before, predisposes to motion sickness. Both the number of meals and their energy content correlate with susceptibility to airsickness2.The position on board the vessel can also be important, with least stimuli if the victim is amidships and using the horizon as a visual reference. Any attempt to read aggravates motion sickness. Psychological factors play a part, especially with seasickness that develops before boarding the vessel. Once one person becomes seasick, there is often a rapid spreading among others present.
AETIOLOGY：Motion sickness is caused by a mismatch or conflict of sensorineural information3. Normally, the vestibular stimuli are consistent with the visual and proprioceptive stimuli, all informing the brain of the position of the body – even when it is in motion. When the environment starts moving as well, the information becomes conflicting. The motion sickness occurs at the onset and cessation of sensory rearrangements; when input of vision, vestibular and proprioception is at variance with the stored patterns of recent stimuli information.