The vestibular system appears to be the sources of the symptoms of motion sickness (airsickness). People who do not have functioning semicircular canals as result of a birth defect, surgical removal, or disease do not experience this problem. Any pilot, subjected to the right combinations of unusual attitudes or flight conditions, can become airsick. In other words, if the vestibular system can be “tumbled” in a way to which the pilot is not adapted, especially when there is a conflict with visual inputs, then he will become sick.
There is no doubt that currency, self confidence, and familiarity with the aircraft and its typical maneuvers is the best deterrent to air sickness.
Passengers (or pilots not in control of the aircraft) are easy subjects. New pilots are also susceptible. Some of the additional common causes include heat discomfort, anxiety, observing or smelling someone else who is airsick, and eating foods that are nauseating.
Etiology of motion sickness
The essential characteristic of stimuli that induce motion sickness is that they generate discordant information from the sensory system that provides the brain with information about spatial orientation and motion of the body.
The principal feature of this discord is a mismatch between the signals provided, principally, by the eyes and inner ear, and those that the central nervous system “experts” to receive and to be correlated.
Several categories of mismatch can be identified. Most important is the mismatch of signals from the vestibular apparatus (labyrinth) of the inner ear, in which the semicircular canals (the specialized receptors of angular accelerations) and the otolith organs (the specialized receptors of translational accelerations) do not provide concordant information. For example, when a head movement is make in a car or aircraft which is turning, both the semicircular canals and the otolith stimulated in an atypical manner and provide erroneous and incompatible information, information that differs substantially from that generated by the same head movement in a stable. 1-G gravity environment. Likewise, low frequency (below 0.5 Hz) linear accelerations, such as occur aboard ship in rough seas or in an aircraft during flight through turbulent air, also generate conflicting vestibular signals and, hence, are potent cause of motion sickness.
The mismatch of visual and vestibular information can also be an important contributory factor. The occupant of a moving vehicle who cannot see out is more likely to suffer from motion sickness that one who has good external visual reference.
The passenger below deck or in an aircraft cabin senses motion of the vehicle by vestibular cues, but he or she receives visual information only of his or her relative movement within the vehicle. The absence of an “expected” and concordant signal in a particular sensory modality is also considered to be the essential feature of visually induced motion sickness, because the visual motion cues are not accompanied by the vestibular signals that the individuals “experts” to occur when subjected to the motion indicated by the visual display.
Signs and symptoms of motion sickness
On exposure to provocative motion, the signs and symptoms of motion sickness develop in definite sequence, the time scale being dependent upon the intensity of the motion stimuli and the susceptibility of the individual.
There are, however, considerable differences among individuals not only in susceptibility, but also in the order in which particular signs and symptoms develop, or whether they are experienced at all.
Typically, the earliest symptom is epigastric discomfort (“stomach awareness”); this is followed by nausea, pallor and sweating, and is likely to be accompanied by feeling of bodily warmth, increased salivation and eructation (belching). These symptoms commonly develop relatively slowly, but with continuing exposure to the motion, there is rapid deterioration in well being, the nausea increase in severity and culminates in vomiting or retching. Vomiting may bring relief, but this is likely to be short lived unless the motion ceases.
There are other more variable features of the motion sickness syndrome. Alteration of respiratory rhythm with sighing and yawning may be an early symptom, and hyperventilation may occur, particularly in those who are anxious about the cause or consequences of their disability. Headache, tinnitus and dizziness are reported, while in those with severe malaise, apathy and depression are not uncommon, and may be of such severity that personal safety and survival are neglected. A feeling of lethargy and somnolence may be dominant following the cessation of provocative motion, and these may be the only symptoms in situations where adaptations to unfamiliar motions takes place without malaise.
Complications of motion sickness
Complications in the real sense of the world do not exist. Falls and aspiration caused by vomiting are sequels of the symptoms themselves.
Diagnosis of motion sickness
Motion sickness is diagnosed very simply, because the coincidence of the inducing motion and the typical symptoms is pathognomonic. Further diagnostic procedures are not necessary.
Prognosis of motion sickness
The prognosis of motion sickness is very favorable. At the least in the moment when the inducing motion ceases, a rapid decrease of the uncomfortable symptoms begins. After some hours, the symptoms of motions sickness disappear.
Differential Diagnosis of motion sickness
A differential diagnosis of motion sickness does not exist. The only exception is that a real vestibular disease can be triggered also be a specific movement or a sensory conflict.
Treatment of motion sickness
Most people know if they are prone to airsickness. Even so, there is no known prevention technique or adequate treatment to help. Much research is being carried out because motions sickness is prime concern in space travel.