Motion sickness, also known as kinetosis and travel sickness, is a condition in which a disagreement exists between visually perceived movement and the vestibular system's sense of movement. Depending on the cause, it can also be referred to as seasickness, car sickness, simulation sickness or airsickness.
Dizziness, fatigue and nausea are the most common symptoms of motion sickness. Sopite syndrome, in which a person feels fatigue or tiredness, is also associated with motion sickness. "Nausea" in Greek means seasickness (naus means ship). If the motion causing nausea is not resolved, the sufferer will usually vomit. Vomiting often will not relieve the feeling of weakness and nausea, which means the person might continue to vomit until the cause of the nausea is treated.
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Cause
The most common hypothesis for the cause of motion sickness is that it functions as a defense mechanism against neurotoxins. The area postrema in the brain is responsible for inducing vomiting when poisons are detected, and for resolving conflicts between vision and balance. When feeling motion but not seeing it (for example, in a ship with no windows), the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still. As a result of the discordance, the brain will come to the conclusion that the individual is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing vomiting, to clear the supposed toxin. Treisman's indirect argument has recently been questioned via an alternative direct evolutionary hypothesis, as well as modified and extended via a direct poison hypothesis. The direct evolutionary hypothesis essentially argues that there are plausible means by which ancient real or apparent motion could have contributed directly to the evolution of aversive reactions, without the need for the co-opting of a poison response as posited by Treisman. Nevertheless, the direct poison hypothesis argues that there still are plausible ways in which the body's poison response system may have played a role in shaping the evolution of some of the signature symptoms that characterize motion sickness.
An alternative theory, also known as the Nystagmus Hypothesis, has been proposed based on stimulation of the vagus nerves resulting from the stretching or traction of extra-ocular muscles [1] co-occurring with eye movements caused by vestibular stimulation. There are three critical aspects to the theory: first is the close linkage between activity in the vestibular system, i.e., semicircular canals and otolith organs, and a change in tonus among various of each eye's six extra-ocular muscles. Thus, with the exception of voluntary eye movements, the vestibular and oculomotor systems are thoroughly linked. Second is the operation of Sherrington's Law describing reciprocal inhibition between agonist-antagonist muscle pairs, and by implication the stretching of extraocular muscle that must occur whenever Sherrington's Law is made to fail, thereby causing an unrelaxed (contracted) muscle to be stretched. Finally there is the critical presence of afferent output to the Vagus nerves as a direct result of eye muscle stretch or traction. Thus, 10th nerve stimulation resulting from eye muscle stretch is proposed as the cause of motion sickness. The theory explains why labyrinthine defective individuals are immune to motion sickness; why symptoms emerge when undergoing various body-head accelerations; why combinations of voluntary and reflexive eye movements may challenge the proper operation of Sherrington's Law, and why many drugs that suppress eye movements also serve to suppress motion sickness symptoms.
Susceptibility
Roughly one third of the population are highly susceptible to motion sickness and most of the rest may get motion sickness under extreme conditions. Several factors influence susceptibility to motion sickness. Statistics indicate that women are more likely to be affected than men and that the risk decreases with advancing age. There is some evidence that people with Asian ancestry get motion sickness more frequently compared with people of European ancestry, and there are situational and behavioral factors, such as whether a passenger has a view of the road ahead, and diet and eating behavior.
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Types
Motion sickness can be divided into three categories:
- Motion sickness caused by motion that is felt but not seen
- Motion sickness caused by motion that is seen but not felt
- Motion sickness caused when both systems detect motion but they do not correspond.
Motion is felt but not seen
In these cases, motion is sensed by the vestibular system and hence the motion is felt, but no motion or little motion is detected by the visual system.
Car sickness
A specific form of motion sickness, car sickness is quite common and often evidenced by the inability to read a map or book during travel. Car sickness results from the sensory conflict arising in the brain from differing sensory inputs. Varying theories exist as to cause. One suggests the eyes view motion while riding in the moving vehicle while other body sensors sense stillness, creating conflict between the eyes and inner ear. Another suggests the eyes mostly see the interior of the car which is motionless while the vestibular system of the inner ear senses motion as the vehicle goes around corners or over hills and even small bumps. Therefore, the effect is worse when looking down but may be lessened by looking outside of the vehicle.
In the early 20th century, Austro-Hungarian scientist Robert Barany observed the back and forth movement of the eyes of railroad passengers as they looked out the side windows at the scenery whipping by. He called it "railway nystagmus." Also called "optokinetic nystagmus." It causes nausea and vomiting. His findings were published in the journal Laeger, 83:1516, Nov.17, 1921.
Air sickness
Air sickness is a sensation which is induced by air travel. It is a specific form of motion sickness and is considered a normal response in healthy individuals. It is essentially the same as car sickness but occurs in an aeroplane. However, some significant differences are that an aeroplane may bank and tilt sharply and due to the small window sizes, and unless the passenger is at a window seat, is likely to see only the stationary interior of the plane. Another factor is that while in flight, the view out of windows may be blocked by clouds, preventing a passenger at the window from seeing the moving ground or moving lower clouds.
Sea sickness
Sea sickness is a form of motion sickness characterized by a feeling of nausea and, in extreme cases, vertigo experienced after spending time on a craft on water. It is essentially the same as car sickness, though the motion of a watercraft tends to be more regular. It is typically brought on by the rocking motion of the craft or movement while immersed in water. As with air sickness, it can be difficult to visually detect motion even if one looks outside of the boat as water does not offer fixed points with which to visually judge motion. Poor visibility conditions, such as fog, may worsen sea sickness. Some sufferers of car sickness are resistant to sea sickness and vice versa.
Centrifuges
Rotating devices such as centrifuges used in astronaut training and amusement park rides such as the Rotor, Mission: Space and the Gravitron can cause motion sickness in many people. While the interior of the centrifuge does not appear to move, one will experience a sense of movement. In addition, centrifugal force can cause the vestibular system to give one the sense that downward is in the direction away from the center of the centrifuge rather than the true downward direction.
Dizziness due to spinning
When one spins and stops suddenly, fluid in the inner ear continues to rotate causing a sense of continued spinning while one's visual system no longer detects motion.
Motion that is seen but not felt
In these cases, motion is detected by the visual system and hence the motion is seen, but no motion or little motion is sensed by the vestibular system. Motion sickness arising from such situations has been referred to as "visually induced motion sickness" (VIMS).
Films and other video
This type of sickness is particularly prevalent when susceptible people are watching films on large screens such as IMAX, but may also occur in regular format theaters or even when watching TV. For the sake of novelty, IMAX and other panoramic type theaters often show dramatic motions such as flying over a landscape or riding a roller coaster. This type of motion sickness can be prevented by closing one's eyes during such scenes.
In regular format theaters, an example of a movie that caused motion sickness in many people is The Blair Witch Project. Theaters warned patrons of its possible nauseating effects, cautioning pregnant women in particular. Blair Witch was filmed with a handheld camcorder, which was subjected to considerably more motion than the average movie camera.
Home movies, often filmed with a hand-held camera, also tend to cause motion sickness in those that view them. The camera-person rarely notices this during filming since their sense of motion matches the motion seen through the camera's viewfinder. Those who view the film afterward only see the movement, which may be considerable, without any sense of movement. Using the zoom function seems to contribute to motion sickness as well, as zooming is not a normal function of the eye. The use of a tripod or a camcorder with image stabilization technology while filming can minimize this effect.
Virtual reality
Motion sickness due to virtual reality is very similar to simulation sickness and motion sickness due to films. In virtual reality, however, the effect is made more acute as all external reference points are blocked from vision, the simulated images are three-dimensional and in some cases stereo sound that may also give a sense of motion. The NADS-1, a simulator located at the National Advanced Driving Simulator, is capable of accurately stimulating the vestibular system with a 360-degree horizontal field of view and 13 degrees of freedom motion base. Studies have shown that exposure to rotational motions in a virtual environment can cause significant increases in nausea and other symptoms of motion sickness.
In a study conducted by the U.S. Army Research Institute for the Behavioral and Social Sciences in a report published May 1995 titled "Technical Report 1027 - Simulator Sickness in Virtual Environments", out of 742 pilot exposures from 11 military flight simulators, "approximately half of the pilots (334) reported post-effects of some kind: 250 (34%) reported that symptoms dissipated in less than one hour, 44 (6%) reported that symptoms lasted longer than four hours, and 28 (4%) reported that symptoms lasted longer than six hours. There were also four (1%) reported cases of spontaneously occurring flashbacks."
Space sickness
Space sickness was effectively unknown during the earliest spaceflights, as these were undertaken in very cramped conditions; it seems to be aggravated by being able to freely move around, and so is more common in larger spacecraft. Around 60% of Space Shuttle astronauts currently experience it on their first flight; the first case is now suspected to be Gherman Titov, in August 1961 onboard Vostok 2, who reported dizziness and nausea. However, the first significant cases were in early Apollo flights; Frank Borman on Apollo 8 and Rusty Schweickart on Apollo 9. Both experienced identifiable and reasonably severe symptoms--in the latter case causing the mission plan to be modified.
Motions that are seen and felt but do not correspond
When moving within a rotating reference frame such as in a centrifuge or environment where gravity is simulated with centrifugal force, the coriolis effect causes a sense of motion in the vestibular system that does not match the motion that is seen.
Sometimes when riding a vehicle for a long time on a badly maintained road at a very slow (10-20 km/h) speed the two senses fail to correspond. Due to the poor road quality the vehicle will jerk too much giving a sense of severe motion to the inner ear, but due to the slow speed, the eye doesn't sense a proportional amount of motion.
Treatment
Many cures and preventatives for motion sickness have been proposed.
Devices
A head-worn, computer device with a transparent display can be used to mitigate the effects of motion sickness (and spatial disorientation) if visual indicators of the wearer's head position are shown. Such a device functions by providing the wearer with digital reference lines in their field of vision that indicate the horizon's position relative to the user's head. This is accomplished by combining readings from accelerometers and gyroscopes mounted in the device (US Patent 5,966,680). This technology has been implemented in both standalone devices and Google Glass. In two NIH-backed studies, greater than 90% of patients experienced a reduction in the symptoms of motion sickness while using this technology.
Activity
One common suggestion is to simply look out of the window of the moving vehicle and to gaze towards the horizon in the direction of travel. This helps to re-orient the inner sense of balance by providing a visual reaffirmation of motion.
In the night, or in a ship without windows, it is helpful to simply close one's eyes, or if possible, take a nap. This resolves the input conflict between the eyes and the inner ear. Napping also helps prevent psychogenic effects (i.e. the effect of sickness being magnified by thinking about it).
Fresh, cool air can also relieve motion sickness slightly, although it is likely this is related to avoiding foul odors which can worsen nausea.
While playing computer games, and mainly in first-person shooter games, some cases of simulation sickness can be resolved by changing the field of view in the game. Some games have a default setting which places a player's vision a small distance ahead of the actual object controlled, which will most likely trigger simulation sickness.
Medication
Over-the-counter and prescription medications are readily available, such as Dramamine/Gravol (dimenhydrinate), scopolamine, Bonine/Antivert (meclizine), promethazine, cyclizine, and Stugeron (cinnarizine). Cinnarizine is not available in the United States, as it is not approved by the FDA. As these medications often have side effects, anyone involved in high-risk activities while at sea (such as SCUBA divers) must evaluate the risks versus the benefits. Promethazine is especially known to cause drowsiness, which is often counteracted by ephedrine in a combination known as "the Coast Guard cocktail.". There are special considerations to be aware of when the common anti-motion sickness medications are used in the military setting where performance must be maintained at a high level.
Scopolamine is effective and is sometimes used in the form of transdermal patches (1.5 mg) or as a newer tablet form (0.4 mg). The selection of a transdermal patch or scopolamine tablet is determined by a doctor after consideration of the patient's age, weight, and length of treatment time required.
Many pharmacological treatments which are effective for nausea and vomiting in some medical conditions may not be effective for motion sickness. For example, metoclopramide and prochlorperazine, although widely used for nausea, are ineffective for motion-sickness prevention and treatment. This is due to the physiology of the CNS vomiting centre and its inputs from the chemoreceptor trigger zone versus the inner ear. Sedating anti-histamine medications such as promethazine work quite well for motion sickness, although they can cause significant drowsiness.
Ginger root is commonly thought to be an effective anti-emetic, but it is ineffective in treating motion sickness.
Electronic
As astronauts frequently have motion sickness, NASA has done extensive research on the causes and treatments for motion sickness. One very promising looking treatment is for the person suffering from motion sickness to wear LCD shutter glasses that create a stroboscopic vision of 4 Hz with a dwell of 10 milliseconds.
Source of the article : Wikipedia
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