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Motion Sickness

April 22, 2015

Motion sickness

Introduction

Motion sickness (kinetosis or travel sickness) occurs when the body is subjected to accelerations of movement in different directions or under conditions where visual contact with the actual outside horizon is lost. Motion sickness is a very common disturbance of the inner ear that is caused by repeated motion such as from the swell of the sea, the movement of a car, the motion of a plane in turbulent air, etc. In the inner ear (also called the labyrinth), motion sickness affects the sense of balance and equilibrium and, hence, the sense of spatial orientation. The balance center of the inner ear then sends information to the brain that conflicts with the visual clues of apparently standing still in the interior cabin of a car, ship or airplane.

Symptoms

Symptoms generally consist of dizziness, fatigue, and nausea which may progress to vomiting. Unlike ordinary sickness, vomiting in motion sickness tends not to relieve the nausea. Fear or anxiety can promote symptoms. Other common signs are sweating and a general feeling of discomfort and not feeling well (malaise). Sopite syndrome in which a person feels fatigue or tiredness is also associated with motion sickness. The list of signs and symptoms mentioned in various sources includes: drowsiness, yawning, disinterest in work, lack of participation in group activities, mood changes, sleep disturbances, mental depression.

Causes

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.

The symptoms of motion sickness appear when the central nervous system receives conflicting messages from the other four systems – the inner ear, eyes, skin pressure receptors, and the muscle and joint sensory receptors. For example, imagine you are riding in an airplane that is being tossed about by air turbulence. Your eyes do not detect all this motion because all you see is the inside of the airplane. Consequently, your brain receives messages that do not coordinate with each other. You might become “air sick.” Or suppose you are sitting in the back seat of a moving car reading a book. Your inner ears and skin receptors detect the motion of your travel, but your eyes see only the pages of your book. You could become “car sick.”

Motion is sensed by the brain through three different pathways of the nervous system that send signals coming from the inner ear (sensing motion, acceleration, and gravity), the eyes (vision), and the deeper tissues of the body surface (proprioceptors). When the body is moved intentionally, for example, when we walk, the input from all three pathways is coordinated by our brain. When there is unintentional movement of the body, as occurs during motion when driving in a car, the brain is not coordinating the input, and there is thought to be discoordination or conflict among the input from the three pathways.

It is hypothesized that the conflict among the inputs is responsible for motion sickness. For example, when we are sitting watching a picture that depicts a moving scene, our vision pathway is telling our brain that there is movement, but our inner ear is telling our brains that there is no movement. Thus, there is conflict in the brain, and some people will develop motion sickness in such a situation (even though there is no motion).

The cause of motion sickness is complex, however, and the role of conflicting input is only a hypothesis, or a proposed explanation, for its development. Without the motion-sensing organs of the inner ear, motion sickness does not occur, suggesting that the inner ear is critical for the development of motion sickness. Visual input seems to be of lesser importance, since blind people can develop motion sickness.

Motion sickness is more likely to occur with complex types of movement, especially movement that is slow or involves two different directions (for example, vertical and horizontal) at the same time. The conflicting input within the brain appears to involve levels of the neurotransmitters (substances that mediate transmission of signals within the brain and nervous system) histamine, acetylcholine, and norepinephrine. Many of the drugs that are used to treat motion sickness act by influencing or normalizing the levels of these compounds within the brain.

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 disconcordance, the brain will come to the conclusion that one of them is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing vomiting, to clear the supposed toxin.

Motion sickness can be divided into three categories:

Motion sickness caused by motion that is felt but not seen.

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. 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 worst when looking down but may be significantly lessened by looking outside of the vehicle. Looking out of the windshield is particularly beneficial.

Motion sickness caused by motion that is seen but not felt.

Motion sickness due to films and other video. This type of sickness is particularly prevalent when susceptible people are watching films on large screens or even when watching TV. Imax and other panoramic type theaters often show dramatic motions such as flying over a landscape or riding a roller coaster. There is little way to prevent this type of motion sickness except to close one’s eyes during such scenes or to avoid such movies.

Watching 3-D movies such as Avatar can cause headaches. Those are related to a difficulties with the focusing, since the clear part of the screen is the single object and not the whole screen as it would be on a conventional screen. The brain tries to bring up the whole picture and it results in a constant attempt to focus what results to headache. Obviating the rest of the objects and focusing on the object that is clear prevents that type of ache.

In regular format theaters, an example of a movie that caused motion sickness in many people is The Blair Witch Project. Theater patrons were warned of its possible nauseating affects, cautioning pregnant women in particular. In this case, a camcorder was used to film the movie. As the camera was hand held, the camera 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 his/her sense of motion matches the motion seen through the camera 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.

Motion sickness caused when both systems detect motion but they do not correspond.

Coriolis effect. Sometimes when riding a vehicle for a long time on a badly maintained road at a very slow (10-20 Kmph) 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 proportional amount of motion.

Source: http://www.medicinenet.com/motion_sickness/page1.htm

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