Motion Sickness

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What is motion sickness?
What causes motion sickness?
Factors contributing to motion sickness
How common is motion sickness?
Symptoms of motion sickness

What is motion sickness?

Motion sickness

Motion sickness, or kinetosis, is a common condition experienced by people travelling on planes, cars, trains, and especially on boats. It can even occur when exposed to moving scenes in virtual reality games. It is characterised by symptoms of dizziness, nausea and fatigue.

There are several types of motion sickness, depending on the underlying cause. These include seasickness, carsickness, airsickness, space sickness and simulation sickness. It can also occur on horseback. Motion sickness is a major issue for astronauts during space travel, and for navymen at sea.

What causes motion sickness?

Motion sickness occurs when parts of the inner ear that help control balance are stimulated too much. This can be caused by excessive repetitive motion, such as that experienced while travelling in a moving car, boat, plane, space craft or on amusement park rides.

Movement is sensed by three systems in the body: the eyes, the vestibular system, and the muscle and joint sensory receptors of the limbs (nerve endings). The motion signals transmitted by these three systems communicate information to the brain about the positioning of the body in space and how it should respond to movement.

The most widely accepted explanation for the symptoms of motion sickness is the sensory conflict hypothesis. This states that motion sickness occurs when there is repeated mismatches between the information received from sensory receptors and what is expected from previous experience. Thus, motion sickness occurs when what is seen does not match with what is felt. The inner ear tells the brain that the body is moving, but the eyes convey information that it is stationary. For example, when at sea, the eyes view the static horizon or unmoving interior of boat, while the body feels the rolling of the waves. While travelling in a car, the limbs and ears feel little movement, but the eyes perceive scenery passing by. If the motion sensing organs of the inner ear do not work, then motion sickness does not occur. This suggests that the inner ear is critical for the development of motion sickness, and supports the hypothesis.

Motion is actually not a requirement for motion sickness to occur. It can be experienced during activities such as virtual reality rides and playing video games, or by watching movies shot on a shaky camera. The illusion of motion created by the virtual world, combined with the absence of motion detected by the inner ear, results in mixed messages being sent to the brain.

The symptoms underlying motion sickness are thought to develop as part of the body’s natural defence mechanism. The mixed signals the brain receives lead the brain to conclude that it is hallucinating, and that this hallucination is due to poisoning by a neurotoxin. The brain responds by inducing nausea and vomiting to clear the toxin.

Factors influencing motion sickness

Factors that can exacerbate motion sickness include poor ventilation, anxiety and fear. Heavy meals of spicy, greasy food before and during a trip can upset the stomach further, as can alcohol. Pregnancy is also known to increase susceptibility to motion sickness.

There is a clear variation in individual susceptibility which is not well understood. Some research suggests that there may be genetic factors involved and that it may be possible to inherit a predisposition to motion sickness. This has not been definitively confirmed, though the fact that motion sickness is more common in some ethnic groups helps to support this theory. It has been observed that persons of Chinese or Japanese origin are more vulnerable to motion sickness than those of British ancestry.

How common is motion sickness?

Whether or not a person develops motion sickness depends on their individual susceptibility and ability to adapt to movement, as well as the nature and severity of the movement. Almost anyone can experience motion sickness if the motion is severe enough. About half of all astronauts suffer from motion sickness during space travel.

Sex and age also play a role in determining susceptibility. Women are more vulnerable than men, particularly during pregnancy and menstruation. Motion sickness is very common in children older than two years. It peaks at around 9-10 years, then begins to decline into the 20s. This decline may be due to habituation, in which a person stops responding to repetitive stimuli. Infants and children under two years appear to be immune to motion sickness.

People with certain medical conditions are more susceptible to motion sickness. This includes migraine sufferers, those with Meniere's disease (a disorder of the inner ear), and any patients who have an inner ear disease or vertigo.