Motion Sickness

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.

According to Medilexicon's medical dictionary:
Motion sickness is the syndrome of pallor, nausea, weakness, and malaise, which may progress to vomiting and incapacitation, caused by stimulation of the semicircular canals during travel or motion as on a boat, plane, train, car, swing, or rotating amusement ride.

In the inner ear (which is also called the labyrinth), motion sickness affects the sense of balance and equilibrium and, hence, the sense of spatial orientation.

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About 33% of people are susceptible to motion sickness even in mild circumstances such as being on a boat in calm water, although nearly 66% of people are susceptible in more severe conditions.

There is no difference between motion sickness and sea sickness. Some people experience nausea and even vomiting when riding in an airplane, automobile, or amusement park ride. This condition is generally called motion sickness. Many people experience motion sickness when riding on a boat or ship. This is commonly referred to as sea sickness (mal de mer), even though it is the same disorder. Individuals and animals without a functional vestibular system are immune to motion sickness.
What are the symptoms of Motion Sickness?

A symptom is something the patient senses and describes, while a sign is something other people, such as the doctor notice. For example, drowsiness may be a symptom while dilated pupils may be a sign.

The symptoms overall of motion sickness include nausea, vomiting, and dizziness (vertigo). Other common signs are sweating and a general feeling of discomfort and not feeling well (malaise).

Mild symptoms are categorized as headache, mild unease and yawning. More serious symptoms include nausea, vomiting, pallor, sweating, drooling, short breath, dizziness and drowsiness.

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. What are the causes of Motion Sickness?

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.

The cause of motion sickness is complex, however, and the role of conflicting input is only a hypothesis 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.

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