What is 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. 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.

What causes 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.

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.

What are the symptoms of motion sickness?

The symptoms 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).

Is there a difference between motion sickness and sea sickness?

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. They are one and the same phenomenon and will subsequently be referred to as motion sickness.

How does our sense of balance work?

Motion sickness relates to our sense of balance and equilibrium. Researchers in space and aeronautical medicine call this sense spatial orientation, because it tells the brain where the body is "in space:" what direction it is pointing, what direction it is moving, and if it is turning or standing still.

Our sense of balance is regulated by a complex interaction of the following parts of the nervous system:

1. The inner ears (also called the labyrinth) monitor the directions of motion, such as turning or forward-backward, side-to-side, and up-and- down motions.


2. The eyes observe where the body is in space (for example, upside down, right side up, etc.) and also the directions of motion.


3. Skin pressure receptors such as those located in the feet and seat sense what part of the body is down and touching the ground.


4. Muscle and joint sensory receptors report what parts of the body are moving.


5. The central nervous system (the brain and spinal cord) processes all the bits of information from the four other systems to make some coordinated sense out of it all.

How does motion sickness affect our sense of 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 during a storm, and the plane is being tossed about by air turbulence. But 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."

Another example illustrates an actual medical condition. Suppose you suffer inner ear damage on only one side from either a head injury or an infection. The damaged inner ear does not send the same signals to the brain as the healthy ear. These conflicting signals about the sensation of rotation can result in a sense of spinning or vertigo, as well as nausea.

Is motion sickness a serious condition?

Not usually. For most people, motion sickness is just a minor annoyance (although it may not feel so minor at the time). Some travelers, however, can be incapacitated by motion sickness.

When do the nausea and vomiting of motion sickness stop?

The distressing symptoms of motion sickness usually stop when the motion that causes it ceases. But this is not always true. There are people who suffer symptoms for even a few days after the trip is over. This is called the "mal d'embarquement" syndrome or, more properly, the "mal de debarquement" syndrome. ("Mal d'embarquement" is embarkment or departure sickness while "mal de debarquement" is disembarkment or arrival sickness.)

What measures can be taken to prevent or minimize motion sickness?

If you know you are prone to motion sickness or if you are suffering from it, we recommend the following:

1. Always ride where your eyes will see the same motion that your body and inner ears feel.
   
   
   • In a car, sit in the front seat and look at the distant scenery.
     
     
   • On a boat, go up on the deck and watch the motion of the horizon.
     
     
   • In an airplane, sit by the window and look outside.
     
     
   • Also, in a plane, choose a seat over the wings where the motion is minimized

2. Do not read while traveling if you are subject to motion sickness, and do not sit in a seat facing backward.

3. Do not watch or talk to another traveler who is having motion sickness.

4. Avoid strong odors and spicy or greasy foods that do not agree with you (immediately before and during your travel). Medical research has not yet investigated the effectiveness of popular folk remedies such as "soda crackers and 7 Up" or "cola syrup over ice," or ginger products.

5. Take one of the varieties of motion sickness medicines before your travel begins, as recommended by your physician.

How is motion sickness treated?

Antihistamine medications are commonly used in the prevention and treatment of motion sickness. These medicines seem to prevent and treat the nausea, vomiting, and dizziness caused by motion sickness by calming the stimulation of the inner ear. Examples of antihistamines used for this purpose include meclizine (Bonine, Antivert, Dramamine). Another class of medications (belladonna) used to prevent motion sickness is represented by the scopolamine skin patch (Transderm Scop). This medicated skin patch is applied behind the ear at least four hours in advance of the motion activity. The medication is slowly absorbed directly into the underlying skin.

When used specifically to prevent motion sickness, regardless of the type of medication, these medicines generally are most effective when administered well before the motion activity takes place.

When should I see a doctor for motion sickness?

Most cases of motion sickness are mild and self-treatable. However, very severe cases and those that become progressively worse deserve the attention and care of a physician with special skill in diseases of the ear, balance (equilibrium), and the nervous system.

Where can I get further information about motion sickness?

Further information can be obtained from the American Academy of Otolaryngology - Head and Neck Surgery, Inc., One Prince Street, Alexandria VA 22314 (which kindly provided some of the information contained in this article).

Motion Sickness At A Glance

• Motion sickness is a disturbance of the inner ear.
  
  
• The sense of balance and equilibrium is affected.
  
  
• The symptoms of motion sickness usually, but not always, stop when the motion that causes it ceases.
  
  
• Medications are available for the prevention and treatment of motion sickness.

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