Motion Sickness

Motion sickness is a form of dizziness with nausea and vomiting that can occur when you are a passenger in rapidly moving vehicles like cars, boats, airplanes, and spaceships. It can come on when using moving devices like swings, or riding on an amusement park ride.  Even watching certain rapidly-rotating objects like a ceiling fan can sometimes set it off.  This sickness only arises in people with functioning inner ear balance systems; people who have had both inner ears destroyed cannot experience seasickness or any of the other forms of motion sickness.  It is more prevalent and easier to trigger in people with a personal or family history of migraine. 

There are many symptoms of motion sickness, although nausea and vomiting are the most dreaded.  Seasickness begins gradually with dizziness, yawning and swallowing.  The sufferer becomes pale and apathetic, and will often want to lie down and avoid movement.  Nausea and retching or vomiting quickly follow this phase.  Many people experience a sense of hopelessness or impending doom.  On prolonged cruises the symptoms tend to diminish after a few days, as the person gradually becomes accustomed to the motion.

There are two general types of situations that are likely to trigger motion sickness: excessive motion, and sensory mismatch.  Excessive motion means exposure to strong, unfamiliar, changing accelerations and directions, especially if these are repetitive.  Boat travel is particularly effective because most people travel only rarely by boat, and the boat can move forward, up, down and tilt sideways in a rhythmic fashion.  A boat traveling in a perfectly straight line at constant speed on a flat lake is much less likely to cause motion sickness than one that is turning, rising and falling on rollers in the ocean. Similarly, riding up and downhill in a car on a winding mountain road requiring constant changes in speed is much more likely to cause symptoms than riding in the same car slowly and steadily along a straight, flat highway. These circumstances are most likely to cause symptoms if you are not used to the activity; once you have become accustomed to similar movements (like a career sailor on a ship) your tendency to motion sickness is reduced.

Sensory mismatch is a second source for motion sickness and adds to the problem of excessive motion.  When you jog, you can feel the bouncing and forward movement through the inner ears, and see the visual environment streaming past you simultaneously.  In this circumstance, the two vestibular sensors, the inner ears and the eyes, receive matching information about movement.  In other situations these two different sensors may receive conflicting information.  For example, if you are resting in bed on a ship, your inner ears will receive information about the up, down and tilting movements the ship is making, but your eyes will see what appears to be a stable room surrounding you.  This mismatch has the effect of accelerating the onset of motion sickness.  Similarly, in a wide-screen theater, your inner ears do not detect motion because the seat remains still, but if a scene of a steeply banking airplane flying over hilly terrain is shown, you may feel a sensation of motion in the head that is generated from the eyes.  This mismatch, if repeated long enough, can also result in motion sickness.

The likelihood of motion sickness can be reduced by controlling its two causes.  If you are prone to motion sickness, you can avoid situations in which it is most likely—choosing a boat trip on a bay in good weather rather than an ocean voyage in a storm.  If  the choice of transportation is out of your control, you can take a medication to reduce the amount of motion information received by your ears. Vestibular suppressants such as meclizine, diphenhydramine  or scopolamine are particularly effective. You should also work to reduce sensory mismatch.  Position yourself so that you are able to see the environment around your vehicle and focus on the horizon in front of you rather than on objects moving quickly past.  Reducing voluntary head movement by gazing steadily ahead also reduces the amount of changing motion information your balance system receives.

Now it’s time to move on to some of the different causes of vertigo. We’ll start by discussing people who have repeated spells of dizziness in our next post.

Published by Vertigone

I translate the medical world of dizziness for non-medical people

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