Given that nearly half the population suffers to some degree from the condition, remarkably little is known about what triggers the sweaty palms and the rising tightness in the throat, or how they should be treated.
Research into motion sickness could have wide implications. When its biological mechanisms are better understood, new ways of treating sickness in early pregnancy and nausea in cancer patients on chemotherapy may be on the cards.
One researcher, Dr Kenneth Koch, a gastroenterologist at the Hershey Medical Center, Pennsylvania, points out that however unpleasant a serious bout of travel sickness may be, it does not kill you and the symptoms disappear when the motion stops. Dr Koch has discovered that although the feelings of nausea are triggered by electrical disturbances in the stomach muscles, the chain of events starts in the eyes and ears, and involves the neurological and hormonal systems centred on the brain.
Motion sickness begins when the ears and eyes receive and process contradictory messages. Imagine the sufferer is travelling in the back of a car, standing in a duty-free shop on a Channel ferry, or strapped into an airline seat when the plane hits a patch of turbulence.
In any of these circumstances, the inner ear - which houses the mechanism that helps us to keep our balance - registers the changes in speed and direction, for example, as the boat pitches and tosses. But the messages the eyes receive are different. The contents of the duty-free shop and the other people in it are all moving with the boat, which to the sufferer looks relatively stable.
This explains why travel-sick people feel better if they look out of the car window, or go on deck and look at the waves or the horizon - and why it is difficult to do anything about it in an aeroplane. It is not that looking at a fixed object helps per se; rather that allowing our eyes to register that we really are moving up and down brings the messages from eyes and ears into harmony.
Dr Koch says that when the messages received by the brain continue to contradict each other, hormones are released which shortly afterwards start the stomach churning - and the ghastly symptoms begin.
The challenge for Dr Koch and his colleagues has been to find out how the hormones are responsible - but first they had to simulate the sort of motion that makes people feel sick. They built a revolving drum that creates the illusion of motion, even though the volunteers are sitting still on a stool. The drum has black and white vertical stripes painted on the inside and rotates around the volunteers about once every six seconds. In less than a minute they are deceived into feeling that they are moving. But their inner-ear mechanisms, and pressure receptors in other parts of the body, remind them they are really sitting still.
Volunteers undergo this torture for up to 15 minutes. They report how they feel at different stages, blood samples are taken and electrodes measure neurological activity.
Dr Koch and his team have looked at two hormones produced in response to stress, epinephrine and norepinephrine, and found that levels of both rose quickly when volunteers started to feel sick. They have also investigated a third hormone, vasopressin, which is involved in a wide range of functions in body organs. After a few minutes in the drum, volunteers' levels of vasopressin start to climb steeply, coinciding with increased electrical activity in the stomach. Instead of the usual three impulses per minute that ripple from the top of the stomach to the small intestine, these electric rhythms 'go bonkers' and their rate triples, Dr Koch says.
At first a motion-sickness sufferer may be only dimly aware that something is happening in the guts. But as the gut continues in overdrive, the body cannot take it and eventually the stomach heaves and the sufferer vomits. 'When the drum stops rotating and the nausea is resolving, the levels of vasopressin fall off,' Dr Koch says. 'We have found, by taking samples every 30 seconds, that the levels of vasopressin are most associated with feelings of motion sickness.'
Now scientists need to find out why high levels of vasopressin cause feelings of nausea. When they have done that, it may be possible to develop a drug to block its activity.
But the role of hormones is not the whole story. Researchers at Sheffield University's Centre for Human Nutrition have built their own drum and are looking into the increased gastric activity. The UK version has only been spinning since the beginning of the year, but one study conducted by Dr Keith Reid and Mohammed Khan, a physiologist, has just been completed.
They have discovered that the mismatch between the signals received from the ears and the eyes delays the rate at which the stomach empties. But volunteers who were about to retch did not necessary have the slowest rates. This appears to confirm the wide variation in individual experience of motion sickness and may explain why eating a starchy, low-fat meal before a journey, or nibbling dry biscuits during it, may stop some people feeling ill.
According to Dr Koch, it is almost as if you are giving the stomach something to do which overrides the other messages it is receiving. This is corroborated by Mr Khan, who points out that independent research has shown that some people feel nauseated when they eat large quantities of fat, which is known to slow down gastric emptying.
There are many lines of research to explore both in motion sickness and other forms of nausea. What, for example, happens to the levels of vasopressin in pregnant women? Do any of the newest drugs available to counteract the nausea associated with chemotherapy have any effect on electrical activity in the gut and gastric emptying? And can a drug be found that will both block the release of vasopressin and maintain gastric emptying?
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