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MOTION SICKNESS

AS NURSE, chef and purser aboard a 160-foot motor yacht called the Michaela Rose, Sheila Moore knows about motion sickness. In one particularly brutal North Atlantic storm, she recalls, a crewman from the Philippines threatened to jump overboard and swim home to escape the cold sweats, nausea and retching that gripped him. During another rough stretch along the coast of Morocco, Moore prepared a sumptuous shipboard Thanksgiving dinner for 10, only to have all but one of the guests flee the table. By midnight, Moore was ministering to that last brave diner, who after several hours of being violently ill was demanding that the yacht be put into port.

Not everybody gets sick at sea. In fact, vulnerability to motion sickness varies widely. Researchers say that for about one-third of us, a simple car trip on a bumpy road is enough to trigger the queasies. Another third require more unstable conditions -- say, a ride aboard a pitching boat -- to become upset. And the rest of us can handle just about anything short of a hurricane at sea.

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The currently favored theory of motion sickness is that the central nervous system can't reconcile the conflicting signals coming from the eyes, the inner ear and the rest of the body.

"You're in a ship's cabin. Your vision says you're not moving, yet your body is constantly pitching and you're having to catch yourself," explains Randall Kohl, senior research associate with the Johnson Space Center in Houston. "The brain is trying to cope with all this until a reaction comes that says, 'I can't deal with coordinating my balance in this environment!' "

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At this point, the central nervous system sends chemical messages, which in turn trigger physical symptoms. These often begin with excessive salivation, yawning, heat rushes and cold sweats, and may lead to pallor, vomiting and drowsiness. You could find yourself with a severe headache, and in the worst case, dehydration, lethargy and collapse.

Advice abounds for people who suffer such symptoms: Look to the horizon. Breathe fresh air. Eat lightly and avoid too much alcohol. If you're in a car, stop, get out and let your body recover.

For those who still suffer, or who don't want to take a chance when heading out to sea, there are a number of remedies -- various over-the-counter antihistamines, introduced in the 1950s and still the most common agents of choice, and the newer patch medication, which is available only by prescription.

For most people, says Robert McCarthy, assistant professor at the Massachusetts School of Pharmacy and Allied Health Sciences, the non-prescription medications are a good place to start. More useful in mild than extreme conditions, the antihistamines include the old standby dimenhydri nate (known popularly as Dramamine), meclizine (sold under the brand names Bonine and Antivert), and cyclizine (Marezine). They work by depressing the signals from the inner ear and by quieting the gastrointestinal tract, decreasing nausea.

"All the antihistamines increase resistance -- not 100 percent, but enough to get most people through most journeys of moderate aircraft turbulence and large vessel voyages," says Charles Wood, a motion sickness researcher working with the National Aeronautics and Space Administration.

Antihistamines must be taken an hour or so before departure, since a stomach already upset will not process the medicine effectively, and another dose taken every four to six hours. There's a choice of strengths and forms -- pills, chewable tablets and liquid. But other than that, says McCarthy, "there's not a whole lot of difference between them."

For children, Wood favors cyclizine, one of the milder antihistamines. For kids 6 to 12, check the dosage on the box. If a drug doesn't list a dosage for children, don't give it to them. And for children younger than 6, motion sickness drugs are not recommended at all.

"If you have a 2- or 3-year-old who is getting carsick," says McCarthy, "try something else first -- putting them in the front seat where they can see, for instance." If that doesn't work, consult your pediatrician.

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The side effects of the antihistamines vary, but may include drowsiness or dizziness. You should not take these drugs if you suffer from glaucoma, emphysema, chronic lung disease or an enlarged prostate. The drugs can aggravate these conditions, says McCarthy. And since the medication has its own sedating effects, it should never be mixed with alcohol or tranquilizers. If you are pregnant, some of the motion sickness drugs, like meclizine and cyclizine, are to be avoided in the early stages, so be sure to check with a doctor. Elderly people, like children, are also more sensitive to medications than the average adult. "Start with the lowest possible dose," says McCarthy.

If the over-the-counter antihistamines don't work for you and stronger stuff seems required, see your doctor. That visit is likely to provide your introduction to the Transderm Scop patch. Stuck on the skin behind the ear, the dime-sized patch gradually releases the drug scopolamine through the skin.

A derivative of belladonna, scopolamine works much like an antihistamine, blocking nerve pathways that would otherwise trigger nausea. Because of the way the drug is delivered to the body, it goes to work more slowly, but lasts far longer than oral remedies. Effective for up to three days, it's been shown to dramatically reduce vomiting at sea. The patch must be put in place three to four hours before embarkation -- but then you can forget about it.

"It's probably the most effective, but it has two problems," says Ken Dretchen, professor of pharmacology and psychiatry at Georgetown University Medical Center. "One is that you can't slap it on a half hour before the plane takes off, because it has to build up to a certain blood level. Second, the side effects, which are uncommon but strong, include blurred vision. So you can't just get off the plane and drive the rental car away."

Some people shouldn't use it at all: Scopolamine is not advised for the elderly, nor has it been proven safe for children. It's not recommended for people with eye or bladder ailments. And touching the patch and then touching your eyes can cause your pupils to dilate.

If you'd rather avoid drugs altogether, you might try a psychological approach. Experts say motion sickness is an illness in which the power of suggestion can be critical. Susceptibility is trained in, often by experience, say researchers, and can in fact be trained out again.

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"You can overcome it," says Tom Dobie, a British physiologist and a visiting scientist at the Naval Biodynamics Laboratory in New Orleans. Part of the trick is to turn your thoughts elsewhere -- concentrate on a task, play a game.

"On a fishing trip, when the fish are biting," says Dobie, "no one gets seasick. If your attention is focused on doing a particular job, then when these motion-mismatch signals get to the brain, they get a busy signal. But if you go in with an inward focus, wondering, 'When am I going to be sick?' you notice every slight thing and dwell on it and it's very destructive."

For some, mind over matter may work fine. If you're not one of these people, and if you're heading someplace where your equilibrium may take a beating, you'll probably do well to try one of the over-the-counter remedies first, or to check with your physician for a recommendation.

Whatever the medication, be sure to follow directions and take it before you step aboard the boat or roller coaster, because it's easier to prevent motion sickness than it is to alleviate it once you've fallen ill. At that point, you may just have to ride it out. But as miserable as you may feel, it won't last forever.

"The one great thing about seasickness," says nurse Moore, "is that it ends just as soon as you reach the dock."

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