James Locke, a flight surgeon at NASA's Johnson Space Center in Houston, has made dozens of people sick in the name of science. When he puts subjects in a spinning chair designed to induce motion sickness, roughly 70 percent of them succumb—and at nearly the exact same point on each ride. Locke has used this research and his work with shuttle astronauts to determine which medications and doses best prevent the nausea and vomiting associated with motion sickness.
Unfortunately, while the chair always goes through precisely the same motions, the real world is less predictable. In a ship at sea or on a small plane in turbulence, for example, the type and amount of motion can vary dramatically—and so can its effects on people.
Researchers like Locke, and those who work with pilots and the military's most frequent flyers, are especially keen to find better ways to treat motion sickness. And the many civilians who face nausea in cars, planes, boats or even the tamest amusement park rides would welcome a cure without the common side effects of current medications, such as sleepiness, or the questionable efficacy of alternative treatments, such as pressure bracelets. The path to those ends remains bumpy and filled with more than a few green faces, but new research is closer to finding the best treatments to keep both side effects and lunch down.
Eyes vs. ears
Despite decades of research, scientists are still not sure exactly why motion sickness occurs—or how. The currently accepted theory is that sensory conflict is to blame.
"Information from both our visual and vestibular systems is processed by the brain to match it all up. Your vestibular system—your inner ear—is tuned to a terrestrial, 1G environment," Locke says. "When you move [yourself] around, changes in your vestibular system match up with what you're seeing. But [riding] in an airplane or car, your inner ear signals that you're moving, but your eye says you're sitting still" because your body is not moving in relation to its immediate environment—such as the seat you're sitting in, the back of the seat in front of you and the floor beneath your feet.
Why this conflicting input actually causes symptoms such as vomiting is also fairly unclear, says Edwin Park, a neurologist at the Naval Aerospace Medical Institute in Pensacola, Fla.. "Most research has only revealed bits and pieces, and it's all speculation on how it goes together."
A number of neural pathways apparently can activate the brain's vomiting center, thought to be located in the medulla. Studies have shown that certain medications—antihistamines, anticholinergics, amphetamines and serotonin agents—are effective in treating motion sickness, which suggests that it involves the related neurotransmitters: histamine, acetylcholine, noradrenalin and serotonin. Locke says some agents used to treat nausea from other causes, such as food poisoning and chemotherapy, curiously fail to work on motion sickness. Thus, these reactions likely do not involve the same brain pathways as motion-induced nausea.
Locke's data suggest that roughly 30 percent of the population is naturally immune to motion sickness, at least in most conditions. Studies as to why some people are susceptible and others are not have been inconclusive, he says: "So far, we're unable to predict who gets it and who doesn't." Most research has focused, instead, on what helps those who do succumb in controlled conditions, which may also help scientists better manage the condition in the real world.
Drugs and other treatments
Unimpressed with the effectiveness of over-the-counter meds, NASA researchers have experimented with combinations of more heavy-hitting drugs to strengthen astronauts' stomachs, so to speak. Through trial and error, Locke has found that a combination of oral scopolamine, to suppress vomiting, and dextroamphetamine, to counteract scopolamine's potential to induce drowsiness, reduced the incidence of motion sickness from 70 percent to about 12 percent among passengers in the "Vomit Comet"—a DC-9 aircraft used to achieve brief periods of zero gravity as part of NASA's Reduced Gravity Program. Oral or injected scopolamine takes effect more quickly and can be administered in higher doses than the patches commonly used by the general public, but the drug is not as readily available to the public in those forms (NASA orders its own supply).
Multiple studies have shown that people with a history of suffering from migraines are more susceptible to motion sickness. Joseph Furman, a professor in the department of otolaryngology at University of Pittsburgh, recently published a study showing that patients who were prone to migraines that are accompanied by dizziness responded to rizatriptan, a serotonin agonist that is often prescribed to help stop migraine headaches in their early stages. But like motion sickness, scientists do not really know why people get migraines, Furman says. And determining whether the drug would have the same vertigo-alleviating effect on people who do not would require a larger study of rizatriptan's application specifically to motion sickness.
Additional factors might complicate the full biophysiology of motion sickness, some experts argue. "To say a certain percentage of the population is susceptible to motion sickness is probably an oversimplification," Park says. "There are so many variables involved, including the type and frequency of motion, and a range of tolerance and frequency." In fact, testing motion sickness in the real world is so difficult precisely because conditions and individuals vary so much on a case-by-case and incident-by-incident basis.
Park thinks anxiety makes people more susceptible, for instance, and having a sense of control over a situation makes them less so—which might explain why people are more likely to get sick riding in a car or aircraft than when they are driving or at the controls. He has used desensitization training, exposure to motion in an artificial environment (the same type of spinning chair Locke uses), and biofeedback, in which subjects learn to control their own breathing, heart rate and other physical responses, to help flyers deal with motion sickness.