Motion sickness sounds trivial until it is experienced. It starts with "stomach awareness". The victim is suddenly aware that they can "feel" their stomach. They weren't thinking about it 5 minutes ago, now they are. Stomach awareness is usually followed by sweating, nausea, a rush of saliva and emesis. Dizziness is also common. This course of events is typical of earth motion sickness. In space, "space adaptation syndrome (SAS)", emesis comes frequently without the warning signs.
The stimulus for motions sickness appears to be conflicting signals from the vestibular and the visual systems. A person sitting below deck in a boat can feel the movement, but their surrounding appear stationary because they are moving with them. Their vestibular system is reporting movement but the visual system is not confirming that movement. The small child sitting in the middle of the back seat of a moving car can see the back of the seat in front, her feet and legs, and the legs of her companions. All of which are moving with the car, vestibular-visual mismatch.
The cause of SAS is thought to be similar. Without other clues the astronauts tend to think of their feet as "down", towards earth. A free floating astronaut may develop SAS on seeing earth out the porthole, apparently above the space craft. In microgravity, the conflict may be between different parts of the vestibular system. When a head movement is made, the semicircular canals signal that a movement has been made to a new head position. The otolith organs, that respond to gravity, are not able in microgravity to confirm the change of position. SAS occurs.
There are a number of drugs that prevent and treat motion sickness. The two classes most commonly used are anticholinergic agents (scopolamine) and antihistamines (dimenhydrinate ). Both of these drug families tend to cause sedation and drowsiness. Interestingly, the non-sedating antihistamines are not effective. Cyclizine and meclizine (long-acting piperazine antihistamines) are reported to cause less sedation and are often reported to be more effective than other antihistamines. Ondansetron, the gold-standard antiemetic in cancer treatment is reported to be ineffective.
Natural treatments for motion sickness are generally reported to have minimal, if any effects under rigorous test conditions. However, there is sound experimental evidence to support the use of ginger, preferably before the onset of discomfort. Ginger can be taken raw, as tea or in capsules. It is even reported to be effective as candy, biscuits or ginger beer. Similar results have been reported for peppermint, but the evidence is not as robust.
Note: If all else fails, an on-line company is offering motion sickness bags in an attractive range of colours and designs.
- Assoc. Prof. Cynthia Darlington