For boat passengers and sailors, acclimatization will develop if progressively increasing periods are spent at sea. Otherwise, it usually takes 2 to 3 days of continuous exposure to adapt to new conditions. The sources of vestibular and proprioceptive stimulation should be reduced to a minimum. This usually means either lying down or being as still as possible. Unnecessary head movements should be avoided. In small craft, staying along the centre line of the craft, toward the stern, incurs the least complex motion. Conflicting visual stimulation is reduced by keeping the eyes closed or by focusing on the horizon. Vulnerable individuals should definitely avoid reading, and noxious stimuli such as smells should be avoided.
Drugs for general use
Most drugs for preventing and treating seasickness are either antihistamines or anticholinergics. This reflects the importance of histaminergic and cholinergic transmission of neural inputs to the vestibular apparatus, the solitary tract nucleus and the vomiting centre itself. Drugs that target the chemoreceptor trigger zone, such as the 5-hydroxytryptamine antagonist ondansetron and the dopamine antagonist droperidol, are not considered particularly effective in motion sickness.
Commonly used antihistamines include cyclizine, dimenhydrinate, promethazine and cinnarizine; and the most commonly used anticholinergic is hyoscine or scopolamine (whose most widely available preparation is a transdermal patch). Various attempts have been made to compare the efficacy of these agents, both within and between the two classes. Graybeil and colleagues4 compared drugs alone and in combinations. They found that the best combination was promethazine hydrochloride 25 mg in combination with 25 mg ephedrine sulphate. Scopolamine was the most effective single drug, but it was more effective also when combined with ephedrine sulphate or d-amphetamine sulphate than as a single drug. In a trial reported by Pyykko and associates5, dimenhydrinate was more effective than one scopolamine patch and about equal to two and had the advantage of needing a shorter period to become effective. Other studies place cyclizine and dimenhydrinate as equal in performance but suggest that cyclizine reduced gastric symptoms and drowsiness6. Use of combined preparations (e.g. hyoscine and dimenhydrinate7) may be more effective than a single compound.
Drugs for divers
The main problem for divers is that all these drugs have some side effects, and none are truly proven safe for diving. Antihistamines may cause dry mouth and drowsiness, whereas anticholinergics also cause dry mouth, variable levels of sedation and occasionally blurred vision. The effects on arousal leave open the possibility of an interaction between motion sickness drugs and nitrogen narcosis. Another frequently cited concern is the question of whether there is any interaction between the drugs and risk of oxygen toxicity. There has been limited investigation of these issues. In hyperbaric chamber dives to 36 metres, transdermal scopolamine was not found to cause decrements in cognitive performance or manual dexterity8. Both scopolamine9 and cinnarizine10 were found not to increase risk of central nervous system oxygen toxicity in rats. Although not a trial in divers or diving, it is perhaps notable that dimenhydrinate was found to induce significant neurocognitive impairment in volunteer subjects, whereas cinnarizine and transdermal scopolamine were not.
In electing to use anti–motion sickness agents, divers must accept that the safety case for use in diving has not been comprehensively made for any drug. However, on balance (which includes consideration of the debilitating effects of seasickness on divers who nevertheless enter the water), the risk versus benefit equation probably favours use of a preventive agent in susceptible divers. At the present time, the evidence suggests that use of a non-sedating antihistamine such as cinnarizine or an anticholinergic in the transdermal form (scopolamine) is acceptable. Unfortunately, both these agents can be difficult to source in Australia and New Zealand. Use of the sedating antihistamines (e.g. cyclizine) or those shown to affect performance significantly (e.g. dimenhydrinate) should be avoided. Any drug used for this purpose must be tried previously to ensure that no untoward reactions occur.