Nausea and vomiting resulting from motion and vestibular disturbances

Motion sickness is observed as a result of exposure to unfamiliar or uncontrolled motion.  This motion can be real or simulated.  The characteristic symptoms are nausea and vomiting and those resulting from extensive autonomic activation such as salivation, pallor and sweating^[1].  Many people are familiar with this in the form of seasickness or travel sickness.  With prolonged exposure to motion, some adaptation normally occurs.  Prolonged motion sickness (with vomiting) can lead to arterial hypotension, dehydration and depression.

Similar mechanisms are involved in nausea and vomiting resulting from disturbances of the vestibular system including Meniere's disease and viral labyrinthitis.  These disorders are often also associated with vertigo.

The aetiology and risk factors for motion sickness

Virtually all people are susceptible to motion sickness if exposed to unfamiliar motion which is of sufficient intensity and duration.  However, the susceptibility of individuals varies^[2].  Motion sickness is rare in the very young (<2years) and in those above the age of 50.  Like most other types of nausea, it is more common in women than men.  Fear and anxiety also increase the chances of an attack.

The causes of motion sickness

The primary cause of motion sickness is thought to be a mismatch between converging sensory signals from the otolith organs and semicircular canals in the inner ear, the eyes and the somatosensory receptors in the skin, joints and muscles^[2].  The afferent pathways from the vestibular apparatus to the vomiting centre may be the most important component since motion sickness only occurs if the otolith organs, semicircular canals and 8^th nerve are intact.

Meniere's Disease is a disease characterised by periodic attacks of rotatory vertigo or dizziness, fluctuating, progressive low frequency hearing loss, tinnitus and aural fullness that cannot be cleared by swallowing as is the case with pressure changes.  It is associated with excessive endolymph in the semi-circular canals.  The incidence of Meniere's Disease is between 0.5 and 7.5/1000, and it is most common in 40-50 year-olds.

The exact cause of Meniere's Disease is unknown. Theories on the causes include viral infections, anatomic abnormalities, biomechanical alterations of the inner ear membrane, inner ear scarring or inflammation from previous bacterial or viral infections.

The consequences of motion sickness

Although motion sickness can be very distressing to individuals and can be debilitating if it lasts for long periods, the consequences are generally not clinically significant. 

Meniere's Disease can be very debilitating to sufferers and warrants treatment.

The management of motion sickness

Prevention of motion sickness is generally considered to be the best approach.  If unavoidable, exposure to motion should be minimised by adopting a position, for example, in the middle of a ship or over the wings in an aeroplane.  Focusing on a stable view, such as the horizon or a distant object ahead often helps.  Reading should be avoided and a supine or semi-recumbent position is best.  A well-ventilated environment is important and excessive food or alcohol consumption should be avoided.  Where appropriate drugs should be given before nausea and vomiting occur.  If vomiting is prolonged, intravenous fluids and electrolytes may be required.

For the prevention of travel sickness there are a number of drugs available over the counter that are effective, including anti-histamines and scopolamine.

For more severe cases of motion disorders the brain-penetrating anti-histamines are effective in both the prevention and treatment of emesis resulting from vestibular disturbances, though the anti-cholinergics are generally less effective after symptoms have developed.^[2] Most other agents, including anti-dopaminergics and 5-HT₃ receptor antagonists are ineffective in motion sickness^[3].

There is no single treatment suitable for conditions such as Meniere's Disease, and drug treatment with anti-histamines, histamine analogues, scopolamine and anxiolytics has been advocated.

Summary

• Motion sickness is observed as a result of exposure to unfamiliar or uncontrolled motion.
• A mismatch between converging sensory signals from the otolith organs, semicircular canal, eyes and somatosensory receptors is thought to be the primary cause of motion sickness.
• Similar mechanisms are responsible for nausea and vomiting in Meniere's disease and labyrinthitis.
• Brain-penetrating anti-histamines are effective in the prophylaxis and treatment of motion sickness, though the anti-cholinergics are generally less effective after symptoms have developed. Most other agents are ineffective.
• Prevention is the best approach.  However, if unavoidable, exposure to motion should be minimised.

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