Motion, or travel, sickness is a form of vertigo in which autonomic symptoms predominate, brought on by ‘passive’ movement, ie, being conveyed by some means of transport (road, sea or air) as opposed to self-propelled motion (walking, running, etc). It can also be caused by amusement park rides and by simulated motion, such as flight simulators and widescreen projected images.

Two, similar, main theories have been put forward to explain how and why motion sickness occurs and autonomic responses come to be triggered:

• the sensory conflict theory1, which proposes that during travel stimuli are received in the brain from the eyes, vestibular apparatus in the ear and receptors in the skin, muscles and joints that sense movement, that conflict with remembered experience of usual forms of movement, such as walking;

• the subjective vertical conflict theory, which proposes that: ‘all situations which provoke motion sickness are characterised by a condition in which the sensed vertical as determined on the basis of integrated information from the eyes, the vestibular system and the non-vestibular proprioceptors is at variance with the expected vertical as predicted on the basis of previous experience.’

Whatever the underlying cause, the sensory mismatch produces effects interpreted in the brain in the same way as those caused by toxic substances, and a similar physiological response as to a poison is initiated, with a number of autonomic nervous system mechanisms activated to reject it.

With prolonged or repeated exposure, the body adapts to unfamiliar types of motion, explaining why sea-sickness, for example, tends to subside after a few days at sea. Also, people in control of a vehicle or even in the front passenger seat of a car, are better able to anticipate unfamiliar movement, eg, going round bends in a road, and are less likely to suffer motion sickness.

The main symptom of motion sickness is vomiting. It is a reflex under the control of the vomiting centre in the brain. This receives input from several sources, including the nucleus solitarius in the medulla, which controls orientation and is responsible for detecting the presence of some poisons. It also receives inputs from the vestibular apparatus, the autonomic nervous system and higher brain centres. Once activated, the vomiting centre transmits stimuli via a cranial nerve to the abdominal musculature, stomach and oesophagus to initiate vomiting.

Other symptoms of motion sickness are produced via the autonomic nervous system, including salivation and stomach emptying via parasympathetic stimulation, and increased heart rate, cold sweats and pallor through sympathetic stimulation.