A misconception of motion sickness leads to false therapeutic expectations

The emetic chemoreceptor trigger zone (CTZ), located in the area postrema of the medulla oblongata, is generally believed to be indispensable for the vomiting in motion sickness and, by extrapolation, also in space sickness. Accordingly, it has been postulated that a "motion vomiting substance" is secreted into the cerebrospinal fluid in the emetic process. Furthermore, certain therapeutic measures against motion sickness are aimed at preventing the presumed chemical stimulation of the CTZ. This concept originated from laboratory experiments in which ablation of the area postrema protected some dogs and monkeys against motion-induced vomiting. More recent experiments, however, showed that verified lesions of the area postrema were not effective in preventing motion sickness in cats. It appears that an indispensable unidentified element close to but separate from the area postrema was fortuitously destroyed in the earlier experiments. The overall evidence leads to the conclusion that the area postrema is not essential for motion-induced vomiting. Therefore, no functional basis exists for the postulation of a motion vomiting substance, and it is irrational for the treatment of motion sickness to seek pharmacologic blocking agents that act at the CTZ.