Motion sickness is a motion-induced syndrome that manifests with some or all of the following signs and symptoms: malaise, flushing or pallor, diaphoresis, headache, gastric hypomotility associated with abdominal awareness, abdominal discomfort and/or vomiting, cardiovascular changes, and endocrine changes. Motion sickness occurs on ships, submarines, aircraft, in automobiles, trains, amusement park rides, simulators, in spacecraft, and with the use of virtual reality devices. The spectrum of severity of symptoms ranges from mild discomfort to severe compromise of function to prostration from continual emesis leading to dehydration. Incapacitation of key personnel can be devastating.

Several theories of the etiology of motion sickness are described in the medical literature. Most of these theories are variations of a theme that asserts that the symptoms of motion sickness arise when the brain is unable to resolve a conflict between the various sensory modalities that provide information regarding position and movement of the body. These modalities include tilt/linear acceleration and angular acceleration from the otolith organs and the semicircular canals, respectively, visual information, and proprioception. A commonly cited example is reading a book while riding in an automobile. The vehicle occupant’s visual reference (the book) is not moving (or is moving minimally) relative to the occupant and therefore the occupant perceives him/herself as stable in space based on visual input. However, vestibular input, in a predictable response to bumpy roads, cornering and hills, is that of variable movement. A naïve Sailor below deck on a ship in rolling seas without visual reference to the horizon experiences a similar phenomenon. Without visual reference to the horizon, the motion of the ship is perceived by the vestibular and proprioceptive systems while the bulkheads do not appear to move relative to the observer. This mismatch of sensory information somehow gives rise to the release of a neurochemical agent (neurotransmitter) that acts in the general region of the Area Postrema, stimulating the nearby vomiting center.

One theory postulates that the sensory conflict described above stimulates a response in the brain stem similar to that caused by some neurotoxins. Proponents of this theory describe the signs and symptoms of motion sickness as the result of the stimulation, by motion, of a "poison-response mechanism." This mechanism evolved to rid the body of toxic ingested substances by emptying the stomach, and countering or minimizing the effects of absorbed toxin via a stress response of the sympathetic nervous system.

Of particular interest is the fact that a functional vestibular system is required for an individual to suffer from motion sickness. In individuals and animals in whom the vestibular apparatus has been ablated or the vestibular pathway interrupted, motion sickness has not been induced, despite extensive effort. Supporting the "poison-response" theory is the fact that toxin-induced emesis can be reduced in dogs by labyrinthectomy.

Diagnosis of motion sickness is straightforward. However, a differential diagnosis should be considered in cases of persistent illness or illness refractory to management.

Most individuals adapt to a motion environment over a period of time that varies in length. Personnel should be advised to minimize their exposure to motion by residing amidships or over the wing in airplanes, and to limit their own head movements to prevent confusing vestibular input. Additionally, maintaining a visual reference to the horizon should be helpful. Alcohol intake in the previous 24 hours has been shown to reduce tolerance to cross-coupled stimulation during desensitization training. Thus, limiting alcohol intake may reduce susceptibility.

Pharmacological intervention can be prophylactic or therapeutic. Medications include parasympathomimetics, antihistamines, often used in combination with stimulants. When administering a drug for prophylaxis, the timing of administration relative to exposure is particularly important. Prophylactic drugs should be given before nausea and vomiting occur. Susceptible individuals should be given meclizine 25 mg orally or diphenhydramine 25 to 50 mg orally, one hour before departure.

In the aviation community, motion sickness is seldom seen in experienced aviators and aircrew. During training, Student Naval Aviators (SNAs) who are experiencing problems with motion sickness severe enough to compromise their performance, undergo a history and physical exam by a flight surgeon to rule out organic disease. If no identifiable pathology is detected, a combination of promethazine (Phenergan) 25mg and ephedrine 25mg PO is given one hour before flight, for up to three flights. The student must be accompanied with an instructor pilot, after ruling out other illness. If the problem persists, the SNA is referred to the Naval Operational Medicine Institute (NOMI) for desensitization, biofeedback, and relaxation training and psychological counseling. The Aeromedical Reference and Waiver Guide cites a success rate of 85 percent of referred individuals.

Interestingly, 70 percent of Shuttle astronauts suffer from Space Motion Sickness (SMS) to some degree. SMS differs from "terrestrial" motion sickness in that typically, there is no diaphoresis, flushing is more common than pallor, and vomiting frequently occurs without nausea or other warning. NASA formerly used a combination therapy consisting of scopolamine 0.4mg and dexedrine 5.0 mg PO as prophylaxis taken before launch. Current NASA policy is to use promethazine (Phenergan) 50mg IM therapeutically when SMS becomes a problem. Unless symptoms are severe, the astronauts are usually instructed to take it before sleep to take advantage of the medication’s depressant effect and minimize its effect on mission tasks.

In one study comparing the efficacy of four medications in reversing severe nausea (diphenhydramine 50mg IM, promethazine 25mg IM, promethazine 50mg IM and scopalamine 0.5mg IM) the scopalamine 0.5mg and promethazine 50mg doses were effective. Little or no benefit was seen with the others. It is felt that IM promethazine is preferred to IM scopalamine in these doses due to the potential toxic effects of the latter drug.

Since these medications act as central depressants they can affect performance and impact safety. Over the counter medications include Dramamine and Bonine. Contraindications in the use of cholinergics should be observed. Other drugs that are used to treat nausea and vomiting that arise from other causes have been ineffective in treating motion sickness. These include metoclopramide, prochloroperazine, and odansetron.

Submitted by LCDR Edwin Y. Park, MC, USNR, Naval Operational Medicine Institute, Pensacola, FL (1999).