TY - JOUR
AU1 - IDF, Orit Samuel, MC
AU2 - IDF, Dror Tal, MC
AB - ABSTRACT Airsickness is one of the forms of motion sickness, and is of significance in both commercial and military flight. Whereas commercial airline passengers may simply feel poorly, the effect of airsickness on military aircrew may lead to a decrement in performance and adversely affect the mission. This is of major importance in the case of flight safety, when a pilot who is incapacitated may endanger the aircraft. The problem is particularly evident in pilot training, because of the high incidence of airsickness at this stage in the pilot's career. The majority of aircrew undergo habituation to airsickness during their service, with a reduction in symptoms and improved function. Although airsickness is a wellknown problem in aviation, we were unable to locate a review of this topic in the literature. This review focuses on the characteristics, clinical evaluation, and treatment of airsickness. It also presents the experience of the Israeli flight academy, and our solution for Navy pilots who have to contend with the risk of seasickness before taking to the air. INTRODUCTION The Motion Sickness and Human Performance Laboratory at the Israel Naval Medical Institute has for many years treated Air Force personnel suffering from airsickness. This also includes pilots engaged in operational activity conducted from Navy vessels at sea. Personnel active in these two fields are required to function effectively in the nonterrestrial environment. The clinical approach to this problem involves making a clear diagnosis for each individual and the provision of treatment that will enable him/her to function efficiently. We conducted a search of the literature for articles on the topic of airsickness. LITERATURE SEARCH—METHODS MEDLINE database searches were conducted in an iterative manner during the year 2013 to retrieve articles related to airsickness. We included documents that discussed airsickness in general, aircrew motion sickness incidence, habituation, and treatment. No specific key words were required as inclusion criteria. A relatively small number of studies exist on the topic, so a “bottom-up” search strategy was required. All searches were limited to studies published in English. No documents were excluded based on date of publication. We also searched the Defense Technical Information Center database for research reports, and used Google Scholar to conduct a more general search for further items not PubMed listed. We located each document and reviewed the abstract or entire document if there was no abstract to determine if the document met our criteria. The reference lists of each article were reviewed in detail to find additional articles. The initial search of PubMed using the keywords “air sickness” and “airsickness” retrieved a total of 3,112 articles. Limiting these results to the English language reduced this to 2,550 articles. Further limiting the search fields to MeSH terms resulted in a total of 1,905 items. A search for the two terms in the title or abstract in all languages retrieved 100 articles. Of these, 74 were in English, and were used for our review, together with 19 nonpeer-reviewed documents (U.S. Department of Defense research reports) and 4 items from the reference lists of the selected articles. Thirty of the articles in this final pool are cited in our reference list. MOTION SICKNESS Background Motion sickness is a universal, age-old phenomenon, having its origins in the intercontinental migration of populations by sea. The word “nausea,” expressing the most prevalent symptom of motion sickness, is derived from the Greek word “naus” for ship.1 It occurs in both humans and animals having a functional vestibular organ, when exposed to provocative acceleration profiles for a sufficient length of time. This physiological condition is the consequence of exposure to an unfamiliar and nonevolutionary motion stimulus, such as road, sea, or air travel, and even space flight. Devices designed to imitate these travel conditions, such as flight simulators, as well as 3D motion pictures and virtual reality, may have similar effects. Each sub-type of motion sickness is characterized by its own specific properties. For example, seasickness studies have demonstrated that vertical acceleration at frequencies of 0.1 to 0.5 Hz is the most provocative stimulus. Etiology Motion sickness symptoms are the result of a conflict between the senses responsible for spatial orientation. This hypothesis is known as the “neural mismatch and sensory rearrangement theory.” It is believed that neural mismatch occurs when there is a difference between current sensory input and past motion experience. According to the theory, a spatial integration center in the brain compares information arriving simultaneously from the vestibular, visual, and proprioceptive systems.1 A mismatch between this information leads to symptoms of motion sickness, while the sensory input from each of the various systems is reweighted.2 In some forms of motion sickness, visual input may lead to the conflict. One way of dealing with this is to focus one's gaze on the horizon, which will help ameliorate symptoms to some extent.3,–5 Clinical Presentation The presenting signs and symptoms of motion sickness include pallor, cold sweating, increased bowel movements, headache, nausea, and vomiting.1 The sequence of symptoms, their severity, and evolution depend on the properties of the motion stimulus, as well as the subject's personal susceptibility and ability to habituate. An airsickness study on civilian flight passengers reported similar symptoms, such as a feverish sensation, drowsiness, headache, and nausea.6 The endpoint of motion sickness, vomiting, is explained as inaccurate triggering of the vomiting center by the nonphysiological motion pattern.7 Motion sickness may also cause cognitive impairment, known as the “sopite syndrome.” This is characterized by symptoms such as somnolence, drowsiness, mood swings (apathy, depression), sleep disturbances, and impaired performance. The syndrome is evidenced by poor execution of objective psychomotor and cognitive tasks, which is of particular importance in airsickness.8 The sopite syndrome may occur independently of other wellknown motion sickness symptoms, without any accompanying nausea or vomiting. AIRSICKNESS Background Airsickness is a subcategory of motion sickness. Documentation of the phenomenon goes back to the World War II, in which armed forces transported by air were “neutralized” and unable to carry out their mission despite reaching their destination.9 Although aviation technology has made significant progress since then, airsickness still represents a major challenge to aviation medicine. Airsickness severity depends on the size and structure of the aircraft, flight speed and profile, and weather conditions. Motion is possible along three translational axes (X, Y, and Z) and around three rotational axes (roll, pitch, and yaw). An important part in the development of airsickness is also played by the relationship among flight direction, the subjective vertical, and the true vertical gravity vector. The perpendicular and lateral axes are noted to be the most provocative for airsickness symptoms.6 In aviation, a visual cue may contribute to the sensory mismatch.9 For example, a coordinated turn, with a fixed speed and over a wide circle, presents the visual system with an image of a tilted horizon, whereas the vestibular organ senses no angular acceleration and reports absolute verticality. The situation becomes more complicated in travel through clouds and air pockets with no visual input. Further suffering can occur in passengers who, unlike the pilots holding the controls, are unable to predict aircraft movement. This resembles the difference between driver and passengers in car sickness.10 Pilots have an additional potential for habituating, as a result of regular and frequent flying. A further factor, particular to the aerospace environment, is the relative hypoxia prevalent in compressed aircraft cabins. It was demonstrated in a laboratory study that motion sickness severity was aggravated by a hypoxic environment.11 Incidence Airline passengers, both tourists and businesspeople, are highly familiar with airsickness. It is a common cause of significant discomfort, mostly due to nausea and sleepiness for which pharmacological assistance is frequently required. The largest comprehensive study in the field was conducted between the years 1946 and 1947 on over a million airline passengers.3 The study revealed that 3 out of 4 passengers reported symptoms related to airsickness. The symptoms were more prevalent in females than in males, and in small rather than large aircraft. Later studies demonstrated a decreasing incidence of airsickness on commercial passenger flights, probably because of the improvement in aviation technology. A British study conducted on 923 national flight passengers, published in 2000, reported that 48% experienced airsickness-related symptoms. Notably, the symptoms were more severe for those sitting at the rear of the aircraft or in close proximity to the wing.6 Airsickness is much more crucial and potentially hazardous in flight crew. Student aviators piloting an airplane or helicopter are required at all times to be at the peak of cognitive function necessary for air transport and combat. Other crewmembers not in control of the aircraft are required to be fully alert and concentrated so that they may provide aircraft technical support, navigation, and search and rescue services. Seventy-one percent of U.S. aircrew report at least one episode of airsickness during training.12,–14 The incidence of airsickness in student aviators is between 10 and 20% according to data collected from the U.S. and German Air Forces, and 31% according to data from the Swiss Air Force, while past records of the Israeli Air Force demonstrate an incidence of 46%.15,16 The variability between these studies could be explained by inconsistency of the criteria for airsickness. Vomiting is often considered as the sole symptom,17 whereas cognitive symptoms are rarely taken into account. Over the years, women have become an integral part of civilian and military aircrew, both as pilots and in all the supporting roles aboard the aircraft. Women are considered to be more airsickness sensitive than men.15 On British civilian flights, the female-to-male symptomatic ratio6 was 7:4. Another study, however, showed no gender differences.17 The overall incidence of airsickness decreases over time as pilots and other aircrew gain experience. However, these data should be considered with certain reservations, because the phenomenon does not always disappear, and student aviators who continue to suffer from airsickness may be disqualified or may drop out of the program. The low exposure of civilian air passengers to flying conditions is not sufficient to induce habituation, and therefore there is no reduction in their airsickness symptoms over time. Habituation In a Swiss Air Force study, 52 student aviators on a basic training course were requested to rate their airsickness symptoms after every flight.15 The number of symptomatic students decreased as the course progressed, and no symptoms were reported after the seventh sortie. In 1965, 188 student aviators from the U.S. Navy participated in a study in which 88% reported airsickness symptoms on one to three of the sorties.18 Three symptomatic peaks were described: the first 3 sorties, flight no. 7 (on which there was an abrupt increase in aircraft acceleration), and flight no. 13 (the aerobatic phase, which followed a break in flying activity). An Italian study of student aviators divided the pilots into “slow adaptors” and “normal adaptors.” The occurrence of vomiting after the sixth flight ascribed a person to the slow group. Of the symptomatic students, 12.2% were proven to be slow adaptors, most of them women, and 87.8% were normal adaptors.17 A study of military parachutists during their transport flight showed a decrease in airsickness incidence from 64% on the first jump to 25% after five consecutive flights.19 Retention of Habituation The changes brought about by the habituation process are assumed to be preserved in the central nervous system. Continued exposure to motion stimuli leads to habituation. Subsequent avoidance of motion stimuli will result in resensitization (the reappearance of symptoms) on a later return to flying activity. However, further exposure to motion will be characterized by accelerated rehabituation. This phenomenon is known as “retention.” An attempt to evaluate the duration of this retention was previously conducted in laboratory studies using an optokinetic drum.20 The authors concluded that adaptation to the motion sickness eliciting stimulus of optokinetic rotation was almost completely retained for 1 month and partially retained for 1 year. The Italian Air Force aviation program includes a 1-year break in flying activity between the different stages. An increase in airsickness symptoms occurred in the first phase compared with the previous, basic training phase, but was followed by a decrease in symptoms over the next two phases (basic training phase—31.4%; phase I—33.3%; phase II—25.4%; phase III—25.4%). The authors concluded that there was no retention between the basic training phase and phase I. Another explanation might be the unique nature of the flights undertaken in the different stages. The aerobatic flights performed in phase I produce stronger motion stimuli than the basic training phase. This would result in students asymptomatic during basic training presenting with symptoms in the subsequent phase.17 Financial Implications Of all branches of the military, Air Force flight academies conduct one of the longest and most expensive training programs. Airsickness during flight training has both direct and indirect financial implications.13 The signs and symptoms of airsickness have a deleterious effect on flight performance, and interfere with the student aviator's ability to prove his or her full potential and qualify as a pilot. Suboptimal performance due to airsickness may necessitate the termination of some flights, with repetition of certain maneuvers at a later date to achieve the required score. This repetition has direct economic implications. It has also been demonstrated that there is a correlation between airsickness severity and training dropout,17 hence the indirect cost of the loss of human resources, when students unable to meet the required criteria, drop out because of airsickness. It is clear that a late dropout will increase the financial loss. Prescription and nonprescription drugs used for airsickness and flight recovery may also represent a financial burden. Finally, post flight-related disability, mostly in the form of somnolence, fatigue, and dehydration, may result in lost working days. However, an ambiguity in the airsickness-incidence reports makes it difficult to obtain a precise financial picture. It is therefore important to develop tools for the prediction of personal susceptibility to airsickness and the ability to habituate, thus enabling the conservation of economic resources. Evaluation of Airsickness Severity To provide appropriate treatment for airsickness, it is necessary to assess the severity of symptoms and their progression over time. A review of the literature failed to disclose a validated-airsickness questionnaire. Most studies in this field have used the simulator sickness questionnaire originally developed for the assessment of motion sickness in simulators, among the flight simulators.21 This survey has been found acceptable for additional forms of motion sickness, such as seasickness and airsickness.22 TREATMENT Nonpharmacological Treatment Desensitization therapy, a treatment that does not involve any pharmacological intervention, has been described in the medical literature. The United Kingdom, Canada, and Italy, among several other nations12,–25 have a special program for airsickness desensitization designed in two stages: ground-based and in-flight training. Both stages expose subjects gradually to situations which present them with a vestibular challenge, while constantly reassessing their status. The British study included pilots, aviation students, and other aircrew, without defining the length of flying experience required to join the program. No correlation was found between subjects' progress during the ground phase and the eventual achievement of airsickness tolerance. It was also difficult to determine whether the airborne phase was successful, even though some of the subjects progressed to become award-winning pilots. Similarly, Lucertini and Lugli24 were unable to reproduce identical flight conditions in the laboratory. In some cases, there was a slight discrepancy between the laboratory results and symptoms observed on subsequent flights. However, their ingenuity was in training patients to face a broad range of effective conditions during future flying activities using a multidisciplinary approach. They combined physical forms of vestibular desensitization with psychological tactics, such as muscle relaxation and deactivation of the emotional response. A recent study from this laboratory presented a success rate of 85% for the Italian airsickness rehabilitation program, with long-lasting benefit of over 8 years.26 Pharmacological Treatment Most of the literature on this topic comes from military sources. A number of armed forces use airsickness medication during flight training. In Britain,12 pharmacological treatment may be administered to pilots and other crewmembers, but is forbidden in solo flying. The accepted treatment is scopolamine (hyoscine hydrobromide 0.3–0.6 mg), and for some long-distance flights Stunarone (cinnarizine 15–30 mg). In U.S. Army and Navy aircrew, promethazine (Phenergan) 25 mg in combination with ephedrine 25 mg taken 1 hour before flight is permitted for up to three flights during training, provided the patient is accompanied in flight by an instructor pilot. In the U.S. Army, this treatment or scopolamine hydrobromide alone or in combination with dextroamphetamine (Scop/Dex), is also used for reacclimation of a rated aviator.27 Scop/Dex is also a well-established treatment in National Aeronautics and Space Administration astronauts. A Canadian study compared the motion sickness medications promethazine, meclizine, and dimenhydrinate, and their impact on psychomotor performance in aircrew. The authors suggest the possible use of promethazine with d-amphetamine in the prevention of airsickness.28 The amphetamine is added to counteract the effect of promethazine on psychomotor performance. An alternative is promethazine in combination with caffeine, which was demonstrated to be effective in preventing airsickness in helicopter passengers.9 A study conducted using an animal model of motion sickness showed a possible effect of different pharmacological agents on the habituation rate. Dimenhydrinate was assumed to have no effect, whereas it was concluded that scopolamine may accelerate the process of habituation to motion sickness.29 AIRSICKNESS IN THE MILITARY SETTING A number of regulations regarding the treatment of airsickness have been drawn up by the Israeli flight academy. The training program includes flights with an instructor in the early stages. Solo flights, without an instructor, begin in the more advanced stages. Trainee pilots are also required to undertake a course of academic study, which results in a long break in flying activity of up to a year between the flight-training periods and often causes recurrence of symptoms on the return to flying. In the past, the drug used for the prevention of airsickness in the flight academy was Travamin (dimenhydrinate 100 mg). This had significant side effects, such as drowsiness and cognitive impairment,28 which can be critical when approaching the question of aviation standards and candidate selection in the course of the program. Nowadays, “per os” or sublingual scopolamine tablets (Kwells, hyoscine hydrobromide 0.3 mg) are the drug of choice, because of high treatment efficacy and minimal cognitive side effects. Kwells is an anticholinergic, antimuscarinic taken 1 hour before exposure to motion. Our recommendation for the aviation training program includes two flights without pharmacological intervention, after which the student aviator is examined in the academy clinic as required. If the physician reaches a diagnosis of airsickness per anamnesis, a review is conducted of the candidate's medical history with emphasis on any contraindication to anticholinergics. Where possible, the candidate is offered the option of prophylactic treatment by scopolamine tablets (Kwells), to be taken “per os” for a limited period. At present, solo flights are not authorized while using scopolamine. An aviation student not habituating in the early stages of training is referred for otoneurological evaluation to rule out vestibular pathophysiology.30 THE NAVY PILOT PARADOX The pharmacological treatment of seasickness is prevalent the world over. A unique problem arises in the case of aircrew based on board sea-going vessels carrying airplanes or helicopters. In such a situation, aircrew may potentially be exposed to seasickness for a prolonged period before their first takeoff, as well as a subsequent possibility of airsickness. The Scopoderm patch (transdermal scopolamine, hyoscine hydrobromide 1.5 mg) is favored in seasickness. It must be applied 6 to 8 hours before departure, and allows constant controlled release of the drug through the skin for 72 hours. The advantage is a low plasma scopolamine level, producing a relatively safe side-effect profile. The paradox is that the pilot controls the aircraft under the influence of the medication, even though not necessarily suffering from airsickness. Our current recommendation is that only one of the pilots in the cockpit be allowed to take medication for seasickness. No such limitation is in force in the case of other members of the aircrew. CONCLUSIONS Airsickness is a common phenomenon in passengers and crew, most of whom are able to undergo habituation with frequent exposure to flying. Infrequent exposure to flying will not enable habituation processes to take place. In our experience, the preferred pharmacological treatment is scopolamine, because of its efficacy and a relatively safe side-effect profile. In the case of a contraindication to the drug, or if it is not available, one should consider cinnarizine or dimenhydrinate. Other motion sickness medications may be considered, although there is a lack of information on this topic in the literature. In pilots, scopolamine is used for symptomatic relief but excluded in solo flights. Our review of the literature has revealed insufficient knowledge in the field of airsickness, and further research is required. ACKNOWLEDGMENTS The authors are grateful to Richard Lincoln for his assistance in the preparation of the manuscript. REFERENCES 1. Reason JT, Brand JJ Motion Sickness . London, Academic Press, 1975. 2. Tal D, Hershkovitz D, Kaminski-Graif G, Wiener G, Samuel O, Shupak A Vestibular evoked myogenic potentials and habituation to seasickness. Clin Neurophysiol  2013; 124( 12): 2445– 9. Google Scholar CrossRef Search ADS PubMed  3. Lederer LG, Kidera GJ Passenger comfort in commercial air travel with reference to motion sickness. Int Rec Med Gen Pract Clin  1954; 167( 12): 661– 8. Google Scholar PubMed  4. 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TI - Airsickness: Etiology, Treatment, and Clinical Importance—A Review
JF - Military Medicine
DO - 10.7205/MILMED-D-14-00315
DA - 2015-11-01
UR - https://www.deepdyve.com/lp/oxford-university-press/airsickness-etiology-treatment-and-clinical-importance-a-review-onYuJcDxHX
SP - 1135
EP - 1139
VL - 180
IS - 11
DP - DeepDyve
ER -