G-induced vestibular dysfunction ('the wobblies') among aerobatic pilots: a case report and review. (Original Article).

Abstract

G-induced vestibular dysfunction (GIVD) is a condition well known to aircraft pilots who experience high positive and negative G loads during unlimited-aerobatic competitions and air-show demonstrations. After landing and walking from their aircraft, pilots with GIVD manifest an extremely unstable gait, which they call the wobblies. This article includes a report of one such case of GIVD, which to the author's knowledge is the first published case report of this condition in the medical literature. The author also discusses what is known and theorized about the pathogenesis of GIVD, and he reviews its diagnosis, treatment, and prevention.

Introduction

If someone called your office and asked for help with the wobblies, you might refer him to a detoxification ward or public health clinic. But what this patient really needs is an ENT evaluation. The wobblies is the term used by high-performance aerobatic pilots to describe the unstable gait they experience during an episode of G-induced vestibular dysfunction (GIVD). While serving as the United States team physician during the World Aerobatics Championships in 1998, the author, who is also a senior Federal Aviation Administration (FAA) medical examiner and otolaryngologist, was struck by not only the high incidence of this condition, but by the paucity of published data regarding its evaluation and treatment.

Human experience with high G forces, especially high negative Gs, is relatively limited. Significant fluctuations in G forces are not routinely experienced by most of the general population, but they are pushed to the limit by high-performance aerobatic pilots. These aviators routinely experience loads in excess of 10 positive and negative Gs during aerobatic training, competition, and air-show maneuvers. The long-term results of this type of activity are unclear because aircraft and engine designs have advanced faster than human studies can test their effects. In this article, the author describes what to his knowledge is the first published report of a case of GIVD in the medical literature. The author also discusses what is known and theorized about its pathogenesis, and he reviews the diagnosis, treatment, and prevention of this condition.

Case report

In 1998, a white 41-year-old man was evaluated for a history of occasional episodes of GIVD, all of which had resolved following extended periods of avoidance of aerobatic flying. The patient's most recent episode had occurred 7 weeks earlier and had resolved after 3 weeks of rest. He had no other history of neurologic, metabolic, otologic, or relevant family disease, and he was on no medication. He had not previously been evaluated for GIVD.

The patient experienced his most recent episode while practicing for the World Aerobatics Championships. Approximately 10 minutes into his flight, while pushing -7 Gs, he felt a sensation of spinning that was accompanied by nausea but no vomiting. He immediately stopped his aerobatic maneuvers and landed. When he exited the aircraft, his gait was very unsteady.

An immediate field evaluation revealed a fine horizontal nystagmus, a positive left head thrust, (1) and a gait lean to the left. Findings were normal on testing of his cranial nerves, central nervous system (CNS) function, cerebellum, tympanic membranes, and hearing. Findings on the Hallpike maneuver, the fistula test, and examination of the sinus, pharynx, and neck were also normal. The patient denied tinnitus and headache, and the results of his mental status examination were normal. His pulse oximetry reading was 98%. The patient scored 14 on the National Aeronautics and Space Administration (NASA) neurologic function rating scale (NFRS). An NFRS score of 11 to 13 is considered normal, and a score of 14 or 15 is suspect. A higher score indicates that consideration should be given to referring the patient for further neurovestibular testing.

Two hours following the field examination, the pilot's subjective symptoms had abated. He exhibited no gross nystagmus, and his gait had returned to normal. His Hallpike test was again negative, but when he turned from the supine to the left or right lateral position, he became vertiginous and exhibited a horizontal nystagmus for 15 to 20 seconds; the nystagmus was accompanied by nausea but not vomiting. His nystagmus in the left head lateral position was left beating and more intense than it was in the right head lateral position. Therefore, a presumptive diagnosis of left horizontal semicircular canal benign positional vertigo was given.

The patient was treated with a repositioning maneuver to move the otolithic material floating in the endolymph into the vestibule. Over a period of 1 to 2 minutes, he was slowly moved in a log-roll fashion from the supine to the right lateral position (the less-affected ear), then prone, then left lateral, then supine. He then sat up and was placed in a soft collar and advised to avoid head and neck motion by turning his head only in unison with his He was administered 10 mg of intramuscular dexamethasone and prescribed a 3-day tapering dose of prednisone, starting at 60 mg. The collar was removed after 48 hours, and the patient began performing head-tilt-left and -right habituation exercises slowly for 4 days. Then, against medical advice, he competed in the aerobatic competition and did not experience a recurrence.

Most findings on examinations after each of his next four flights were normal, and no symptoms of GIVD were evident. The patient did exhibit a far-left peripheral-gaze horizontal fatiguing nystagmus, and he had difficulty hopping on his nondominant leg with his eyes closed. His NFRS score again was 14. All abnormalities reverted to normal within 6 hours of each flight.

The author compared the findings in this patient with those of other U.S. pilots who had admitted to a history of GIVD in the remote past and noted that results were similar. Other pilots recorded immediate postflight NFRS scores in the range of 12 to 14 and a 2-hour postflight mean score of 11. The improvement in these pilots' conditions during the 2-hour postlanding period could be the result of their adaptation to normal G loading, an unloading of perilymphatic/endolymphatic pressure, or a settling of the otoconia or canaliths. (2)

Discussion

Clinical features. GIVD usually occurs during high negative G maneuvers (beyond-4 G); it can also occur when a pilot experiences a high positive G load immediately following a negative G load (push-pull phenomenon). During an episode, a pilot is unable to perform aerobatic maneuvers but is able to control the aircraft without assistance (one exception occurred when ground-radioed assistance became necessary to assist a single-seat aircraft pilot maintain control and land).

The clinical picture of GIVD is consistent with vertigo. During flight or shortly after landing, a pilot affected by GIVD typically experiences a whirling sensation in which the horizon appears to spin. There is no loss of consciousness, but nausea without vomiting is generally reported. On departing the aircraft, the affected pilot is usually very unsteady and will often lean against the aircraft or immediately sit down. Symptoms are generally exacerbated by moving the head into specific positions. The duration of symptoms ranges from days to months. The pilot usually experiences no subjective ear pressure, ear pain, tinnitus, loss of hearing, fever, headache, or neck pain. The pilot is properly oriented with respect to person, place, and time.

Etiology. The etiology of GIVD in the patient described in this report was the displacement of canaliths that occurred during a period of high negative G load. However, this conclusion is based solely on physical examination findings during a field evaluation; because this episode took place in a relatively remote area of eastern Europe, no audiologic, vestibular, balance, laboratory, or radiologic testing was possible. Other conditions that can contribute to spatial disorientation include drug reactions, vascular disease, (3) hyperviscosity syndrome, (4) perilymphatic hypertension or fistula, (5) otosclerosis, a CNS or neurologic abnormality, inflammatory or traumatic disease, cervical spine disease, middle ear or sinus disease, and cardiac, metabolic, or allergic conditions. The author has observed many of these conditions while evaluating patients with GIVD. Unfortunately, most thorough evaluations are not performed until long after the inciting flight has concluded.

Given the high negative G loads that these pilots experience, it is not surprising to find a perilymphatic fistula. In such cases, it is possible that a patent or semipatent cochlear aqueduct or modiolus would allow for hydrodynamic changes in pressure to occur concurrently with volume changes between the perilymph and spinal fluid in the adjacent subarachnoid space, but this is difficult to confirm. A clear relationship between barotrauma and either sinusitis or otitis has also been observed in patients with GIVD, some of whom required surgical intervention. In some countries, permanent grounding has been reported for as many as 12% of air crew members as a result of ENT-related etiologies. (6)

Research. Problems related to pilot disorientation were reported as far back as World War I, (7) and these problems have persisted well into the space shuttle era. NASA has found that more than two-thirds of all shuttle crews have experienced difficulties associated with microgravity and motion sickness. Spatial disorientation in flight usually results from misperception of visual, vestibular, or proprioceptive cues. (8) According to the Naval Aerospace Medical Institute, loss of spatial awareness has been implicated as a direct causal factor in 4 to 10% of serious aircraft mishaps and in 10 to 20% of fatal mishaps. (9)

The unique environment in which unlimited-aerobatic pilots perform has been only superficially evaluated. The U.S. military's evaluation of G tolerance has primarily focused on positive G forces, including the prolonged and gradual onset of positive G load environments during centrifuge training. Positive G forces have been found to be associated with musculoskeletal symptoms (10) and spinal shrinkage. (11) The military has used this information to develop G suits and to design aerial maneuvers that allow pilots to better tolerate high G loads.

Less is known about negative G forces. In limited studies, very low negative G loads have been associated with bradycardia. (12) Studies of cardiovascular changes in a microgravity state have identified several adverse effects, including intravascular contracture, decreased oxygen-carrying capacity, a decrease in heart rate, and abnormal sympathetic autonomic responses. (13) Even so, we still have far more questions than answers with respect to human tolerance of high negative G loads, which appear to trigger the clinical features of GIVD.

Incidence. GIVD is common among high-performance aerobatic pilots. An unofficial survey taken during the World Aerobatics Championships in 1998 revealed that more than 75% of team members from the U.S., Britain, Australia, Russia, Switzerland, Hungary, and Slovakia had experienced at least one episode of GIVD. Only France reported that GIVD was infrequent, although a top French flyer has since retired because of an inner ear disorder. Discussions with flyers and team physicians revealed that although some affected pilots are treated with sedating medications and antihistamines, most are simply grounded for 1 to 3 months. This would explain why most pilots traditionally avoid medical evaluation whenever possible.

Prevention. To keep their pilots in top physical shape, the most successful aerobatic teams have developed excellent training programs. Their regimens include moderate nonaerobic exercises (e.g., fencing and karate), avoidance of alcohol, a balanced diet, and good hydration. Flying-specific practices include a gradual build-up of G tolerance, limits on the duration and frequency of practice sessions, a ban on flying while ill, and head-positioning maneuvers during high G loads--all carefully monitored by a team coach. Engineers could also modify cockpit and seat designs so that pilots sit as close as possible to the aircraft's center of gravity or center of pressure and so that exposure to vibration is kept to a minimum. However, new developments in military aircraft design, including vectored thrust, are going to lead to further G load challenges for military air crews. New aircraft and engine designs are also increasing the amount of G load that can be placed on aerobatic pilots.

Further countermeasures need to be developed for civilian and military pilots in the unique setting of single-pilot high-performance operations. In order to design these countermeasures, it will be necessary to test affected and nonaffected pilots immediately after they land their aircraft. Also, upon the onset of GIVD, all pilots must be trained to immediately reduce all G loads and land as soon as possible to avoid becoming an accident statistic.

Recently, a visually driven neck reflex was identified as a cause of head tilt toward the horizon. (14) This optokinetic cervical neck reflex has an effect on aircraft control and might be related to GIVD via the vestibulospinal or vestibulo-ocular reflexes. Whatever the cause, this dangerous condition needs further evaluation, particularly with respect to the acute and chronic effects that high negative G loads have on the vestibular system. Possible countermeasures might include vestibular rehabilitation or habituation exercises, physical conditioning, head positioning or restraint/stabilization in flight, and other modifications in aircraft design.

In the space program, possible countermeasures for vestibular dysfunction have been developed. They include head restraints, pneumatic cuffs, medications, and sensoriadaptive and habituation training. (15)

Treatment. Some successful treatment measures have already been discussed in this case report, but it is important to identify and treat other concomitant conditions. Steroids are directed at reducing any underlying inflammation and in accelerating the repositioning of canaliths. Steroids might not be necessary for pilots who have adequate time to recover before they resume further flight duties. Other medications that reduce perilymphatic pressure might be helpful, and so might other classes of drugs (e.g., antihistamines) that treat possible allergic conditions. Of course, any medication use by pilots must adhere to FAA guidelines.

The possibility of anterior or posterior canal involvement should also be considered and evaluated by positional maneuvers. Given the three-dimensional flight envelope and high G loads in which pilots function, it is possible that more than one ear or canal might be involved. If so, appropriate repositioning maneuvers would have to be performed.

In conclusion, further study of not only acute events, but recurrent and chronic episodes would be helpful in evaluating this select and limited population. Studies should also assess unaffected pilots in the immediate postflight environment. To this end, it would be safer to develop a negative-G centrifuge for controlled testing rather than relying on field evaluations following actual flights. It would be especially interesting to record perilymphatic pressure. Another helpful focus of study would be the evaluation of space shuttle crews after they have experienced microgravity and military pilots after they have experienced vectored thrust and nontraditional G load environments.

References

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(10.) Kikukawa A, Tachibana S, Yagura S. G-related musculoskeletal spine symptoms in Japan Air Self Defense Force F-15 pilots. Aviat Space Environ Med 1995;66:269-72.

(11.) Hamalainen O, Vanharanta H, Hupli M, et al. Spinal shrinkage due to +Gz forces. Aviat Space Environ Med 1996;67:659-61.

(12.) Banks RD. Gray G. "Bunt bradycardia": Two cases of slowing of heart rate inflight during negative Gz. Aviat Space Environ Med 1994;65:330-1.

(13.) Short HD. Cardiovascular effects of microgravity: Evolution of understanding. Otolaryngol Head Neck Surg 1998;l18:S52-54.

(14.) Smith DR, Cacioppo AJ, Hinman GE, Jr. Aviation spatial orientation in relationship to head position, altitude interpretation, and control. Aviat Space Environ Med 1997;68:472-8.

(15.) Holt GR. Countermeasures for vestibular dysfunction. Otolaryngol Head Neck Surg 1998;118:S29-30.

From the University of Texas Medical Branch, Galveston, and providing medical operational support for the National Aeronautics and Space Administration, Houston; and Wyle Laboratories, Houston.

Reprint requests: Thomas Muller, MD, PO Box 580304, Houston, Texas 77258. Phone: [phone]; fax: [phone]; e-mail: [e-mail] or [e-mail]