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1 Introduction Imperfect vision is one of the most common amictions of the modern world. Over half of all Americans more than 130 million people (Bennett, 1988)wear some type of corrective lenses. The great majority of these use spectacles, but a growing numberestimated at over 21 million (Bennett, 19~)have chosen contact lenses to correct their refractive error. Most contact lens wearers adopt lenses for cosmetic reasons. But contact lenses can also offer functional advantages over spectacles in many areas, such as in sports, work in humid environments, and in occupations in which spectacles are inconvenient or incompatible with required equipment, like special headgear. The military services the United States Army, Navy, Air Force, and Marines draw their personnel from this general population; it therefore follows that many military personnel have imperfect vision. No estimate is available of the current number of military personnel with corrected vision. However, advances in military technology have created situations in which personnel wearing spectacles have found their glasses incompatible with equipment requirements. Military service offers many scenarios in which contact lenses might present advantages over spectacles; military aviation represents one of the most significant of these. Driving current military interest in contact lenses are technical advances involving equipment or visual requirements that conflict with traditional spectacle use. For instance, spectacles interfere with new electronic visual aids designed to help fly attack helicopters and target their weapons; spectacles are also incompatible with recently developed protective masks for chemical warfare (personal communication, R.W. Wiley, 1988~. 1
2 CONTACT l:F:NS USE UNDER ADVERSE CONDITIONS However, along with the recognized advantages of contact lenses come some disadvantages of great potential significance. Safe, effective deploy- ment of contact lenses in several distinct military aviation settings requires careful consideration of these advantages and risks in relation to each particular application. This report: outlines the salient features of different military aviation settings of interest, including helicopters, fighter-attack-reconnaissance (F-A-R) aircraft, and tanker-transport-bomber (T-T-B) aircraft; · defines the advantages offered by contact lenses in each of these settings and determines their relative importance; · details the likely complications or adverse effects for the aviation use of contact lenses in general and for each of these settings in particular; determines the risk factors responsible for likely complications, ranks these risks as to severity, and detail possible strategies to ameliorate them; and · makes recommendations regarding contact lens use in each of the pertinent settings. In addition to examining lens types, care procedures, and wear strate- gies for specific settings of interest to the military, our broader aim is to point up those factors critical to the success of a comprehensive contact lens policy for military aviation, as well as to outline the risks inherent in such a policy. In undertaking our examination, we adopted two assumptions. First, the working group assumed that contact lenses offer a definite functional advantage over spectacles for some aspects of military aviation. That is, use of contact lenses in military aviation allows the superior performance of important mission-related tasks. Absent this postulated advantage, the working group might well have found nothing to recommend contact lens use over spectacles, given the risks involved in their use in military settings. In a related vein, we assumed that combat-ready air personnel rep- resent a critical element of overall military readiness, as well as a major investment in terms of time and money toward their training. Thus, to en- sure peak performance, the health and training of these personnel must be carefully stewarded. As such, the working group assumed that risks to ocu- lar health must be conned to those situations essential to discharging mission duties. In effect, the eyes of aviation personnel must be zealously guarded, and any risks to Hem carefully balanced against known advantages.
INTRODUCTION 3 CONTACT LENS DEVELOPMENT Since their introduction to the general public in the late 1950s, contact lenses have gained steadily in popularity with advances in lens materi- als, designs, fitting, and care. Early contacts were made of polymethyl methacrylate (PMMA), a rigid polymer impermeable to the transmission of gases both the oxygen the cornea consumes as it respires and the carbon dionde it produces. Moreover, the sensitivity of the eye to lens edges and also to airborne particles meant that many patients could not tolerate their discomfort. "Soft" lenses made of pliable, water-absorbing polymers became avail- able in lg71 and quickly exceeded hard lenses in popularity. They transmit oxygen relatively well because of their high water content (up to 75 per- cent in some), and they shape themselves to the cornea, allowing quick adaptation by the eye and increased comfort. Unfortunately, soft lenses, known as hydrogels, are less durable than hard lenses and require greater care and more frequent replacement. With them the eye is more prone to infection and a number of other complications and, while well suited for correcting myopia (nearsightedness), they are often unable to correct astigmatism adequately. Rigid gas-permeable lenses (RGPsy, available since 1979, combine good gas transmissibility and the superior optical properties of rigid mate- rials. However, they suffer from some of the same problems as the older PMMA lenses, including greater sensitivity to foreign particles in the eye and increased risk of lens displacement or loss. Both hydrogel and RGP lenses available today offer the possibility of extended wear for periods longer than a day. About a third of the hYdro~el lenses in use are of this extended-wear variety (Schein, 1990), ~ cat ., ~ , ,, with a typical wear tune or one weed Nonetheless, all contact lenses, either daily wear or extended wear, must be removed at regular intervals to prevent complications from oxygen deprivation, bacterial infection, and mechanical irntation. CONTACT LENS USE IN MILITARY AVIATION Military Ophthalmic Policies Despite increasing electronic sophistication in the cockpit, an aviator's eyesight remains a critical asset. Although vision standards for military personnel are quite high, the natural vision changes that accompany aging result in decreased uncorrected visual acuity among a significant percentage of military aviators as their careers proceed. Often this decrease in visual performance does not take long to man- ifest. Upon graduation, about half of the Air Force Academy cadets who
4 CONTACT LENS USE UNDER ADVERSE CONDITIONS had been pilot-qualified when they entered the Academy can no longer qualify for entrance into pilot training without spectacles (O'Neal and Can- non, 1987~. Thus, a large number of aviators begin their flying careers with corrective lenses or require them soon afterward. Surveys show that 27.4 percent of all Air Force pilots (approximately 7,000 out of 25,600 total), 51.5 percent of Air Force navigators (about 5,500 out of 10,600 total), and 40.2 percent of other Air Force flight crew members, including crew chiefs, boom operators, and others (about 6,000 out of 15,000 total), currently wear spectacles (Dennis, 1990~. Among Navy aviators, about 19 percent of pilots and 90 percent of naval flight officers (e.g., navigators and tactical bombardiers) require some correction (Markovits, 1990~. Among Army aviators approximately 18 percent wear corrective lenses (Bachman, 1989~. One option for dealing with these relatively high levels of vision defects would be to further tighten military vision standards, but this would have the effect of greatly limiting the available pool of qualified personnel an eventuality that evokes considerable reluctance among military planners. The other remaining option involves correction of refractive errors through the means currently available: spectacles and contact lenses. It should be noted that surgical correction of refractive error radial keratotomyis currently not an allowable option among military aviators due to the risks involved. Currently, there is no uniform policy across the military services re- garding the aviation use of contact lenses, but no service allows their unrestricted use aboard military aircraft. In general, spectacle use is man- dated for correcting refractive error unless a waiver is obtained for the medical use of contact lenses or for their use under special circumstances. Only recently has Air Force policy authorized the use of contact lenses as a replacement for spectacles. Their use until summer 1989 had been allowed only for medical indications not amenable to correction by spec- tacles (such as keratoconus, irregular astigmatism, aphakia, anisometropia, etc.) and certain special indications, such as incompatibility with helmets, instruments, or night vision goggles (lledici and Flynn, 1986; Sanders, 1989~. All authorized contact lenses had been fit by the Air Force School of Aerospace Medicine. Certain air crew may now purchase contact lenses from civilian specialists, but such lenses must be approved by the flight surgeon (as the responsible physician) before use in flight. Soft lenses are now permitted if their use is considered advantageous. The Army prohibits contact lens use by all aircraft crew members unless medically indicated or unless crew members are subject to certain special environments that preclude spectacle use (Lattimore, 1990~. Naval and Marine aviators are subject to slightly different regulations. Pilots are restricted to spectacle use unless contact lenses are medically
INTRODUCTION s indicated. However, Class 2 aviation personnel so-called backseaters, such as navigators, radar intercept officers, tactical bombardiers, and flight surgeons are currently permitted to wear contact lenses at their own expense for cosmetic purposes (Markovits, 1990~. Spectacles Versus Contact Lenses Traditionally, spectacles have been used to accommodate vision defects among aviators and to control glare. The well-known military style of "aviator glasses" has a substantial history of development and successful use, not to mention significant popularity today, even among civilians (~edici, 1976~. The visual acuity offered by eyeglasses is usually excellent, and the combination of frames and lenses has proven itself over time as a rugged appliance with the added benefit of providing a measure of protection from foreign objects, wind, and other dangers. Moreover, spectacles are not subject to limitations on wear time, provide complete prescription specification (e.g., tint, bifocal), and are easy to don and remove Edict, 1988; Brennan and Girvin, 1985~. However, spectacles can give rise to several problems in the context of military flight, some that are simply inconvenient and others with serious implications for mission success and aircrew safety. First, as mentioned ear- lier, spectacles present compatibility problems with many advanced optical systems, life support equipment, night vision or laser protective goggles, chemical protective hoods, and other personal protective gear. This ranges from complete incompatibility with some helmet-mounted optical systems to simple discomfort or less-than-optimal performance with personal safety gear. For instance, spectacles worn under a helmet on long flights can create hot spots above the ears that are very uncomfortable (Dennis, 1990; Edict, 1988), nor is their weight insubstantial, especially during aircraft acceleration. Fogging of spectacles can occur from a combination of body heat and aircraft air conditioners, with obvious effects on vision. So too, spectacles can interfere with peripheral vision needed for target sighting and other tasks. Vision can also be blurred by sweat beads on spectacle lenses, or by displacement of eyeglasses due to high gravitational (G) forces or vibrations. In certain instances, reflections from spectacle lenses can obscure vision, especially at night (Dennis, 1990; Medics, 1988~. In light of these problems, military interest in contact lens use aboard aircraft has been keen, for contact lenses could eliminate many of these concerns. ~ summarize the advantages of contact lenses Edict and Flynn, 1986; Brennan and Girvin, 1985~: compatibility with optical systems, personal protective equipment, and special headgear of all types; .
6 CONTACT IONS USE UNSI)ER ADVERSE COADmONS no fogging or sweat problems; no reflections; · an increased visual field, including unobstructed peripheral vision weight and wear "hot spots" eliminated; good vision in foul weather; and treatment for certain ocular conditions. As stated before, there are also disadvantages to contact lens use that have kept them from being universally adopted across the military services Predict and Flynn, 1986; Brennan and Girvin, 1985~: limited allowable wear time; · variable visual acuity or acuity sometimes poorer than that attain- able with spectacles; · difficulty in correcting some types of refractive error, most notably, moderate to severe astigmatism; · not well tolerated by some; · present increased risk of serious ocular complications; · may be dislodged (hard lenses) or torn (soft lenses); · some ocular conditions preclude their use; · careful fitting and follow-up care by qualified ophthalmic profes- sionals (i.e., optometrists or ophthalmologists) required; high cost, relative to spectacles, for initial fitting, follow-up care, lens replacement, and lens care solutions; · bubbles may form beneath lenses at high altitudes (especially with hard lenses); · foreign bodies may lodge under the lens, compromising lens wear (especially with hard lenses); necessary lens hygiene difficult to maintain under field conditions; may modify the shape of the cornea ("molding"), resulting in tem- porary distortion upon removing lenses and returning to spectacles. Content Experience Despite policies against general use of contact lenses for aviation purposes, the various services have gained considerable experience in their use among aircrews over the years. Air Force and Army aviators with medical waivers for contact lens use, as well as Navy backseaters (nonpilots) who have the option of wearing contact lenses, have provided an informal demonstration that contact lenses can be successfully employed in military settings by many individuals. In addition to this informal experience, the military has also undertaken many formal studies of the ejects of contact lens wear in military aviation environments. In addition to ongoing laboratory studies, both the Air Force
INTRODUCTION 7 and the Army are currently conducting field studies of soft contact lens use in various aviation settings. Some of the principal findings of this collective research are summarized below. In trials in the early 1970s, hard (PMMA) lenses performed poorly under the effects of high acceleration as might be experienced in fighter aircraft. Tests conducted on a centrifuge showed that lenses decentered under high acceleration, with consequent degradation of visual acuity. In contrast, recent centrifuge trials with soft lenses show much better performance. Some decrease in acuity was experienced at +6 and +8 Gz, but the acuity of spectacle wearers was also reduced at these accelerations (Dermis, 1990; Flynn et al., 1985b). Bubble formation beneath contact lenses at high altitudes (low atmo- spheric pressures) has been documented for both hard and soft lenses. Early studies found bubble formation beneath PMMA lenses at altitudes above 18,000 ft. with some effects on visual acuity as well as the potential for corneal molding (changes in the shape of the cornea). Recent trials with rigid gas-permeable lenses showed bubble formation at altitudes above 20,000 ft. but these bubbles dissipated rapidly after several blinks and both visual acuity and the cornea's integrity were unimpaired. Bubbles were also noted beneath soft contact lenses at elevations as low as 6,000 ft. but they were all at the cornea's edge and had no effect upon visual acuity or corneal integrity. These bubbles did not disappear upon blinking, but dissipated over a few minutes time (Flynn et al., 1987~. The effect of decreased oxygen availability to the cornea (hypoxia) due to reduced atmospheric pressures at high altitudes has been the subject of much interest. Air Force studies of soft contact lens wear at high altitude have shown that a combination of low atmospheric pressure and low humidity, as commonly experienced on many military aircraft, results in no degradation of visual performance after 4 hours. However, these conditions do produce a significant increase in physiological stress as indicated by increased tear debris, conjunctival injection, and uptake of stain by the corneal epithelium. Investigators cautioned that, while results suggest that soft contact lenses may be worn during flight, prolonged or repeated exposure, combined with additional environmental factors, might stress the cornea (Dennis et al., 1988; Flynn et al., 1985a, 1986~. The behavior of contact lenses in chemical warfare scenarioswhether the lens will act to protect the eye from chemical agents or enhance the chemical exposurehas also been of interest to military investigators. An Air Force study showed that soft contact lenses act as a barrier to the chemical irritant for about an hour, protecting the cornea. After that, the lenses act as a chemical sink for about the next 8 hours, supplying a steady dosage of the chemical to the eye (Dennis, 1990~.
8 CONTACT LENS USE UNDER ADVERSE CONDITIONS The Air Force has recently field-tested soft lenses in military transport aircraft during missions consisting of long flights repeated over several con- secutive days. Visual acuity of all subjectsboth with and without contact lensesdecreased slightly as the mission progressed, while the volume of tear debris increased. However, contact lens wearers suffered consider- ably more conjunctival injection than those without lenses. Investigators postulated that all of these signs of physiological stress may have been precipitated by the low relative humidity present in the aircraft (10-15 per- cent). In any case, visual performance was not degraded enough to obviate the use of soft lenses in transport aircraft (Dennis et al., 1988~. Army helicopter crews have also recently tested rigid gas-permeable and soft lenses under rigorous field conditions. After 6- to 24-month trials, more than a third of the subjects exhibited both mild vascularization Hess than 2 mm of in-growth) and mild injection. Several subjects were required to temporarily suspend lens use due to various causes from conjunctivitis to foreign body involvement, but no pilot was grounded for lens-related complications. Further, several subjects discontinued lens use due to dis- comfort (RGP users) or dissatisfaction with acuity (soft lens users). Overall success rates were 86 percent for RGP lenses and 93 percent for soft lenses. Subjects were uniformly positive in their assessments of contact lenses and their effect on job performance (Bachman, 1989~. CONTACT LENS USE IN CIVIL AVIATION The Federal Aviation Administration (FAA) has allowed the use of contact lenses in civil aviation both among commercial and private pilots- for well over 20 years. Prior to December 1976, however, civil pilots wishing to wear contact lenses could do so only after obtaining a special medical waiver a time-consuming process. Consequently, before 1976 the number of civil pilots using contact lenses was small. In 1967, for example, less than 1 percent of all pilots wore contact lenses, although over 25 percent of all pilots exhibited refractive error (i.e., required corrective lenses of some type) (Nakagawara, 1989~. In December 1976, the FAA ruled that contact lenses could be used routinely in the cockpit to satisfy distance vision requirements. At that time, the results from multiple FAA-sponsored studies on whether wearing contact lenses affected accident rates had been found to be inconclusive, and the agency stated its belief that air safety would not be adversely affected by discontinuing the waiver process (Nakagawara, 1989), clearing the way for broader use of contact lenses by pilots. ~day, use of contact lenses has increased to over 3 percent of the nearly 700,000 active civil pilots about 22,000 people. Significantly, the most dramatic increase in contact lens use has been among pilots holding
INTRODUCTION 9 First Class medical certificates those whose aviation environment most closely parallels that of the military. In 1967, less than 3 percent of civil pilots wearing contact lenses held First Class medical certificates; today, almost 19 percent of civil pilots who wear contact lenses belong to this group, which is primarily comprised of airline pilots, many with prior military aviation experience (Nakagawara, 1989~. The FAA does not routinely collect data on dropout rates, complication rates, lens types in use, or mode of wear among contact lens-wearing pilots. Nonetheless, the FAA has not become aware of any adverse trends associated with contact lens wear from the medical examiners who work with and evaluate civil pilots. However, while it may be concluded that contact lenses can be worn successfully in a civil aviation setting by some aviators, little can be said about the particulars or limits of this success. It should also be noted that the military aviation environment may dif- fer substantially from that experienced by commercial and private pilots even those holding air-transport pilot licenses in terms of the length of time contact lenses must be worn, the ease of lens removal during flight in case of discomfort or other problems, the presence of Articulates, and the level of lens hygiene that can be maintained. Nonetheless, the experience of civil aviators over the last 20 years demonstrates that contact lenses can comprise a viable part of routine aviation gear under many circumstances.
