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4 Evaluating Risks in the Military Aviation Environment The environmental extremes faced by military aviators pose many risks to the successful wearing of contact lenses, as has been amply demonstrated in earlier chapters; however, these risks are not all equivalent. Some pose a greater threat of disrupting lens wear or causing long-term complications than others, and some risks change in relative importance in the various aviation scenarios. The working group divided the principal risk factors into three groups: high risk factors those most likely to render contact lens use unacceptable or dangerous; moderate risk factors those of definite concern but unlikely to cause irremediable problems; and minimal risk factorsthose with little possibility for causing lens failure or long-term complications. Military aviators generally represent a relatively young and healthy group, with greater than average immunological resiliency and high mo- tivation. Such a group would undoubtedly show a lower rate of contact lens-induced complications than the public if subjected to identical en- vironmental stresses. In reality, however, the stresses these aviators will encounter are much more severe than those encountered by civilian contact lens wearers, and the risks are consequently much greater. HIGH RISK FACTORS The panel has identified five factors most likely to render contact lens use as dangerous or unacceptable: . low humidity and high air flow; · extended wear or overwear; · unhygienic conditions for lens care; 30
EVALUATING RISKS 31 · particulates; · lack of regular ophthalmic follow-up care by qualified specialists. These five risks act as the principal determinants in decisions regarding military lens wear policy for aviators, involving lens type, wear mode, care regimen, and follow-up care schedule. Low Humidity and High Air Flow Low cockpit humidity and the related problem of air flow directed in the vicinity of the eyes can be regarded as the single greatest threat to the aviator's ability to wear lenses comfortably and avoid complications. Good data do not exist describing the humidity level below which dry-eye symptoms become a problem or at what level they become unbearable, but clinical experience clearly demonstrates that the 5 to 15 percent humidity levels present in aircraft are sufficiently low to cause difficulties. Humidity-induced dryness will undoubtedly cause lens discomfort for many aviators perhaps a third or moreand may make some dysfunc- tional, depending on the length of exposure. Dehydration of hydrogels will also decrease oxygen transmission through the lens and may result in acuity changes as the lens shrinks and the fit changes. Repeated exposure may precipitate more severe complications if substantial irritation of ocular surfaces occurs. High air flow or air blowing across the face is also worrisome, since it can exacerbate the drying effect. This may be of special concern when chemical-protective hoods are employed, some of which have air volumes of as much as 4 ft3/min passing through them. It should be noted that Oc- cupational Health and Safety Administration (OSHA) regulations prohibit contact lens use with full-face respirators because of evidence that air flow in these respirators irritates contact lenses. In those cases in which the air stream is adjustable, contact lens wearers should be warned to direct it away from the eyes. Extended Wear or Overwear Extended wear of contact lens is known to be associated with increased levels of ocular pathology. When the bounds of a given extended-wear regime are exceeded, the risk of complications increases even more. In all the military settings considered here, the nature of the missions dictates that extended wear will either be the predominant wear mode or will at least be experienced occasionally. Moreover, there is every chance that in some scenarios, such as he- licopter missions, overwear beyond approved wear times will become a distinct possibility. In time of war, wear time for aviators involved in
32 CONTACT I INS USE UNDER ADVERSE CONDITIONS combat will probably be dictated by factors beyond the wearer's control. In all of these cases, the risk of complications is greatly magnified, and the potential severity of these complications may eventually ground some aviators or result in their inability to continue contact lens wear. Simply put, the eye of the contact lens wearer requires occasional rest. There is good reason to expect that successful contact lens wear depends to a great extent on the recovery process the eye undergoes when the lens is removed. Some believe this normalization period is as important to lens- wearing success as is proper lens fit or good lens hygiene. Evidently, the eye needs sufficient time without the lens in place to cope with the inevitable ocular insult occurring with lens wear. If this rest time is eliminated, complications may result. Unhygienic Conditions for Lens Care Hygiene is always a consideration in lens wear, particularly for hydro- gel lenses, which can be easily contaminated and can act as fertile ground for the culture of infectious organisms. Even under the best of circum- stances, incomplete sterilization of lenses, contamination of lens solutions, or contamination of the lens upon insertion can occur. In unsanitary field conditions, the chances are many times greater that some form of ocular contamination will occur, with a corresponding increase in the likelihood of infection. The subject of unhygienic conditions is of particular importance to assault helicopter missions conducted by the Army and the Marines. In them, field conditions may prevail for one to many weeks, with no obvious means of ensuring sterile lens insertion or removal or even adequate lens cleaning, especially in combat situations. As stated in the previous chapter, the consequences of ocular contami- nation are magnified in extended-wear scenarios, yet helicopter missions are precisely those likely to require the greatest use of the extended-wear mode because of their length. Further compounding the situation is the high level of particulate contamination that helicopter aircrews are subjected to. If this constant particle bombardment causes corneal or lid irritation, the chances for infection after contamination are again increased. It has been suggested that the use of disposable lenses might answer some of the concerns about hygiene. Certainly it would eliminate some potential sources of contamination by obviating the need for lens cleaning and disinfection. However, it would not ensure sterile insertion, unless additional measures were employed, such as the use of sterile finger cots to apply the lens. More importantly, use of disposable lenses might itself be a source of substantial complications when used in a field environment.
EVALUATING RISKS 33 Currently available disposable lenses have unfinished edges, which may irritate the cornea and lid. Participates The presence of high levels of Articulates in the cabin air is prevalent in every aviation scenario the panel considered. Particulate contamination in helicopters is especially severe, but F-A-R and T-T-B aircraft also show high particle counts. The cockpit is a dirty place and, according to military representatives, this cannot be easily remedied with the installation of filters. Fortunately, hydrogel lenses minimize the problems caused by particu- late contamination and may even act as a protective barrier against particle irritation of the cornea. Rigid lenses are much more sensitive to foreign body involvement. The safety of the aircrew and the completion of the mis- sion could be jeopardized by a severe reaction to a foreign body, especially in a high-performance aircraft. Smaller particles may simply cause milder ocular irritation, but if experienced on a routine basis, it may precipitate other acute or chronic complications. In light of this risk, the working group feels that rigid lenses are impractical in all of the aviation settings considered. It is important to realize that even the use of hydrogel lenses does not render particle-laden environments benign. Rather, high-particulate environments are rightly regarded as stressful to any contact lens wearer. Indeed, little is known about the effects of a constant barrage of Articulates on the hydrogel lens wearer, as is expected in the helicopter setting. It is likely that such a high particle count simply adds to the physiological burden with which the eye must cope. Inadequate Follow-Up Care For the majority of military aviators wearing contact lenses, the avail- ability of regular and competent care may not pose any risk at all. But for those based on aircraft carriers or with extended field assignments who do not have access to professionals with considerable experience in caring for contact lens patients, the risk may be high. Flight surgeons and other med- ical professionals are unlikely to have training and experience in contact lens fitting and care. Careful monitoring of contact lens-wearing aviators is essential to both preventing complications and ensuring that those that do arise do not become debilitating. If this monitoring cannot be ensured, then the probability of problem-free wear declines markedly.
34 CON1;4CT IONS USE UNDER ADVERSE CONDITIONS MODERATE RISK FACTORS The following three factors, though of definite concern, are unlikely to cause irremediable problems for contact lens wearers. Hypoxia Contact lens-wearing aviators in high performance aircraft will almost certainly be subjected to moderately or severely hypoxic conditions, al- though they will ordinarily encounter these conditions for only a limited time. For this reason, it is expected that comeal edema or other hyponc effects, even when they occur, will not pose a great risk to these aviators. So too, T-T-B aviators with contact lenses may encounter hypoxic conditions for more extended periods, but the expected hypoxia is mild enough to be of little concern if adequate rest time with lenses out is available after the flight. Of greater concern is the possibility of repeatedly experiencing hypoxic conditions while using contact lenses in an extended-wear mode when inadequate recovery time is allowed. In any case, no consequences to visual acuity are expected but, as mentioned before, frequent assault by hypoxic conditions may compromise corneal health and ultimately lead to more serious complications. Noxious Fumes Surprisingly, there is a lack of good data on the levels of cockpit contamination by noxious fumes or vapors. It is therefore difficult to determine the level of risk they pose. Nonetheless, the panel believes that the possibility for the presence of compounds that can act as lens irritants is severe. Even if the only such irritants are cigarette smoke and ozone, the potential irritation they can cause should not be discounted. Happily, cigarette smoke is one irritant that can be easily eliminated in the aviation environment by mandate. Overmotivation The pilot's self-image and fear of being grounded may result in no reported symptoms until a full-blown complication is evident. How much of a risk this poses is not known, but even a small percentage of unnecessary complications is a serious matter. It is quite possible that this risk can be minimized through careful education of contact lens wearers, a program of regular ophthalmic monitoring, and a few judicious regulations aimed at lessening the fear of reporting symptoms or imposing penalties for doing so.
EVALlL4lING RISKS 35 ANIMAL RISK FACTORS In this section, we address factors with little possiblity for causing lens failure or long-term complications. Bubble Formation From Vapid Decompression Bubble formation is not expected to be a problem with lens materials that allow gas exchange, such as those used in hydrogel or RGP lenses. Though bubbles are known to form beneath both these lens types, in neither case do the bubbles persist very long, nor is visual acuity adversely affected. High G-Forces Expenencing high acceleration is part of piloting high-performance aircraft. The G-forces exerted in this process are substantial and the decentering effect they have on small-diameter PMMA lenses is well doc- umented, as well as its consequences to vision. However, there is evidence that hydrogel and large RGP lenses tolerate high G-forces well without sub- stantial Recentering. With lens movement thus minimized, no complications are expected to result. Temperature Extremes Extremes in temperature are not expected to pose any particular risk to contact lens wearers. In fact, it is possible that contact lenses may act to insulate the eye from temperature extremes. In any case, clinical experience shows that contact lens wearers can participate in both cold winter and hot summer outdoor activities with no hindrance from their lenses.
