Corneal Effects of Extreme Environments: Considerations for Pilots Wearing Contact Lenses
Joshua E. Josephson
A review of the literature revealed several areas of concern that may be applicable for considering the ocular risks of military pilots wearing contact lenses in various extreme environments. However, there were a few limitations to the implications or interpretations of these reports in the literature. Many of the studies did not use controls. In addition, others involved animal experiments without human follow-up.
Gauvreau's (1976) study of free-fall-jumping parachutists fitted with soft lenses revealed that, when lenses were blown off the eye, corneal epithelial punctate staining and temporarily reduced visual acuity occurred.
ULTRAVIOLET RADIATION AT HIGH ALTITUDES
Increased ultraviolet radiation is a risk at higher altitudes. The effects of ultraviolet radiation are primarily a long-term risk. Ocular protection may be achieved by wearing a visor or by wearing ultraviolet-absorbing contact lenses, which are available in both hard and soft lens materials. Because a soft lens covers the diameter of all of the cornea, limbus, and a portion of the bulbar conjunctiva, it is more protective than the smaller hard lens.
Brennan and Girvin (1985) tested subjects wearing soft contact lenses for 1 hour in an environment where the temperature was kept at 50 °C, the relative humidity was 20 percent, and the wind speed was 1.5 meters per second−1. They found no significant subjective or objective changes in vision or comfort.
Lovsund et al. (1979) investigated temperature changes in contact lenses on rabbit eyes when directly exposed to radiation from infrared heaters. When the rabbit eyes were kept open, the lenses tended to dry on the eye. When rabbits wearing soft lenses were exposed to radiation from certain types of welding (such as manual metal arc, tungsten inert gas, and metal inert gas), a great increase in contact lens temperature was noticed, especially with manual metal arc welding. It should be noted that the rabbits did not have the ability to turn away from the stimulus and that their blink rate is significantly less than that of humans. Although these experiments provide information, the situations may not be a realistic representation of the human situation. They may be even less representative of cockpit environment.
Socks (1982, 1983) examined the effects of extreme cold on the eyes of rabbits wearing contact lenses. One eye of each rabbit was fitted with a hard contact lens, while the other eye served as the control. The rabbits were then exposed to a temperature of −28.9°C with winds up to 78 miles per hour for 3 hours. Throughout the cold exposures no permanent harmful effects were produced. Minor epithelial damage, which cleared within a few hours after exposure, was detected in three animals. Socks suggested that contact lenses may be acceptable in an extremely cold environment and may protect the eye from wind-driven ice and snow.
The average comfortable indoor humidity is between 35 and 45 percent. An increase in ambient humidity has been reported to have no significant effect on vision or ocular response (Eng et al., 1982).
Brennan and Girvin (1985) reduced relative humidity to 21 percent and found that it had no significant effect on vision or ocular health. Eng (1979), using a survey of flight attendants on commercial aircraft, where the ambient relative humidity is in the range of between 7 and 11 percent,
related that 50 percent of the flight attendants reported significant discomfort within 2 hours of takeoff and that 85 percent reported significant discomfort after 5 hours. Jagerman (1973) and Corboy (1980), ophthalmologists at hospital emergency rooms near airports, have encountered a conspicuous frequency of corneal epithelial damage and edema in lens wearers who had just completed long nonstop high-altitude flights. They have anecdotally associated these problems with the dry cabin environments.
Zantos et al. (1986) observed subjects wearing high-water-content hydrogel lenses of various thicknesses in normal- and low-humidity environments. In low-humidity environments, some subjects developed a localized central or inferior central epithelial breakdown. This was particularly noted with very thin extended-wear lenses. Others have observed similar effects with very thin hydrogel lenses (Holden et al., 1986; McNally et al., 1987).
Low humidity and moving air that induces evaporation from the lens surface have been implicated as the cause of central and inferior central keratitis associated with soft lens wear, especially with thin extended-wear lens.
EFFECTS OF ALTITUDE ON THE CORNEA
Two studies that investigated the effects of reduced atmospheric pressure on subjects wearing contact lenses showed significant adverse ocular effects. All other studies reported minimal adverse ocular effects.
Hapnes (1980), working with 38.5 percent hydroxyethylmethacrylate (HEMA) lenses worn by subjects in a low-pressure chamber, decreased the atmospheric pressure “fairly rapidly” to an altitude equivalent to 18,000 feet for a partial pressure of oxygen of 80 mmHg. Ambient temperature was maintained at 21°C and the relative humidity 42 percent (an unrealistically high figure for any useful investigation of a high-altitude cabin environment). Air circulation was “below draught” conditions. After 3 hours all subjects developed debris in the tear film, and three to five subjects experienced ocular irritation in both eyes. After 4 hours one subject had significant discomfort, pronounced ciliary injection, photophobia, and abundant lacrimation. At this point the test monitors were obligated to temporarily stop the test. Hapnes found that there was no change in corneal thickness in these subjecs; however, visual acuity was reduced from one to three lines in 6 out of 10 eyes. There was no change in visual acuity in 4 eyes.
Castren (1984) tested subjects in a hypobaric chamber at an atmospheric pressure equivalent to 13,000 feet. The relative humidity varied between 42 and 64 percent. This humidity was unrealistically high for the investigation of a cabin environment. All soft lens wearers developed injection. Corneal “erosions” were observed in 4 eyes and stromal “opacities” in 10 eyes. Fluorescein staining was “pathological” in 4 out of 14 eyes.
Eng et al. (1978) tested subjects wearing Bausch and Lomb soft lenses at 20,000 feet and 30,000 feet and found no change in visual acuity, refraction, or keratometry. At the conclusion of their study the subjects reported that their eyes felt tired and dry but that there was no severe ocular discomfort.
Flynn et al. (1985) observed polymethylmethacrylate (PMMA) rigid lens wearers up to 40,000 feet and found that 66 percent of eyes showed central bubbles, primarily at altitudes greater than 18,000 feet. Two subjects with large bubbles were subjectively aware of blurry vision. Rigid gas-permeable lens wearers were tested to a maximum of 25,000 feet. Central bubbles were found in 2 out of 10 eyes, particularly at 20,000 feet. These bubbles dissipated rapidly. All subjects had bubbles under the edge of the contact lens, which cleared rapidly with blinking. There were no adverse effects on vision or comfort, and the epithelium was unaffected. Soft lens wearers were also tested, and 33 percent had bubbles up to 25,000 feet. However, the bubbles were present at the limbus only and dissipated over several minutes. There were no adverse effects on the central cornea or on vision. The occurrence of subcontact lens bubble formation and the duration of bubbles may be related to the gas transmissibility of the contact lens. In the case of rigid contact lens a further variable may be the affectivity of the lens design in blink-induced tear pumping.
Flynn et al. (1988) examined subjects fitted with low-, medium-, and high-water-content soft lenses and with spectacles worn in a hypobaric chamber. The temperature was maintained at 21 to 25°C. Simulating a fighter-attack-reconnaissance (FAR) aircraft cabin environment, subjects breathed supplemental oxygen through oro-nasal masks. The relative humidity was maintained at 40 to 50 percent (this was too high to accurately represent the FAR aircraft cabin environment). Atmospheric pressure was varied from 8,000 feet for 30 minutes to 25,000 feet for 30 minutes, at a simulated ascent of 5,000-feet per minute. Descent was 5,000 feet per minute with stops at each 5,000-foot level. There were no changes in visual acuity or subjective vision over time. There were no physiological changes from base line or in the ability to wear soft lenses. Flynn et al. also simulated a tanker-transport-bomber (TTB) aircraft cabin environment. The relative humidity was first maintained at 50 percent at ground level and then at 5 percent at ground level. Subjects were then observed at 10,000 feet, and the relative humidity was first maintained at 50 percent and then 5 percent. Visual acuities were 20/20 or better throughout the flight simulation. Although visual acuity fluctuations occurred 19 times in 6 of 8 soft lens wearers and 12 times in 4 of 8 subjects wearing spectacles, these were not considered to be significant and could not be attributed to the con atmospheric pressure. A reduction in contrast sensitivity in subjects wearing contact lenses compared with subjects wearing spectacles was reported only at the highest spatial frequency. However, contrast sensitivity of the subjects wearing contact lenses after 4 hours was not statisti-
cally different from base line. There was an increase in corneal staining at high altitudes and in dry environments. There was no significant difference in response between the low-water-content and high-water-content lens wearers, especially at the 50 percent relative humidity level. There were no subjective visual changes in all environments tested.
The physiological stress on the cornea presented by such signs as tear debris, conjunctiva injection, and corneal epithelial staining was greater at higher altitudes with contact lens wear. Flynn et al. (1988) suggested that any conjunctival or corneal staining may be the result of low atmospheric pressure, although other factors such as “dry air” may play a role. The results of this study confirm that the physiological response of the cornea to soft lens wear is subject to higher levels of stress at altitude than at ground level. However, the higher stress levels occurred without measurable visual degradation or significant adverse ocular effects. Therefore, the authors suggested that, although the exposure was limited and prolonged repeated exposures combined with additional aircraft environmental factors may hypothetically severely affect the physiological response of the cornea to adversely affect vision, soft contact lenses can be worn while flying.
Brennan and Girvin (1985) simulated military situations with subjects wearing Snowflex 50 percent water content contact lenses and Scanlens 75 percent water content contact lenses. The first altitude simulations were at 12,000 feet. There were no physiological changes or changes in contrast sensitivity at this altitude. All subjects achieved satisfactory visual acuity; however, two had minor decreases in visual acuity that were not considered to be significant. There was a minimal increase in flare and edema that produced no significant adverse visual effects. An atmospheric pressure of 27,000 feet was then simulated. At this level 16 of 17 subjects had satisfactory visual acuity. There was no change in contrast sensitivity. Subjects were also tested with rapid decompression of 6,500 feet per second. All subjects had satisfactory visual acuity, although there was a minor reduction in visual acuity in two subjects. No bubble formation was observed. There was no increase in flare and no changes in contrast sensitivity. Brennan and Girvin also tested the effects of an aircrew respirator worn by subjects in this situation. Air flow was directed across the eyes at a rate of 50 liters per minute for 2 hours. There were no changes in visual acuity or contrast sensitivity. There was a minor insignificant increase in flare after 2 hours. In summary, Brennan and Girvin found that in all instances the visual performance and ocular response of the aircrew wearing contact lenses did not differ significantly from that of spectacle wearers and was not degraded significantly by any of the environmental stresses of the study. In addition, there was no difference in performance between a 75 percent water content lens and a 50 percent one.
Forgie (1981) examined subjects wearing soft contact lenses in a hypobaric
chamber with atmospheric pressure equivalent to 25,000 feet over a period of 2.5 hours. He observed no significant changes in vision or corneal thickness. There were no subcontact lens gas bubbles observed at any time. After 6 hours of testing at 10,000 feet, there were no significant changes in the vision of the test subjects and no gas bubbles were observed under the lenses at any time. Minor changes were noted in tear film debris in both controls and in lens wearers. There was no clear evidence that the tear debris increased significantly as the runs progressed. After 6 hours at 10,000 feet, there was no fluorescein staining in the four control eyes. In the 18 test eyes, 3 showed no staining, 11 showed absolutely minimal staining, 2 showed minimal staining and 2 showed mild staining. The tear film was sampled at 2, 4, and 6 hours into the run. The epithelial cells, the polymorphonuclear leukocytes and the lymphocytes were identified, counted, and compared with samples taken of the tear film prior to the 6-hour run. The results were evaluated statistically and found to be so widely variable that there were no significant differences observed in the lens wearers and the controls at any time between the beginning and the end of the experiment. Forgie concluded that in the situations tested there was no problem sufficient to significantly interfere with aircraft control or visual acuities. He concluded that selected aircrew can wear soft lenses and operate without any problems in a wide variety of military aircraft.
IMPRESSIONS OF PREVIOUS RESEARCH AND RECOMMENDATIONS
From the available studies it would seem likely that aircraft pilots can wear soft and hard lenses in FAR and TTB aircraft cabin environments with minimal risk of ocular or visual complications. However, the errors in the test parameters—such as 40 percent more or humidity when the typical relative humidity in a cabin environment is much lower, lack of controls in some studies, and conflicting results —make it advisable that further controlled testing be completed. Such testing should represent both real and simulated environments. Although visual acuity (Snellen) was monitored in many studies, more sensitive high- and low-contrast log acuity tests may be needed. The protocols should be reviewed and approved by a panel of experts prior to the initiation of the studies.
Better controls of lens materials and physical parameters are needed to correlate the results of necessary future studies. Lens water content should be monitored on the eye with an Atago device using the method recommended by Efron. The thickness of lenses should be standardized for soft lens studies, and four thicknesses should be used: 0.035, 0.07, 0.12, and 0.15 millimeters. If any subjects wear toric lenses, other variables also must be controlled, such as the design used to control meridional orientation.
All testing should be performed on three classes of designs and in two water contents (38 and 55 percent): thin zone torics, back toric, prism ballast, and front toric prism ballast. In addition, if testing prism ballast designs are tested, two wedge designs require testing: 0.75Δ and 2.0Δ.
For rigid lens testing, two standard diameters should be used: 8.7 millimeter and 9.5 millimeter; thicknesses of 0.08 and 0.17 millimeters should be considered. Also, prescriptions of +4.00, −2.00, −6.00 diopters; low oxygen permeable (24 percent × 10−11); and high oxygen permeable (70 × 10−11) materials should be used.
Brennan, D.H., and J.K. Girvin 1985 The flight acceptability of soft contact lenses: An environmental trial. Aviation, Space, and Environmental Medicine 56 (1):43–48.
Castren, J. 1984 The significance of low atmospheric pressure on the eyes with reference to soft contact lenses. Acta Ophthalmologica 161 (Suppl.):123– 127.
Corboy, P.M. 1980 Contact lens dehydration syndrome. Paper read before aviation medical seminar, Federal Aviation Administration, U.S. Department of Transportation, Jackson Hole, Wyoming.
Eng, W.G. 1979 Survey on eye comfort in aircraft: I. Flight attendants. Aviation, Space, and Environmental Medicine 50(4):401–404.
Eng, W.G., J.L. Rasco, J.A. Marano 1978 Low atmospheric pressure effects on wearing soft contact lenses. Aviation, Space, and Environmental Medicine 49(1):73–75.
Eng, W.G., L.K. Harada, and L.S. Jagerman 1982 The wearing effects of hydrophilic contact lenses aboard a commercial aircraft: humidity effects of fit. Aviation, Space, and Environmental Medicine 53(3):235–238.
Flynn, W.J., R.E. Miller, and T.J. Tredici 1985 Subcontact lens bubble formation at altitude. Poster No. 39 of the American Academy of Optometry, December, Atlanta, Ga.
Flynn, W.J., R.E. Miller, T.J. Tredici, and M.G. Block 1988 Soft contact lens wear at altitude: effects of hypoxia. Aviation, Space, and Environmental Medicine 44:48.
Forgie, R.E. 1981 Problems arising from the wearing of head equipment. Advisory Group for Aerospace Research and Development, North Atlantic Treaty Organization. Paper reprinted from lecture series number 115: personal visual aides for air crew. June. 3
Gauvreau, K.D. 1976 Effects of wearing the Bausch and Lomb Soflens® while skydiving. American Journal of Optometry and Physiological Optics 53(5):236–242.
Hapnes, R. 1980 Soft contact lenses worn at a simulated altitude of 18,000 feet. Acta Ophthalmologica 58:90–94.
Jagerman, L.S. 1973 Effects of air travel and contact lens surfaces. American Journal of Ophthalmology 75:533.
Lovsund, P., S.E. Nilsson, and P.A. Oberg 1979 Temperature changes in contact lenses in connection with radiation from infra-red heaters. Scandinavian Journal of Work, Environment, and Health 5:271–279.
Socks, J.F. 1982 Contact lenses in extreme cold environments: response of rabbit corneas American Journal of Optometry 59:297–299. 1983 Use of contact lenses for cold weather activities: results of a survey International Contact Lens Clinic 10:82–91.
Zantos, F.G., G.N. Osborn, H.C. Walter, and H.A. Knoll 1986 Studies on corneal staining with thin hydrogel contact lenses. Journal of th e British Contact Lens Association 9(2):61–64.
Alfonso, E.S. Mandelbaum, M.J. Fox, and R.K. Forster 1986 Ulcerative keratitis associated with contact lens wear. American Journal of Ophthalmology 101:429–433.
American Optometric Association 1988 Position Paper on the Risk of Contact Lens Use in Industrial Environments Contact Lens Section, American Optometric Association, Alexandria, Va.
Chalupa, E., H.A. Swarbrick, B.A. Holden, and J. Sjostrand 1987 Severe corneal infections associated with contact lens wear. Ophthalmology 94:17–22.
Chevaleraud, J.P 1976 Aptitude Au Vol Et Lentilles De Contact Souples. AGARD Conference Proceedings #191, April, 1976.
Clark, C. 1975 Contact lenses at high altitude: experience on Everest South-west Force. British Journal of Ophthalmology 60:479–480.
Cohen, E.J., P.R. Laibson, J.J. Arentsen, and C.S. Clemens 1987 Corneal ulcers associated with cosmetic extended wear contact lenses Ophthalmology 94:109–114.
DeHann, W.V. 1982 The Optometrists' and Opthalmologists' Guide to Pilots' Vision. Boulder, Colorado: The American Trend Publishing Co.
Dille, J.R., and C.F. Booze 1982 The prevalence of visual deficiences among (1979) general aviation accident airmen. Aviation, Space, and Environmental Medicine 53(2):179– 182.
Donnenefeld, E.D., E.J. Cohen, J.J. Arentsen, G.W. Genvert, and P.R. Laibson 1986 Changing trends in contact lens associated with corneal ulcers: an overview of 116 cases. Contact Lens Association Ophthalmology Journal 12:145–149.
Draeger, J., U. Schroder, and L. Vogt 1980 Unter Suchungen uber die Inlosigkeit von Kontaklinsen in der Lust-und Rahmfahrt. Klinische Monatsblatter fur Augenheilkunde 176:421–426.
Efron, N., and N.A. Brennan 1985 Simple measurement of oxygen transmissibility. Australian Journal of Optometry 68(1):27–35.
Galentine, P.G., E.J. Cohen, P.R. Laibson, C.P Adams, R. Michaud, and J.J. Arentsen 1984 Corneal ulcers associated with contact lens wear. Archives of Ophthalmology 102:891–894.
Hart, L.G. 1985 Wearing contact lenses in space shuttle operations. Aviation, Space, and Environmental Medicine 56:1224–1225.
Mandell, R.B., and R. Farrell 1980 Corneal swelling at low atmospheric oxygen pressures . Investigative Ophthalmology and Visual Science 19:697–702.
Mauger, T.F., and R.M. Hill 1985 Can soft lenses save eyes? Contact Lens Forum May:19–20.
Mondino, B.J., B.A. Weissman, M.D. Farb, and T.H. Pettit 1986 Corneal ulcers associated with daily-wear and extended-wear contact lenses. American Journal of Ophthalmology 102:58–65.
Nilsson, K., and R.H. Rengstorff 1979 Continuous wearing of Duragel® contact lenses by Swedish airforce pilots. American Journal of Optometry and Physiological Optics 56(6):356– 358.
Ormerod, L.D., and R.E. Smith 1986 Contact lens-associated microbial keratitis. Archives of Ophthalmology 104:79–83.
Polishuk, A., and D. Raz 1975 Soft hydrophilic contact lenses in civil and military aviation. Aviation, Space, and Environmental Medicine 46(9):1188–1190.
Randolph, S.A., and M.R. Zavon 1987 Guidelines for contact lens use in industry. Journal of Occupational Medicine 29(3):237–242.
Van Norren, D. 1984 Contact lenses in the military service. American Journal of Optometry and Physiological Optics 61(7):441–447.
Weissman, B.A., B.J. Mondino, T.H. Pettit, and J.D. Hofbauer 1984 Corneal ulcers associated with extended wear soft contact lenses American Journal of Ophthalmology 97:476–481.