Contact Lens Wear in the Aerospace Environment
Visual standards for pilot flight training have been updated in Air Force Regulation 160-43 as of October 1, 1988. Congress has mandated that 70 percent of the entering Air Force Academy cadets meet these standards. All pilot candidates entering the Reserve Officers Training Corps (ROTC) and the Officers Training School (OTS) also must meet the standards. The standards limit refractive error to no more than 2 diopters of hyperopia or 0.25 diopters of myopia. Candidates must have 20/20 acuity in both eyes with no more than 0.75-diopter astigmatism and 2 diopters of anisometropia. Candidates cannot wear contact lenses within 3 months of the entry examination or have undergone radial keratotomy.
The standards are relaxed after entry for Academy and ROTC cadets and for other Air Force personnel entering pilot training. Presently, pilot candidates are limited to 2 diopters of hyperopia and 1.50 diopters of myopia with 20/ 70 acuities. They are also allowed 1.50 diopters of cylinder correction. After pilot training they are limited to 3.50 diopters of hyperopia, 2.50 diopters of myopia (20/200 acuities), 2 diopters of cylinder, and 2.50 diopters of anisometropia. Refractive errors over these limits are waiverable for pilots.
INCIDENCE OF SPECTACLE WEAR IN THE U.S. AIR FORCE
In 1980 a survey by the U.S. Air Force School of Aerospace Medicine (USAFSAM) indicated that 20 percent of the USAF pilot population and 50 percent of the navigators wore spectacles. A more recent study (September 1988) by Robert Miller at USAFSAM has shown the percentage to be even higher. Over 27 percent of the USAF pilots and 51 percent of the navigators are spectacle wearers. These percentages are distributed uniformly across
the four major commands, that is, Air Training Command (ATC), Tactical Air Command (TAC), Strategic Air Command (SAC), and Military Airlift Command (MAC).
There are a number of significant problems with spectacle wear by aircrew. Spectacles are prone to fogging from a combination of body heat and aircraft air conditioners. Vision can be blurred from sweat beads streaming down the lenses, and spectacles can easily be dislodged under high +Gz forces and vibrations. Another major problem is the restricted field of view with the current spectacle frame when trying to locate a peripheral target. Spectacles can be very uncomfortable when worn under a helmet on long missions, and the current frame has a tendency to create hot spots above the ears. Compatibility with life support equipment, night vision goggles, and personal protective devices has always been a problem for the spectacle wearer.
How do we solve the spectacle compatibility problem? The USAF could tighten its visual standards and eliminate waivers, but by doing so the USAF would decrease its candidate pool and possibly the quality of its candidates. With the long-term trend toward myopia, only those candidates with a reserve of hyperopia would probably remain nonspectacle wearers. Other alternatives would be to make the equipment compatible with spectacles or to design a new aircrew combat spectacle that is universally compatible. The former concept, although an arduous task, is currently being examined by Bill Woessner at USAFSAM. The most pragmatic solution to the spectacle compatibility problem may be contact lenses.
CONTACT LENS PERFORMANCE
We have been doing research on the performance of contact lenses in the aerospace environment for several years at USAFSAM. Polymethylmethacrylate (PMMA) lenses were tested on the centrifuge and in the altitude (hypobaric) chamber in the early 1970s. They performed very poorly under the effects of acceleration (+Gz). At the +6-Gz level, the PMMA lenses were pushed far enough down the cornea to have a significant effect on visual acuity. PMMA material is relatively heavy, with a specific gravity of 1.24, and these particular lenses were small-diameter (8.2-millimeter) tricurve lenses. PMMA lenses also performed poorly in the altitude chamber. Bubbles, which had an effect on visual acuity, formed under the lenses above 20,000 feet.
In 1982 a seven-phase program was initiated to evaluate the feasibility of soft contact lens wear in the aerospace environment. A number of centrifuge riders wearing various kinds of spherical and toric lenses were subjected to acceleration forces up to +8 Gz. Because of the fitting characteristics of hydrogel lenses, the maximum decentration of the lenses, including torics,
was only 2 millimeters. Visual acuity was reduced somewhat at +6 Gz and +8 Gz, but it was also reduced during the spectacle control rides.
Subjects with soft lenses were exposed to altitudes up to 25,000 feet to see if there was any bubble formation or decrement in visual acuity. Although bubbles formed as low as 6,000 feet, they were all near the limbus and had no effect on visual acuity or corneal integrity. Explosive decompressions from 8,000 feet to 25,000 feet also failed to demonstrate any significant bubble formation or loss of corneal integrity. Visual acuity and contrast sensitivity were unaffected during 75-minute rides at 25,000 feet and 4-hour rides at 10,000 feet. However, there was an increase in corneal physiological stress during the 4-hour rides at 10,000 feet, especially when low humidity was added.
Several subjects wearing soft contact lenses were challenged with physostigmine, which was used as a chemical warfare agent simulant. The data indicated that a soft lens acts as a barrier to the chemical agent for the first hour and then as a sink as it spreads the dosage out over time.
Subjects wearing soft contact lenses from USAFSAM participated in a field study on C-130 and C-5 flights to Hawaii and the Orient. They were monitored on the flights by a slit lamp and a military vision-testing device. Although visual acuity and contrast sensitivity were again unaffected during the flights, the low relative humidity (10–15%) in the aircraft cabin may have been the reason for the increased physiological stress (tear debris, conjunctival injection, and corneal epithelial staining) on the cornea. A soft contact lens field study using TAC aircrews is an ongoing project with the USAF Tactical Air Warfare Center at Eglin Air Force Base, Florida.
The USAF has several concerns with contact lens wear in the aerospace environment. A major concern is that of soft contact lens dehydration due to the low humidity of the cockpit (5–15%) and the drying effect of aircraft air conditioners. The air-conditioning systems of many high-performance aircraft create drying problems by blowing across the contact lens and cornea. Most of the crew members in the TAC field test carry rewetting drops with them. We are interested in what this group believes might be the best type of rewetting agent for our environment. Also, what might be the ideal water content for a hydrogel lens in the low-humidity/air-conditioned cockpit environment? Lastly, would rigid gas-permeable (RGP) materials offer the Air Force any advantages under these conditions?
Another concern is that of foreign body incursion under contact lenses while flying. Aircraft air conditioners are a source of particulate matter in the cockpit. As an example, the F-lll's air conditioner blows out particles from the aircraft's insulation. Second, the cockpits of many high-performance
aircraft are dirty, and when pulling a negative Gz force, this dirt rises to the top of the canopy. This may not be a problem with soft contact lens wearers, as we have had no incidences of foreign bodies interrupting a mission during the TAC soft lens test. However, it may be enough of a problem with RGPs to lead to a two-tier system for aircrew (i.e., only soft lenses for high-performance aircraft aircrew, with RGP use limited to multiplace aircraft).
The issue of wearing contact lenses in a hypoxic environment is especially germane to the Air Force. Tanker-transport-bomber aircraft are pressurized to approximately 5,000 to 10,000 feet but have longer missions (some over 12 hours in length). Fighter-attack-reconnaissance aircraft must contend, under certain circumstances, with much higher altitudes. Routinely, high-performance aircraft are pressurized to around 10,000 feet, but under rapid decompression may be exposed to altitudes of 20,000 to 22,000 feet The Air Force is interested in determining which materials, hard and soft, have the Dk values and other characteristics to support the cornea at these altitudes for this length of time.
Another concern is the effect of wind blast on vision-correcting devices when ejecting from aircraft. This is one situation where the advantages of contact lens wear may definitely outweigh those of spectacle wear. However, we do not have any jump data on fliers wearing contact lenses. Do the Army's Golden Knights wear contact lenses during their jumps?
We also have a number of logistical concerns if contact lens wear is approved for Air Force fliers. A cleaning and disinfecting system that is simple, effective, and mobile is needed. How many more eye care professionals will have to be recruited to manage a contact lens program? How many times a year do aircrew members need to be seen? Is an annual exam sufficient? Do disposable contact lenses offer the Air Force any advantages? These are just a few of the issues that we would like the panel to help us with.