Extended-Wear Lenses: The U.S. Navy's Experience
The submariner working in a clean environment with humidity of approximately 50 percent will have a greater choice of contact lens types than the helicopter pilot who is exposed to dust and wind. The risks associated with extended-wear contact lenses are significantly different for each group, with the latter also having the heightened probability of foreign bodies under the lens, lens displacement, and lens loss.
The high-performance jet pilot, the astronaut, and the deep-sea diver will each experience markedly different environments that will affect their ability to wear contact lenses and the performance of the lenses in those particular environments.
What drives the use of contact lenses in the military and where we are heading? Basically, it is the current regulations, the vision standards, relative to each specialty community. We have heard there is a manpower shortfall. A closer look at refractive error and the relationship of visual acuity I think will drive home the point.
The vision of our aviators, or whatever the specialty happens to be, with a little bit of myopia, begins to drop off rather dramatically and particularly when astigmatic error is included, as shown by Peters (1961) in Figure 1 The aviator, then, has a great deal of difficulty performing the assigned task.
There is, of course, the problem of man/machine interface. Machines are built without man in mind, which is a major problem. Hardware developers will build the periscopes, the night vision devices, the gas masks, and then present each to the military community and say “use it.” Then somebody says, “but the man can't see.” Then it comes back to us to resolve the problems.
Wearing contact lenses—be they hydrogel or rigid gas-permeable (RGP) materials—coupled with decreased humidity reduces the available oxygen to the cornea, which results in corneal swelling. Furthermore, the wearing of those lenses for an extended time results in further decrease in oxygen availability over the course of the wearing period. That results in increased swelling and a high probability of contact-lens-related complications.
In addition to the Air Force and Army's operational scenarios, we must keep in mind that special combat units operating at high mountain altitudes in the cold are exposed to a reduced partial pressure of oxygen as well as the effects of the cold. Furthermore, the U.S. Navy has given consideration to reducing the ambient partial pressure of oxygen aboard nuclear fleet ballistic missile and nuclear attack submarines in order to decrease combustibility —thus, the ever-present danger of fire that exists on submarines.
In each of these situations ocular tissues have a decreased concentration of oxygen available, and it is likely that increased insult to the cornea will result from wearing contact lenses, especially extended-wear lenses.
COMPRESSION AND DECOMPRESSION
In studies of decompression we found that bubbles formed under hydrogel lenses during decompression (Molinari and Socks, 1987). Those bubbles left pitting of the cornea near the limbus. When we did the same decompression studies with RGP lenses in hyperbaric chambers, there was central bubbling (Socks et al., 1987). Again, that confirmed the findings of Flynn
et al. (1986) that pressure changes can lead to pitting of the corneal epithelium.
The bubbles that formed underneath the lenses grew rapidly in size during the decompression, then fractured into multiple small bubbles as we reached the surface. Over a period of time, 2 to 3 hours, the dimpling disappeared. It is not known what the repeated effects of exposure in hypobaric or hyperbaric environments would have on the cornea during extended operations.
Of particular interest to us was the fact that formation of bubbles occurred from depths as shallow as 37.5 feet, which is well within the limits of the no-decompression-dive tables set by the U.S. Navy.
Extreme cold might be experienced by mountain troops or possibly even by aviators at high altitudes. But it is unlikely that there will be a negative impact on contact lens wear (Socks, 1981, 1982), except for the fact that those environments are usually very dry. Actually, the wearing of contact lenses in extreme cold—and we are supposed to be talking about adverse environments even though we have spent most of our time speaking about the hypobaric high-altitude situation—will provide protection from wind-driven ice and snow (Kolstad and Opsahl, 1969).
The question was also raised about contaminants in the atmosphere of the cockpit and in any enclosed environment, and we have noted that many chemicals and particulate matter can be found in the submarine environment (Knight et al., 1983). Reports from submariners say that discomfort can occur at times when wearing the contact lenses aboard ship. This discomfort is often related to excess gaseous chemicals in the environment of the submarine, those that are not scrubbed out ( Table 1).
I will not examine the risks of contact lens in detail because we are all aware of the specific risks of extended-wear lenses. I would like to say that the contraindications are the same for the military environment as they are for the civilian sector: inflammation, dry eye, corneal disease, and systemic disease with ocular overtones. Careful screening and patient education are absolutely vital to avoid problems during an operational situation.
Regardless of whether hydrogel or RGP lenses are used by military personnel on an extended-wear basis, they need to have spare lenses, lens care solutions that are appropriate, and access to appropriate health care professionals. Critical personnel wearing extended-wear contact lenses cannot afford to be without spare lenses during key operations. Therefore, the spare lenses must be readily available in case of loss or damage to the first pair of lenses. Disposable lenses may be an alternative.
We also need to look at the lens care systems that are available. Contact
TABLE 1 Actual or Potential Sources of Contaminants in Submarine Atmospheres
Expiration (acetone, isopren, acetonitrile) Other excretions (methane, H2O)
Cooking (aldehydes) Drinks (ethanol)
Leaks of diesel fuel (hydrocarbons) Oils and greases (hydrocarbons, esters)
By emission, leak, thrown out:
Frigorific fluids (halons)
Paints, glues, cements, adhesives, sealers
Heat insulation (styrene)
Electric batteries (hydrogene, arsine, stibine)
Infirmary (ether, halothan)
Cleaning materials (ammoniac, halogenated hydrocarbons)
Normal (HCl, HF, CO) Undesigned (HCN)
SOURCE: Naval Submarine Medical Research Laboratory, Special Report83-1, June 1, 1983.
lens care systems play a vital role in the success of the contact lens wearer. For military operations simplicity is the key to compliance in the field. Storage/disinfecting solutions should possess adequate antimicrobial activity, be nonsensitizing and noncytotoxic, and have a high degree of patient acceptability. Because of limited availability of supplies and storage space in the field, it is imperative that the care regime be limited to as few components as possible and that it be simple in design and use. To this end, solutions should be multipurpose; that is, they should fill the needs of storage, soaking, disinfecting, and rinsing. The same solution might even be considered for use as a rewetting/comfort drop. Cleaners must possess adequate efficacy to rapidly remove the complex deposits that form on the lenses.
CURRENT MILITARY EXPERIENCE WITH EXTENDED-WEAR LENSES
My comments below are based on work performed at the Naval Submarine Medical Research Laboratory in Groton, Connecticut. Keep in mind that the submariner is presented with a particular problem, the ability of the periscope operator to interface with the periscope itself because of the limitations of the optics to correct both spherical and astigmatic refractive error.
Figure 2 shows the distribution of spectacle corrections of the 154 officers and 20 enlisted men we studied. The Type 18 periscope, which is found on the Los Angeles Class 688 attack submarines, corrects to plus or minus approximately 4.5 diopters.
The entrance requirements to submarine service allow refractive spherical equivalent to plus or minus 5.5 diopters. Already the man who is at the limits is handicapped because he is 1 diopter undercorrected by the Type 18 periscope. Add to that the allowable limit on astigmatic error of 2 diopters.
It was decided that a feasibility study would be conducted. We ran this study from 1981 to 1984, primarily on fleet ballistic missile (FBM) submarine crews whose home port was Groton, Connecticut (Socks, 1985).
FBM ships may not be where the crews are home ported, and in our case in New London the ships are in the Holy Loch, Scotland. The crews leave for 3 months and relieve the opposite crew when they arrive in the Holy Loch. They go out on station for 70 days, but they are away from the base area for 3 months.
Remember 1981 was just after the first Hydrocurve and Cooper Permalenses were approved, so the materials were early-generation extended-wear materials.
We found some problems but not very many in terms of lens replacement and tearing. These findings were not unexpected.
The other complications we found were minor, including four subjects with infiltrates and nine with minor neovascularization. Today we probably would not even put in stippling and infiltrates.
As a matter of fact, there have been no significant medical events in the time period that this program has been a fleetwide program —from about 1985, when it was initiated. In a study of submariners fit for contact lenses both under the Navy program and by non-Navy fitters, it was concluded that the risks associated with contact lens wear by submariners were low (Ulrich, 1987). Contact lenses are available at government expense to all submariners who are qualified periscope operators. The Navy will provide the care, the lenses, and the solutions. It has been a very successful program.
It has been successful, in part, because the submariners are enthusiastic; they are highly educated; they comply very strictly with the lens care instructions. I think this grows out of the Rickover era, which influenced the type of person who is accepted and trained in the nuclear submarine navy. The submariners receive regular follow-up visits; hospital corpsmen were put on the ship to ensure that they return to the clinics. We have a strong patient education program and in-depth training of Navy hospital corpsmen.
I think contact lenses in the cockpit are inevitable. The Army, Navy, and Air Force will all have them. The equipment, the manpower, the vision standards will demand so.
We completed the pilot program and it was successful. The Air Force is doing a pilot program feasibility study, and to date it appears to be successful. I challenge the Navy air community and the Army air community and special communities within the three services to also conduct pilot programs and see what the findings are.
Flynn, W.J., R.E. Miller, T.J. Tredici, M.G. Block, and E E. Kirby 1986 Subcontact Lens Bubble Formation Under Low Atmospheric Pressure Conditions. USAF School of Aerospace Medicine. August 21. Report no. USAFSAM-TR-86-21. NTIS no. AD-A173.
Knight, D R., K.R. Bondi, and J.P. Malaspina 1983 The Analysis of French Submarine Atmosphere Containments. Proceedings of the Third Tripartite Conference on Submarine Medicine, France,
United Kingdom, and United States Naval Submarine Medical Research Laboratory. Special Report 83-1, June. NTIS no ADA 133150.
Kolstad, A., and R. Opsahl, Jr. 1969 Cold injury to corneal epithelium: A cause of blurred vision in cross-country skiers. Acta Ophthalmologica 47(3):656–659.
Molinari, J.F., and J.F. Socks 1987 Effects of Hyperbaric Conditions on Corneal Physiology With Hydrogel Contact Lenses. Naval Submarine Medical Research Laboratory. Report No. 1102. October. NTIS no A189152.
Peters, H.B. 1961 Relationship between refractive error and uncorrected visual acuity American Journal of Optometry and Physiological Optics 38:194–198.
Socks, J.F. 1981 Use of Contact Lenses for Cold Weather Activities. Naval Submarine Medical Research Laboratory. Report No. 969. December. NTIS no. ADA 110020. 1982 Contact lenses in extreme cold environments: response of rabbit corneas American Journal of Optometry and Physiological Optics 59(4):297– 300. 1985 Contact Lenses on Submarines. Naval Submarine Medical Research Laboratory Report No. 1048. April. NTIS no. A156900.
Socks, J.F., J.F., Molinari, and J.L. Rowery 1989 Rigid Gas Permeable Contact Lenses in Hyperbaric Environments. Naval Submarine Medical Research Laboratory. Report No. 1100. August. NTIS no. A187744.
Ulrich, S. 1987 An overview of contact lens use aboard submarines. Thesis for designation as submarine medical officer. Naval Undersea Medical Institute, Naval Submarine Base, Groton, Conn.