Ординатура / Офтальмология / Английские материалы / Considerations in Contact Lens Use Under Adverse Conditions Proceedings of a Symposium_Flattau_1991

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Visual acuity results for four contact lenses and spectacles at reduced luminance

tivity function is better with contact lenses than with spectacles. Other studies

show just the opposite. Usually the differences between results obtained by the

two forms of correction are so small that it makes no practical difference. When

there is a "significant" difference between the two types of corrections—contact

lenses and spectacles—it can usually be attributed to the fact that there was

residual astigmatism present when the subjects were wearing contact lenses. I

think that the result is very meaningful, however, in terms of what results might

be anticipated with contact lenses worn by military personnel.

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Figure 4

(2.5 CD/M2). Based on data from Guillon et al. (1988).

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Some contact lens manufacturers have found that certain aspherical designs

produce enough spherical aberration that visual acuity is actually degraded. Most

of the time this problem is not one to be concerned about because the

manufacturer does not make the lens available. Unfortunately, much of the time

this information has been restricted to within the organization and the data are

thus never published.

RESIDUAL ASTIGMATISM

Residual astigmatism which remains when the contact lens is in place, is a

very real problem with regard to fitting military personnel with contact lenses.

The task is to fit a group of subjects with an unknown distribution of refractive

errors, and without knowing precisely the incidence of astigmatism. If these

subjects are to be fit with soft contact lenses, there will be a significant number of

people who are going to have enough residual astigmatism that it will cause a

reduction in visual acuity. Even if we knew the population numbers, they would

not necessarily apply to the military sample of pilots. It certainly must be

recognized ahead of time that this will be a significant problem.

If you decide to fit all military personnel needing a refractive correction with

soft contact lenses, there are going to be, based on the occurrence of residual

astigmatism, a certain number of people who will have to have an alternative

lens.

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Figure 5

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Figure 5 shows the distribution of spherical refractive error against

astigmatism and the amount of reduction that is to be expected in visual acuity.

This information permits prediction of the amount of visual acuity loss that can

be expected to occur in a certain percentage of pilots. Residual astigmatism is

something that the military will have to contend with in the selection of contact

lenses. It will almost have to be an individualized situation as far as fitting is

concerned. For an individual who has a certain amount of astigmatism, it cannot

always be predicted what type of lens will produce the best visual acuity for that

person.

In conclusion, the military's policy toward contact lenses cannot really

restrict the lens type to soft contact lenses and expect to provide an optimal

correction for the majority of potential pilots. On the other hand, it certainly

seems that the military should restrict the number of lenses to certain types. There

is going to have to be a broad representation if the needs of all pilots are to be

addressed.

REFERENCES

Bailey, I. no date. Contrast Sensitivity Visual Acuity Chart. Available from Multimedia Productions,

School of Optometry, University of California.

Blum, H.L., et al. 1959. Vision Screening for Elementary Schools: ''The Orinda Study." Berkeley:

University of California Press.

Ginsburg, A.P., D.W. Evan, N.W. Cannon, Jr., C. Owsley, and P. Mulvanny 1984. Large-sample

norms for contrast sensitivity. American Journal of Optometry and Physiological Optics

61:80–84 .

Guillon, M., D.P.M. Lydon, and R.T. Solman 1988. Effect of target contrast and illuminance on soft

contact lens and spectacle visual performance. Current Eye Research 7(7):635–648 .

Long, G.M., and D.L. Penn 1986. Normative contrast sensitivity functions: the problems of

comparison. American Journal of Optometry and Physiological Optics 64(2):131–135 .

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Copyright © National Academy of Sciences. All rights reserved.

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AN OVERVIEW OF U.S. ARMY AVIATION AND CONTACT LENS ISSUES

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In order to develop relative safety patterns in established

rotary-wing

An Overview of U.S. Army Aviation and

Contact Lens Issues

Morris R. Lattimore, Jr.

While U.S. Army aircraft can be either fixed-or rotary-wing, this discussion

will address the rotary-wing or helicopter segment of the Army aviation

community. The Army's arsenal of helicopters encompasses a number of types of

aircraft, each designed for a specific mission. These include cargo, observation,

utility, and attack helicopters. As an institution, Army aviation has been seriously

interested in contact lens wear by its ametropic pilots since the early 1970s, when

it was shown that dust and debris did not cause a significant problem for soft

contact lens wearers in an aviation environment (Crosley et al., 1974). However,

with the soft lenses commercially available then, an unacceptable variation in

visual acuity was documented.

At present, Army Regulation (AR) 40-63 prohibits contact lens wear by all

aircraft crew members; the only waivers to that restriction have been associated

with investigative protocols. The advent of new lens materials spurred further

interest within the aviation community, leading to the development of a

preliminary investigation of extended contact lens wear by a small group of

volunteer aviators (Bachman, 1988). The results are in the process of being

published, so only a brief summary will be supplied here.

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PRELIMINARY STUDY

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The extended-wear lenses were worn on a 7-day/6-night schedule. That is,

after the initial fitting, the lenses were worn continuously for 7 days and 6 nights.

The lenses then were removed prior to retiring on the seventh night, and were

reapplied the following morning after using an appropriate disinfection and lens

care regimen. Postfitting follow-up examinations were provided on day 1, day 8,

and every 30 days thereafter. The study ran for 6 months with an 86 percent

success rate. Six subjects withdrew from the study because of acuity problems (2)

or discomfort (4).

Prior to the initial contact lens fitting, the mean flying time for the subject

population was 2,136 hours. Over the 6-month period of the study the mean

flying time for successful contact-lens-wearing subjects was 294 hours. During

the course of the study, there were no groundings for contact-lens-related

reasons, and there were no aircraft incidents or accidents related to the wearing of

contact lenses. Subjective performance assessments rated the contact lenses used

as being superior to spectacle wear by a vast majority of the subjects for preflight

(68 percent), takeoffs (83 percent), routine flight (83 percent), nap-of-the-earth

flight (89 percent), night vision goggles flight (88 percent), instrument flight (88

percent), and mission-oriented protective posture IV conditions (100 percent).

Temporary discontinuances of contact lens wear were incurred by six pilots a

total of nine times. Causes of discontinuance were conjunctivitis (8), abrasion

(2), foreign body (2), facial trauma (1), and meibomitis (1). Again, none of the

contact lens wear discontinuances led to grounding; the affected aviators merely

wore their spectacles in lieu of the contact lenses. In summary, this initial

feasibility study demonstrated the safe use, both in medical and flight terms, of

extended-wear contact lenses by AH-1 and UH-1 pilots.

DRIVING FORCE

Immediate interest is being directed at the AH-64 Apache attack helicopter

(Figure 1). The Apache's integrated helmet and display sighting system

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(IHADSS) uses a virtual imaging system to provide visual input to the pilot and

copilot from a closed-circuit video system. The image can be in a daytime TV

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AN OVERVIEW OF U.S. ARMY AVIATION AND CONTACT LENS ISSUES

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Figure 1

AH-64 Apache attack helicopter. NOTE: The AH-64 Apache attack helicopter

represents the utmost in modern air weaponry. The two-man airframe system is

designed to fly in literally any weather conditions.

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Figure 2

original work has been recomposed from XML files created from the original paper book, not from the original

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AN OVERVIEW OF U.S. ARMY AVIATION AND CONTACT LENS ISSUES

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inserted. Please use the print version of this publication as the authoritative version for attribution.

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Figure 3

originalwork has been recomposed from XML files created from the original paper book, not from the original

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AN OVERVIEW OF U.S. ARMY AVIATION AND CONTACT LENS ISSUES

146

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onto the external surface of the eye bubble. Visual complications induced by

such a revision include image magnification and distortion. In addition, the

peripheral HDU symbology is obscured because of a decreased field of view that

stems from the increased physical distance imposed on imager placement (Figure

4). Consequently, ametropic aviators may be unable to fly the AH-64 under

chemically contaminated conditions.

Contact lens wear constitutes an alternative to the M-43 glue-on optics and

represents an attractive short-term solution to the aviation community. However,

before even advocating contact lens wear as a viable option, the Army's medical

community wishes to ensure that all potential hazards and costs are fully

documented so that an informed decision can be reached.

REFERENCES

Bachman, W.G. 1988. Extended-Wear Soft and Rigid Contact Lenses: Operational Evaluation Among

Army Aviators. Contact Lenses in the Aviation Environment. USAARL Technical Report

No. 88-17, Fort Rucker, Alabama.

Crosley, J.K., E.G. Braun, and R.W. Bailey 1974. Soft (hydrophilic) contact lenses in U.S. Army

aviation; an investigative study of the Bausch & Lomb "Soflens." American Journal of

Optometry and Physiological Optics 5:470–477 .

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