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

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PREFACE

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reviewed limitations of contemporary materials with respect to selection criteria

and task performance, including follow-up care. The program offered ample

opportunity for formal and informal group discussion. The edited proceedings

of the discussion together with the formal papers of the participants are the con-

tents of this report.

In addition to the specialists who participated in the symposium, a number

of people contributed in important ways to this project. Robert Miller of the

Brooks Air Force Base staff facilitated arrangements for the symposium. Roger

Wiley and his staff at the U.S. Army Aeromedical Research Laboratory assisted

the working group in arranging for presentations by U.S. military personnel

both at the symposium and at meetings of the working group. Pamela Ebert Flat-

tau, the committee's study director, provided valuable assistance in organizing

the symposium and preparing the proceedings report. As always, Carol Metcalf,

the committee's administrative secretary, provided efficient and skillful support.

KENNETH POLSE, CHAIR

WORKING GROUP ON CONTACT LENS USE UNDER ADVERSE CON-

DITIONS

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CONTENTS

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CONTENTS

Contact Lenses and the Eye: Basic Considerations

1

Environmental Gases and Contact Lens Wear

Gerald E. Lowther

3

Hypoxia

George W. Mertz

14

Contact Lenses and Corneal Energy Metabolites in the

Rabbit

Morris R. Lattimore, Jr.

24

Environmental Conditions and Tear Chemistry

Leo G. Carney

34

Tear Evaporation Considerations and Contact Lens Wear

Miguel F. Refojo

38

Mechanical Aspects of Soft Contact Lenses

James T. Jenkins

44

Contact Lenses and the Eye: Complications

51

Medical Problems Associated With Contact Lens Use

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Robert P. Green, Jr.

53

Adverse Reactions Associated With Contact Lens Use

Oliver Schein

58

Corneal Topography and Contact Lenses

Stephen D. Klyce

68

Treatment of Giant Papillary Conjunctivitis

Mathea R. Allansmith

74

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CONTENTS

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Endothelial Effects From Contact Lens Wear

John P. Schoessler

84

Contact Lenses and the Eye: Practical Considerations in

Everyday and Military Life

95

Corneal Effects of Extreme Environments: Considera-

tions for Pilots Wearing Contact Lenses

Joshua E. Josephson

97

Effect of Aircraft Cabin Altitude and Humidity on Oxy-

gen Tension Under Soft and Hard Gas-Permeable Con-

tact Lenses

Melvin R. O'Neal

106

Ocular Occupational Health Concerns: Considerations

for Pilots Wearing Contact Lenses

Joshua E. Josephson

119

Lens Performance Considerations

Gerald E. Lowther

128

Vision Performance With Contact Lenses

Robert B. Mandell

135

An Overview of U.S. Army Aviation and Contact Lens

Issues

Morris R. Lattimore, Jr.

142

Contact Lens Wear in the Aerospace Environment

Richard Dennis

148

Use of Soft Contact Lenses by Tactical Aircrews

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Richard Dennis

152

Job Demands in Naval Aviation

Andrew Markovits

156

Extended-Wear Lenses: The U.S. Navy's Experience

James Socks

159

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CONTACT LENSES AND THE EYE:

BASIC CONSIDERATIONS

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ENVIRONMENTAL GASES AND CONTACT LENS WEAR

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soft (hydrogel) lens less than 2 percent exchange occurs (Polse, 1979; Wagner et

Environmental Gases and Contact Lens

Wear

Gerald E. Lowther

OXYGEN

Oxygen has been the most studied gas with respect to contact lens wear

because the avascular cornea must obtain oxygen from the atmosphere. Without

oxygen from the air, corneal swelling will occur, with resultant corneal clouding,

decreased visual acuity, discomfort, and other long-term adverse effects. The

cornea requires about 5 µl O /mm2 cornea/hour (Hill and Fatt, 1963; Larke et al.,

2

1981). This value varies with individuals from about 3 to 10 µl O /mm2

cornea/

2

hour (Larke et al., 1981).

Under open-eye conditions at sea level the anterior surface of the cornea has

available about 20.9 percent oxygen (O2 in air) or about 155 mmHg partial

pressure. With the eye closed the oxygen level drops to about 7 percent. With

contact lens wear the lens will impede oxygen from reaching the cornea.

Therefore, considerable effort has gone into developing lenses that will supply

oxygen to the cornea. Oxygen can be supplied under a lens by designing the lens

such that tear exchange occurs with each blink, bringing in oxygen dissolved in

the tears. This usually does not supply enough oxygen. An exchange of about

10–15 percent of the tear volume under a rigid lens can occur with each blink

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(Figure 1) (Fatt and Hill, 1970; Fatt, 1969; Cuklanz and Hill, 1969), but with a

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ENVIRONMENTAL GASES AND CONTACT LENS WEAR

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value, termed permeability, is determined. The higher the Dk value the more

oxygen will diffuse through a unit thickness of material. To determine the

amount of oxygen that will diffuse through a given contact lens, the Dk value is

divided by the lens thickness (L) to give a Dk/L value, which is called

transmissibility. Clinically the transmissibility is the important value. The higher

the value, the more oxygen will reach the cornea. Permeability is given as a

number × 10-11 (cm22/sec)(ml O /ml mmHg). Transmissibility is given as a

2

number × 10-9 (cm/sec)(ml O /ml mmHg). In both cases only the initial number

2

is quoted with the units and power of 10 assumed.

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ENVIRONMENTAL GASES AND CONTACT LENS WEAR

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0 percent and 20.9 percent. The higher the value the more oxygen transmitted

through lens to the cornea. Plots are usually given as percent oxygen against lens

thickness (Figure 2) (Hill and Mauger, 1980).

Figure 2

The relative equivalent oxygen percentage (EOP) of six hydrophilic contact lens

materials over various ranges of lens thickness. A sample Dk value (see text)

appears in parentheses for each in units multipled by 10-11 (cm2/sec) ml O /ml ×

2

mmHg). SOURCE: Adapted from Hill and Mauger (1980).

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An important question is the minimum oxygen that the cornea requires.

Another important factor is how much of the required oxygen does the available

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ENVIRONMENTAL GASES AND CONTACT LENS WEAR

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EOP 9.9% Dk/L 24.1

EOP 12.1% Dk/L 34.3

EOP 17.1% Dk/L 87.0

With present hydrogel lenses the Dk value is related to the water content

(Figure 4) (Sarver et al., 1981). The highest Dk value possible is about 50.

Transmissibility is therefore related to only the water content and lens thickness.

Thus, with the present hydrogel materials the amount of oxygen that can be

supplied is limited. Table 2 gives the Dk/L values for different water contents and

lens thicknesses.

TABLE 1 Estimates of the Critical Oxygen Requirements (COR) of the Cornea

Author

Year

Criterion

COR (%)

Epithelium: Biochemical

Uniack et al.

1972

Glycogen depletion

5

Uniack et al.

1972

LDH concentration

5

Hill et al.

1974

SDH reactivity

5

Hamano et al.

1983

Lactate

<13.2

Epithelium: Structure/Function

Fatt

1968

Oxygen consumption

2.7

Uniacke et al.

1972

Epithelial thickness

5

Millodot and O'Leary

1980

Sensitivity

>7.7

Hamano et al.

1983

Mitosis

<13.2

Masters

1984

Mitochondria

>10

Benjamin and Hill

1985

Oxygen consumption

15.6

Stroma

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Polse and Mandell

1970

Corneal edema

1.5–2.5

Carney

1974

Corneal edema

2

Mandell and Farrell

1980

Corneal edema

3.1

Mizutani et al.

1983

Corneal edema

15

Holden et al.

1984

Corneal edema

10.1

Mandell

1985

Corneal edema

7.4

Brennan et al.

1987

Corneal edema (gas)

10.9

Brennan et al.

1987

Corneal edema (EOP)

18.0

Endothelium

Williams

1986

Bleb response

16.6

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ENVIRONMENTAL GASES AND CONTACT LENS WEAR

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Figure 3

Mean corneal swelling for the group after 8 hr exposure to the various oxygen

concentrations. The curve is an exponential fit. The error bars represent ±1

standard deviation. SOURCE: Holden et al. (1984). Reprinted by permission.

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Figure 4