The Effect of Corneal Contact Lenses on the Oxygen ...

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chamber of rabbit eyes was measured continuously with a ... The eye was allowed to stabilize about 1 hr .... Bank and University of Wisconsin Eye Bank within 6.

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INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / October 1987

As far as the annual course is concerned, the first occurrence of keratitis Solaris was observed during March. Thus, radiation exposures in January and February are below the threshold dose. However, the individual behaviors of people in winter in response to the lower temperatures, such as wearing spectacles more frequently, may contribute to the non-appearance of keratitis Solaris in winter months. All of the above considerations lead to a rough estimation of the threshold radiant exposure for keratitis Solaris to be less than the lowest calculated radiant exposure by a factor of 2 to 4. Thus, a threshold radiant exposure of 300 to 600 Jm~2 is estimated. So far, threshold radiant exposures for keratitis quoted in the literature1"312 are based on extremely intensive short-term radiation of a few minutes' duration, such as occurs during welding work. The threshold radiant exposure for such short-term radiation is quoted as 40 Jm"2. This value is several times lower than the threshold dose for solar radiation exposure estimated in this study, even if all the above discussed influences are considered. It is therefore concluded that the threshold radiant exposure for keratitis may depend on the intensity of the radiation and increases with decreasing intensity. Further clinical observations will be carried out to permit a more accurate assessment of threshold dose for solar radiant exposure. Key words: keratitis solans, action spectrum, solar radiation, threshold radiant exposure From the "Institute of Medical Physics at the University of Innsbruck, Innsbruck, Austria, and the f University Clinic for Ophthal-

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mology, Innsbruck Regional Hospital, Innsbruck, Austria. Submitted for publication: December 9, 1986. Reprint requests: Dr. M. Blumthaler, Institute of Medical Physics at the University of Innsbruck, Muellerstrasse 44, A-6020 Innsbruck, Austria.

References 1. Pitts DG: The human ultraviolet action spectrum. Am J Optom Physiol Optics 51:946, 1974. 2. Pitts DG: The ocular effects of UV-radiation. Am J Optom Physiol Optics 55:19, 1978. 3. DIN 5031: Deutsche Normen, Strahlungsphysik im optischen Bereich und Lichttechnik. Teil 10 (Vornorm). Berlin, Beuth Verlag, 1979, pp. 1-8. 4. Bener P: Approximate values of intensity of natural ultraviolet radiation for different amounts of atmospheric ozone. Final Technical Report, Contr. DAJA37-68-C-1017, London, European Research Office, United States Army, 1972. 5. Valko P: Vereinfachtes Auswerteverfahren fuer die Schueppsche Methode zur Bestimmung der optischen Truebung. Arch Met Geophy Biokl Ser B 11:81, 1961. 6. Blumthaler M, Ambach W, and Canaval H: Seasonal variation of solar UV-radiation at a high mountain station. Photochem Photobiol 42:147, 1985. 7. Blumthaler M and Ambach W: Neuere Messungen der Albedo verschiedener Oberflaechen fur erythemwirksame Strahlung. Annalen der Meteorologie Nr. 22. Offenbach a. Main, Deutscher Wetterdienst, 1985, p. 114. 8. Dirmhirn I: Das Strahlungsfeld im Lebensraum. Frankfurt/ Main, Akademische Verlagsgesellschaft, 1964, pp. 124-134. 9. Sliney D: Physical factors in cataractogenesis: Ambient ultraviolet radiation and temperature. Invest Ophthalmol Vis Sci 27:781, 1986. 10. Livingston W: The landscape as viewed in the 320 nm ultraviolet. J Opt Soc Am 73:1653, 1983. 11. Sliney D and Wolbarsht M: Safety with Lasers and Other Optical Sources. New York, Plenum Press, 1980, pp. 101-245. 12. Sliney D: The merits of an envelope action spectrum for ultraviolet radiation exposure criteria. Am Ind Hyg Assoc J 33:644, 1972.

The Effect of Corneal Contact Lenses on the Oxygen Tension in the Anterior Chamber of the Rabbit Eye Einor Srefonsson,*t Gary N. Foulks,* and Richard C. Hamilton* The oxygen tension in the aqueous humor in the anterior chamber of rabbit eyes was measured continuously with a polarographic electrode. The normal oxygen tension in the anterior chamber was 23 ± 2 mm Hg (mean ± SD, n = 4). A contact lens was then placed on the cornea for at least 10 min and the drop in oxygen tension recorded. A hard polymethylmethacrylate lens reduced the oxygen tension by 16 ± 4 mm Hg, and a larger hydroxyethylmethacrylate soft lens (Soflens®) decreased oxygen tension by 17 ± 4 mm Hg (mean ± SD, n = 4). Comparable statistically significant decreases were seen with the Permalens®, Polycon II®, and Silcon® lenses. Only the elastofllcon A lens (Silsoft®) did not decrease the oxygen tension in the anterior chamber significantly. Invest Ophthalmol Vis Sci 28:1716-1719, 1987

Previous studies have shown that corneal contact lenses can reduce the oxygen tension in the anterior chamber of rabbits1 and cats.2 However, the contact lenses used in previous experiments are not comparable with those in current clinical use. The aim of the present study was to determine whether corneal contact lenses, which are used clinically, affect the intraocular oxygen tension. Six types of lenses were tested ranging from a hard polymethylmethacrylate lens with poor oxygen permeability to the highly oxygenpermeable silicone lenses. Materials and Methods. Four adult New Zealand red rabbits (2-4 kg, either sex) were anesthetized with

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Table 1. Physical characteristics of contact lenses used in this study

Corneal contact lens

Manufacturer

PMMA® Polycon II® Soflens® Silcon® Permalens® Silsoft®

Conforma Laboratories Syntex Ophthalmics Bausch and Lomb Conforma Laboratories Cooper Vision Dow Corning Ophthalmics

Diameter power (mm) (m-'J 10.0 10.0 14.0 10.0 14.0 11.3

+ 15 + 15 + 15 + 15 + 15 + 15

Oxygen permeability* (Dk X 10") (cm cm3/cm2 sec mm Hg) 0.2 8.7 9.9

14.3 32.0 340.0

Center thickness (mm)

Central oxygen transmissibility (Dk/L X 109) (cm3/cm2 sec mm Hg)

0.45 0.51 0.50 0.45 0.43 0.30

0.04 1.7 1.9 3.2 7.4

113.0

intramuscular ketamine hydrochloride (20-30 mg/kg) and anesthesia was maintained with intravenous alphachloralose (80-120 mg/kg) through an intravenous line. A femoral artery was cannulated for continuous blood pressure measurements and intermittent measurements of arterial blood oxygen tension, carbon dioxide tension, pH and hematocrit. A rectal thermometer was placed and the temperature maintained close to 38°C with the help of a heating pad. The electrocardiogram was monitored continuously. The rabbit was placed in a stereotactic holder. A self-sealing beveled stab incision was made through the cornea at the limbus and the polarographic oxygen electrode placed into the center of the anterior chamber. The reference electrode was placed subcutaneously. The oxygen tension measuring system consists of a chemical microsensor, polarographic oxygen electrode (Model 760), silver-silver chloride reference electrode, and a calibration cell (Model 1251), all made by Diamond Electrotech, Inc. (Ann Arbor, MI). The system was calibrated in pure nitrogen and 20% oxygen/80% nitrogen before and after each experiment. The eye was allowed to stabilize about 1 hr after insertion of the electrode, to establish a stable baseline oxygen tension in the anterior chamber. Continuous topical applications of 0.9% saline at the rate of 4 drops per min kept the cornea moist.

blood oxygen tensions 102 ± 13 mm Hg, carbon dioxide tension 40 ± 9 mm Hg, and arterial blood pH 7.38 ± 0.06, hematocrit 32 ± 2%, and rectal temperature 36.8 ± 1.2°C (mean ± standard deviation, n = 4). At the end of each experiment the rabbit was euthanized with an intravenous overdose of phenobarbital. All animals were treated in accordance with the ARVO Resolution on the Use of Animals in Research. Results. The average oxygen tension in the aqueous humor in the anterior chamber of the New Zealand red rabbit was found to be 23 ± 2 mm Hg (mean ± 1 standard deviation, n = 4). The average decrease in anterior chamber oxygen tension 10 min after the lens had been placed on the eye was recorded. A significant decrease in anterior chamber oxygen tension was found with each of the lenses tested, with the exception of the Silsoft lens. The results are listed in Table 2 and shown in Figure 1. Discussion. Several corneal contact lenses in current clinical use are capable of reducing the oxygen tension in the aqueous humor of the rabbit eye. Since the aqueous humor supplies the corneal endothelium with oxygen, it is inevitable that the aqueous humor hypoxia also leads to hypoxia of the corneal endothelium.

Once a stable baseline oxygen tension was reached, one of the contact lenses was placed on the eye for 10 to 60 min and the oxygen tension monitored continuously. The contact lens was then removed and a new baseline oxygen tension again established before the next lens was applied. Six clinically available contact lenses were tested in random order. All of them were + 15 diopter in refractive power and the radius of the base curve was 6.75 mm. We used a hard polymethylmethacrylate lens, a soft hydroxyethylmethacrylate lens (Soflens®) and a soft elastofilcon lens (Silsoft®) as well as a silafilcon lens (Silcon®), silafocon lens (Polycon II®) lens and perfilcon A Permalens®. The characteristics of the lenses are shown in Table 1. The arterial blood pressure was 53 ± 9 mm Hg, arterial

Table 2. Anterior chamber oxygen tension fall with corneal contact lenses

Corneal contact lens

Decrease in anterior chamber oxygen tension (mm Hg)

P-value

Poly methyl methacry late Soflens® Permalens® Polycon II® Silcon® Silsoft®

16.2 ±4.1 17.4 ±3.8 14.8 ±6.7 12.9 ± 1.0 15.6 ±2.9 2.6 ± 2.4

0.01 0.01 0.05 0.01 0.01 Not significant

Anterior chamber oxygen tension decrease from baseline with each corneal contact lens (mm Hg, mean ± one standard deviation, n = 4). The P-value on a two-tailed paired t-test is given. All except the Silsoft lens show a statistically significant decrease in oxygen tension. The baseline aqueous humor oxygen tension was 23 ± 2 mm Hg.

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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / October 1987

SILSOFT

60

SOFLENS

80

PERMOLENS

100

120

POLYCONII

160

180

200

Time (mins)

Fig. 1. Anterior chamber oxygen tension (mm Hg) in one experimental rabbit is shown as a function of time (minutes). One of the six corneal contact lenses is placed on the cornea for the time interval indicated. The drop in anterior chamber oxygen tension occurs immediately following the placement of the contact lens and the anterior chamber oxygen tension recovers when the lens is removed. All the lenses except the Silsoft lens cause a significant drop in anterior chamber oxygen tension.

A corneal contact lens serves as a barrier between the atmosphere and the anterior surface of the corneal epithelium and anterior stroma. Poise and Decker4 showed that the oxygen tension under a contact lens can indeed reach zero. It is likely that in this situation the oxygen tension of the corneal epithelium and anterior stroma falls. This hypoxia would cause an increase in the oxygen flux entering the cornea from the aqueous humor as the oxygen tension gradient steepens. The aqueous humor would now be supplying a larger portion of the cornea than it normally does. An increased oxygen flux from the aqueous humor to the cornea is the most likely explanation for the reduced oxygen tension in the aqueous humor that occurs with contact lens wear. The oxygen intake from the atmosphere is impaired by a corneal contact lens and the continuing epithelial oxygen demand is partially compensated by an increased oxygen flux from the aqueous humor resulting in a decrease in the aqueous humor oxygen tension. It has previously been recognized that corneal contact lenses place a burden on corneal oxygen supply and metabolism.5 Several authors have shown accumulation of lactic acid in the cornea and aqueous humor in contact lens wearers.6 Lactate accumulation may well be a consequence of the tissue hypoxia indicated by our data. Barr and Silver1 and Stefansson et al2 have previously shown that corneal contact lenses can reduce the aqueous humor oxygen tension, and this has recently been confirmed in vitro.7 While these studies were performed with contact lenses different from those in current clinical use, they demonstrated the effect of contact lenses on the oxygen ten-sion in the anterior chamber1 and also indicated some recovery in oxygen tension in the anterior

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chamber after several hours of contact lens wear.2 This might indicate that compensatory mechanisms, such as increased iris blood flow, increased aqueous humor flow or decreased oxygen consumption may be important in the long-term regulation of oxygen tension in the aqueous humor of contact lens wearers; further research is needed to clarify this point. While the present report shows, for the first time, that corneal contact lenses in current clinical use can cause aqueous humor hypoxia, the differences between the experimental and the clinical setting have to be kept in mind when these data are interpreted. The experimental rabbits do not blink under anesthesia. To simulate blinking, physiologic saline at body temperature was dripped on the cornea or contact lens at the rate of 4 drops per min. The +15 diopter lenses used in this study are for aphakic patients and are thicker, and thus have less oxygen transmissibility than the corresponding lenses that myopic patients would use. Relative hypoxia at the outer corneal surface in the face of an actively respiring corneal epithelium may create a gradient that lowers oxygen tension in the anterior chamber. Hypoxic effects on the endothelium may explain changes in endothelial morphology and corneal stromal hydration. Kamiya8 was able to demonstrate corneal endothelial changes in contact lens wearers and produce similar changes by blowing nitrogen over the cornea to produce hypoxia. It is possible that aqueous humor and endothelial hypoxia caused by corneal contact lenses may play a role in increased corneal hydration and endothelial changes observed in contact lens wearers. In addition, the prolonged relative hypoxia, especially with extended wear, may play a role in peripheral corneal vascularization. Further research is needed to fully understand the clinical effect of corneal and aqueous humor hypoxia caused by corneal contact lenses. Key words: contact lens, cornea, oxygen, rabbit, polarography, hypoxia From the *Duke University Medical Center, Department of Ophthalmology, Durham, North Carolina, and the |National Eye Institute, Clinical Branch, National Institutes of Health, Bethesda, Maryland. Supported by a Career Development Award from the Veterans Administration to ES. Submitted for publication: October 13, 1986. Reprint requests: Einar Stefansson, MD, PhD, Duke University Eye Center, Box 3802, Durham, NC 27710.

References 1. Barr RE and Silver IA: Effects of corneal environment on oxygen tension in the anterior chambers of rabbits. Invest Ophthalmol 12:140, 1973. 2. Stefansson E, Wolbarsht ML, and Landers MB: The corneal contact lens and aqueous humor hypoxia in cats. Invest Ophhalmol Vis Sci 24:1052, 1983.

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3. Hamano H, Kawabe H, and Mitsunaga S: Reproducible measurements of oxygen permeability (Dk) of contact lens materials. CLAO 11:221, 1985. 4. Poise KA and Decker M: Oxygen tension under a contact lens. Invest Ophthalmol Vis Sci 18:188, 1979. 5. Burns RP, Roberts H, and Rich LF: Effect of silicone contact lenses on corneal epithelial metabolism. Am J Ophthalmol 71:486, 1971. 6. Praus R, Brettschneider I, and Dreifus M: Study of the effect of hydrophilic gel contact lenses on the cornea: I. Lactic acid in

HLA-DR /T6

the cornea and aqueous humor and cornea hydration after application of contact lenses geltakt in rabbits. Ophthalmologica 159:398, 1969. 7. Hamano H, Mikami M, Mohri H, Mitsunaga S, and Kotani S: Measurement of oxygen tension in anterior ocular segments via platinum micoelectrode: 1. Preliminary experiments in vitro. Journal of Japan Contact Lens Society 28:47, 1986. 8. Kamiya C: A study of corneal endothelial response to contact lenses. Contact and Intraocular Lens Medical Journal 8:92, 1982.

Langerhans Cells of the Human Cornea

Samuel K. Sero,* Thomas E. Gillette, and John W. Chandler* We have investigated normal human corneas for the presence of T6-marker on Langerhans cells. With the exception of one pair of newborn corneas and two pairs of very young infant corneas, all HLA-DR-positive cells in central and peripheral corneal epithelium were T6-negative by doublelabeled immunofluorescence. In contrast, epidermal sheets from normal human eyelid skin displayed positive staining for T6 on most of the HLA-DR-positive Langerhans cells. Since T6 antigen is considered to be a specific Langerhans cell differentiation marker, we interpret this finding to indicate a nonactivated or undifferentiated state of Langerhans cells in normal human corneas. Invest Ophthalmol Vis Sci 28:1719-1722,1987

Studies of normal human epidermal Langerhans cells have demonstrated heterogenic populations of such cells. HLA-DR + /T6 + , HLA-DR + /T6~, and HLA-DR~/T6+ Langerhans cells have all been detected in human skin.1"3 The discovery of human thymocyte antigen (T6) on Langerhans cell membranes has led to subsequent convictions that T6 is a more specific marker for human Langerhans cells than previously used markers such as la antigen, ATPase, and Fc and C3b receptors.45 One study has shown T6 antigen to be present on Langerhans cells of normal human conjunctiva.6 However, there have been no published studies as yet on the presence or absence of this marker on corneal Langerhans cells. We have investigated normal human corneas of donors of various ages for the presence of T6 antigen. All corneas, with the exception of one pair of newborn corneas and two pairs of young infant corneas, contained HLA-DR-positive Langerhans cells that were T6 negative. Materials and Methods. Whole globes were obtained from the University of Washington Lions Eye Bank and University of Wisconsin Eye Bank within 6 hr of the donor's death. Corneas were dissected with approximately 2 mm of scleral rim. After a brief rinse

in normal saline, the corneas were incubated in 20 raM EDTA in phosphate buffered saline (PBS, pH 7.3) for 45-60 min at 37°C. The epithelial sheets were separated from the corneal stromata with fine dissecting forceps and fixed in cold acetone for at least 10 min. Eyelid epidermal sheets used as controls were similarly isolated from tissue obtained from blepharoplasties. Prior to staining, each epithelial sheet was washed and rehydrated in PBS containing 1% bovine serum albumin (BSA-PBS) for 10 min. The tissues were then incubated in suspension with a 1:50 dilution of OKT6 monoclonal antibody (Ortho Diagnostic Systems, Raritan, NJ) at 4°C overnight. Following this incubation the epithelial sheets were washed twice in BSA-PBS for 10 min and incubated in a 1:50 dilution of rhodamine-conjugated sheep antimouse IgG (H&L) (Cappel Laboratories, West Chester, PA) at 4°C overnight. The tissues were washed twice again in BSA-PBS and then incubated in a 1:50 dilution fluorescein-conjugated anti-HLADR antibody (Becton Dickinson, Mountain View, CA) at 4°C overnight. After two final washes in BSAPBS, radial incisions were made through each epithelial sheet to allow a flat mount onto a microscopic slide. The tissues were coverslipped with Aquamount (Lerner Laboratories, New Haven, CT) and the edges of the coverslip sealed with clear nail polish to prevent evaporation. Additionally, a number of corneas and human skin epidermis specimens were stained in a one-step process with only a fluorescein-conjugated OKT6 (Ortho Diagnostic Systems). Results. Twenty-six pairs of corneas were examined by double fluorescent-labeled antibody staining for HLA-DR and T6. The age ranges of these corneas are given in Table 1. Staining of these corneas was compared to that of identically processed human skin epidermis. Of these 26 pairs of corneas, 23 pairs were completely negative for T6 antigen (Fig. la) but dis-

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