Search

Purpose: To determine the refractive and corneal topographic recovery after the use of reverse-geometry contact lenses for overnight orthokeratology., Methods: Both eyes of 15 subjects were fitted with reverse-geometry contact lenses that were worn by the subjects for 1 month. Uncorrected visual acuity, refractive correction (sphere and spherical equivalent), corneal curvature, and corneal thickness were measured during this time and for 2 weeks after discontinuation of lens wear., Results: Ten subjects completed the investigation. Uncorrected visual acuity, refractive correction, and corneal curvature had changed significantly (P = 0.01) after 1 month of lens wear. By the end of 1 month, central corneal thickness was significantly thinner than the baseline value (P = 0.01), but it recovered fully after one night of no lens wear. Recovery of corneal curvature was complete 1 week after lens wear was discontinued. Refractive correction and binocular uncorrected visual acuity recovered fully after 2 weeks. Monocular uncorrected visual acuity remained significantly (P = 0.01) different from baseline acuity 2 weeks after lens discontinuation., Conclusions: Full effect of overnight orthokeratology in low myopes is achieved within 1 week of initiating use of reverse-geometry lenses. Recovery after short-term use of reverse-geometry lenses is rapid for corneal thickness and corneal curvature. Refractive correction and binocular uncorrected visual acuity recovered fully after 2 weeks. Monocular uncorrected visual acuity was the slowest to recover and had not achieved full recovery after 2 weeks.

Purpose: To achieve an optimal fit with reverse geometry Contex OK lenses and to determine a time course for and the stability of visual and corneal changes in achieving maximal refractive, corneal curvature, and corneal thickness changes after overnight wear of OK B and D series lenses., Methods: This investigation was conducted under an Food and Drug Administration IDE G000059. Both eyes of 10 subjects were fitted with the lenses, and uncorrected visual acuity, refractive correction, contrast sensitivity, corneal curvature, and corneal thickness were measured at baseline and at 1 day, 1week, 1 month, and 3 months after lenses were worn. Except for baseline, data were collected at four different times during the day, immediately following lens removal and 4, 8, and 12 hours after lens removal., Results: The results from eight subjects showed that uncorrected visual acuity, refractive correction, contrast sensitivity, and corneal curvature all changed significantly (P=0.01) overnight. By the end of 1 week, all corneal and visual changes had reached a maximal level and remained fairly stable during the day. These changes were sustained at 3 months. The epithelial thickness data from four subjects showed that the corneal epithelial thickness was reduced by approximately 19 microm after 3 months of lens wear., Conclusions: Successful fitting of OK B and D series lenses requires a thorough understanding of the lens-cornea relationship. Full effect of overnight orthokeratology is achieved by the end of 1 week. The visual and corneal changes remain stable for all waking hours of the day and allow patients to enjoy excellent device-free vision (20/20).

Purpose: To determine if the variability in contact lens-induced corneal swelling is associated with variability in corneal oxygen consumption (Q(C)) or corneal endothelial function., Methods: Corneal swelling was induced in 30 non-contact lens wearers by 2 hours of closed-eye contact lens wear, using thick (oxygen transmission [Dk/t] = 4.0 x 10-9) and thin (Dk/t = 12 x 10-9) hydrogel lenses of identical design. Following the induction of swelling, corneal thickness was continually measured by pachymetry until open-eye steady-state (OESS) thickness was achieved. The percentage of recovery per hour was calculated as a measure of endothelial function. Tear oxygen tension (PO(2)) beneath the hydrogels was measured in the open eye and after 5 minutes of eye closure to obtain estimates of Q(C). A change in corneal pH during eye closure while wearing the hydrogels was used as a measure of hypoxic acidosis. Associations between corneal swelling and endothelial cell density or corneal epithelial thickness were also tested., Results: There were modest but significant (P < 0.05) correlations between thick-lens corneal swelling and thick-lens closed-eye PO(2) (r = -0.36); thin lens corneal swelling and thin-lens closed-eye and open-eye PO(2) (r = -0.40 and -0.39, respectively). Corneal swelling also increases with increasing Q(C) (r = + 0.29 to + 0.33). Corneal swelling was associated with the decrease in pH during lens wear (r = + 0.30 and + 0.20 for thick and thin lenses, respectively). Thick- and thin-lens corneal swelling was significantly associated with percentage of recovery per hour (r = -0.40 and -0.34, respectively). Multiple regression analysis of corneal swelling with PO(2) and percentage of recovery per hour suggested an additive effect, however the effects were not significant. There was a modest inverse association between corneal swelling and OESS, however there were no associations between corneal swelling and endothelial cell density or epithelial thickness., Conclusions: The variability in contact lens-induced corneal swelling is associated with both corneal metabolic activity (Q(C) and pH) and endothelial function (percentage of recovery per hour). Our interpretation is that individuals with larger Q(C) produce more lactic acid (i.e., more swelling) whereas stronger endothelial function resists swelling. The modest correlations, however, suggest that other factors also are involved in explaining the phenomenon of corneal swelling.

Contact lens wear can cause a number of physiological changes in the cornea. Two areas of interest in my laboratory have been contact lens effects on the endothelium and, more recently, the role of metabolic activity in predicting corneal swelling. The first part of this review focuses on the function of the corneal endothelium, the nature of its fluid pump, and the effects of contact lens-induced hypoxia and corneal pH changes on corneal endothelial function. In the second part, the etiology of hypoxia-induced corneal swelling is reviewed in relation to new studies on the causes of intersubject corneal swelling variability. The results indicate that corneal swelling is influenced by both corneal metabolic activity and endothelial function.

Purpose: (1) To update Fatt's mathematical model of the distribution of oxygen tension (pO2) across the cornea and contact lens (CL) to include the recent finding that corneal oxygen consumption increases with the acidification that occurs with CL wear. (2) To estimate the minimum transmissibility (CL Dk/t) to avoid epithelial anoxia or to avoid stromal anoxia., Methods: A five-layer static and one-dimensional mathematical model of oxygen diffusion through the cornea based on Fatt's models was used. The relationships between acidosis and increased QO2, and acidosis and CL Dk/t were used to estimate corneal QO2 for a given CL Dk/t., Results: (1) Revised model predictions are in agreement with direct tear pO2 measurements beneath CLs in the rabbit. (2) For the human eye, the minimum CL Dk/t for oxygen delivery to the basal epithelial cells was determined to be 23 for the open eye and 89 for the closed eye. To prevent anoxia throughout the entire corneal thickness the Dk/t requirements are 35 for the open eye and 125 for the closed eye., Conclusions: (1) Model predictions of the oxygen distribution beneath contact lenses are significantly lower than previous models that did not include the effect of acidosis on corneal QO2. (2) Minimum Dk/t values that allow oxygen delivery to the basal epithelium are in agreement with the Dk/t needed to avoid corneal edema.

Purpose: Previous studies have shown that contact lens oxygen transmissibility correlates with binding of Pseudomonas aeruginosa to the rabbit cornea after overnight lens wear. Studies of human lens wear stratified by oxygen transmissibility will be required to validate these animal results. In humans, bacterial binding to shed cells obtained through corneal irrigation cytology may provide an indirect measure of in vitro binding. The purpose of this study was to establish the relationship between binding to shed cells and to the residual corneal surface in an animal model of lens wear prior to initiation of human studies., Methods: The test contact lenses used were: rigid lens A (Dk/L = 10 x 10(-9) [cm/ sec][mL O2/mL mmHg]); rigid lens B (Dk/L = 97); soft lens A (Dk/L = 9); soft lens B (Dk/L = 20); and, soft lens C (Dk/L = 39). There were six rabbits in each group, except for the soft lens C group, which had seven rabbits. After overnight lens wear, the corneal surface was irrigated with a corneal irrigation chamber to collect surface cells before exposure to a bacterial suspension (1 x 10(7) CFU/mL) for 30 minutes. The number of bacteria adherent to the residual corneal surface was then assessed by CFU determination. Cells collected from the corneal surface (9 mL) were incubated with 1 mL bacterial suspension containing 10(8) (CFU/mL) for 30 minutes. The number of bacteria adherent to shed cells was assessed by staining with acridine orange and direct counting by epifluorescence microscopy., Results: The differences in the number of bacteria adhering to shed epithelial cells between the treated and the control eyes were 2.90 +/- 1.20 and 0.23 +/- 0.41 for rigid lenses A and B, respectively, and 5.97 +/- 1.54, 3.67 +/- 2.32, and 0.90 +/- 1.45 (bacterial/cell) for soft lenses A, B, and C, respectively. Overnight contact lens wear induced a significant increase in bacterial binding to shed corneal epithelial cells for rigid lens A and for soft lenses A and B. There were significant differences among lens groups (P = 0.00017, ANOVA), with significant differences between rigid lenses A and B, soft lenses A and C, and soft lenses B and C. The binding of bacteria to shed cells was significantly correlated with the binding of bacteria to the residual corneal surface, both confirming and extending previous results (R = 0.78, P < 0.001)., Conclusion: These results demonstrate a positive correlation between P. aeruginosa adherence to shed corneal cells and to the residual corneal surface in the rabbit eye following contact lens wear. In light of the results from prior animal studies, examination of the behavior of P. aeruginosa binding to exfoliated cells appears to be a promising and valid method for future assessment of similar lens-induced increases in bacterial binding in prospective human clinical studies.

Purpose: To develop a noninvasive measurement of tear oxygen tension and to determine tear oxygen tension and calculated oxygen flux rates during wear of contact lenses of varying oxygen transmissibility (Dk/L)., Methods: The basis of the tear oxygen tension (PO2) measurement is phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl) porphine by oxygen. A fiber-optic bundle, positioned approximately 2 mm from a cuvette or from the corneal surface, delivers an excitation flash (< 4 microseconds, 539 +/- 23 nm) and collects the phosphorescence emission (> 645 nm) of the dye at 1 MHz for 2 ms. Porphine (500 microM) + 1% albumin is instilled directly into the tears of a sedated rabbit, and phosphorescence is measured before and after contact lens insertion., Results: Phosphorescence intensity decay lifetimes are related inversely to PO2 and follow the Stern-Volmer relationship. Calibration control experiments revealed a temperature effect on lifetime of 1% per degree, dye concentration independence over a 100-fold dilution, and no significant pH effect between 6.8 and 7.9. Phosphorescence lifetimes indicated that tear PO2 beneath contact lenses reached steady state within 3 to 8 minutes. Mean steady state tear PO2 (mm Hg +/- SE; N > 7) beneath contact lenses was: polymethylmethacrylate (Dk/L = 0), tear PO2 = 0.58 +/- 0.22; Oxyflow f30 (Dk/L = 15.3), tear PO2 = 27.7 +/- 3.4; Fluorex 700 Dk/L = 30), tear PO2 = 52.5 +/- 3.8; Oxyflow 151 (Dk/L = 51.9), tear PO2 = 78.7 +/- 23.9. Steady state tear PO2 beneath an Oxyflow f30 after application of topical anesthetic and 10 minutes of lens wear was 41 +/- 1.2 mm Hg, significantly higher than without anesthetic., Conclusions: Phosphorescence-based measurements allow the direct, sensitive, and noninvasive assessment of oxygen availability beneath a contact lens. Oxygen levels beneath permeable contact lenses found by direct measurement are significantly lower than what mathematical models predict. The measured values, however, are close to PO2 estimates using the equivalent oxygen percentage (EOP) technique. Tear PO2 measurements may be used to examine metabolic disturbances as shown by reduced oxygen consumption rates after application of topical anesthetic.

Purpose: Changes in pH in the corneal epithelium, stroma, endothelium and aqueous humor following contact lens wear were evaluated., Methods: Intracellular pH, stromal pH, and aqueous humor pH were measured either in vivo or in vitro in humans and in rabbits using pH-sensitive fluorescent dyes. Fluorescence was measured before and during lens wear using a slit lamp fluorimeter (in vivo experiments) or a microscope fluorimeter (in vitro experiments)., Results: The pH of corneal epithelium, stoma epithelium and aqueous humor decreased significantly with lenses up to Dk/L = 100. Hypoxia had large effects on the corneal epithelium and stroma, but had no effect on the endothelium or aqueous. Conversely, carbon dioxide accumulation caused significant acidosis in all compartments measured., Conclusions: Corneal acidosis during contact lens wear is significant under a variety of wearing conditions and, thus, must be considered when evaluating the morphological or functional changes that occur following long-term contact lens wear.

Contact lens wear causes significant epithelial and stromal acidosis. In this study, we tested whether lens wear can cause endothelial acidosis as well. Rabbit corneas were isolated and perfused in vitro. The endothelial intracellular pH (pHi) was measured with a pH sensitive fluorescent probe (BCECF). Three conditions were examined: 1) Polymethylmethacrylate (PMMA) and rigid gas-permeable (RGP) contact lens wear using a range of oxygen transmissibility (Dk/L) from 0 to 121, 2) epithelial hypoxia produced by exposure to oligomycin/sodium azide solution or epithelial perfusion with 100% N2 equilibrated Ringer's solution, and 3) epithelial exposure to Ringer's equilibrated with 5% CO2, balance air. PMMA and RGP contact lens wear acidified endothelial cells by 0.23 +/- 0.01 (n = 23) and 0.11 +/- 0.01 pH units (n = 23), respectively, within twenty min of lens insertion. Epithelial hypoxia, induced by sodium azide and oligomycin, reversibly acidified the endothelium by 0.04 +/- 0.01 pH units (n = 4). However, epithelial hypoxia induced by perfusion with 100% N2 equilibrated Ringer's did not have a significant effect on endothelial pHi. Introduction of 5% CO2 to the epithelium, acidified the endothelium by 0.15 +/- 0.02 pH units (n = 7) within 10 min. We conclude that contact lens wear can significantly acidify corneal endothelial cells. The endothelial pHi change is caused almost exclusively by a build up of CO2 behind the lens; hypoxia having very little contribution. As expected, RGP contact lenses induced less endothelial acidosis than PMMA controls.

Purpose: Based on contact lens-induced stromal acidification of the cornea, it has been suggested that the corneal epithelial and endothelial cells also become acidotic during contact lens wear. This alleged acidification may have a role in altered cell appearance and metabolism during contact lens wear. This study investigated the effects of anoxia, carbon dioxide retention, and contact lens gas transmissibility on the epithelial and aqueous humor pH in living rabbits., Methods: Epithelial intracellular pH (pHi) and aqueous humor pH were fluorophotometrically measured with a pH sensitive-dye (BCECF) during contact lens wear or exposure to various gas mixtures., Results: Polymethylmethacrylate (PMMA) lens wear acidified epithelial cells by preventing CO2 efflux and by inducing hypoxia. Increasing lens oxygen transmissibility decreased epithelial acidification. After initiation of rigid, gas-permeable (RGP) lens wear or CO2-air exposure, pHi dropped transiently and then recovered partially. This recovery of pHi was not observed during anoxia, whether induced by PMMA lens wear or exposure to 100% N2. The aqueous humor also acidified during PMMA lens wear, a phenomenon not observed during RGP lens wear. Changes in aqueous pH were smaller, slower, and delayed when compared to their epithelial counterparts., Conclusions: Hypoxic contact lens wear acidifies the corneal epithelium and aqueous humor. The aqueous humor pH change indicates a probable endothelial acidification during hypoxic contact lens wear; the pH changes are caused by two separate and additive effects, CO2 retention and hypoxic acidosis. Increases in the oxygen transmissibility of the lens decrease the cellular acidosis, which might minimize cellular complications arising from contact lens wear. We estimate that a lens with an oxygen transmissibility (Dk/L) of 300 x 10(-11) (cm/sec)(ml O2/ml x mm Hg) is needed to prevent epithelial pHi changes in the open eye. In contrast, lenses with Dk/L as low as 18 x 10(-9) (cm/sec)(ml O2/ml x mm Hg) can prevent aqueous humor pH changes.

The effects of tear-film hypoxia and contact lens wear on human in vivo stromal pH was tested using a non-invasive fluorometric technique. Hypoxia was produced by exposing the normal open eye to 100% nitrogen gas passed through tight-fitting goggles. Stromal pH dropped from 7.53 +/- 0.02 to 7.34 +/- 0.03 (n = 12, +/- SD) within 90 min of nitrogen gas exposure, t1/2 = 20 min. After removing the goggles, stromal pH returned to baseline in 35 min, t1/2 = 10 min. Wearing a thick hydrogel contact lens which caused a tear PO2 less than or equal to 2 mm Hg with the eyes open, reduced stromal pH from 7.55 +/- 0.02 to 7.15 +/- 0.04 (n = 12, +/- SD) in 80 min, t1/2 = 9.5 min. After removing the lens, baseline pH was reached in 40 min, t1/2 = 4.5 min. The stromal pH differences between hypoxia (N2 only) and contact lens wear were not due to differences in tear temperature between the two procedures (contact lens wear 32 +/- 1.5 degrees C, goggles 33 +/- 1.0 degrees C). However exposing the eye to 95% nitrogen-5% carbon dioxide reduced stromal pH to 7.16 +/- 0.05 (n = 7, +/- SD) in 80 min, t1/2 = 8 min, which was similar to that produced during contact lens wear. These experiments show that contact lens wear causes corneal acidosis by: (1) the production of protons from hypoxic metabolism, and (2) the accumulation of carbon dioxide behind the lens due to low lens CO2 transmissibility.

Corneal stromal pH was measured fluorometrically in nine human subjects after 1.5 hours of eye closure while wearing hard gas permeable contact lenses. Six lens types providing a wide range in oxygen transmissibilities (Dk/L)O2 from 0.15 to 55.0 X 10(-9) (cm ml O2/seconds of ml mmHg) were used. Stromal pH upon opening the eyes was directly related to (Dk/L)O2 and ranged from 7.01 to 7.26. These pH values were all significantly lower than the control (no lens), which had a pH of 7.38 (P less than 0.005). After eye opening (lens still on) stromal pH increased and reached a new steady-state (range, 7.11-7.45) in approximately 30 minutes; however, these pH levels were still below the control (no lens) (pH 7.54; P less than 0.005). The rate of pH change after eye opening also increased directly with (Dk/L)O2. The authors conclude that the lenses presently available provide insufficient gas exchange to prevent alteration of corneal pH and suggest that the measurement of stromal pH may provide a sensitive index for evaluating the metabolic effects of contract lens wear.