Predicting extended wear complications from overnight corneal swelling.

PURPOSE: To examine the hypothesis that the corneal overnight swelling response (ONSR) is a predictor of ocular complications in contact lens extended wear (EW). METHODS: The Berkeley Contact Lens Extended Wear Study (CLEWS) was a randomized, concurrently controlled clinical trial in which more than 200 subjects in EW with rigid gas-permeable (RGP) lenses were observed for 1 year. After adapting to EW, subjects were randomized to either medium or high oxygen-permeable (Dk) RGP lenses and underwent clinical assessments, keratometry, and corneal pachometry at 3-month intervals. RESULTS: The ONSR was directly related to lens Dk (P = 0.01) and exhibited substantial variability across subjects. The probability of remaining free of complications over time was not significantly lower for subjects with a mild ONSR compared with those with greater edema (P = 0.84). The risk of development of keratopathy was not significantly related to the ONSR (relative risk = 1.00). CONCLUSIONS: The corneal ONSR is not a good predictor of ocular complications in 1 year of RGP EW. Lenses that cause little or no corneal edema are not necessarily safer for overnight wear.

Oxygen tension beneath piggyback contact lenses and clinical outcomes of users.

PURPOSE: Patients with keratoconus sometimes wear a rigid contact lens as a piggyback over a countersunk soft one. This geometry restricts movement of the rigid lens over the central cornea and may also reduce corneal oxygenation. This aim of this study was to estimate the equivalent oxygen percentage beneath such lenses and to review the clinical records of users to determine their success rate. METHODS: The right eyes of four normal subjects were covered for 5 minutes with a combination of rigid gas permeable (Boston ES or 7 [Polymer Technology Corp., Rochester, NY]) and soft (Acuvue [Johnson & Johnson, Jacksonville, FL], Keraflex [KF] 3 or 5 [Blanchard, Sherbrooke, QC, Canada]) contact lenses. After removal of the lenses, oxygen tension was measured with a Clark electrode. In order to determine whether measurements of oxygen tension were supported by clinical data, the clinical files of 11 patients fitted with KF 5-Boston ES piggyback lenses were reviewed for the presence of corneal edema. RESULTS: Mean relative equivalent oxygen percentage ranged from 6.9% to 11.2% beneath the following lens combinations: KF 3-Boston ES; KF 3-Boston 7; KF 5-Boston ES; KF 5-Boston 7; Acuvue-Boston ES; and Acuvue-Boston 7. Of the ten patients followed after dispensing, none had any sign of corneal edema. Nine had worn their contact lenses for periods up to 6 years. CONCLUSIONS: Based on the measured equivalent oxygen percentage and on the absence of corneal edema in patients, the combination of countersunk KF 5 with Boston ES lenses provides enough oxygen to fulfill corneal needs in oxygen during daily wear.

On-eye dehydration of proclear, resolution 55G and acuvue contact lenses.

In order to compare the extent of dehydration of Proclear, Resolution 55G and Acuvue contact lenses overtime, 13 patients wore each lens during 4, 6 and 8 h. Three observers, initially shown to have a high inter-observer reliability, measured water content, with a soft contact lens refractometer before and after each period of wear. Absolute dehydration of Proclear and Resolution 55G lenses were similar after 4, 6 and 8 h of wear, but were smaller than for Acuvue lenses (1.2% compared to 5.7% after 8 h). The loss of lens mass following dehydration of Resolution 55G lenses, calculated with the change in lens relative water content was found to correlate very well with an in vitro study of the same material. Lens dehydration should be taken into account by clinicians when making their choice of soft contact lens.

Dorzolamide and corneal recovery from edema in patients with glaucoma or ocular hypertension.

PURPOSE: To investigate whether dorzolamide alters corneal hydration control in patients with glaucoma or ocular hypertension. METHODS: Pachymetry, tonometry, and endothelial cell density were measured by a masked observer in 19 subjects with bilateral glaucoma or ocular hypertension. They were treated with 2% dorzolamide in one eye, and with saline in the other, before wearing contact lenses under patched eyes. Corneal thickness, measured each 30 minutes up to 4.5 hours after contact lens removal, enabled estimation of percentage recovery per hour and time for 95% of corneal thickness recovery for both eyes. Seven patients repeated this test after 1 year of dorzolamide use, and their results were compared with those of the preceding year. RESULTS: After induction of hypoxic corneal edema, there was no significant difference between paired corneas in swelling levels (60.0+/-11.8 and 59.8+/-12.9 microm) (P = .94), time to 95% recovery (440.6+/-255.8 and 445.4+/-186.7 minutes) (P = .93), and percentage recovery per hour (38.1%+/-10.9% and 36.1%+/-9.6%) (P = .40). Subjects followed up after 1 year of dorzolamide use did not differ significantly in values of endothelial cell density, percentage recovery per hour, or time to 95% recovery from those obtained a year before. One subject developed persistent corneal edema after his stress test in the eye treated with dorzolamide. CONCLUSION: There is no significant difference in the recovery from induced corneal edema after either a short-term or 1-year use of dorzolamide in patients with glaucoma or ocular hypertension with a normal corneal endothelium. One patient had persistent corneal edema after the stress test was performed on the dorzolamide-treated eye.

Short-term oxidative status of lens and aqueous humor after excimer laser photorefractive keratectomy.

PURPOSE: Oxidation in the anterior ocular segment is associated with cataractogenesis and possible complications to corneal endothelium. We investigated whether oxidation occurred in the rabbit anterior ocular segment shortly after excimer laser photorefractive keratectomy (PRK). METHODS: Rabbits were treated unilaterally with PRK, the other eye serving as a control. Aqueous humor sampled shortly after treatment was assayed spectrophotometrically for hydrogen peroxide using ferrous oxidation in xylenol in the presence (group 1; n=10), or absence of oxygen (group 2; n=8). Oxidized glutathione and malondialdehyde levels were measured in lenses by spectrophotometry and HPLC. RESULTS: Hydrogen peroxide concentration of aqueous humor was not different between treated (77 +/- 36 microM) and control eyes (88 +/- 34 microM) in the oxygen group or the nonoxygenated group (treated eyes: 6.7 +/- 5.4 microM and control eyes: 5.5 +/- 4.7 microM). Peroxide levels did not correspond to endogenous H2O2 but presumably reflected action on ascorbic acid. There was no difference in the percent of oxidized glutathione between experimental and control eyes. Malondialdehyde could not be detected in the lens of treated or control eyes, despite good sensitivity in recovery assays. CONCLUSION: Based on these assays, there is no evidence that PRK oxidizes the aqueous humor or the lens of treated rabbits within 10 minutes of treatment.

Non-steady-state diffusion in a multilayered tissue initiated by manipulation of chemical activity at the boundaries.

Diffusion of ionic and nonionic species in multilayered tissues plays an important role in the metabolic processes that take place in these tissues. To create a mathematical model of these diffusion processes, we have chosen as an example hydrogen-bicarbonate ion pair diffusion within the mammalian cornea. This choice was based on the availability of experimental data on this system. The diffusion coefficient of the hydrogen-bicarbonate ion pair in corneal stroma and epithelium is calculated from the observed change in pH in the stroma when conditions at the corneal anterior epithelial surface are changed while the posterior surface is continually bathed with a Ringer's solution in equilibrium with a CO2-gas air mixture. Matching experimental results to a mathematical model of the cornea as a two-layer diffusion system yields, at 37 degrees C, a diffusion coefficient of the hydrogen-bicarbonate ion pair of 2.5 x 10(-6) cm2/s in the stroma and 0.4 x 10(-6) cm2/s in the epithelium. Application of the Nernst-Einstein equation to these data gives the following diffusion coefficients in the two layers: 1) stroma, D(H+) = 11.8 x 10(-6) cm2/s; D(HCO3-) = 1.5 x 10(-6) cm2/s; and 2) epithelium, D(H+) = 1.9 x 10(-6) cm2/s; D(HCO3-) = 0.22 x 10(-6) cm2/s.