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.

The Berkeley Contact Lens Extended Wear Study. Part I : Study design and conduct.

OBJECTIVE: The primary aim of the Berkeley Contact Lens Extended Wear Study (CLEWS) was to test the hypotheses that extended wear of rigid gas-permeable (RGP) contact lenses with greater oxygen permeability (Dk) reduces the incidence of contact lens-associated keratopathy (CLAK) and increases the survival rate in RGP extended wear (EW). In this article we describe the clinical trial design in detail, present the results of subject recruitment and retention, and provide the baseline demographic and ocular characteristics of the CLEWS subjects, whose data will be analyzed to address the study aims in a companion article. DESIGN: A randomized, concurrently controlled clinical trial. INTERVENTION: Subjects were fitted with day wear (DW) high-Dk RGP lenses and then adapted to EW. Subjects who adapted to EW were then randomly assigned to either high- or medium-Dk RGP lenses for 12 months of 6-nights/week EW. MAIN OUTCOME MEASURES: Slit-lamp assessment and grading of 17 possible keratopathies, measurement of refractive error and corneal curvature, and symptoms. Follow-up data were collected every 3 months. RESULTS: From 545 subjects entering the DW adaptation phase, 201 adapted to EW and were randomly assigned to medium- or high-Dk lenses for 12 months of EW. The baseline characteristics of the two study groups were similar and did not differ from the 344 DW subjects who failed to adapt to EW. The distributions of oxygen transmissibility for the two study groups were disjoint, indicating that each group received distinctly different levels of hypoxia. CONCLUSIONS: We show that CLEWS was appropriately designed to address the study hypotheses, was conducted with regard for the safety of the subjects, and adhered to rigorous protocols designed to control for bias and ensure the integrity of study data. We establish the internal validity of between-group statistical comparisons and characterize our study population to permit informed evaluation of the applicability of our results to the contact lens-wearing population in general.

The Berkeley Contact Lens Extended Wear Study. Part II : Clinical results.

OBJECTIVE: To describe the principal clinical outcomes associated with 12 months use of rigid gas-permeable (RGP) extended wear contact lenses and address two primary study questions: (1) does extended wear (EW) of high oxygen transmissibility (Dk/t) RGP lenses reduce the incidence of ocular complications, and (2) does the wearing of high-Dk/t lenses reduce the rate of failure to maintain 6-night RGPEW over 12 months? DESIGN: A randomized, concurrently controlled clinical trial. INTERVENTION: Subjects who adapted to EW with high Dk (oxygen permeability) RGP lenses were randomized to either high Dk or medium-Dk RGP lenses for 12 months of 6-night EW. MAIN OUTCOME MEASURES: Contact lens-associated keratopathies (CLAK), changes in refractive error and corneal curvature, and survival in EW. RESULTS: Two hundred one subjects were randomized to medium or high-Dk lenses for 12 months of EW. Sixty-two percent of the subjects in each group completed 12 months of EW; however, the probability of failure was significantly greater for the medium-Dk group. Although the risk of complications was similar for the two groups, the number of CLAK events that led to termination were 16 versus 5 for the medium-Dk and high-Dk groups, respectively. This suggests that the type of adverse response or the inability to reverse an adverse event was different for the group being exposed to the lower oxygen dose. CONCLUSIONS: The level of oxygen available to the cornea has a significant impact on maintaining successful RGP extended contact lens wear, but not on the initial onset of CLAK. The number of clinical events leading to termination was substantially higher for the medium Dk group, which suggests that corneal hypoxia is an important factor in the development of CLAK. Although overnight contact lens wear should be recommended with caution and carefully monitored for early detection of ocular complications, it appears that high-Dk RGP lenses can be a safe and effective treatment for correction of refractive error for most individuals who can adapt to EW.

The effects of one-hour wear of high-Dk soft contact lenses on corneal pH and epithelial permeability.

PURPOSE: Previous studies have shown that 1-hour closed-eye contact lens wear with a low-Dk lens causes a significant reduction in corneal pH and an increase in epithelial permeability. In the present study, we evaluated the effects of super-high-Dk/t soft lenses on corneal epithelial barrier function and stromal pH. METHODS: Corneal thickness was measured by optical pachometry, while epithelial permeability and stromal pH were measured by fluorophotometry. A paired-eye design was used in which one eye was randomly allocated to wear a high-oxygen-permeable soft lens (CIBAVision Focus/NIGHT & DAY (Dk/t= 175) while the other eye did not wear a lens. RESULTS: After 1-hour closed-eye lens wear, neither the difference in corneal swelling (P = 0.206) nor the permeability (P = 0.055) between both eyes was significantly different. The mean pH values under open-eye conditions were 7.66 vs. 7.57 for the treatment and control eyes, respectively (P = 0.082), dropping to 7.27 vs. 7.25 after 1-hour eye closure (P = 0.283). CONCLUSIONS: Although our results are limited to a 1-hour observation period, they do provide evidence that high-Dk materials may eliminate corneal acidosis and reduced epithelial barrier function that accompany closed-eye contact lens wear with lower-Dk soft lens materials.

Measurement of post-lens tear thickness.

PURPOSE: A method to measure the tear film beneath a soft contact lens, referred to as post-lens tear thickness (PLTT), would have many applications to contact lens research. In this study a noninvasive technique for measuring the PLTT is presented. METHODS: The feasibility of measuring the tear layer by optical pachometry was first assessed using a model eye. The baseline corneal thickness (B) of both eyes of 21 subjects was measured, etafilcon-A ionic disposable soft contact lenses (58% water) were inserted, and the total thickness (T) of the cornea, contact lens, and PLTT were measured. After the pachometry readings the lenses were removed and their center thickness (C) determined. The PLTT was calculated using the equation: PLTT = T-(B+C). Two sets of measurements of T were performed at 15 and 25 minutes after lens insertion. The entire procedure was repeated at a second visit. RESULTS: The pachometry measurements of the small aqueous reservoir between the model eye and the lens closely matched those obtained by direct microscopic measurement. For human PLTT, the mean values (and 95% confidence intervals) for right eyes on visits 1 and 2 were 11 (8, 13) and 12 (10, 15) microm, respectively, and for left eyes were 12 (10, 15) and 11 microm (8, 14) microm, respectively. CONCLUSIONS: It is possible to measure the post-lens tear thickness using optical pachometry. The variability between repeated measurements suggests that with careful sample size planning, the technique is sufficiently precise to be useful in group assessments of PLTT.

Predicting RGP daily wear success.

PURPOSE: While most clinicians agree that fitting of rigid gas permeable (RGP) lenses is the preferred treatment strategy for some types of patients, many patients have difficulty adapting to full-time daily wear of these lenses. The Contact Lens Extended Wear Study (CLEWS) is a randomized, controlled clinical trial in which subjects are first adapted to full-time RGP daily wear prior to randomization to either high or medium oxygen transmissibility (Dk/L) RGP lenses for 12 months of extended wear. The CLEWS pre-randomization data provide an opportunity to examine the reasons some patients fail to adapt to RGP daily wear and to determine whether a patient's demographic, historical, and ocular characteristics can be used to predict RGP daily wear success. METHODS: From 1,809 individuals who expressed interest in RGP lenses, 411 subjects passed the CLEWS screening criteria and were fit with high Dk lenses (mean Dk = 92 x 10(-11) [cm2/sec][mL 02/mL x mmHg]). The fitting strategy included minimum apical alignment, lid attachment, centration, and average lens diameters of 9.2 mm. Success was defined as a minimum of 14 hours per day without adverse ocular responses that would contraindicate either full-day daily wear or extended wear. RESULTS: Of the 411 subjects, 286 (69.6%) were successful with RGP extended wear. Among the 125 failures, 50 were based on clinician findings (e.g., cornea, vision, compliance), while 75 subjects chose to discontinue wear because of unacceptable comfort or poor vision. Multivariate linear logistic regression modeling shows that younger patients with greater corneal curvature (K) and less predicted residual astigmatism have a higher probability of RGP daily wear success. CONCLUSIONS: Our study showed that RGP lenses can be fit with a relatively high rate of success and that many of those patients who elect to discontinue RGP daily wear report unacceptable comfort as the primary reason. We propose a Bayesian statistical method that will assist clinicians in selecting those candidates for RGP daily wear who are most likely to adapt successfully.

Tear mixing under a soft contact lens: effects of lens diameter.

PURPOSE: Tear exchange under a soft contact lens is modest, and higher exchange rates may be necessary to reduce extended-wear complications; what is not known is the optimal soft lens design to increase tear mixing. We explored the effect of lens diameter on tear mixing. METHODS: Twenty-three subjects wore four different soft contact lenses with diameters of 12.0, 12.5, 13.0, and 13.5 mm. Tear mixing was quantified by placing fluorescein isothiocyanate-dextran on the posterior lens surface, inserting the lens, and monitoring the changes in fluorescence intensity in the postlens tear film. Tear mixing, expressed as the percentage decrease in fluorescence intensity per blink, was estimated using an exponential model. Lens movement was videotaped and lens comfort was graded on a 50-point scale (50 = excellent comfort). Subjects reporting a comfort level of less than 35 were excluded. RESULTS: The mean +/- SE tear mixing rates were 1.82% +/- 0.17%, 1.61% +/- 0.16%, 1.34% +/- 0.17%, and 1.24% +/- 0.17% per blink for the 12.0-, 12.5-, 13.0-, and 13.5-mm diameter lenses, respectively. By regression analysis we found that, on average, mixing under the 12.0-mm lens was 0.59% per blink greater than with the 13.5-mm lens (P = .0024). Lens diameter was a significant predictor of lens comfort, and adjusting for the effects of comfort weakened the relationship between diameter and tear replenishment rate, although the mean rate under the 12.0-mm lens was still 0.43% per blink greater than with the 13.5-mm lens (P = .0468). CONCLUSIONS: These data suggest that smaller-diameter soft lenses provide substantially better tear mixing than larger lenses; however, even small lenses provide modest tear mixing compared with rigid contact lenses.

Effects of hypoxia on corneal epithelial permeability.

PURPOSE: Epithelial permeability to fluorescein (Pdc) increases with closed-eye soft contact lens wear. Possible mechanisms responsible for this change include corneal hypoxia (and acidosis) during overnight wear and interference of the lens with normal tear exchange. In this study we used a local environmental chamber to examine the effects of hypoxia on Pdc. METHODS: After baseline slit-lamp examination, autofluorescence, and central corneal thickness measurements, 35 subjects were fitted with a pair of airtight swimming goggles. One eye was exposed to a humidified gas mixture containing 95% nitrogen (N2) and 5% carbon dioxide (CO2), and the fellow (control) eye was exposed to air. The experimental eye and the eye measured first were assigned using a randomized block design. After the subjects wore the goggles for 1 hour, corneal thickness measurements were repeated and Pdc was assessed as previously described. A final slit-lamp examination was performed by a masked examiner to assess epithelial integrity. RESULTS: There was no significant difference in the mean ln(Pdc) (95% confidence interval) for eyes exposed to N2/CO2 when compared with paired eyes exposed to air (-2.70 [-2.99, -2.41] vs -2.51 [-2.72, -2.30] ln[nm/sec], P = .272). Although there was also no substantial difference in the slit-lamp appearance of the experimental and control eyes, the mean (95% confidence interval) change in corneal thickness for the hypoxic eyes was 19.78 (15.72, 23.84) microm compared with 3.40 (-0.67, 7.47) microm (P < .0001) in the control eyes. CONCLUSION: One hour of hypoxia caused a significant increase in corneal thickness with no apparent change in Pdc. This suggests that other factors, either alone or in combination with hypoxia, may be responsible for increased Pdc during closed-eye contact lens wear.

Soft lens extended wear affects epithelial barrier function.

OBJECTIVE: To explore the impact of eye closure and soft contact lens extended wear (SCLEW) on epithelial permeability to fluorescein (Pdc). DESIGN: A prospective cohort study. PARTICIPANTS: Thirty-one noncontact lens (CL) wearers participated. INTERVENTION: The effects of eye closure on Pdc were evaluated by comparing morning (AM) and afternoon (PM) measurements on non-CL subjects. The effects of SCLEW on Pdc were determined by measuring Pdc before beginning SCLEW and then after 2, 4, and 12 weeks of SCLEW. MAIN OUTCOME MEASURES: Pdc measured in the morning versus the afternoon and before versus after SCLEW was examined. RESULTS: Analyses of Pdc were done using the natural logarithm (ln). The mean +/- standard error (SE) ln(Pdc) measured in the AM versus PM on 16 non-CL wearers did not differ significantly (-2.56+/-0.16 vs. -2.69+/-0.15, respectively; P = 0.46). In contrast, the mean +/- SE ln(Pdc) in 15 subjects after 2 (-1.73+/-0.183, P < 0.001), 4 (- 1.59+/-0.188, P < 0.001), and 12 weeks (-1.99+/-0.206, P = 0.02) of SCLEW was substantially greater than that measured before lens wear (-2.42+/-0.159 ln(nm/sec)). Interestingly, the mean+/-SE ln(Pdc) measured in the afternoon on seven subjects after 13 weeks of SCLEW was similar to their average baseline ln(Pdc) (-2.62+/-0.27 vs. -2.52+/-0.243, respectively; P = 0.54). Further analysis showed that Pdc was highest in the morning and decreased approximately 12.5%/hour (P < 0.001) during the day. CONCLUSIONS: Sleeping without CLs does not appear to alter Pdc; however, 2 weeks of SCLEW caused a 99% increase in permeability without observable changes by slit-lamp examination. Increases in Pdc appear greatest in the morning after SCLEW and then decrease exponentially during the day. Whether changes in Pdc will predict CL-associated keratopathy needs further study.

Fluorophotometry in contact lens research: the next step.

BACKGROUND: Fluorophotometry can be used to quantify changes in epithelial permeability (Pdc), corneal pH, and tear exchange (T95; time to deplete 95% of a fluorescent dye from beneath a contact lens) associated with contact lens wear. Using fluorometric procedures, we present previously reported data in order to review the effects of contact lens wear on Pdc and pH. We also introduce a new method for measuring tear exchange beneath a soft contact lens and present preliminary data. METHODS: Pdc was assessed on 32 subjects after 1 h of closed-eye soft contact lens wear. Stromal pH was assessed on 21 subjects wearing lenses with a range of different oxygen transmissibilities (Dk/L). T95 was assessed on 7 subjects who wore disposable lenses. Pdc estimates were derived by measuring the rate at which topically applied fluorescein crossed the epithelial barrier from the tears; corneal pH was quantified by alternately exciting a fluorescein-loaded cornea with two wavelengths of blue light and calculating the fluorescence intensity ratio (490/450 nm), which is pH-sensitive; and the T95 was estimated by applying 2 microliters of FITC-Dextran to the posterior surface of a soft contact lens and the monitoring the exponential decay of dye under the lens. RESULTS: On average, 1 h of closed-eye contact lens wear caused a 41% increase in Pdc compared to the control eye. Corneal pH varied directly with a decrease in Dk/L. On average the T95 under contact lenses (mean = 29 min) was slow compared to normal rates with no lens (approximately 5 min). CONCLUSIONS: Fluorophotometry can be used to quantify some interesting effects associated with contact lens wear, and the use of these techniques may provide new information about the impact of contact lens wear on corneal structure and function.

Epithelial permeability reflects subclinical effects of contact lens wear.

AIMS: Recently, it was reported by the authors that a single drop fluorophotometric technique for estimating corneal epithelial permeability (Pde) to fluorescein is not sufficiently precise for monitoring permeability changes in individual patients., but may be useful for evaluating mean differences in Pdc in population based research. To determine whether this technique provides a more sensitive index of epithelial integrity compared with conventional clinical assessments, the effects of mild corneal trauma on Pdc, the slit lamp appearance of the cornea, and corneal thickness (CT) were assessed. METHODS: After baseline slit lamp examinations (SLE) and CT measurements, one randomly chosen eye of each of 32 normal subjects underwent 1 hour of closed eye soft contact lens (CL) wear while the fellow eye served as a control (no CL). After removing the CL, the SLE and CT measurements were repeated. Then, Pdc to fluorescein was assessed using a single drop fluorophotometric method refined to enhance feasibility, precision, and accuracy. RESULTS: The mean (95% confidence interval) difference in natural log (Pdc) between 32 pairs of eyes (CL minus no CL) was 0.341 (0.069, 0.613), p = 0.016. By contrast, none of the 32 subjects exhibited corneal epithelial disruption upon SLE with white light following the closed eye period. Also, no substantial differences were apparent in the corneal swelling response between paired eyes, mean delta CT (95% CI) = -2.31(-7.53, 2.91) microns, p = 0.37. CONCLUSIONS: Pdc measurements, used in studies of modest sample size, appear capable of detecting average differences in corneal barrier function that remain undetectable by SLE or pachymetry.

The etiology of refractive changes at high altitude after radial keratotomy. Hypoxia versus hypobaria.

OBJECTIVE: Refractive changes at high altitude that occur after radial keratotomy (RK) may be caused by hypoxia or hypobaria. DESIGN: A prospective study was performed to evaluate the effects of hypoxia on RK and non-RK corneas. PARTICIPANTS: There were 20 RK and 20 control eyes. INTERVENTION: These eyes were subjected to ocular surface hypoxia using an air-tight goggle system at sea level for 2 hours. MAIN OUTCOME MEASURES: Keratometry, cycloplegic refraction, and pachymetry were evaluated using repeated measures analysis of variance. RESULTS: A significant hyperopic shift (P < 0.0001) and corneal flattening (P < 0.0013) occurred in all subjects with RK compared with those of control subjects. Corneal thickening occurred symmetrically in both groups. CONCLUSIONS: These results suggest that refractive changes in subjects with RK occur at high altitude as a direct result of corneal hypoxia.

Corneal function during normal and high serum glucose levels in diabetes.

PURPOSE: To assess corneal structure and the effects of acute hyperglycemia on corneal function in subjects with type 1 diabetes. METHODS: Twenty-one diabetic and 21 nondiabetic volunteers of similar age were recruited. Baseline measurements of intraocular pressure (IOP), corneal thickness (CT), corneal autofluorescence (CAF), corneal sensitivity (CST), central and temporal endothelial cell density (DenC and DenT), and coefficient of variation in cell area (CVC and CVT) were taken. Corneal edema was induced, and the percent recovery per hour (PRPH) from hypoxic edema and endothelial permeability to fluorescein were determined. These procedures were done twice in the diabetic subjects under controlled euglycemic (EG) and hyperglycemic (HG) conditions, and once in control subjects while they were fasting. RESULTS: Substantial differences in baseline measurements were found for IOP, CT, CAF, CST, DenC, and CVT. The mean +/- SE corneal swelling in the HG diabetic subjects (51.6 +/- 2.3 microm) was less when compared to the swelling in the EG diabetic subjects (56.2 +/- 1.87 microm, P = 0.05) and the control subjects (58.9 +/- 1.56 microm, P = 0.011). During euglycemia, the mean +/- SE PRPH was less in diabetic subjects than in control subjects (65.0 +/- 3.20 versus 73.8 +/- 1.81%/hour, P = 0.02) but did not differ in diabetic subjects under EG and HG conditions (65.0 +/- 3.20 versus 67.7 +/- 3.1%/hour, P = 0.56). No significant differences were noted between groups in endothelial permeability. CONCLUSIONS: In addition to differences in baseline corneal structure, diabetic subjects showed less corneal swelling and reduced corneal recovery from hypoxia than did control subjects. During acute hyperglycemia, corneal swelling was less than during euglycemia in diabetic subjects, which suggests that hyperglycemia affected corneal hydration control.

Measurement of corneal epithelial permeability to fluorescein. A repeatability study.

PURPOSE: Permeability (Pdc) to sodium fluorescein (F) is a characteristic of the barrier function of the corneal epithelium. The repeatability of several in vivo fluorophotometric methods used to measure permeability in humans remains largely undocumented. This study examines the repeatability of a method based on topical instillation of a single drop of F for the quantitative assessment of Pdc. METHODS: Nine healthy subjects with no history of ocular disease provided 1 (n = 1), 2 (n = 1), or 3 (n = 7) repeated measurements of each eye at successive visits. After making 3 baseline fluorescence scans centrally through the tear film and cornea, 2 microliters of 0.35% F were instilled and 10 fluorescence scans were obtained at approximately 2-minute intervals immediately after instillation. Subsequently, the eyes were rinsed three times with nonpreserved saline and four additional scans were performed. RESULTS: Pdc was calculated by dividing the baseline-corrected postrinse stromal fluorescence by the time integral of the tear film fluorescence calculated over the 20-minute exposure period. After applying a logarithmic transformation to the Pdc estimates, a mixed-model analysis was used to assess measurement repeatability. On the Pdc scale, there is an estimated 95% chance that a second measurement could be as much as 2.88 times higher or 0.35 times lower than a first measurement. CONCLUSIONS: This substantial variability between repeated measurements indicates that the single-drop procedure is unreliable for monitoring individual patient changes. However, with careful sample size planning, this technique can be used in population-based research to compare differences in treatment effects between groups of subjects.

Sequential staining: the effects of sodium fluorescein, osmolarity, and pH on human corneal epithelium.

BACKGROUND: Previous reports have suggested that sequential applications of sodium fluorescein (NaF) to the ocular surface cause loss of epithelial cells. In those experiments the solutions were free of preservatives, but delivered a hypertonic and alkaline load. It is possible that either the hyperosmolarity, NaF, and/or alkalinity may have contributed to the epithelial cell loss. Our study explored the possible impact of these three factors on epithelial integrity. METHODS: We used a paradigm in which we designed four test solutions to isolate the cytotoxic effects of the three factors. Fifteen subjects were exposed to one of the solutions on separate visits. One solution was instilled in both eyes, every 3 min, for a total of seven applications, and a slitlamp examination was performed. Staining was graded on a scale of 0 to 3 for each of five corneal sectors. RESULTS: Application of solution 1 (NaF, hyperosmotic, and alkaline) resulted in staining for all subjects. For the other three solutions (without NaF), insignificant staining occurred. CONCLUSIONS: The epithelium tolerates changes in pH and osmolarity between 7.2 to 7.8 and 290 to 350, respectively. However, NaF when applied in multiple doses may be cytotoxic.

Effects of hypoxia and hypercapnia on contact lens-induced corneal acidosis.

It has been assumed that contact lens wear (CLW) may induce stromal acidosis, which is a result of corneal hypoxia and the accumulation of CO2 (hypercapnia) at the tear-lens interface. However, it has not been directly shown whether hypoxia and hypercapnia are the only causes of CL-Induced corneal acidification. In this study, we provide preliminary data about the relative contributions of hypercapnia and hypoxia to CL-induced stromal acidification by monitoring pH while the cornea was exposed to a hyperbaric oxygen atmosphere. This paradigm minimized if not eliminated the pH effects of lens-induced hypoxia on all but one subject without altering the pH effect of hypercapnia. Seven subjects were fitted with hydrogel lenses; 5 with low O2 transmissibility (Dk/LO2 = 14.0 x 10(-9) (cm/s) (ml O2/[ml x mm Hg)]), and 2 with medium O2 transmissibility (Dk/LO2 = 17.2 x 10(-9) (cm/s) (ml O2/[ml x mm Hg])) lenses. After lens insertion, modified goggles were fitted to control the corneal environment by exposing 1 eye to 20%O2 and 80%N2 (air), and the contralateral eye to 80%O2 and 20%N2 (hyperbaric O2). Corneal thickness (CT) was measured before CL insertion and over 120 min of wear. We assumed that corneal hypoxia was present if CT increased during the test period. Stromal pH was measured using a slitlamp fluorophotometer before lens insertion and at 20-min intervals for a total of 80 min. After 80 min of wearing the low Dk/L lens under hyperbaric exposure, 4 of 5 subjects showed reduced pH (mean delta pH = 0.23 +/- 0.05) and no increase in CT, suggesting that only hypercapnia was contributing to acidosis. For the same lens, but with exposure to air, 4 of 5 subjects showed a larger drop in pH (mean delta pH = 0.62 +/- 0.48) compared to hyperbaric exposure and an increase in CT, indicating that both hypoxia and hypercapnia reduced pH. Subjects wearing the medium Dk/L lens showed a small but equal drop in pH under both air and hyperbaric conditions without changes in CT, suggesting that only hypercapnia was contributing to acidosis. These preliminary results suggest that both mechanisms contribute to the pH shift accompanying CLW and that the contribution of hypercapnia is approximately 30%. Finally, the effects of hypercapnia and hypoxia are dependent on individual metabolic requirements and the transmissibility of the lens to O2 and CO2.

Recovery from induced corneal edema and endothelial morphology after long-term PMMA contact lens wear.

PURPOSE: To study the effect of long-term polymethyl methacrylate (PMMA) contact lens wear (CLW) on corneal endothelial morphology and corneal hydration control (CHC). METHODS: Sixteen long-term PMMA CL wearers (mean duration of wear 20.8 +/- 8.39 SD years; mean age 40.37 years +/- 10.47 SD) served as subjects and 14 age-matched, non-lens wearing subjects (mean age 41.29 years +/- 10.15 SD) served as controls. Corneal function was assessed by measuring the corneal thickness recovery rate (PRPH) after induced edema from a hypoxic stress load. Endothelial morphology was assessed by taking specular microscopy photographs, digitizing the images, and calculating the cell density, size, and coefficient of variation of cell size (CV). RESULTS: Morphologic analysis showed a significant decrease in mean endothelial cell density (2147 vs. 2865, p < 0.01), an increase in mean cell size (582 vs. 381, p = 0.001), and an increase in polymegethism (CV, 0.32 vs. 0.26, p < 0.01) for PMMA lens wearers compared to agematched controls. Corneal function as measured by PRPH was not significantly different for PMMA lens wearers compared to controls when taken as a whole. However, a subgroup of PMMA lens wearers (seven eyes, four subjects) did show a substantial reduction in PRPH. These subjects also showed a strong relation between reduced PRPH, morphology, and years of lens wear (r2 = 0.99). CONCLUSIONS: Longterm PMMA CLW leads to increased polymegethism in most, if not all, cases and results in a substantial decrease in corneal function in some cases.

The association between pH level and corneal recovery from induced edema.

Corneal acidosis has been shown to reduce corneal hydration control (CHC) as measured by the rate, expressed as the percent recovery per hour (PRPH), at which the thickness of the cornea decreases exponentially after an increased hydration load. Since the effect of pH on corneal function is of scientific interest and may have clinical implications, we explored the relationship between pH and PRPH in greater detail by examining the effect of different stromal pH levels on corneal hydration control. Corneal edema was induced using a 90-min exposure to wearing a hypoxic contact lens (CL). Following removal of the CL, random assignment over four eye-test combinations of either 0, 3, 5, and 7% CO2 were made while pH and corneal thickness were monitored using slit lamp fluorophometry and optical pachometry to measure corneal pH and corneal thickness, respectively. From these measurements we determined the pH-dose/PRPH relationship. The average stromal pH +/- 1SD resulting from exposure to either the 0, 3, 5, and 7% CO2, was 7.65 +/- 0.11, 7.30 +/- 0.09, 7.15 +/- 0.08 and 7.04 +/- 0.07 (p < 0.001), respectively. Analysis based on a quadratic model of the dose-response relationship between PRPH and corneal pH indicates that PRPH is relatively unchanged for pH in the physiological range (pH = 7.40-7.65) and then decreases notably below the physiological range.

Corneal hydration control in diabetes mellitus.

PURPOSE: To assess the effects of diabetes mellitus on corneal structure and function. METHODS: The authors measured endothelial permeability to fluorescein and corneal deswelling for 7.5 hours after 2 hours of hypoxic contact lens wear in 20 patients with diabetes who had nonproliferative retinopathy and 21 age-matched control subjects. Central corneal endothelial photographs were also taken. Corneal deswelling rates, expressed as percent recovery per hour (PRPH), and open eye steady state (OESS) thickness were estimated by nonlinear regression techniques. RESULTS: The OESS thickness was greater in patients with diabetes than in controls (562 +/- 35 microns versus 539 +/- 24 microns, P = 0.02). During hypoxia, the diabetic corneas swelled less (7.7% +/- 1.8% versus 9.9% +/- 1.6%, P < 0.001) and had less endothelial permeability (3.55 +/- 0.83 x 10(-4) cm/min versus 4.14 +/- 0.68 x 10(-4) cm/min, P = 0.02) than the controls. During normoxia after contact lens removal, however, diabetic and control corneas had similar deswelling rates and permeabilities. Corneal autofluorescence was increased in the patients with diabetes (8.1 +/- 3.1 versus 6.0 +/- 1.9 ng/ml fluorescein equivalents, P = .005). The endothelial cells of the two groups were morphologically similar. Within the group with diabetes, however, those with moderate nonproliferative retinopathy had larger coefficients of variation of cell area and smaller percentages of hexagonal cells than those with mild nonproliferative retinopathy. CONCLUSIONS: Although the diabetic corneas were thicker and more autofluorescent than control corneas, during hypoxia they swelled less and had decreased endothelial permeability. During normoxia, however, no difference was found in endothelial permeability or deswelling rate. The effects of diabetes on endothelial cell morphologic features appear to be related to the severity of the diabetes.