| dc.description.abstract | Purpose: To compare central corneal swelling (CS) after eight hours of sleep in eyes wearing 12 different silicone hydrogel (SiHy) contact lenses (CLs) and to model between-subject variability in CS and deswelling. Methods: 29 neophytes wore 12 SiHy lenses with a central transmissibility range of 31 to 211 Dk/t units (4 SiHy CLs x 3 different powers) on separate nights, in random order, and in one eye only. Central corneal thickness in both lens-wearing and no-lens contralateral eyes was measured using digital optical pachymetry before lens insertion, immediately after lens removal on waking, then 20, 40 minutes, 1, 2 and 3 hours later. Descriptive analysis and Repeated Measures ANOVA (Re-ANOVA) were conducted to verify the distribution of individual CS and average differences in CS among CL types, respectively. Simultaneous analysis of group and between-subject effects for CS vs. Dk/t as well as for deswelling vs. time was carried out using mixed modeling. The following hypotheses were tested:
• Average CS in lens-wearing or in control eyes is normally distributed.
• There is a correlation in average CS between lens-wearing and control eyes.
• There are statistically significant differences in overall CS between the lens types.
• There are constant between-subject differences in CS over the range of SiHy Dk/ts.
• CS on eye-opening (intercept) can explain most of the between-subject differences in corneal deswelling.
• Individual CS or deswelling response can be predicted by lens Dk/t.
• Individual CS or deswelling response can be predicted by their age, sex or refractive error.
Results: Distribution of corneal swelling in both lens and control eyes, both on average and for each CL, was not significantly different from a normal curve (p>0.20 for all). When averaged over CL powers, CS with lotrafilcon A was significantly higher than galyfilcon A (Re-ANOVA, p<0.001). Mixed modeling of CS vs. Dk/t showed a significant effect of Dk/t (p<0.001) only in lens-wearing eyes (and no significant effect for any other/subject-related predictors in either eyes). However, mixed modeling also showed constant between-subject differences in CS, irrespective of SiHy Dk/t differences. More than 90% of between-subject differences in corneal deswelling vs. time (in both lens-wearing and no-lens eyes) was explained by between-subject variability in CS (intercept) compared to <10% of between-subject variability in the slope of CS over time. Although Dk/t was a significant predictor of the average corneal deswelling response in lens-wearing eyes, the contribution of Dk/t to between-subject differences in corneal deswelling (intercept/slope) was trivial. In lens-wearing eyes only, age was inversely related to the rate of corneal deswelling. Conclusions: Although descriptive analysis showed the CS among study subjects was normally distributed this initial analysis was incapable of providing any useful insight into the structure and/or predictors of between-subject variability in CS response. In addition, the average group analysis (ANOVA) showed a difference in the average CS between the highest and lowest oxygen transmissible SiHy materials (averaged over lens power for each CL type). However, due to its averaging nature, this traditional group analysis masked the largest source of variability in CS that is the individual-specific differences in corneal response to hypoxia. In contrast, mixed modeling showed that, despite the strong inverse relation between CS and CL DK/t, between-subject differences in CS is the largest source of CS variability but it is not dependent on CL Dk/t. Therefore, the results of this novel CS analysis suggest that the individual differences in closed-eye CS, and NOT the average differences in CS response among different SiHy CLs, should be the main consideration in clinical decision-making. | en |
