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T. Leung, W. S. Tierney, H. French, M. R. Nguyen, M. C. Lin; Corneal Epithelium as a Diffusion Barrier in Human Subjects: Evidence of Changes During Overnight Sleep. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4615.
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21 healthy subjects (age: 18 ± 28) with no history of contact lens wear within 12 months prior to the study. Corneal thickness (CT) and corneal epithelial permeability (Pdc) were measured by optical pachometry and scanning fluorometry, respectively. Pdc was measured using the single-drop technique with 2 µl of 0.35 w/v of fluorescein. Corneal staining status was examined by slit lamp immediately after completing CT and Pdc measurements. All measurements were repeated before and after a minimum of 6 hours of sleep. Data from subjects who exhibited central corneal staining greater than grade 1 (less than 5 punctate stains in central 4 mm corneal zone) were excluded from analysis.
A total of 42 eyes were measured. Assuming that status of CT and Pdc were at least equal or worse than the baseline, a total of 38 eyes were included for analysis. Further elimination of data from the eyes that exhibited central corneal staining greater than grade 1 was necessary due to their confounding effects on fluorometric measurements, resulting in a total of 21 eyes for final analysis. The mean ± SD of CT before and after sleep were 520 ± 33 µm and 546 ± 39 µm, respectively (p <0.001). The mean ± SD of Pdc before and after sleep were 0.093 ± 0.063 nm/s and 0.21 ± 0.15 nm/s, respectively (p < 0.001). The mean ± SD normalized CT and Pdc were 0.053 ± 0.024 and 1.106 ± 1.010, respectively. The regression line showed that Pdc Normalized = -13.13 CT Normalized + 2.0411 (p-value of slope = 0.1342).
The increase of CT and Pdc between before and after sleep was statistically significant. Additionally, there appeared to be some evidence that the amount of relative change in CT after sleep was inversely proportional to the amount relative change in Pdc. However, this trend was not statistically significant. We suggest that the driving force for water diffusion from aqueous to tear-film is the chemical-potential gradient that forces water to diffuse from the aqueous within an eye into the tear compartment when corneal epithelium tight junctions were compromised.
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