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W. S. Tierney, M. M. Enos, J. E. Lim, H. Y. Wier, T. Leung, A. Graham, M. C. Lin; Diurnal Variation of Corneal Epithelial Permeability of Non-Contact-Lens Wearers. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2554.
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Corneal epithelial permeability (Pdc) is a measure of epithelial barrier function that can be assessed by established fluorometric techniques. Previous studies of contact lens wearers have suggested that Pdc changes significantly after initial eye opening, and that the nature of these changes depends on the type of contact lens material. It has been hypothesized that these changes are due to interactions among the lids, contact lens and ocular surface; however, it is not known to what extent epithelial barrier function fluctuates naturally throughout the human diurnal cycle. In this study, we analyze Pdc as it compares to the time since eye opening (TO), with TO values ranging from a few minutes to 12 hours.
Pdc examinations were administered on 463 neophytes at various TO values using a scanning fluorophotometer (Ocumetrics, CA). After measurement, all subjects were examined by slit lamp and the degree of central corneal staining was graded (0=none, 4=severe). Pdc values measured from corneas that exhibited central staining were excluded to avoid bias.
Scatterplot smoothing techniques revealed that immediately upon awakening Pdc rises from 0.125 nm/s to 0.16 nm/s after 60 minutes of awake time. Pdc then decreases to a minimum of 0.09 nm/s at approximately 4 hours after eye-opening, then steadily increases throughout the rest of the day.
In the absence of contact lenses, corneal epithelial barrier function appears to follow a diurnal cycle. Corneal permeability worsens immediately upon awakening, recovers over the first four hours of the day, and then steadily worsens as the day progresses. In studies of corneal epithelial barrier function it is, therefore, crucial either to set an appropriate awake time as part of the study protocol, or adjust for awake time in subsequent statistical analyses. Further study will be required to determine whether this increase in epithelial permeability correlates with dry eye symptoms commonly reported at the end of the day.
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