Using Fluorometry to Assess Corneal Epithelial Barrier Function
New diagnostic methods are helping researchers learn how lens care solutions affect the ocular surface.
By Meng Lin, OD, PhD, FAAO

he corneal epithelium is a critical part of the eye's defense system. The upward differentiation of corneal epithelial basal cells to the superficial cells establishes tight junctions, forming a barrier between the inner corneal structures and the outside environment and preventing potentially harmful substances from entering the eye.

Clinical slit lamp examination provides qualitative information about corneal integrity, but may not be sensitive enough to detect subtle quantitative changes in corneal integrity induced by contact lenses or lens care systems. A more sensitive tool for quantifying corneal integrity is a fluorometer, which measures the rate at which topically applied fluorescein enters the cornea. This simple technique can detect and quantify potentially critical changes in epithelial integrity that are not detectable by standard slit lamp assessment.

ASSESSING CORNEAL PERMEABILITY

In a healthy eye, intact epithelial cell membranes and tight intercellular junctions resist the passage of fluorescein and other hydrophilic substances. Therefore, increased uptake in fluorescein dye reflects increased corneal permeability and reduced epithelial barrier function.

At the University of California Berkeley Clinical Research Center, my colleagues and I investigated how contact lenses affect the corneal epithelium. In a series of 1-hour studies, we investigated the effect of open- vs. closed-eye contact lens wear on epithelial permeability in normoxic and hypoxic conditions. Our results suggest that 1 hour of contact lens wear with low- or high-Dk/t materials in the open-eye condition does not alter corneal epithelial permeability. However, closed-eye lens wear, along with lens-induced hypoxia, can compromise corneal epithelial barrier function. To learn more about the mechanisms behind altered epithelial barrier function, we used fluorometry to measure the effect of extended lens wear — between 8 hours and overnight — on corneal permeability.^1-4

Previous studies of the effect of 1-night overnight silicone hydrogel lens wear on the corneal epithelium used conventional lens materials as a control. One study⁵ showed that 43% of silicone hydrogel and 57% of conventional soft lenses significantly increased corneal epithelial permeability after only 1 night of overnight wear.

Another study⁶ compared the effect of gas permeable (GP) siloxane-fluorocarbon polymer lenses with different Dk/t values during 1 to 12 months of overnight wear. The data showed that increased corneal epithelial permeability is directly proportional to the amount of lens-induced hypoxia, and that epithelial barrier function is impaired after GP overnight lens wear compared to no lens wear.

These studies suggest that dose-dependent hypoxia contributes to increased epithelial permeability during closed–eye contact lens wear, but isn't solely responsible. Despite their increased oxygen transmissibility, ultra-high Dk/t silicone hydrogel lenses worn overnight also can alter epithelial junction integrity.

COMPARING LENS CARE PRODUCTS

Additional study paradigms developed at our lab examine how lens care products affect corneal epithelium integrity. Recently, we compared the effect of Complete MoisturePlus multipurpose solution (CMP) and Opti-Free Express (OFX) multipurpose solution on corneal permeability in non-contact lens wearers.⁷ Our primary endpoint was to determine differences between the mean permeability score of each solution at each visit, and the scores between baseline and follow-up visits with each solution type.

We examined study participants at baseline (day 1) and the following day (day 2). On day 1, eligible subjects came to the research center after being awake for a minimum of 4 hours. After a baseline anterior segment examination, we delivered 2 μL of 0.35% sodium fluorescein to the superior bulbar conjunctiva of both eyes. Each eye was scanned immediately after instillation. On day 2, we repeated the anterior segment
examination and instilled 40 μL of CMP or OFX into the subjects' eyes over eight applications and repeated the fluorometry measurements.

Mean baseline permeability was similar in both groups: –2.26 nm/sec in CMP and –2.28 nm/sec in OFX subjects (P = 0.89). On day 2, we noted a statistically significant difference in epithelial permeability between eyes exposed to different lens care solutions. Permeability in the CMP group was –2.38 nm/sec, compared with –2.14 nm/sec in the OFX group (P = 0.04). The overall difference between solutions was 27%, with eyes exposed to OFX demonstrating a higher average epithelial permeability (weaker barrier function).

The CMP eyes showed slightly less permeability on day 2, but permeability in the OFX eyes increased within the same time frame. Although neither change from baseline was statistically significant, the positive direction in the OFX group indicated more disruption of the epithelium and decreased barrier function, whereas the negative trend in the CMP group indicated less disruption and increased barrier function. However, further investigation into the interaction between soft contact lenses and lens care products is needed to illustrate the cumulative effects of these solutions on corneal
epithelial barrier function and to understand the clinical implications of fluorometry for measuring corneal epithelial integrity.

PROTECTING THE BARRIER

Fluorometry is a sensitive and useful tool for quantifying the effects of contact lens wear and lens care solutions on the corneal epithelium. This technology may help us understand the fundamental factors that contribute to altered epithelial barrier function in contact lens patients. CLS

Contact Lens Spectrum, Issue: November 2005