What Does Overnight Lens Wear Do to the Corneal Epithelium?

Antimicrobial compounds in tears

The tear film coats the cornea and conjunctiva and serves several important functions. It provides lubrication, prevents drying of the ocular surface epithelia, helps provide a smooth surface for refracting light, supplies oxygen and is an important component of the innate defense system of the eye providing protection against a range of potential pathogens. This review describes both classic antimicrobial compounds found in tears such as lysozyme and some more recently identified such as members of the cationic antimicrobial peptide family and surfactant protein-D as well as potential new candidate molecules that may contribute to antimicrobial protection. As is readily evident from the literature review herein, tears, like all mucosal fluids, contain a plethora of molecules with known antimicrobial effects. That all of these are active in vivo is debatable as many are present in low concentrations, may be influenced by other tear components such as the ionic environment, and antimicrobial action may be only one of several activities ascribed to the molecule. However, there are many studies showing synergistic/additive interactions between several of the tear antimicrobials and it is highly likely that cooperativity between molecules is the primary way tears are able to afford significant antimicrobial protection to the ocular surface in vivo. In addition to effects on pathogen growth and survival some tear components prevent epithelial cell invasion and promote the epithelial expression of innate defense molecules. Given the protective role of tears a number of scenarios can be envisaged that may affect the amount and/or activity of tear antimicrobials and hence compromise tear immunity. Two such situations, dry eye disease and contact lens wear, are discussed here.

Agreement and repeatability of the Sonogage ultrasound pachometer compared with a Mitutoyo micrometer

PURPOSE

The agreement and repeatability of the Sonogage ultrasound pachometer was tested by comparing it with a Mitutoyo micrometer.

METHODS

The thickness of five polymacon disks with thickness and ultrasonic properties similar to the human cornea and corneal epithelium were measured using both instruments. The mean values obtained using both instruments were compared and analyzed using a two-way analysis of variance, Bland-Altman analysis, and coefficient of variation.

RESULTS

The measurements of the Sonogage were found to be statistically indistinguishable from the micrometer for all disks combined (p = 0.25) and for the disks with thicknesses similar to the corneal epithelium (p = 0.10), Bland-Altman analysis reveal limits of agreement for all thicknesses with a lower limit of -4.23 microm and upper limit of 5.27 microm. The coefficient of variation was <4% for each thickness. Bland-Altman plots of thin and thick disks exhibit no systematic difference between the average micrometer and Sonogage measurements.

CONCLUSION

The measurements taken with the Sonogage pachometer are comparable with those taken with the micrometer when measuring polymacon disks with properties similar to corneal epithelium and total corneal thickness.

Histomorphometric profile of the corneal response to short-term reverse-geometry orthokeratology lens wear in primate corneas: a pilot study

PURPOSE

To investigate the histological changes in primate cornea induced by short-term overnight orthokeratology (OK).

METHODS

Nine young adult primates were used. One animal served as negative control. The remaining 8 animals wore reverse-geometry OK lenses for periods of 4, 8, 16, and 24 hours on 1 eye with the other eye as control. Central and midperipheral corneal thickness, as well as ultrastructural changes in corneal epithelium, stroma and endothelium in response to OK lenses, were evaluated.

RESULTS

OK significantly reduced the thickness of the central cornea in all treatment groups. The central corneal thinning was both stromal and epithelial in origin. Substantial midperipheral corneal thickening was seen in 16-hour and 24-hour lens-wear groups and this effect was both stromal and epithelial in origin as well. Histology evidence indicated the primary epithelial response in the central cornea was compression of cells that resulted in wing cells becoming shorter and basal cells being squatted rather than lost or migration of cell layers. These pronounced cell shape changes occurred without compromising the structural integrity of the desmosomes. The thickened corneal epithelium has normal cell layers. The squamous cells have larger surface sizes and are composed of oval instead of flattened nuclei. This implied delayed surface cell exfoliation at the thickened midperipheral epithelium. Physical presence of OK lens over the cornea did not influence the microstructures of microvilli and microplicae, endothelium, and collagen distribution.

CONCLUSIONS

The primate cornea, particularly the corneal epithelium, responds rapidly to the application of reverse-geometry OK lenses with significant epithelial cell shape alterations with short-term OK lens wear. This finding suggests that the corneal epithelium is moldable in response to the physical forces generated by the OK lenses.

In vivo confocal microscopic evaluation of langerhans cell density and distribution in the corneal epithelium of healthy volunteers and contact lens wearers

PURPOSE

To examine and compare the density and distribution of Langerhans cells (LCs) in the corneal epithelium of healthy volunteers and contact lens wearers.

METHODS

A total of 225 eyes of 130 healthy volunteers (age, 17-81 years) without history of ocular inflammation, trauma, or surgery and 98 eyes of 55 contact lens wearers (age, 13-76 years) were examined in vivo with the combination of the Heidelberg Retina Tomograph II and in-house-invented Rostock Cornea Module.

RESULTS

In healthy volunteers, in vivo confocal microscopy revealed LCs in 31% of all volunteers, with 37 of these 43 volunteers presenting LCs both in the center and the periphery of the cornea with densities of 34 +/- 3 and 98 +/- 8 cells/mm, respectively. In the group of contact lens wearers, 55% of all corneas presented with LCs, and 11 of these 33 corneas revealed LCs at central and peripheral locations. Although LC densities were markedly higher in both the central (78 +/- 25 cells/mm) and the peripheral cornea (210 +/- 24 cells/mm) of contact lens wearers, the gradient of LC density from peripheral to central cornea was found almost identical in both groups. In the central cornea, LC density decreased with duration of contact lens wear. LCs were located at the depth of 35 to 60 microm (ie, the level of lower intermediate cells, basal cells, and subepithelial nervous plexus). LCs presented as either large cells bearing long processes or smaller cells lacking cell dendrites, most supposedly indicating mature and immature phenotype, respectively.

CONCLUSIONS

In vivo confocal microscopy enables evaluation of LC density and distribution in corneal epithelium. LCs were found present both in the center and the periphery of the cornea without difference in distribution between healthy volunteers and contact lens wearers. However, contact lens wearers revealed almost twofold higher LC densities in both locations, implying chronic mechanical irritation of the cornea in response to the contact lens as foreign body. Taken together, analysis of LC using in vivo confocal microscopy provides helpful information for a better understanding of contact lens-disturbed ocular homeostasis.