Short-term adaptation of the human corneal endothelium to continuous wear of silicone hydrogel (Lotrafilcon A) contact lenses after daily hydrogel lens wear

Purpose. The purpose of this study is to assess whether improved oxygen availability to the cornea resulted in changes in the corneal endothelium. Methods. Eighteen adult (average age 25.3 +/- 5.1 years) hydrogel lens wearers (average of 5.5 years prior daily lens wear, range 3-9 years) were refitted with silicone hydrogel lenses (Focus Night and Day) for continuous wear over 30 days and nights. They were assessed in detail immediately before the refit (baseline measures) and again after 6 months of wear with lens replacement every 30 days. Assessments included slit biomicroscopy (for grading of limbal and bulbar redness), corneal staining with fluorescein, and then by noncontact specular microscopy for central corneal thickness (CCT) and endothelial cell layer images. The endothelial images exported as JPEG files and printed at 7000x magnification. The cell-cell borders were marked and then the areas of an average of 255 cells/image measured with a digitizer pad in stream mode. The number of cell sides was also counted. Results. After 6 months of silicone hydrogel lens wear, most subjects showed an improvement in mean bulbar (1.1 to 0.9) and limbal redness (1.0 to 0.6, p < 0.001) and epithelial fluorescein staining (0.5 to 0.3) grades, and the mean CCT values decreased slightly in most subjects (0.527 to 0.520 mm), although the decrease was not statistically significant (p = 0.565). The mean endothelial cell area also increased slightly (358 to 363 mu m(2); p nonsignificant [NS] = 0.701), whereas the mean coefficient of variation (COV) decreased slightly (30.2 to 29.1%, p NS = 0.357). The calculated mean endothelial cell density (ECD; area/1000000 mu m(2)) also decreased slightly from 2821 to 2774 cells/mm(2), but this change also was not statistically significant (p = 0.620). However, the changes in ECD showed a very substantial relationship to the changes in CCT, i.e., as CCT decreased, so the apparent ECD decreased; this change was highly significant (r = 0.747, p < 0.001). This relationship was slightly stronger still when possible image magnification differences were corrected for. The percentage of six-sided (hexagonal) cells increased slightly (58.3 to 60.1%). Conclusion. The results indicate that, in silicone hydrogel lens wearers exhibiting positive external eye signs thought to be associated with improved oxygen availability, subtle morphologic changes (marginally decreased polymegethism and pleomorphism) in the central region of the corneal endothelium can occur. It remains to be established, however, whether these changes can be directly attributed to oxygen effects (reduced hypoxia and hypercapnia altering the endothelial cells) or to a mechanical effect (in which changes in corneal thickness result in a reorganization of the corneal endothelium).