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C. E. Morin, D. M. Grzybowski, C. J. Roberts; Reflectivity and Pachymetry Response to Corneal Swelling in Normal and Post LASIK Eyes With the SL-OCT. Invest. Ophthalmol. Vis. Sci. 2008;49(13):658.
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To evaluate changes in reflectance and pachymetry of normal and post-LASIK corneas in response to induced swelling.
17 subjects (34 eyes) underwent induction of corneal swelling for 2 hours by exposure to a humidified nitrogen (N2) environment through modified swim goggles. 7 subjects had previously undergone bilateral LASIK (mean 39±9.9yrs) at least 1 year prior to their participation; 10 were non-surgical controls (mean 28±4.7yrs). Corneal health was assessed using endothelial cell (EC) count from the Noncon ROBO PACHY. Pachymetry in the paracentral (1.5mm from center) and peripheral (4.5mm from center) zones and associated reflectivity data were measured with the SL-OCT [Heidelberg Engineering]. All data were acquired on each eye prior to and immediately after induction of swelling. A Reflectivity to Background Ratio (RBR) was calculated from the reflectivity data at each pachymetry measurement location. T-tests were used for analysis.
Corneal thickness increased significantly (p<0.05) after exposure to N2 in both zones for both groups. Peripheral thickness was greater than the paracentral thickness for all corneas both pre- and post-swelling. Unlike normal corneas, LASIK corneas exhibited a significant (p<0.05) change in RBR between pre- and post-swelling measurements in both regions. The ratio of paracentral to peripheral RBR was significantly (p<0.05) different between the pre-swelling normal and LASIK corneas and between pre- and post-swelling for LASIK corneas. Additionally, the percent change in RBR decreased 12% peripherally while it increased 9.5% paracentrally, on average, for LASIK corneas. No significant relationship was found between RBR and thickness. No differences in EC density, size or shape were measured.
The significant differences in RBR for LASIK corneas imply that optical properties are altered after LASIK and are more sensitive to change as a result of swelling, both of which may impact vision. These findings also indicate that the RBR is not significantly related to thickness for induced swelling in either group within the tested regions with the Heidelberg SL-OCT.
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