We thank Gatzioufas and associates for their interested in our case series of persistent corneal edema after collagen cross-linking for keratoconus and for initiating a discussion of the various methods to prevent this condition in the future. Gatzioufas et al proposed various causes for the persistent corneal edema after CXL treatment with epithelium off, including corneal dehydration during the ultraviolet A exposure, imprecise estimation of preoperative and intraoperative corneal pachymetry, and poor calibration of equipment.

We agree that this potential complication has been documented only as a single case report. Because endothelial damage and corneal edema can affect vision significantly, we believed it was important to report the additional 10 cases of the same condition. As stated in the study, we could only speculate on the exact cause of the findings. We do concur with Gatzioufas and associates regarding the variables that may prevent endothelial cell toxicity.

We agree that accurate measurements of preoperative and intraoperative corneal thickness are vital to avoid endothelial damage. It is possible that the ultrasonic devices may miss the thinnest corneal point, so we obtained our preoperative pachymetry readings from a Pentacam (Pentacam Oculyzer; Oculus Optikgerate GmbH, Heidelberg, Germany).

Calibration of the equipment is of paramount importance to prevent inadvertent delivery of excessive energy and resultant ultraviolet light toxicity. We are very conscientious of this fact and checked the irradiance using a calibrated ultraviolet meter to confirm 3.0-mW/cm ² emissions before each treatment session.

With respect to the issue of possible corneal dehydration during the ultraviolet A emission, we agree that the corneal thickness measurement should have been rechecked during ultraviolet A exposure to confirm hydration and stability of the corneal thickness. In a recent study, an intraoperative corneal thickness decrease from more than 400 to 350 μm in 80% of eyes during a 60-minute epithelium-off CXL treatment has been reported. However, despite the decreased thickness, corneal edema or endothelial cell damage did not occur. In another study by Kymionis and associates, they did not find a statistically significant change in the corneal thickness during ultraviolet A irradiation or any endothelial cell loss. The mean preoperative pachymetry reading in our study was 472.6 ± 17.5 μm. Considering the preoperative pachymetry readings and the results of these studies, even with some corneal dehydration, the intraoperative CCT may not have decreased to less than 350 μm. This hypothesis cannot be confirmed secondary to the retrospective nature of the study. Because studies have shown some corneal dehydration with ultraviolet A exposure, we do agree that CXL protocols should include checking pachymetry during the ultraviolet exposure stage of the procedure and corneal rehydration or cessation of the procedure if the corneal thickness drops substantially. We thank Gatzioufas and associates once again for their interest and good insight.