We thank Dr Zhang for his interest in our article. We agree that especially in keratoconic corneas, the thickness of the cornea may be very different on different areas of the cornea, and that is why measurements based on handheld instruments may be demanding. Perhaps optical methods to explore the entire cornea could be better. Yet, during corneal cross-linking (CXL), this is not feasible. Accordingly, we state in the Methods, “Corneal thickness was evaluated preoperatively and postoperatively and during the CXL treatment using an ultrasound pachymeter by measuring the corneal thickness at its thinnest point (based on topography) 5 times and calculating the average of these readings.” Naturally, we are not able to measure the thickness at exactly the thinnest point, but we aimed to do this as well as possible using topography to guide our measurements. Obviously, this will cause some fluctuation in the results. However, it does not jeopardize our results because the trend in decreasing corneal thickness can be seen easily. Importantly, we included in our study the first 30 patients treated in our department with CXL who had keratoconus (24 patients), pellucid marginal degeneration (2 patients), ectasia after laser in situ keratomileusis (3 patients), and bullous keratopathy (1 patient). In all cases, a marked decrease in the corneal thickness during CXL was seen.

The purpose of our study was to show and warn the ophthalmological community of the corneal thinning effect during CXL, which could threaten the health of the endothelium.