LAUSR

Dear Editors, We thank Nicola Rosa and Maddalena De Bernando for their crucial comments and observations. Central cornea thickness (CCT) measurement in health and disease is a difficult task, subjected to numerous patientand device-related factors. In agreement with our work, several studies have reported Pentacam CCT values to be generally higher in healthy and especially keratoconus patients [1–3]. However, as further stated in the meta-analysis by Wu [4], these differences frequently do not reach statistical significance. In addition, some authors also report the Pentacam to generate thinner CCTs than ultrasound pachymetry (USP) [5, 6]. Most of these studies were performed in healthy corneas without edema; therefore, the results are not completely interchangeable. We are currently working on a larger study addressing the question, if a significant difference in between the previously reported devices exists in different degrees of corneal edema. Because of the differences in measurements between healthy and diseased corneas, establishing a universal regression formula could be difficult. To date, we have not considered computing regression formulas in our research, but we will take this interesting approach into consideration for future projects. The application of topical anesthetic eye drops is a confounding factor in the contact USP measurements and, as a limitation of our work, no predetermined interval was adhered to between application of the eye drops and the start of USP measurements. However, we agree with Rosa and De Bernardo that it is highly unlikely that the corneal thickness decreases after application of topical anesthesia. It is likely that the thickness should increase after application of eyedrops or remain unaffected, as the authors have reported in their interesting work [7]. We believe that the comparison of the large difference between our two investigators regarding USP measurements highlights the number of confounding factors for this measuring technique (including application of topical anesthesia without a fixed Bwait time^) in particular. We feel that the user dependency, especially the correct placement on the corneal apex, which often might not completely concur with the central cornea, is a major limitation of the technique. Furthermore, we do agree with Rosa and De Bernando once again that the US measurements, in theory, should result in fake increased CCT results in cases of corneal edema. Silverman et al. reported of slower speed-of-sound in edematous bovine corneas [8]. The authors speculated that the found speed-of-sound error values and the use of a standard speed-of-sound constant would lead to an overestimation of corneal thickness determination, in corneas with 10–20% edematous swelling, of 4 to 7 μm. This error is lower than 1% of the total corneal thickness and, in our experience, variations of that amount can easily occur between repeated USP CCT measurements in healthy corneas as well. Nevertheless, we wanted to verify that the lower values in our work are not a result of a Bflawed^ study design with two measurement sessions with four single measurements each. We decided on this course of action, as we expected the highest variability in the hand-held USP measurements and wanted to minimize the influence of possible outliers as much as possible in preparation of our work. Therefore, we thank Rosa and De Bernando for the opportunity to revisit our results. To see if the cornea was compressed during the measurement sessions * D. Kuerten [email protected]