We warmly welcome the interest by the scientific community in regard to in-vivo measurement of epithelial thickness in clinical practice. Our interest in this matter predates optical coherence tomography (OCT); we have investigated epithelial thickness via scanning high-frequency ultrasound, and our study was renewed with the advent of OCT-enabled epithelial imaging in keratoconic patients, epithelial remodeling following cataract surgery, lamellar keratoplasty, cross-linking, myopic laser-assisted in situ keratomileusis (LASIK), and high-myopic LASIK with concurrent prophylactic cross-linking.
The recent ability of 3-dimensional epithelial mapping offers excellent repeatability, especially at the central and mid-peripheral zones up to 5 mm. Standard deviation in normal eyes is reported by Ma and associates to be 0.7 (0.6 to 0.9) μm and 0.7 μm by Li and associates. Our own investigation reports similar repeatability (0.88 ± 0.71 μm). These values offer confidence in the minimum spotted difference of 1 μm.
The value of axial resolution of 5 μm is reported by the manufacturer, and we merely report the same. To ascertain this value, access to A-scan raw data is required, but it is not currently available to end-users. As Ge and associates report, the axial resolution of OCT may play a role in determining the precision with which the epithelial thickness is measured, although it may not affect the measurements.
In regard to further comments:
We enrolled patients aiming to form as closely as possible age-matched groups. This resulted in a ± 2 years of age mean difference, which we do not view as a discrepancy.
We performed 4 acquisitions per eye. The mean of the 4 values in center, minimum, maximum, and average has been included in the study.
The receiver operating characteristic analysis was performed with the best cut-off value for the center, 56.25, and for the average, 56.20 μm.
In closing, we believe that the clinical difference reported, despite the noted overlap between healthy eyes and dry eyes, may be important in clinical practice, acting as a flag for more specific dry-eye investigation. The proposed screening offers an easy-to-document procedure that maybe helpful in the diagnosis and dry eye treatment assessment. We have particularly employed this clinical pearl in the management of post LASIK dry eye and its correlation to postoperative refraction and quality of vision. The anticipated clinical ramifications appear very encouraging, as evidenced in recent peer-reviewed literature, and of course, in the current correspondence.