The PRK procedure damages the epithelial, nerves, and upper layers of the cornea . Thus, the PRK procedure may cause various symptoms, such as pain in patients, despite the use of postoperative medications . According to previous studies that predict symptoms after PRK, gender, anxiety, procedural knowledge, and spherical equivalent refractive error may preoperatively determine pain . Epithelial hyperplasia after myopic laser ablation may contribute to regression in myopia. However, the relevance of epithelial hyperplasia to refractive change is not clear. While there are studies showing that myopic regression after PRK is related to the increase in corneal epithelial thickness, there are also studies that argue that it is not [17–18]. Moreover, Kang et al.  revealed a correlation between the increase in corneal epithelial thickness after PRK and myopic shift in controls one year after surgery. In recent literature, little is known about the effect of preoperative dry eye on post-PRK CET, OSDI scores, and uncorrected visual acuity. Thus, the aim of the present study was to investigate the effects of these variables. According to the results, statistically significant differences were found between the groups in terms of the CET values and OSDI scores. Thus, the null hypothesis that no differences in the post-PRK CET, OSDI scores, and uncorrected visual acuity existed between the groups was partially rejected.
Although the CET values and OSDI scores were similar in the control group at the third month after the operation and at the preoperative period, the mean OSDI score and CET value increased significantly in the dry eye group at the third month after the operation compared to the preoperative period. This study is the first to compare preoperative dry eye on the CET values, OSDI scores, and uncorrected visual acuity. Thus, a direct comparison cannot be made with the findings of previous reports. However, the postoperatively increased OSDI scores and CET values in the dry eye group can be explained by the presence of preoperative dry eye. Therefore, it can be suggested that dry eye should be treated preoperatively, followed by PRK.
Since there is no gold standard test, there are difficulties in the evaluation of dry eye . To overcome these difficulties, various diagnostic tools have been developed for the evaluation of dry eye. For instance, the questionnaire method has been used frequently in previous studies for the evaluation of dry eye [16, 20–22]. Furthermore, the OSDI has a sensitivity of 60% and a specificity of 79%, and provides reliable information for dry eye in terms of severity, history, and effects . Thus, in the present study, OSDI was used.
The Schirmer test alone is insufficient to determine the severity of dry eye disease. This is due to its poor repeatability . A low Schirmer test value is not a contraindication for refractive laser surgery. Therefore, patients with low Schirmer test results were also included in this study.
The Schirmer test is an indicator of the aqueous component of the tear film layer. Healthy corneal epithelium and stable tear film layer are required to high-resolution vision. Tear film dysfunction can reduced tear volume and cause changes in tear composition, including elevated concentrations of inflammatory mediators and osmolarity. High osmolarity and free radicals induced by osmotic stress produce inflammatory mediators that further amplify corneal epithelium inflammation causses corneal epithelial edema. We think that the increase in CET value after PRK in the dry eye group may be related to corneal epithelial edema. In our study, this level of change in corneal epithelial thickness did not affect post-prk UCVA.
Various studies have shown that the measurement of CET with spectral-domain optical coherence tomography, which measures without contact with the eye, is reliable and reproducible [25–26]. Many studies using spectral-domain optical coherence tomography have shown that CET changes after LASIK and PRK are not statistically different from each other [27, 28].
Reinstein et al.  measured the mean CET values in the normal corneas of healthy individuals as 53.4 ± 4.6 µm. In the CET measurement program used in the present study, the normal value was between 50 µm and 60 µm. Spadea et al.  showed that the CET value started to increase from the first week after refractive laser surgery, and the thickness increase reached the highest level at approximately 6.5 µm in the third month. In the present study CET measurement was performed at the third month after PRK.
In our opinion, the most important limitation of our study is that we could not clearly demonstrate whether the change in corneal epithelial thickness has an effect on the OSDI score.