We appreciate the interest that Dr. Roy has shown in our research. He is correct in that axial length changes after refractive surgery due to changes in corneal thickness. He is also correct that refractive surgery causes a change in the magnification of the optical coherence images acquired postoperatively. However, we have found that the valuables would not significantly impact the accuracy of retinal thickness measurements in our cohort.
The commonly used avenue to correct the image magnification after refractive surgery is Bennett's formula [scaling factor = 3.3823 × 0.0130623 × (axial length − 1.82)].1–3 To check whether our assessment of retinal thickness is correct, we carefully reviewed the changes in corneal thickness after small-incision lenticule extraction. Based on the changes in corneal thickness (∆Cor), we recalculated the retinal thickness based on the following formula: corrected retinal thickness = primary retinal thickness × . In our cohort, the changes in corneal thickness from PRE to postoperative day 1 (POD1) and PRE to POD7 were 87 ± 18 μm (range 54 to 127) and 92 ± 19 μm (range 56 to 131), respectively. Thus, these changes in corneal thickness at POD1 and POD7 would result in the magnification of 0.0038 ± 0.0008 (range 0.0024 to 0.0056) and 0.0041 ± 0.0008 (range 0.0025 to 0.0058), respectively, changes that unlikely significantly affect the accuracy assessment of retinal thickness.
Shpak AA et al. found that in patients with very high myopia, a decrease of over 10 diopters (D) in corneal refractive power can increase the average thickness of the peripapillary retinal nerve fiber layer (RNFL) and ganglion cell-inner plexiform layer by 2 to 3 μm acquired by optical coherence tomography.4 It is unclear whether this effect is mediated by magnification or by anatomical physiological change. However, in our cohort, there were no patients with such an extreme change in corneal refractive power, with an average spherical equivalent of −4.50 ± 1.77 D (range −8.00 to −1.75).
In Table 5 of our manuscript, we discussed 7 papers that studied RNFL changes after refractive surgery.4–10 We carefully reviewed them again, and we found that none of the 7 papers adjusted the axial length after refractive surgery. Furthermore, our findings are in agreement with those of 7 studies, which also did not find any significant changes in RNFL thickness after refractive surgery.
Taken together, these statements and supporting evidence suggest that the assessment of retinal thickness after small-incision lenticule extraction in this study was correct and reliable.
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