In this study, both the anterior segment and retina changed after the surgery. Results from previous researches suggested that anterior segment changes would last for a long time while posterior segment changes only can be observed in a short time and then recovered. Prior articles have reported that PME and ACD decreased after the surgery and even several years later[10, 11], and this change was more significant in younger patients[12]. It also has been reported that changes in elevation correlated with residual bed thickness[13]. In this study, the 𝜟ACD was negatively correlated with sphere, this suggested that higher myopia was with more ablation depth and less residual bed thickness, leading to decreased cornea biomechanics and ACD. Besides, a negative correlation between 𝜟ACD and 𝜟LT affirmed that thickened LT also attributed to the reduced ACD.
AL shortened about 0.1 mm after surgery due to the ablation part. Corneas were usually edema after SMILE so that CCT was thinner in 1 week than 1 day after surgery, and another article has the same result[14].
In a previous study of treatment of presbyopia using a femtosecond laser, they found the crystallines lens moved axially and laterally, and it seemed to be affected by suction [8]. While the effect of suction usually lasts a transient time. Our results showed LT thickened in 1 day and even thicker 1 week, so suction maybe not the predominant factor. Other researchers found LT increased after LASIK with four different instruments, and the pupil was dilated with 0.5% tropicamide before each measurement. The authors believed that residual accommodation might contribute to the LT increase[15]. In our study, all patients were with natural pupils and accompanied with thicker LT, PD became smaller than preoperation. This may prove the hypothesis that accommodation enhanced after refractive surgery. Another research found that amplitude of accommodation (AA) significantly decreased postoperatively. In our study, a slightly positive correlation between 𝜟LT and sphere suggested that higher myopia had less AA, which may explain that some patients complained of accommodation hysteresis after surgery, especially in high myopia patients. The poor accommodative facility, slow accommodative responsiveness, and increased accommodation demand may attribute to the result [16].
There are few articles reporting retina or choroid changes after SMILE, but changes in LASIK have studied a lot. In previous LASIK studies, MT was observed thickened [9], or total macular volume increased [17]. 1 day after surgery, all parameters returned to the baseline [18], and more extended observation did not see any significant changes [19]. While in our research, MT decreased after surgery, but GCL was unchanged. With the decreased M1, M2, M3, and M5, M8 turned increased in 1 week, and that is quite different from prior studies about LASIK. Why MT decreased and which layer was thinner need more deep studies.
Since 20 years ago, RNFL changes after LASIK attracted doctors' interests, and different results were concluded. RNFL was found changed with scanning laser polarimetry (SLP) but unchanged with OCT[20]. This result was caused by corneal birefringence[21], not the real RNFL changed. While other researchers believed RNFL did reduce but only in a very short time after surgery and soon recovered. Suction during surgery and high IOP caused disorder of optic nerve axoplasm and malnutrition of retinal ganglia cells[9]. Nevertheless, research in children revealed that MT was thicker 1 day after surgery, but RNFL unchanged [22]. Another research found RNFL was thicker 3 months after LASIK, especially in the inferior-temporal sector [23], which was similar to our results, but the possible mechanism in it was still not clear.
CT was observed thicker postoperatively [16], and there is research believing CT was affected by ciliary muscle contraction, which may explain why LT thickened. While in our research, results were similar, but not every measure point was statistically significant.
Vessel density was studied a lot in glaucoma and retina disease since OCTA came up. Vessel density changes were assumed by the suction effects during surgery on retinal microcirculation, and instantaneous changes of suction may cause ischemia-reperfusion injury[9]. IOP Elevation during surgery also caused a decrease in ocular blood flow[24]. In a paper studied healthy people by increasing IOP, researchers found transient elevation of IOP altered optic nerve head topography[25]. Other articles reported different results by increasing adult primates and found nothing meaningful clinical impact. In this study, macular vessel density and peripapillary vessel density reduced. The recovery of macular vessel density was behind peripapillary. Different retina structures and sensitivity may attribute to this. Reduced superficial vessel density also diminished vessel infusion, which may lead to the thinned MT.
During the observation, none of the patients was with severe complications, and all of them got ideal visual acuity. However, at 1 day after the surgery, corneas were not as clear as before surgery due to mild cornea edema, and it is difficult to capture high-quality images of the retina. For the limitation of the OCTA soft version, only superficial vessel density was analyzed. Those were the shortcomings of the study, and longer follow-up was still needed.