In this study, we performed a quantitative assessment of macular vascular density using OCTA and compared the macular blood perfusion before and after phacoemulsification cataract surgery in Chinese normal eyes. The patient’s axial length, age, and IOP were limited to a small range, because these factors may affect the fundus blood flow . Also, there was no statistical difference was detected in the blood pressure and heart rate of all patients during each examination. Notably, we found that the blood flow parameters in the macula area increased gradually and stabilized about one week after the surgery. FAZ-related parameters are stable before and after surgery. The main parameters of phacoemulsification surgery did not show any statistically significant correlations with changes in macular hemodynamics. Also, no statistically significant correlations were established between postoperative BCVA and macular vascular densities.
The inner retina was supplied by the central retinal vascular system and segmented into two layers on the OCTA image. One is the superficial capillary plexuses (SCP), mainly located in the retinal nerve fiber layer, including arterioles, venules, and capillaries. The other is the deep retinal vascular plexus (DCP), which is mainly located in the inner nuclear layer of the retina and consists of a capillary network [12, 13]. Previous studies used OCTA, which indicated that fundus vessel density is negatively correlated with the axial length or age . Abnormal vessel densities of SCP and DCP have been observed in the eyes or various systemic diseases, and the changes are often closely related to the decline of visual function [12，19].
In this study, vascular densities in the macula area increased gradually from 1 day to 1 week after the surgery, which was consistent with the results of the study by Yu et al. . The study used OCTA to investigate 13 cataract eyes and observed an increase in either perfusion or vessel density on SCP and DCP in a 3 mm×3 mm en face image one week after the surgery . However, Yu et al. attributed this increase to the result of the different refractive media before and after the surgery, wherein the patients’ ocular blood flow status was measured immediately after the surgical intervention, which ignored the impact of cataract surgery on macular hemodynamics. Moreover, Zhao et al. studied 32 cases of uncomplicated phacoemulsification surgeries by using OCTA . A Significant increases was noted in macular vessel density and macular thickness were found at 1 and 3 months postoperatively. This phenomenon was basically consistent with the conclusions of our large-sample study. Previous studies have pointed out that fundus perfusion will eventually improves due to the IOP-lowering effect of cataract surgery, resulting from the widening of the anterior chamber [20-23]. This finding is consistent with the current study since patients’ IOP decreased significantly at 1 week after the surgery. A previous study indicated that several inflammatory factors could be released due to the destruction of the blood-aqueous barrier, and many presented a vasodilator effect [24, 25]. The interpretation of these studies supports our findings. In addition, most of the subjects presented earlier cataracts (LOCS scale, median: N3, C4, P3) in the current study in order to ensure the imaging quality of larger en face images (6mm×6mm). The scan quality of all the included patients was >6 points, which was considered acceptable. We also manually excluded the images with good scan quality but more artifacts and speculated that the effect of lens opacities were diminished; thus, this study's results should be reliable.
Previous clinical studies of FAZ have many limitations due to the invasiveness, equipment complexity, and long scanning time of the method [26-29]. Some clinical studies using OCTA to analyze FAZ [30-34] were mainly focused on diabetic retinopathy(DR) and have shown a decrease in total retinal blood flow associated with an increase in FAZ area with increasing age [35, 36]. These studies have also showed that the size of FAZ was negatively correlated to both the macular vascular density and BCVA [31, 37]. Therefore, FAZ parameters may detect the impairments of macular micro-vessels and visual function to some extent. In the current study, the influence of age has been avoided, and the FAZ area of the subjects after cataract surgery increased first, followed by a decrease; however, the differences were not significant (P>0.05). This finding could be ascribed to the lack of systemic vascular disease in our participants. The postoperative microcirculation change was only transient and mild, with no qualitative vascular changes, which was consistent with the findings of Yu et al. , wherein no significant differences were detected in the FAZ area and perimeter between preoperative and measurements one week after cataract surgery. However, unlike our findings, Zhao et al.  found a decrease in the foveal avascular zone after cataract surgery in 32 patients with the axial length between 20 mm and 25 mm. This difference may arise due to the inconsistency in the axial length of enrolled patients included in the two studies. Since the axial length is the influencing factor of fundus blood vessel density  and the eye volume is small in short eyes,the IOP fluctuation is rather severe in the surgery. In addition, fundus vessels are straight and fragile in long eyes and stretched due to eyeball extension . Also, because of the large eye volume, the pressure on the eyeball wall is uneven during the operation .
The effects of various surgical parameters on ocular structures have been a primary concern and have been evaluated [40, 41]. However, there is a dearth of literature on the effect of different flow parameters on the posterior segment. Previous studies  demonstrated that phacoemulsification ultrasound energy induces some cytokines, affecting ocular hemodynamics. This prompts us to explore the correlations between surgical parameters and changes in ocular blood flow. In this study, no statistically significant correlations were observed between the two parameters (EPT and CDE) and the rate of change in both superficial and deep macular vascular densities thereby suggesting that the main parameters of phacoemulsification surgery may not be the key factors affecting macular hemodynamics in normal eyes. The degree of intraoperative IOP increases and the unstable status of the intraocular fluid flow system could be the main influencing factor of macular hemodynamics [43-44]. However, the LOCS nuclear opalescence score of our subjects was mainly concentrated between 2+ and 3+, which might result in a very small fluctuation of the surgical parameter and reduce its influence. Also, the included patients did not present any vascular disease, and hence, the good vascular elasticity might cause a rapid recovery after surgery.
Previous studies on various ocular vascular diseases, such as retinal vein occlusion (RVO), DR, and macular telangiectasia, have demonstrated that the macular vascular densities are positively correlated with BCVA [18-19, 45]. However, in this study, no correlation was established between BCVA and macular vascular density at each follow-up after the surgery. This difference might be caused by the different vascular conditions of the included patients. Unlike the above studies, the physiological function of the fundus blood vessel in patients included in this study was intact and did not have organic changes. However, the effect of fundus vascular density on the visual function in patients after surgery needs further investigation
The main limitation of this study is the lack of research on patients with more severe cataracts. However, this study is the basis for exploring the effects of phacoemulsification on macular hemodynamics in different populations in the future. Future studies in the future are needed to focus on abnormal eyes, especially in high myopia, glaucoma, and systemic vascular disease. This will provide a reference for clinical practice. For example, a highly myopic patients, who have long eyes, weak eyeball wall, and liquefying vitreum, have significant intraoperative IOP fluctuations. The comparison of ocular vasculature changes between long eyes and normal eyes before and after the surgery can guide the clinicians to optimize surgical parameters and clinical medication further. This study can also form a basis for further studies of fundus hemodynamic changes after vacuum application in femtosecond laser-assisted cataract surgery.