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. We limited the patient's axial length, age and IOP to a small range, because these factors may affect the fundus blood flow [9]. And there was no statistical difference in blood pressure and heart rate of all patients during each examination. Of note, we found that blood flow parameters in the macula area increased gradually and stabilized in about one week after surgery. FAZ related parameters are stable before and after surgery. Main parameters of phacoemulsification surgery have no statistically significant correlations with changes of macular hemodynamics. No statistically significant correlations were found between postoperative BCVA and macular vascular densities.
With the development of phacoemulsification technology, surgeons use maximum vacuum to reduce direct damage by the power of phacoemulsification. But it brings the effect of high perfusion pressure at the same time. Previous reports have reported that the instability of the fluidics system during cataract surgery results in a longer operation time and an increase in CDE, which in turn will lead to longer recovery time and even poor visual function in postoperative patients [1-2]. So we wonder whether the fluctuation of intraocular perfusion pressure during phacoemulsification affect fundus microcirculation and then affect the patient's postoperative vision. Many literatures have reported the effects of phacoemulsification cataract surgery on ocular hemodynamics by using various techniques [10-13]. Nevertheless, due to the limitations of these devices, there is no consistent conclusion currently. Meanwhile, the emergence of OCTA provides us with new ideas to explore.
A non-invasive and quantitative research of fundus blood vessels could be obtained by OCTA in seconds[6-8]. Good reproducibility and repeatability of OCTA on vascular and FAZ measurements have already been demonstrated[14-15]. The inner retina which supplied by the central retinal vascular system can be segmented into two layers on 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 capillary network[6,7].Researches used OCTA has indicated that fundus vessel density is negatively correlated with the axial length or age[16]. Abnormal vessel densities of SCP and DCP has been observed in various eye or systemic diseases. And the changes are often closely related to the decline of visual function[6,17].
In this study,vascular densities in the macula area increased gradually in 1 day to 1 week after surgery, which is consistent with the results of Siqing Yu[18]. They used OCTA to study 13 cataract eyes and observed that there was a increase of either perfusion or vessel density on SCP and DCP in a 3 mm×3 mm en face image one week after surgery[18]. But Siqing Yu et al. attributed this increase as the result of the different refractive medium before and after surgery, they did not measure patients’ ocular blood flow status immediately after the surgical intervention, which ignored the impact of cataract surgery on the macular hemodynamics. Moreover, Zhennan Zhao et al. studied 32 cases of uncomplicated phacoemulsification surgeries by using OCTA[19]. Significant increases in macular vessel density and macular thickness were found in one month and three months postoperatively. This is basically consistent with the conclusions of our larger sample study. Previous studies have pointed out that fundus perfusion will eventually improve due to the IOP-lowering effect of cataract surgery, which is believed to result from the widening of the anterior chamber[20-23]. This is consistent with our research since patients' IOP decreased significantly one week after surgery. Previous study indicated that several inflammatory factors could be released as the result of the destruction of the blood-aqueous barrier and many of them had vasodilator effect[24, 25]. The interpretation of these studies is in support of our findings. In addition, most of the subjects had earlier cataracts (LOCS scale,median: N3, C4, P3) in current study in order to ensure the imaging quality of larger en face images (6mm×6mm). The scan qualities of all included patients were more than 6, which was considered acceptable. We also manually excluded images with good scan quality but more artifacts, so we believed that the effect of lens opacities were diminished and the results of this study should be reliable.
Previous clinical studies of FAZ have many limitations due to the invasiveness, equipment complexity and long scanning time[26-29]. Some clinical studies using OCTA to analyze FAZ[30-34] are mainly focused on diabetic retinopathy(DR) have shown a decrease in total retinal blood flow associated with an increase in FAZ area with increasing age[35, 36]. And they have showed that the size of FAZ was negatively correlated with 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 our study, the influence of age had been avoided and the FAZ area of subjects after cataract surgery increased first and then decreased, while the differences were not significant (P>0.05). This difference may be due to participants in our study have no systemic vascular disease. The postoperative microcirculation change was only transient and mild, with no qualitative vascular changes. Our result is consistent with the findings of Siqing Yu et al.[18] that there were no significant differences in FAZ area and perimeter between preoperative measurements and one week after cataract surgery. However, unlike our findings, Zhennan Zhao et al[19] found a decrease in the foveal avascular zone after cataract surgery in 32 patients with the axial length between 20mm and 25mm. We analyzed this difference may be contributed 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 [3] and the eye volume is smaller in short eyes, so the IOP fluctuation is much more severe in surgery. Besides, fundus vessels are straight and fragile in long eyes, being stretched as the result of the extension of eyeball[38]. Moreover, because of the large eye volume, the pressure on the eyeball wall is uneven during the operation[39].
The effects of various surgical parameters on ocular structures have been of concern and efforts have been made to evaluate the same[40, 41]. However,there is a shortage of literature on the effect of different flow parameters on the posterior segment. Previous studies[42] demonstrated that phacoemulsification ultrasound energy can induce the production of some cytokines, which in turn affects ocular hemodynamics. This prompts us to explore the correlations between surgical parameters and changes in ocular blood flow. In our study, no statistically significant correlations were observed between the two parameters (EPT and CDE) and rate of change in both superficial and deep macular vascular densities. It suggests that the main parameters of phacoemulsification surgery may not be the key factors affecting macular hemodynamics in normal eyes. However, the LOCS nuclear opalescence score of our subjects were mainly concentrated between 2+ and 3+, which may result in a very small fluctuation of surgical parameter and reduce its influence. And the included patients had no vascular disease, so the good vascular elasticity may cause a rapid recovery after surgery.
Previous studies on various ocular vascular diseases such as retinal vein occlusion (RVO), diabetic retinopathy (DR) and macular telangiectasia have demonstrated that the macular vacular densities is positively correlated with BCVA [16-17, 43]. However, in this study, there was no correlation between BCVA and macular vascular density at each follow-up after surgery. This difference may be caused by the different vascular conditions of the included patients. Unlike the above studies, physiological function of the fundus blood vessel in patients included in this study was intact and did not have organic changes. The effect of fundus vascular density on 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. Furthermore, studies in the future are needed to focus on abnormal eyes, especially patients who with high myopia, glaucoma and systemic vascular disease. This will give clinicians more reference for practice. For example, a highly myopic patients who have long eyes, weak eyeball wall and liquefying vitreum, will have greater intraoperative IOP fluctuations. Comparison of ocular vasculature changes between long eyes and normal eyes before and after surgery can guide clinicians to further optimize surgical parameters and clinical medication. This study can also be a basis for further studies of fundus hemodynamic changes after vacuum application in femtosecond laser-assisted cataract surgery.