This report, using commercially available OCTA system and integrated automated techniques, showed lower amounts of VD in macular area and all various subsegments of SCP and DCP in diabetic patients compared with normal controls. There was a constant and significant decrease from the normal cases to NDR, NDR to NPDR, and NPDR to PDR groups in both SCP and DCP, with the exception of SCP foveal VD and DCN complete VD, which increased moderately. In the CC, the VD decreased insignificantly from normal to NDR and then decreased significantly in NPDR stage and then rose marginally (not statistically significant) in the PDR stage but never reached the normal quantities. BCVA was correlated mainly with parafoveal VD in SCP and foveal VD in CC. VD of all macular area subfields except the VD of foveal DCP showed lesser levels in DME patients compared with those without DME.
VD in retinal microvessels
The present study showed lower amount of VD in macular area and all various subsegments of SCP and DCP in diabetic patients compared with normal controls. Although structurally different, previous studies including ours have revealed reduced VD parafoveal VD in DR without considering the staging of the DR 36,18,37-39. But our results for the central 1mm and whole image VD in SCP and DCP differs with their results. Some qualitative or quantitative studies of microvascular changes showed increased non-perfused areas from normal cases to the higher stages of the DR 18,35,37,38,40.
After adjusting for hypertension, age, sex, and duration of DM our research showed that VD in SCP and DCP healthy subjects were higher than that in NDR group. Nesper et al adjusted their data to the hypertension and obtained the same result as ours. This finding supports VD changes being a potential biomarker of DR before retinopathy becomes clinically apparent. Previous studies showed increased blood flow in the early stages of DR, but the flow decreases in the advanced stages of the disease 41-43. A study that evaluated retinal blood circulation, using video fluorescein angiography, has described decreased mean circulation time in diabetic patients without DR 44. This incongruence could be due to special retinal microvascular blood flow autoregulation and nonlinear correlation between large vessels flow and capillary flows in the retina.
On the other hand, it has been shown that early retinal vascular dysregulation occurs in DR. By Doppler method, the maximum or centerline velocity of RBCs was shown to be slower than normal in the eyes with DR 41. In Grunwald’s study, the maximum velocity of red blood cells was significantly lower than normal in eyes with diabetic retinopathy. Calculated volumetric blood flow rate, however, was not significantly different from normal in eyes with NDR, NPDR, and PDR, and was significantly decreased from normal in patients with PDR who had been treated by panretinal photocoagulation 41. Although hard to be contemplated, the slow rate of RBC movements could explain the less detectability of the microvessels by OCTA. It is known that acute elevation of fibrinogen, haptoglobulin and other globulin may increase thixotropy and viscoelasticity of the blood 45. There are diabetic retina changes that may trigger hypoxia at some point, diabetes, including leukostasis and sluggish circulation 48-50, excitotoxic damage to glial cells 51, which could contribute to impaired neurovascular coupling and hypoperfusion 52, and endothelial cell and pericyte loss that destroys capillaries 46,47. This indicates that slower capillary blood flow below the lowest detection level for retinal vasculature will result in undetectable vessels or lower VD on OCTA at least in the earlier phase of diabetic retinopathy or best to say NDR.
Unlike our result, Nesper et al. showed the strongest correlation between the DCP vessels density and with DR severity not the full retina, SCP, and choriocapillaris VD 35. Unlike our quantitative results of uniform decreasing of VD in both SCP and DCP, some qualitative studies showed more defective VD of DCP in NDR 20. Previous reports have found microaneurysms to be present in a larger extent in DCP than in the superficial plexus 48,49.
The contradictory findings found in the studies may be due to different imaging and measurement techniques, the various phases of the disease being investigated and the varying length of DM in the studies, and animal rather than human researches.
VD in CC
Consistent with our series, Forte et al showed that in CC, the VD was not more different in Type 1 and Type 2 DM with NDR than controls in any of the evaluated quadrants 36,50.
Some studies described diabetic choroidopathy in histology, electron microscopy, and FA and ICGA studies in the earlier stages of DR 51-55. Our results fit with other studies that showed significantly decreased posterior ciliary arteries, using color Doppler imaging, in patients with background DR 20.
In addition, the severity of VD changes in SCP and DCP and CC alteration were evidently correlated except for foveal VD in both retinal layers (P>0.05). This study provides new information on CC alterations that occur in the course of DR stages from NDR to PDR. Which of the retina VD or choroid VD has more impacts on DR remains to be determined in larger studies.
Although there are many studies 51-55, with conflicting findings on the role of CC in DR, a meta-analysis is required to arrive at a definitive result in this period.
Correlation of VD and visual acuity
After adjusting to several conflicting variables, this study found that vision was directly associated with parafoveal VD at SCP and subfoveal VD at CC. The result was independent of the stage of the DR. Dissimilar to our result; previous researches showed that VD of DCP has more impact on the VA than SCN 56.
In diabetic patients, the reasons for reduced VA include macular ischemia57, photoreceptor dysfunction 58 and accumulated subfoveal hard exudates 59. There is a correlation between retinal nonperfusion and the integrity of the photoreceptors 35,60. There was a strong correlation between external limiting membrane (ELM) and ellipsoid zone (EZ) integrity for DME patients 61. On the other hand, a recent study reported similar correlations between the photoreceptor layer integrity and VA among patients with DME and implied that the extent of macular edema does not match the severity of visual dysfunction 61,62.
Using oxygen-sensitive microelectrodes and oximetry, Linsenmeier and Zhang discovered that the choroidal and retinal circulation could supply the metabolic demand of photoreceptors in various proportions under both dark and light conditions. 63. It is important to know that, while most of the outer retina including photoreceptor is nourished with choroidal circulation, the contribution of the retinal circulation to photoreceptor metabolism is modest 63. They calculated that the photoreceptor demand, met by the retinal circulation, was different in dark and lighted conditions (10 to 18% in darkness) 63.
In a cohort of DR patients using adaptive optics and OCTA, Nesper et al. observed changes in capillary dropout areas in the DCP, indicating that DCP integrity is required for the metabolism of the photoreceptor 64. Incongruously, the VD of parafoveal SCP not DCP in our patient only had some effect on the vision.
Recently, by using spectral-domain OCT (SD-OCT), the concept of disorganization of retinal inner layers (DRIL) has been described to characterize retinal thinning with the loss of identifiable borders between retinal cellular layers in the background of capillary non-perfusion 65,66. Areas of DRIL were significantly correlated with disturbance of the photoreceptor (P=0.035) relative to adjacent DRIL-free areas. 66
The decrease in VD in the SCP was significantly associated with the decrease in inner retinal thickness in DR, a notorious finding in mild DR, which may contribute to DRIL in the case of more serious capillary dropout 65. On the contrary, there was no association between the decrease in VD in the DCP and the thickness of the inner nuclear / outer plexiform layer (INL / OPL). The explanation for this disparity, also seen in healthy eyes, is unknown. 67,68. May be the SCP is much important for the inner layer nourishment.
Onishi et al reported that DRIL is due to a defect in DCP with involvement of the MCP, SCP, or both in 28 cases of well developed DR (NPDR and PDR cases) 69, although it is also reported in early stages of DR cases 70. Moeini et al reported capillary flow impairment in the SCP on OCTA may be a predictor of visual acuity in patients with DRIL 71. It is believed that the SCP, whose vascular supply is mainly arterial 72, has a standardized measurement, and is better to reveal ischemia data than the DCP 73. Other researchers measured parafoveal VD as our study. It also decreased in both the SCP and the DCP in their analysis 20,39.
DME
Macular edema can develop at any stage of DR. We found that in all subfields, the patients with DME had less VD than the patients without DME, except for the foveal VD at DCP. Inconsistent with our findings, Tang et al. did not observe any statistically significant association between OCTA metrics and the presence of DME on SCP 74. Lee et al. recently demonstrated that eyes with DME have lower VD only at DCP, compared with eyes without DME 75.
A limitation of this study was a modest age difference between control group and other groups, although in all stages of the analytic part of the study, the age matching was performed. The small field of OCTA imaging and the use of both eyes in the analysis were other limitations. The automated programmed algorithm in the AngioVue system only segments two retinal capillary plexuses: the SCP and the DCP, and the middle capillary plexus (MCP) that is defined by swept source OCT could not be calculated.
Despite previous similar study 35, the advantage of present study was the inclusion of treatment naïve cases and an almost large sample size as well as the use of statistical models with adjustments for covariates.
In assumption, our study showed that VD in OCTA is correlated significantly and linearly with disease severity in eyes with DR. The results support VD of retinal vessels and CC to be a potential surrogate for DR before clinical signs development. We introduced parafoveal VD of SCP and foveal VD of CC as biomarkers to predict the VA loss in the diabetic patient. As a noninvasive and rapidly acquired technique, OCTA may be a tool for early detection of microvascular abnormalities in the retina and choroid, elucidating the pathogenesis of retinopathy, and treatment response monitoring in patients with diabetes.