Reversed Neurovascular Coupling on Optical Coherence Tomography Angiography Is the Earliest Detectable Abnormality before Clinical Diabetic Retinopathy

Abstract

Diabetic retinopathy (DR) has traditionally been viewed as either a microvasculopathy or a neuropathy, though neurovascular coupling deficits have also been reported and could potentially be the earliest derangement in DR. To better understand neurovascular coupling in the diabetic retina, we investigated retinal hemodynamics by optical coherence tomography angiography (OCTA) in individuals with diabetes mellitus (DM) but without DR (DM no DR) and mild non-proliferative DR (mild NPDR) compared to healthy eyes. Using an experimental design to monitor the capillary responses during transition from dark adaptation to light, we examined 19 healthy, 14 DM no DR and 11 mild NPDR individuals. We found that the only structural vascular abnormality in the DM no DR group was increased superficial capillary plexus (SCP) vessel density (VD) compared to healthy eyes, while mild NPDR eyes showed significant vessel loss in the SCP at baseline. There was no significant difference in inner retinal thickness between the groups. During dark adaptation, the deep capillary plexus (DCP) VD was lower in mild NPDR individuals compared to the other two groups, which may leave the photoreceptors more susceptible to ischemia in the dark. When transitioning from dark to ambient light, both diabetic groups showed a qualitative reversal of VD trends in the SCP and middle capillary plexus (MCP), with significantly decreased SCP at 5 min and increased MCP VD at 50 s compared to healthy eyes, which may impede metabolic supply to the inner retina during light adaptation. Mild NPDR eyes also demonstrated DCP dilation at 50 s and 5 min and decreased adjusted flow index at 5 min in light. Our results show altered neurovascular responses in all three macular vascular plexuses in diabetic subjects in the absence of structural neuronal changes on high resolution imaging, suggesting that neurovascular uncoupling may be a key mechanism in the early pathogenesis of DR, well before the clinical appearance of vascular or neuronal loss.

Keywords: OCT angiography; dark adaptation; diabetic retinopathy; neurovascular coupling.

Figure 1

Schematic of dark and light adaptation image acquisition. Images were acquired at baseline in ambient light, after 45 min of dark adaptation, and at several time points (50 s, 2 min, 5 min, and 15 min) after transitioning from dark to ambient light. An asterisk (*) denotes a timepoint of optical coherence tomography angiography (OCTA) image acquisition.

Figure 2

OCTA image segmentation. Panels (A–C) show the respective en-face angiograms of the superficial, middle, and deep capillary plexuses with cross-sections shown in (D–F) with angio overlay.

Figure 3

Schematic of OCTA parameters. (A) Full retinal thickness OCTA image of a healthy control showing the foveal avascular zone (FAZ) outlined in yellow. The FAZ was used to establish the background threshold for both vessel length density (VLD) and adjusted flow index. (B) Superficial capillary plexus (SCP) angiogram of the same individual showing delineation of the parafovea area between two rings of 1 mm and 3 mm in diameter. All OCTA parameters were obtained for the parafovea. (C) Skeletonized SCP image for the calculation of vessel length density (VLD) after binarization based on a threshold determined by the FAZ and subsequent skeletonization of all vessels to 1-pixel wide.

Figure 4

Parafoveal vessel density and adjusted flow index on OCTA at baseline and after dark adaptation. (A) Absolute vessel density (VD) and (C) adjusted flow index (AFI) at baseline in ambient light before any adaptation. At baseline, the mild NPDR group had significantly decreased superficial capillary plexus (SCP) VD compared to the control and DM no DR groups. The DM no DR group also had significantly higher VD than controls. VD and AFI after dark adaptation in (B,D) respectively are shown as values relative to baseline, which was set to zero. (B) shows significantly lower change in deep vascular plexus (DCP) VD in the mild NPDR group compared to the control and DM no DR groups who had increased DCP VD during dark adaptation. All timepoints with statistically significant differences (p < 0.05) between groups are marked with an asterisk (*). Error bars represent standard errors. Abbreviations: VD—vessel density, AFI—adjusted flow index SCP—superficial capillary plexus, MCP—middle capillary plexus, DCP—deep capillary plexus.

Figure 5

Parafoveal vessel density on OCTA during ambient light transition. Vessel density (VD) in ambient light at each timepoint in (A–D) are shown as values relative to the dark, which was set to zero. (A,C,D) show VD changes in the superficial (SCP), middle (MCP), and deep (DCP) vascular plexuses, respectively, at consecutive timepoints in ambient light after dark adaptation and comparing the control, DM no DR, and mild NPDR groups. (A) SCP VD is significantly increased at 5 min of ambient light in the control compared to the DM no DR and mild NPDR groups, which continues until 15 min in the control compared to the DM no DR group. (B) Vessel length density (VLD) measurements indicate significantly elevated VLD in the control compared to the DM no DR group at 50 s in ambient light. (C) In the middle capillary plexus (MCP), the two diabetic groups showed significantly increased VD at 50 s in ambient light compared to controls. (D) In the deep capillary plexus (DCP), the mild NPDR group showed significantly increased VD at 50 s and 5 min in ambient light compared to controls and to DM no DR. All timepoints with statistically significant differences (p < 0.05) between the diabetic and control conditions are marked with an asterisk (*) and the color of the asterisk corresponds with the respective diabetic group that is different from the control. Error bars represent standard errors. Abbreviations: SCP—superficial capillary plexus, MCP—middle capillary plexus, DCP—deep capillary plexus, s—seconds, min—minutes.

Figure 6

Parafoveal adjusted flow index on OCTA during ambient light transition. Adjusted flow index (AFI) in ambient light at each timepoint in (A–C) are shown as values relative to the dark, which was set to zero. (A) demonstrates ambient light AFI changes in the superficial capillary plexus of the three groups. (B,C) show a general trend of an increase in AFI in ambient light that peaks earlier and higher (at 5 min) in the middle, and deep capillary plexuses of the control group and later and lower in the diabetic groups. In the deep capillary plexus (DCP), the peak AFI at 5 min in the control group is significantly higher than the DCP AFI of the mild NPDR group. All timepoint(s) with statistically significant differences (p < 0.05) between the diabetic and control conditions are marked with an asterisk (*) and the color of the asterisk corresponds with the respective diabetic group that is different from the control. Error bars represent standard errors. Abbreviations: SCP—superficial capillary plexus, MCP—middle capillary plexus, DCP—deep capillary plexus, s—seconds, min—minutes.