Regional morphology and pathophysiology of retinal vascular disease

The relationship between ON-OFF function and OCT structural and angiographic parameters in early diabetic retinal disease

PURPOSE

This study measured associations between ON and OFF functional indicators and structural optical coherence tomography (OCT) and OCT angiography (OCTA) markers in diabetic retinal disease.

METHODS

Fifty-four participants with type 1 or type 2 diabetes (mean age = 34.1 years; range 18-60) and 48 age-matched controls (mean age = 35.4 years, range 18-59) underwent visual psychophysical testing, OCT and OCTA retinal imaging. Psychophysical tasks measuring (A) contrast increment and decrement sensitivity and (B) response times to increment and decrement targets were assessed as surrogate measures of ON and OFF retinal ganglion cell function.

RESULTS

The group with diabetes had worse foveal contrast increment and decrement thresholds (p = 0.04) and were slower to search for increment and decrement targets relative to controls (p = 0.009). Individuals with diabetes had a less circular foveal avascular zone (FAZ) (p < 0.001) but did not differ from controls in foveal vessel density and FAZ area. Functional and structural outcome measures related to the peripheral retina were also comparable between those with and without diabetes. Functional responses to increments and decrements were not significantly correlated with FAZ circularity or vessel density in individuals with diabetes.

CONCLUSIONS

Diabetic retinal disease results in impaired performance on measures of inferred ON and OFF pathway function in addition to vascular deficits measurable with OCTA. Future longitudinal studies may determine the temporal relationship between these deficits, and whether they predict future diabetic retinopathy.

AI-based fully automatic analysis of retinal vascular morphology in pediatric high myopia

PURPOSE

To investigate the changes in retinal vascular structures associated with various stages of myopia by designing automated software based on an artificial intelligence model.

METHODS

The study involved 1324 pediatric participants from the National Children's Medical Center in China, and 2366 high-quality retinal images and corresponding refractive parameters were obtained and analyzed. Spherical equivalent refraction (SER) degree was calculated. We proposed a data analysis model based on a combination of the Convolutional Neural Networks (CNN) model and the attention module to classify images, segment vascular structures, and measure vascular parameters, such as main angle (MA), branching angle (BA), bifurcation edge angle (BEA) and bifurcation edge coefficient (BEC). One-way ANOVA compared parameter measurements between the normal fundus, low myopia, moderate myopia, and high myopia groups.

RESULTS

The mean age was 9.85 ± 2.60 years, with an average SER of -1.49 ± 3.16D in the right eye and - 1.48 ± 3.13D in the left eye. There were 279 (12.38%) images in the normal group and 384 (16.23%) images in the high myopia group. Compared with normal fundus, the MA of fundus vessels in different myopic refractive groups was significantly reduced (P = 0.006, P = 0.004, P = 0.019, respectively), and the performance of the venous system was particularly obvious (P < 0.001). At the same time, the BEC decreased disproportionately (P < 0.001). Further analysis of fundus vascular parameters at different degrees of myopia showed that there were also significant differences in BA and branching coefficient (BC). The arterial BA value of the fundus vessel in the high myopia group was lower than that of other groups (P = 0.032, 95% confidence interval [CI], 0.22-4.86), while the venous BA values increased (P = 0.026). The BEC values of high myopia were higher than those of low and moderate myopia groups. When the loss function of our data classification model converged to 0.09, the model accuracy reached 94.19%.

CONCLUSION

The progression of myopia is associated with a series of quantitative retinal vascular parameters, particularly the vascular angles. As the degree of myopia increases, the diversity of vascular characteristics represented by these parameters also increases.

The retinal oxygen metabolism and hemodynamics as a substitute for biochemical tests to predict nonproliferative diabetic retinopathy

Predicting the occurrence of nonproliferative diabetic retinopathy (NPDR) using biochemical parameters is invasive, which limits large-scale clinical application. Noninvasive retinal oxygen metabolism and hemodynamics of 215 eyes from 73 age-matched healthy subjects, 90 diabetic patients without DR, 40 NPDR, and 12 DR with postpanretinal photocoagulation were measured with a custom-built multimodal retinal imaging device. Diabetic patients underwent biochemical examinations. Two logistic regression models were developed to predict NPDR using retinal and biochemical metrics, respectively. The predictive model 1 using retinal metrics incorporated male gender, insulin treatment condition, diastolic duration, resistance index, and oxygen extraction fraction presented a similar predictive power with model 2 using biochemical metrics incorporated diabetic duration, diastolic blood pressure, and glycated hemoglobin A1c (area under curve: 0.73 vs. 0.70; sensitivity: 76% vs. 68%; specificity: 64% vs. 62%). These results suggest that retinal oxygen metabolic and hemodynamic biomarkers may replace biochemical parameters to predict the occurrence of NPDR .

Folate and retinal vascular diseases

Folate, a pteroylglutamic acid derivative, participates in fundamental cellular metabolism. Homocysteine, an amino acid, serves as an intermediate of the methionine cycle and can be converted back to methionine. Hyperhomocysteinemia is a recognized risk factor for atherosclerotic and cardiovascular diseases. In recent decades, elevated plasma homocysteine levels and low folate status have been observed in many patients with retinal vascular diseases, such as retinal vascular occlusions, diabetic retinopathy, and age-related degeneration. Homocysteine-induced toxicity toward vascular endothelial cells might participate in the formation of retinal vascular diseases. Folate is an important dietary determinant of homocysteine. Folate deficiency is the most common cause of hyperhomocysteinemia. Folate supplementation can eliminate excess homocysteine in plasma. In in vitro experiments, folic acid had a protective effect on vascular endothelial cells against high glucose. Many studies have explored the relationship between folate and various retinal vascular diseases. This review summarizes the most important findings that lead to the conclusion that folic acid supplementation might be a protective treatment in patients with retinal vascular diseases with high homocysteine or glucose status. More research is still needed to validate the effect of folate and its supplementation in retinal vascular diseases.

Increased Oxygen Saturation in Retinal Venules During Isometric Exercise Is Accompanied With Increased Peripheral Blood Flow in Normal Persons

Purpose

A recent study has shown that an increase in the arterial blood pressure of approximately 10 mm Hg in healthy persons can increase the oxygen saturation in venules from the retinal periphery but not from the macular area. The purpose of the present study was to investigate whether a higher increase in blood pressure has further effects on oxygen saturations and whether this is accompanied with changes in retinal blood flow.

Methods

In 30 healthy persons, oxygen saturation, diameter, and blood flow were measured in arterioles to and venules from the retinal periphery and the macular area. The experiments were performed before and during an experimental increase in arterial blood pressure of (mean ± SD) 18.3 ± 6.2 mm Hg.

Results

A higher number of venules than arterioles branching from the temporal vascular arcades to the macular area was balanced by a smaller diameter of the venules. Isometric exercise induced significant contraction of both peripheral and macular arterioles (P < 0.01 for both comparisons) and significant increase in oxygen saturation in both peripheral and macular venules (P < 0.001 for both comparisons). This was accompanied with a significant increase in the blood flow in the peripheral arterioles and venules (P = 0.4 for both comparisons), but not in their macular counterparts (P > 0.06 for both comparisons).

Conclusions

Increased systemic blood pressure leading to arterial contraction and increased venous oxygen saturation in the retina in normal persons can increase peripheral blood flow without significant effects on macular blood flow. This may contribute to explaining regional differences in the response pattern of retinal vascular disease.

Dopamine signaling from ganglion cells directs layer-specific angiogenesis in the retina

During central nervous system (CNS) development, a precisely patterned vasculature emerges to support CNS function. How neurons control angiogenesis is not well understood. Here, we show that the neuromodulator dopamine restricts vascular development in the retina via temporally limited production by an unexpected neuron subset. Our genetic and pharmacological experiments demonstrate that elevating dopamine levels inhibits tip-cell sprouting and vessel growth, whereas reducing dopamine production by all retina neurons increases growth. Dopamine production by canonical dopaminergic amacrine interneurons is dispensable for these events. Instead, we found that temporally restricted dopamine production by retinal ganglion cells (RGCs) modulates vascular development. RGCs produce dopamine precisely during angiogenic periods. Genetically limiting dopamine production by ganglion cells, but not amacrines, decreases angiogenesis. Conversely, elevating ganglion-cell-derived dopamine production inhibits early vessel growth. These vasculature outcomes occur downstream of vascular endothelial growth factor receptor (VEGFR) activation and Notch-Jagged1 signaling. Jagged1 is increased and subsequently inhibits Notch signaling when ganglion cell dopamine production is reduced. Our findings demonstrate that dopaminergic neural activity from a small neuron subset functions upstream of VEGFR to serve as developmental timing cue that regulates vessel growth.

SegR-Net: A deep learning framework with multi-scale feature fusion for robust retinal vessel segmentation

Retinal vessel segmentation is an important task in medical image analysis and has a variety of applications in the diagnosis and treatment of retinal diseases. In this paper, we propose SegR-Net, a deep learning framework for robust retinal vessel segmentation. SegR-Net utilizes a combination of feature extraction and embedding, deep feature magnification, feature precision and interference, and dense multiscale feature fusion to generate accurate segmentation masks. The model consists of an encoder module that extracts high-level features from the input images and a decoder module that reconstructs the segmentation masks by combining features from the encoder module. The encoder module consists of a feature extraction and embedding block that enhances by dense multiscale feature fusion, followed by a deep feature magnification block that magnifies the retinal vessels. To further improve the quality of the extracted features, we use a group of two convolutional layers after each DFM block. In the decoder module, we utilize a feature precision and interference block and a dense multiscale feature fusion block (DMFF) to combine features from the encoder module and reconstruct the segmentation mask. We also incorporate data augmentation and pre-processing techniques to improve the generalization of the trained model. Experimental results on three fundus image publicly available datasets (CHASE_DB1, STARE, and DRIVE) demonstrate that SegR-Net outperforms state-of-the-art models in terms of accuracy, sensitivity, specificity, and F1 score. The proposed framework can provide more accurate and more efficient segmentation of retinal blood vessels in comparison to the state-of-the-art techniques, which is essential for clinical decision-making and diagnosis of various eye diseases.

Comprehensive retinal vascular measurements: time in range is associated with peripheral retinal venular calibers in type 2 diabetes in China

AIM

Retinal vascular parameters are biomarkers of diabetic microangiopathy. We aimed to investigate the relationship between time in range (TIR) assessed by continuous glucose monitoring (CGM) and retinal vascular parameters in patients with type 2 diabetes in China.

METHODS

The TIR assessed by CGM and retinal photographs were obtained at the same time from adults with type 2 diabetes who were recruited. Retinal vascular parameters were extracted from retinal photographs by a validated fully automated computer program, and TIR was defined as between 3.9-7.8 mmol/L over a 24-h period. The association between TIR and caliber of retinal vessels distributed in different zones were analyzed using multivariable linear regression analyses.

RESULTS

For retinal vascular parameters measurements, the peripheral arteriovenous and middle venular calibers widen with decreasing TIR quartiles (P < 0.05). Lower TIR was associated with wider peripheral venule after adjusting for potential confounders. Even after further correction for GV, there was still a significant correlation between TIR and peripheral vascular calibers (CV: β = - 0.015 [- 0.027, - 0.003], P = 0.013; MAGE: β = - 0.013 [- 0.025, - 0.001], P = 0.038) and SD: β = - 0.013 [- 0.026, - 0.001], P = 0.004). Similar findings were not found for the middle and central venular calibers or arterial calibers located in different zones.

CONCLUSIONS

The TIR was associated with adverse changes to peripheral retinal venules but not central and middle vessels in patients with type 2 diabetes, suggesting that peripheral retinal vascular calibers may be affected by glycemic fluctuations earlier.

Effects of semaglutide and empagliflozin on oxygenation, vascular autoregulation, and central thickness of the retina in people with type 2 diabetes: A prespecified secondary analysis of a randomised clinical trial

AIMS

Semaglutide and empagliflozin have shown cardiovascular protection. In SUSTAIN-6, semaglutide was associated with an increased risk of diabetic retinopathy. We investigated whether changes in retinal oxygenation, retinal vascular autoregulation, and central retinal thickness are altered by semaglutide, empagliflozin or the combination.

METHODS

This study was a prespecified, secondary outcome from a randomised, 32 weeks partly placebo-controlled, partly open-label, clinical trial on the effects of semaglutide and empagliflozin on arterial stiffness and kidney oxygenation. A total of 120 participants with type 2 diabetes, established or high risk of cardiovascular disease and age ≥50 years were randomised into four parallel groups (semaglutide, empagliflozin, the combination or tablet placebo, n = 30 for each group). We primarily hypothesized that semaglutide would increase venular oxygenation.

RESULTS

We found no changes in retinal arteriolar, venular or venular-arteriolar oxygenation nor in retinal vessel diameter regardless of treatment group. Semaglutide increased central retinal thickness compared to placebo with ~1 % (3.8 μm 95 % CI [0.9;6.7], p = 0.009) with no changes in the empagliflozin or combination group.

CONCLUSION

Neither semaglutide, empagliflozin nor the combination alters markers of retinal function. The effect of semaglutide on central retinal thickness was small, but the clinical significance is uncertain.

Studies of the retinal microcirculation using human donor eyes and high-resolution clinical imaging: Insights gained to guide future research in diabetic retinopathy

The microcirculation plays a key role in delivering oxygen to and removing metabolic wastes from energy-intensive retinal neurons. Microvascular changes are a hallmark feature of diabetic retinopathy (DR), a major cause of irreversible vision loss globally. Early investigators have performed landmark studies characterising the pathologic manifestations of DR. Previous works have collectively informed us of the clinical stages of DR and the retinal manifestations associated with devastating vision loss. Since these reports, major advancements in histologic techniques coupled with three-dimensional image processing has facilitated a deeper understanding of the structural characteristics in the healthy and diseased retinal circulation. Furthermore, breakthroughs in high-resolution retinal imaging have facilitated clinical translation of histologic knowledge to detect and monitor progression of microcirculatory disturbances with greater precision. Isolated perfusion techniques have been applied to human donor eyes to further our understanding of the cytoarchitectural characteristics of the normal human retinal circulation as well as provide novel insights into the pathophysiology of DR. Histology has been used to validate emerging in vivo retinal imaging techniques such as optical coherence tomography angiography. This report provides an overview of our research on the human retinal microcirculation in the context of the current ophthalmic literature. We commence by proposing a standardised histologic lexicon for characterising the human retinal microcirculation and subsequently discuss the pathophysiologic mechanisms underlying key manifestations of DR, with a focus on microaneurysms and retinal ischaemia. The advantages and limitations of current retinal imaging modalities as determined using histologic validation are also presented. We conclude with an overview of the implications of our research and provide a perspective on future directions in DR research.

Complement 3a Mediates CCN2/CTGF in Human Retinal Pigment Epithelial Cells

Background. Complement 3 (C3) is the crucial component of the complement cascade when retina was exposed to external stimulus. Cellular communication network 2/connective tissue growth factor (CCN2/CTGF) is important in response of retinal stress and a fulcrum for angiogenesis and fibrosis scar formation. Our study aims to explore the interaction between C3 and CCN2/CTGF via bioinformatics analyses and in vitro cell experiments. Methods. The GSE dataset was selected to analyse the chemokine expression in human retinal pigment epithelium (ARPE-19) cells under stimulus. Then, RPE cells were further transfected with or without C3 siRNA, followed by C3a (0.1 μM or 0.3 μM) for 24, 48, and 72 hours. Reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) were used to measure CCN2/CTGF mRNA and protein levels. Results. The GSE36331 revealed C3 expression was significantly elevated in RPE under stimulus. Compared with negative control, CCN2/CTGF mRNA was increased with all types of C3a treatments, whereas a significant increase of protein level was only observed with high concentration of 0.3 μM C3a for a prolonged 72-hour time. Compared with nontransfected cells, significant reductions of CCN2/CTGF mRNA were observed in the C3 siRNA transfected cells with 0.3 μM C3a for 24, 48, and 72 hours, and a significant reduction of CCN2/CTGF protein was observed with 0.3 μM C3a for 48 hours. Conclusions. C3 was elevated in RPE under environmental stimulus and long-term exposure to specified concentration of C3a increased CCN2/CTGF expression in RPE, which could be partially reversed by C3 siRNA.

Segmenting Retinal Vessels Using a Shallow Segmentation Network to Aid Ophthalmic Analysis

Retinal blood vessels possess a complex structure in the retina and are considered an important biomarker for several retinal diseases. Ophthalmic diseases result in specific changes in the retinal vasculature; for example, diabetic retinopathy causes the retinal vessels to swell, and depending upon disease severity, fluid or blood can leak. Similarly, hypertensive retinopathy causes a change in the retinal vasculature due to the thinning of these vessels. Central retinal vein occlusion (CRVO) is a phenomenon in which the main vein causes drainage of the blood from the retina and this main vein can close completely or partially with symptoms of blurred vision and similar eye problems. Considering the importance of the retinal vasculature as an ophthalmic disease biomarker, ophthalmologists manually analyze retinal vascular changes. Manual analysis is a tedious task that requires constant observation to detect changes. The deep learning-based methods can ease the problem by learning from the annotations provided by an expert ophthalmologist. However, current deep learning-based methods are relatively inaccurate, computationally expensive, complex, and require image preprocessing for final detection. Moreover, existing methods are unable to provide a better true positive rate (sensitivity), which shows that the model can predict most of the vessel pixels. Therefore, this study presents the so-called vessel segmentation ultra-lite network (VSUL-Net) to accurately extract the retinal vasculature from the background. The proposed VSUL-Net comprises only 0.37 million trainable parameters and uses an original image as input without preprocessing. The VSUL-Net uses a retention block that specifically maintains the larger feature map size and low-level spatial information transfer. This retention block results in better sensitivity of the proposed VSUL-Net without using expensive preprocessing schemes. The proposed method was tested on three publicly available datasets: digital retinal images for vessel extraction (DRIVE), structured analysis of retina (STARE), and children’s heart health study in England database (CHASE-DB1) for retinal vasculature segmentation. The experimental results demonstrated that VSUL-Net provides robust segmentation of retinal vasculature with sensitivity (Sen), specificity (Spe), accuracy (Acc), and area under the curve (AUC) values of 83.80%, 98.21%, 96.95%, and 98.54%, respectively, for DRIVE, 81.73%, 98.35%, 97.17%, and 98.69%, respectively, for CHASE-DB1, and 86.64%, 98.13%, 97.27%, and 99.01%, respectively, for STARE datasets. The proposed method provides an accurate segmentation mask for deep ophthalmic analysis.

Dry-Lensectomy Assisted Lensectomy in the Management for End-Stage Familial Exudative Vitreoretinopathy Complicated With Anterior Segment Abnormalities

Purpose

To report a modified technique of dry-lensectomy assisted lensectomy in the management of end-stage familial exudative vitreoretinopathy (FEVR) complicated with capsule-endothelial, iris-endothelial adhesion and secondary glaucoma.

Methods

24 eyes of 16 patients with severe complications of advanced pediatric total retinal detachment caused by FEVR who received limbus-based dry-lensectomy were studied retrospectively. Preoperative and postoperative clinical information was collected and reviewed.

Results

Among the 24 eyes, three eyes (12.50%) underwent lensectomy combined with vitrectomy and membrane peeling simultaneously. 21 (87.50%) eyes underwent lensectomy without membrane peeling due to severe corneal opacity or retinal vascular activity, of which eight underwent another vitrectomy combined with membrane peeling. At the last visit (mean:13.86 ± 5.24 months of follow-up), all eyes had a reconstructed anterior chamber with normal depth. Among 21 eyes having preoperative corneal opacity, 15 (71.43%) had a clearer cornea with reduced opacity, 5 (23.81%) showed similar corneal opacification without deterioration. Among 11 eyes undergone retrolental fibroplasia peeling, seven (63.64%) eyes showed partial retinal reattachment in open-funnel type.

Conclusion

Dry-lensectomy offered a simple way to lower the intraocular pressure and simplified the surgery, which helped to solve the severe anterior segment complications and offer a chance for following retrolental fibroplasia peeling and potential visual gain for selected end-stage FEVR patients.

Artery vein classification in fundus images using serially connected U-Nets

BACKGROUND AND OBJECTIVE

Retinal vessels provide valuable information when diagnosing or monitoring various diseases affecting the retina and disorders affecting the cardiovascular or central nervous systems. Automated retinal vessel segmentation can assist clinicians and researchers when interpreting retinal images. As there are differences in both the structure and function of retinal arteries and veins, separating these two vessel types is essential. As manual segmentation of retinal images is impractical, an accurate automated method is required.

METHODS

In this paper, we propose a convolutional neural network based on serially connected U-nets that simultaneously segment the retinal vessels and classify them as arteries or veins. Detailed ablation experiments are performed to understand how the major components contribute to the overall system's performance. The proposed method is trained and tested on the public DRIVE and HRF datasets and a proprietary dataset.

RESULTS

The proposed convolutional neural network achieves an F₁ score of 0.829 for vessel segmentation on the DRIVE dataset and an F₁ score of 0.814 on the HRF dataset, consistent with the state-of-the-art methods on the former and outperforming the state-of-the-art on the latter. On the task of classifying the vessels into arteries and veins, the method achieves an F₁ score of 0.952 for the DRIVE dataset exceeding the state-of-the-art performance. On the HRF dataset, the method achieves an F₁ score of 0.966, which is consistent with the state-of-the-art.

CONCLUSIONS

The proposed method demonstrates competitive performance on both vessel segmentation and artery vein classification compared with state-of-the-art methods. The method outperforms human experts on the DRIVE dataset when classifying retinal images into arteries, veins, and background simultaneously. The method segments the vasculature on the proprietary dataset and classifies the retinal vessels accurately, even on challenging pathological images. The ablation experiments which utilize repeated runs for each configuration provide statistical evidence for the appropriateness of the proposed solution. Connecting several simple U-nets significantly improved artery vein classification performance. The proposed way of serially connecting base networks is not limited to the proposed base network or segmenting the retinal vessels and could be applied to other tasks.

Retinal oximetry does not predict 12-month visual outcome after anti-VEGF treatment for central retinal vein occlusion: A multicentre study

PURPOSE

Central retinal vein occlusion (CRVO) entails retinal hypoxia that often causes visual impairment. It has been shown that oxygen saturation in larger retinal vessels correlates with the visual acuity at the time of diagnosis of CRVO but has no predictive value for the visual outcome in patients treated with anti-VEGF medication after 3 months. However, assessing the predictive value of retinal oxygen saturation after 12 months is essential because this is when the main restitution after CRVO occurs.

METHODS

Retinal oximetry was performed in 117 patients referred with CRVO to three European centres. The correlation between oxygen saturation and visual acuity at baseline and the predictive value of oxygen saturation in larger retinal vessels for the 12-month visual outcome after treatment with anti-VEGF medication were studied.

RESULTS

In the affected eye, the oxygen saturation was significantly higher in the arterioles, significantly lower in the venules, and the arterio-venous (A-V) significantly higher than in the unaffected eye (p < 0.001 for all comparisons). Correlations between best-corrected visual acuity (BCVA) and oxygen saturations were moderate and negative for arterioles (p < 0.001), positive for venules (p = 0.03) and negative for the A-V difference (p = 0.001). Best-corrected visual acuity (BCVA), but not oxygen saturation or the other explanatory variables at baseline, contributed significantly to predicting BCVA after 12 months.

CONCLUSION

Retinal vessel oxygen saturation is affected in CRVO, and saturation correlates with BCVA. However, retinal oximetry cannot replace measures of visual function as a predictor of visual outcome after 12 months of anti-VEGF treatment for CRVO.

ATP induced calcium signaling activity in perivascular cells differ at different vascular branch levels in the porcine retina

BACKGROUND

The purine adenosine triphosphate (ATP) plays a significant role in retinal blood flow regulation and recent evidence suggests that the vasoactive effect of the compound differs in vessels at different branching level. However, the cellular basis for the regulation of retinal blood flow mediated by ATP has only been scarcely studied.

METHODS

Perfused porcine hemiretinas (n = 60) were loaded with the calcium-sensitive fluorophore Oregon Green ex vivo. Spontaneous oscillations in fluorescence were studied in perivascular cells at five different vascular branching levels ranging from the main arteriole to the capillaries, before and after the addition of intra- and extravascular ATP alone or in the presence of a P2-purinergic receptor antagonist.

RESULTS

Intravascular ATP induced an overall significant (p < 0.01) constriction of (mean ± SD) -9.79 ± 13.40% and extravascular ATP an overall significant (p < 0.01) dilatation of (mean ± SD) 19.62 ± 13.47%. Spontaneous oscillations of fluorescence in perivascular cells were significantly more intense around third order arterioles than around vessels at both lower and higher branching levels (p < 0.05 for all comparisons). ATP increased intracellular fluorescence in perivascular cells of first and second order arterioles after extravascular application, and the increase correlated with the accompanying vasodilatation (p < 0.03). Blocking of P2-receptors reduced oscillating fluorescence in pre-capillary arterioles secondary to intravascular ATP (p = 0.03).

CONCLUSIONS

Spontaneous oscillations of calcium-sensitive fluorescence in perivascular retinal cells differ at different vascular branching levels. Extravascular ATP increases fluorescence in cells around the larger retinal arterioles exposed to the retinal surface. Future studies should investigate calcium signaling activity in perivascular retinal cells during interventions that simulate retinal pathology such as hypoxia.

Quantitative assessment and determinants of foveal avascular zone in healthy volunteers

Objectives

To evaluate the foveal avascular zone (FAZ) area in healthy volunteers using optical coherence tomography angiography (OCTA) and identify factors that influence the FAZ.

Methods

This single-center cross-sectional study included 526 eyes of 263 healthy volunteers who underwent macular scanning by Zeiss OCTA. A linear mixed model was used to investigate the effects of systemic factors (age, sex, blood pressure, height, and weight) and ocular factors (intraocular pressure, biometric parameters, and central macular thickness) on FAZ.

Results

In total, 520 eyes of 262 healthy volunteers were included in the analysis. The mean volunteer age was 38.59 ± 22.03 years (range, 5–84 years); 124 volunteers were male (47.33%) and 138 volunteers were female (52.67%). The mean FAZ area was 0.30 ± 0.03 mm² (95% confidence interval [CI], 0.29–0.31 mm²). Univariate analysis showed that FAZ area was associated with age (β = 0.0011), anterior chamber depth (β = −0.0513), and axial length (β = −0.0202). Multivariate analysis showed that FAZ area was negatively correlated with axial length (β = −0.0181).

Conclusions

The mean FAZ area in healthy volunteers, measured using Zeiss OCTA, was 0.30 ± 0.03 mm². Furthermore, FAZ area was negatively associated with axial length; this relationship should be considered in clinical practice.

eNOS-induced vascular barrier disruption in retinopathy by c-Src activation and tyrosine phosphorylation of VE-cadherin

Background

Hypoxia and consequent production of vascular endothelial growth factor A (VEGFA) promote blood vessel leakiness and edema in ocular diseases. Anti-VEGFA therapeutics may aggravate hypoxia; therefore, therapy development is needed.

Methods

Oxygen-induced retinopathy was used as a model to test the role of nitric oxide (NO) in pathological neovascularization and vessel permeability. Suppression of NO formation was achieved chemically using L-NMMA, or genetically, in endothelial NO synthase serine to alanine (S1176A) mutant mice.

Results

Suppression of NO formation resulted in reduced retinal neoangiogenesis. Remaining vascular tufts exhibited reduced vascular leakage through stabilized endothelial adherens junctions, manifested as reduced phosphorylation of vascular endothelial (VE)-cadherin Y685 in a c-Src-dependent manner. Treatment with a single dose of L-NMMA in established retinopathy restored the vascular barrier and prevented leakage.

Conclusions

We conclude that NO destabilizes adheren junctions, resulting in vascular hyperpermeability, by converging with the VEGFA/VEGFR2/c-Src/VE-cadherin pathway.

Funding

This study was supported by the Swedish Cancer foundation (19 0119 Pj ), the Swedish Research Council (2020-01349), the Knut and Alice Wallenberg foundation (KAW 2020.0057) and a Fondation Leducq Transatlantic Network of Excellence Grant in Neurovascular Disease (17 CVD 03). KAW also supported LCW with a Wallenberg Scholar grant (2015.0275). WCS was supported by Grants R35 HL139945, P01 HL1070205, AHA MERIT Award. DV was supported by grants from the Deutsche Forschungsgemeinschaft, SFB1450, B03, and CRU342, P2.

Reduced Oxygen Extraction in the Retinal Periphery When the Arterial Blood Pressure Is Increased by Isometric Exercise in Normal Persons

Purpose

Recent evidence suggests that the smaller retinal vessels are significantly involved in the regulation of retinal blood flow and that this regulation may differ among the macular area and the retinal periphery. An alternative to studying blood flow regulation in smaller retinal vessels that are difficult to resolve is to assess the metabolic consequences of changes in the microcirculation using oximetry.

Methods

In 20 normal persons aged (mean ± SD, range) 30.1 ± 3.8 (24–37) years, the oxygen saturation and diameter of retinal arterioles and venules to the macular area and the retinal periphery were studied before and during an increase in the arterial blood pressure induced by isometric exercise.

Results

The isometric exercise increased the mean arterial blood pressure by (mean ± SEM) 10.0 ± 1.1 mm Hg but induced no significant changes in the diameter of the arterioles (P = 0.83). The isometric exercise had no significant effect on the oxygen saturation in the arterioles supplying the macular area and the retinal periphery (P > 0.42 for both comparisons). However, there was a significant increase in the oxygen saturation in venules draining the retinal periphery to reduce the oxygen extraction from (mean ± SEM) 36.0% ± 2.3% to 30.6% ± 2.1% (P = 0.002) but no significant change in the preexisting low oxygen extraction in the macular area that changed from (mean ± SEM) 18.2% ± 3.0% to 16.2% ± 1.9% (P = 0.37).

Conclusions

Minor changes in the arterial blood pressure can induce changes in retinal rheology with significant regional variation. The finding may help explain regional variations in manifestations of retinal vascular disease such as hyperpermeability in the macular area and capillary occlusion in the retinal periphery.

Systemic risk factors contribute differently to the development of proliferative diabetic retinopathy and clinically significant macular oedema

AIMS/HYPOTHESIS

The purpose of screening for diabetic retinopathy is to detect either of the two sight-threatening complications: proliferative diabetic retinopathy (PDR) or clinically significant diabetic macular oedema (DME). The aim of the study was to evaluate whether systemic risk factors affect the risk of developing these two complications differently.

METHODS

Survival analysis with death as a competing risk was used to describe the effect of sex, age and time of onset of diabetes, systolic (SBP) and diastolic (DBP) BPs, and the weighted exposure and CV of HbA_1c for the development of PDR and DME from all 2773 patients treated for diabetic retinopathy in a defined population from the Aarhus area, Denmark, between 1 July 1994 and 1 July 2019.

RESULTS

Increasing HbA_1c above normal increased the risk of developing both PDR and DME (p < 0.04), and values below normal increased the risk of developing PDR (p < 0.013). Increasing DBP increased the risk of developing both PDR and DME (p < 0.0001), whereas increasing SBP increased the risk of developing DME (p < 0.0001), but not PDR (p > 0.08). The risk of developing PDR increased with decreasing age of onset of diabetes (p < 0.0001), whereas the risk of developing DME was maximal for a known onset of diabetes at about 30 years of age and decreased significantly for both lower and higher ages of onset (p < 0.0001). The risk of developing both PDR and DME was lower in women than in men (p < 0.004) and was reduced with lower variability of repeated HbA_1c measurements (p < 0.0001).

CONCLUSIONS/INTERPRETATION

Systemic risk factors such as metabolic regulation, arterial BP and the age of onset of diabetes contribute differently to the development of PDR and DME. The overall risk of developing treatment-requiring diabetic retinopathy should be calculated from the risks of reaching each of the two complications separately.

Retinal arteriolar oxygen saturation predicts the need for intravitreal aflibercept in patients with diabetic macular oedema

Objective

Given the increasing burden of repetitive intravitreal injections in diabetic macular oedema (DMO) treatment, non-invasive markers of treatment outcome are needed. Hence, we aimed to examine retinal oximetry parameters as markers of need for intravitreal aflibercept in patients with DMO.

Methods

This study was based on data from a 12-month clinical trial including 35 eyes of 25 patients with centre involving DMO. Retinal oximetry, visual acuity (VA) and central retinal thickness (CRT) were performed at baseline (BL). Patients then received 3 monthly injections of aflibercept followed by focal/grid laser photocoagulation. From month 4 (M4) through 12 (M12), patients were followed monthly and additional injections were given pro re nata if criteria of retreatment were met. We evaluated the difference in need for intravitreal aflibercept in groups of eyes with the highest and lowest retinal arteriolar and venular oxygen saturations, respectively.

Results

From BL-M12, overall VA letter score improved by 8.7 (7.2–10.2). Likewise CRT reduced by 100.7 (68.2–133.3) µm and the mean number of injections was 4.3 (3.8–4.8). Overall retinal arteriolar and venular oxygen saturations were 95.7 (93.0–98.4)% and 62.7 (59.4–65.9)% at BL. Eyes with the highest retinal arteriolar oxygen saturations had significantly more injections between BL and M12 compared with eyes with the lowest retinal arteriolar oxygen saturations (5.0 (4.2–5.8) vs 3.6 (3.1–4.0), p=0.002).

Conclusion

Higher retinal arteriolar oxygen saturation independently predicted the need for more intravitreal aflibercept during the first year of DMO treatment and may serve as a valuable adjunctive to established procedures for retinal imaging in terms of individualised treatment plans.

Trial registration number

NCT02554747

Eyeing the Extracellular Matrix in Vascular Development and Microvascular Diseases and Bridging the Divide between Vascular Mechanics and Function

The extracellular matrix (ECM) is critical in all aspects of vascular development and health: supporting cell anchorage, providing structure, organization and mechanical stability, and serving as a sink for growth factors and sustained survival signals. Abnormal changes in ECM protein expression, organization, and/or properties, and the ensuing changes in vascular compliance affect vasodilator responses, microvascular pressure transmission, and collateral perfusion. The changes in microvascular compliance are independent factors initiating, driving, and/or exacerbating a plethora of microvascular diseases of the eye including diabetic retinopathy (DR) and vitreoretinopathy, retinopathy of prematurity (ROP), wet age-related macular degeneration (AMD), and neovascular glaucoma. Congruently, one of the major challenges with most vascular regenerative therapies utilizing localized growth factor, endothelial progenitor, or genetically engineered cell delivery, is the regeneration of blood vessels with physiological compliance properties. Interestingly, vascular cells sense physical forces, including the stiffness of their ECM, through mechanosensitive integrins, their associated proteins and the actomyosin cytoskeleton, which generates biochemical signals that culminate in a rapid expression of matricellular proteins such as cellular communication network 1 (CCN1) and CCN2 (aka connective tissue growth factor or CTGF). Loss or gain of function of these proteins alters genetic programs of cell growth, ECM biosynthesis, and intercellular signaling, that culminate in changes in cell behavior, polarization, and barrier function. In particular, the function of the matricellular protein CCN2/CTGF is critical during retinal vessel development and regeneration wherein new blood vessels form and invest a preformed avascular neural retina following putative gradients of matrix stiffness. These observations underscore the need for further in-depth characterization of the ECM-derived cues that dictate structural and functional properties of the microvasculature, along with the development of new therapeutic strategies addressing the ECM-dependent regulation of pathophysiological stiffening of blood vessels in ischemic retinopathies.

Short-time effect of intravitreal injections on retinal vascular oxygenation and vessel diameter in patients with diabetic macular oedema or neovascular age-related macular degeneration

PURPOSE

To investigate the short-time effect of intravitreal injections (IVI) of the vascular endothelial growth factor inhibitors ranibizumab and aflibercept on retinal arterial and venous oxygen saturation (SO2a and SO2v), arteriovenous oxygen saturation difference (AVD) and vessel diameter (VDa and VDv) in patients with diabetic macular oedema (DME) and patients with choroidal neovascularization (CNV) due to age-related macular degeneration.

METHODS

Uncontrolled prospective observational study in 100 eyes. Retinal vessel oxygen saturation and diameters were assessed using a retinal oximeter before and minutes after IVI of ranibizumab or aflibercept.

RESULTS

40 eyes with CNV and 34 eyes with DME were included in the analysis. At baseline, SO2a and SO2v were significantly higher in DME (p = 0.043 and p = 0.009, respectively). After IVI, SO2a significantly decreased in CNV and DME eyes by 2.6% (p = 0.016) and 4.6% (p = 0.002) and SO2v decreased by 14.0% (p = 0.004) and 12.4% (p = 0.017), respectively. However, a significant increase in AVD was only found in CNV (15.7%, p = 0.001). VDa decreased significantly only in DME by 5.7% (p = 0.010). No medication-specific disease effect was found and vice versa.

CONCLUSIONS

The observed changes can be interpreted as signs of increased metabolic demand during the physiological stress after an IVI. The abnormal arterial constriction and the abolished increase in AVD seen only in eyes with DME indicate an impairment of vascular autoregulation and oxygen distribution and a reduced neuroretinal metabolism in the diabetic retina with a significant impact on inner retinal oxygen consumption shortly after IVI.

Modeling ZNF408-Associated FEVR in Zebrafish Results in Abnormal Retinal Vasculature

Purpose

Familial exudative vitreoretinopathy (FEVR) is an inherited retinal disease in which the retinal vasculature is affected. Patients with FEVR typically lack or have abnormal vasculature in the peripheral retina, the outcome of which can range from mild visual impairment to complete blindness. A missense mutation (p.His455Tyr) in ZNF408 was identified in an autosomal dominant FEVR family. Little, however, is known about the molecular role of ZNF408 and how its defect leads to the clinical features of FEVR.

Methods

Using CRISPR/Cas9 technology, two homozygous mutant zebrafish models with truncated znf408 were generated, as well as one heterozygous and one homozygous missense znf408 model in which the human p.His455Tyr mutation is mimicked.

Results

Intriguingly, all three znf408-mutant zebrafish strains demonstrated progressive retinal vascular pathology, initially characterized by a deficient hyaloid vessel development at 5 days postfertilization (dpf) leading to vascular insufficiency in the retina. The generation of stable mutant lines allowed long-term follow up studies, which showed ectopic retinal vascular hyper-sprouting at 90 dpf and extensive vascular leakage at 180 dpf.

Conclusions

Together, our data demonstrate an important role for znf408 in the development and maintenance of the vascular system within the retina.

Factors Affecting the Foveal Avascular Zone Area in Healthy Eyes among Young Chinese Adults

Purpose

To evaluate the influence of systemic and ocular factors on the foveal avascular zone (FAZ) area in young Chinese subjects' healthy eyes.

Methods

The current observational, cross-sectional study included 344 eyes from 172 healthy individuals (103 women, 69 men). Optical coherence tomography angiography realized with the split-spectrum amplitude-decorrelation angiography (SSADA) algorithm was used to assess the area of superficial FAZ. To determine the related factors and to reveal their potential correlations with the FAZ area, comprehensive examinations including both systemic and ocular ones were executed. Systemic examination involved factors including age, gender, and body mass index, while ocular examination involved factors including BCVA, refractive error, intraocular pressure, axial length (AL), anterior chamber depth, and central corneal thickness. Especially for fundus examination, central macular thickness (CMT), retinal volume, mean retinal thickness, macular blood flow area/vessel density in the superficial retinal layer (SRL) and deep retinal layer (DRL), mean retinal nerve fiber layer (RNFL) thickness, ganglion cell layer (GCL) thickness, C/D rate, rim area, and subfoveal choroid thickness were assessed, using mixed-effects regression models to appropriately account for intereye correlation. Subgroup analyses were performed based on gender and high myopia categories.

Results

The mean FAZ area was 0.30 ± 0.11 mm² and varied significantly across gender (P = 0.0024). AL, CMT, and RNFL thickness were found significantly correlated with the FAZ area in the univariate regression analysis (AL, P = 0.0005; CMT, P < 0.0001; and RNFL thickness, P = 0.0461). According to the multivariate results, CMT and macular blood flow in SRL were negatively correlated with FAZ (CMT: P < 0.0001; macular blood flow in SRL: P = 0.00223). Mean retinal thickness, mean GCL thickness, and macular blood flow in DRL were positively correlated with FAZ (mean retinal thickness: P = 0.0005; mean GCL thickness: P < 0.0001; and macular blood flow in DRL: P = 0.0099). Correlation results among these filtered factors and FAZ were more pronounced in non-high-myopic eyes than in high-myopic eyes and had a significant difference when data of male and female subjects were processed separately from each other.

Conclusion

The present cross-sectional study performed comprehensive systemic and ocular examinations in young Chinese adults and filtered factors affecting FAZ. We indicated that among all the assessed candidate factors, gender, AL, retinal thickness, macular blood flow, RNFL, and GCL thickness affected the FAZ area most significantly. Such findings would facilitate future research concerning the role of FAZ variation in fundus diseases.

Retinal vessel reactivity is not attenuated in patients with type 2 diabetes compared with matched controls and is associated with peripheral endothelial function in controls

BACKGROUND AND AIMS

Attenuated retinal vasoreactivity in patients with type 2 diabetes preceding diabetic retinopathy development has been proposed to reflect local endothelial dysfunction. Whether retinal vessel reactivity is associated with peripheral endothelial dysfunction and large artery stiffness in patients with type 2 diabetes remains to be elucidated.

METHODS

Twenty patients with type 2 diabetes without retinopathy and 20 sex- and age matched controls (diabetes duration: 9.9 years (range 6.0;12.4), 40% male, age: 66.5 ± 7.3 (diabetes) and 65.2 ± 7.6 years (controls)) were included. Endothelial function was assessed using EndoPAT. Arterial stiffness was assessed by carotid-femoral pulse wave velocity using the SphygmoCor. Retinal blood supply regulation was examined by retinal arteriolar diameter change during 1) isometric exercise (hand-weight lifting), 2) exposure to flickering lights, and 3) a combined stimulus of 1) + 2) using the Dynamic Vessel Analyzer.

RESULTS

No significant differences were observed in retinal vessel reactivity in T2DM patients compared to controls. Endothelial function was associated with mean arteriolar diameter change during only the combination intervention, (Beta = 0.033 [0.0013;0.064], p = 0.042) in the overall population of patients and controls. When groups were analyzed separately, the associations was statistically significant only in controls. However, formal test for interaction was not statistically significant, p = 0.40. No association was observed between pulse wave velocity and retinal arteriolar %-diameter change in patients or controls.

CONCLUSION

Peripheral endothelial function was associated with retinal arteriolar diameter change in the combined sample. The association seemed to be driven primarily by the controls. Our findings indicate that peripheral endothelial function is reflective of endothelial function in the retina mainly in subjects without T2DM, whereas an association in T2DM without retinopathy was not observed. Further studies are needed in T2DM patients with more advanced retinopathy.

Translational research in retinal vascular disease. An approach

The clinical presentation of the most frequent vision threatening retinal diseases is dominated by lesions indicating that disturbances in retinal blood flow are involved in the pathogenesis of these diseases. The present review describes the experience from a translational strategy pursued to investigate retinal vascular diseases with diabetic retinopathy as the main object. The normal regulation of retinal blood flow is investigated in porcine retinal vessels in vitro and ex vivo. Subsequently, the in vitro findings are translated to clinical studies in normal persons in vivo, and it is investigated whether the mechanisms are disturbed in retinal vascular disease. This is followed by clinical intervention studies on these diseases. The approach has been used to investigate pressure autoregulation, metabolic autoregulation and vasomotion in retinal vessels. The investigations have shown that retinal vascular tone can be regulated by receptor-specific agonists and antagonists to vasoactive compounds such as purines, prostaglandins and nitric oxide synthesis and that the vasoactive effects can be modulated by the concentration and the mode of administration of these compounds. Additionally, it has been shown that retinal precapillary arterioles and capillaries not visible by ophthalmoscopy may play an important role for the pathophysiology of retinal vascular disease and its treatment. Future studies should focus on investigating normal and pathological regulation of retinal blood flow in these smaller vessels.

Insights into the pathogenesis of cystoid macular edema: leukostasis and related cytokines

Cystoid macular edema (CME) is the abnormal collection of intraretinal fluid in the macular region, especially in the inner nuclear and outer plexiform layers. CME leads to severe visual impairment in patients with various retinal diseases, such as diabetic retinopathy, retinal vascular occlusion, choroidal neovascularization, and uveitis. Although various retinal conditions lead to CME, a shared pathogenesis of CME is involved in these diseases. Accordingly, the pathogenesis of CME based on vasogenic mechanisms is first discussed in this review, including vascular hyperpermeability, leukostasis, and inflammation. We then describe cytotoxic mechanisms based on retinal Müller cell dysfunction. This comprehensive review will provide an understanding of the pathogenesis of CME for potential therapeutic strategies.

Reduced baseline diameter and contraction of peripheral retinal arterioles immediately after remote ischemia in diabetic patients

PURPOSE

Remote ischemic conditioning (RIC) implies that transient ischemia in one organ can affect blood flow and protect from ischemia in another remote organ such as the retina. The purpose of the present study was to investigate the effect of RIC on the diameter of retinal arterioles in patients with diabetic retinopathy and whether this effect differs among peripheral and macular vessels.

METHODS

In twenty type 1 diabetes patients aged 20-31 years, the Dynamic Vessel Analyzer (DVA) was used to measure diameters of peripheral and macular arterioles during rest, isometric exercise, and flicker stimulation. Measurements were obtained before, immediately after, and 1 h after RIC, and were compared to responses obtained from normal persons.

RESULTS

The reduced baseline diameter (p < 0.009) and contraction of peripheral retinal arterioles during isometric exercise (p = 0.025) observed immediately after RIC in normal persons were absent in the studied diabetic patients.

CONCLUSIONS

RIC affects the diameter of peripheral but not macular arterioles in normal persons, but the response is abolished in diabetic patients.

TRIAL REGISTRATION

NCT03906383.

Retinal Vasculature in Development and Diseases

The retina is one of the most metabolically active tissues in the body, consuming high levels of oxygen and nutrients. A well-organized ocular vascular system adapts to meet the metabolic requirements of the retina to ensure visual function. Pathological conditions affect growth of the blood vessels in the eye. Understanding the neuronal biological processes that govern retinal vascular development is of interest for translational researchers and clinicians to develop preventive and interventional therapeutics for vascular eye diseases that address early drivers of abnormal vascular growth. This review summarizes the current knowledge of the cellular and molecular processes governing both physiological and pathological retinal vascular development, which is dependent on the interaction among retinal cell populations, including neurons, glia, immune cells, and vascular endothelial cells. We also review animal models currently used for studying retinal vascular development.

Near infrared adaptive optics flood illumination retinal angiography

Image-based angiography is a well-adapted technique to characterize vasculature, and has been used in retinal neurovascular studies. Because the microvasculature is of particular interest, being the site of exchange between blood and tissue, a high spatio-temporal resolution is required, implying the use of adaptive optics ophthalmoscopes with a high frame rate. Creating the opportunity for decoupled stimulation and imaging of the retina makes the use of near infrared (NIR) imaging light desirable, while the need for a large field of view and a lack of distortion implies the use of a flood illumination-based setup. However, flood-illumination NIR video sequences of erythrocytes, or red blood cells (RBC), have a limited contrast compared to scanning systems and visible light. As a result, they cannot be processed via existing image-based angiography methods. We have therefore developed a new computational method relying on a spatio-temporal filtering of the sequence to isolate blood flow from noise in low-contrast sequences. Applying this computational approach enabled us to perform angiography with an adaptive optics flood illumination ophthalmoscope (AO-FIO) using NIR light, both in bright-field and dark-field modalities. Finally, we demonstrate the capabilities of our system to differentiate blood flow velocity on a retinal capillary network in vivo.

Retinal capillary perfusion: Spatial and temporal heterogeneity

The central role of the cardiovascular system is to maintain adequate capillary perfusion. The spatially and temporally heterogeneous nature of capillary perfusion has been reported in some organs. However, such heterogeneous perfusion properties have not been sufficiently explored in the retina. Arguably, spatial and temporal heterogeneity of capillary perfusion could be more predominant in the retina than that in other organs. This is because the retina is one of the highest metabolic demand neural tissues yet it has a limited blood supply due to optical requirements. In addition, the unique heterogeneous distribution of retinal neural cells within different layers and regions, and the significant heterogeneity of intraretinal oxygen distribution and consumption add to the complexity. Retinal blood flow distribution must match consumption of nutrients such as oxygen and glucose within the retina at the cellular level in order to effectively maintain cell survival and function. Sophisticated local blood flow control in the microcirculation is likely required to control the retinal capillary perfusion to supply local retinal tissue and accommodate temporal and spatial variations in metabolic supply and demand. The authors would like to update the knowledge of the retinal microvessel and capillary network and retinal oxidative metabolism from their own studies and the work of others. The coupling between blood supply and energy demands in the retina is particularly interesting. We will mostly describe information regarding the retinal microvessel network and retinal oxidative metabolism relevant to the spatial and temporal heterogeneity of capillary perfusion. We believe that there is significant and necessary spatial and temporal heterogeneity and active regulation of retinal blood flow in the retina, particularly in the macular region. Recently, retinal optical coherence tomography angiography (OCTA) has been widely used in ophthalmology, both experimentally and clinically. OCTA could be a valuable tool for examining retinal microvessel and capillary network structurally and has potential for determining retinal capillary perfusion and its control. We have demonstrated spatial and temporal heterogeneity of capillary perfusion in the retina both experimentally and clinically. We have also found close relationships between the smallest arterioles and capillaries within paired arterioles and venules and determined the distribution of smooth muscle cell contraction proteins in these vessels. Spatial and temporal heterogeneity of retinal capillary perfusion could be a useful parameter to determine retinal microvessel regulatory capability as an early assay for retinal vascular diseases. This topic will be of great interest, not only for the eye but also other organs. The retina could be the best model for such investigations. Unlike cerebral vessels, retinal vessels can be seen even at the capillary level. The purpose of this manuscript is to share our current understanding with the readers and encourage more researchers and clinicians to investigate this field. We begin by reviewing the general principles of microcirculation properties and the spatial and temporal heterogeneity of the capillary perfusion in other organs, before considering the special requirements of the retina. The local heterogeneity of oxygen supply and demand in the retina and the need to have a limited and well-regulated retinal circulation to preserve the transparency of the retina is discussed. We then consider how such a delicate balance of metabolic supply and consumption is achieved. Finally we discuss how new imaging methodologies such as optical coherence tomography angiography may be able to detect the presence of spatial and temporal heterogeneity of capillary perfusion in a clinical setting. We also provide some new information of the control role of very small arterioles in the modulation of retinal capillary perfusion which could be an interesting topic for further investigation.

The vasodilating effect of glucose differs among vessels at different branching level in the porcine retina ex vivo

Diabetic retinopathy is characterized by retinal lesions related to disturbances in retinal blood flow. The metabolic dysregulation in diabetes involves hyperglycemia which in both clinical and experimental studies has been shown to induce dilatation of larger retinal vessels, which has been suggested to be mediated by nitric oxide (NO). However, the effects of glucose on the diameter of smaller retinal vessels that are the site of development of diabetic retinopathy are unknown. Diameter changes in porcine retinal arterioles, pre-capillary arterioles and capillaries were studied ex vivo during acute changes in intraluminal glucose concentrations that mimicked changes in plasma glucose in diabetic patients. The experiments were repeated during blocking of NO-synthesis. Intravascular application of 2 mM glucose dilated arterioles and capillaries significantly, while 20 mM glucose dilated precapillary arterioles significantly. Intravascular application of 20 mM glucose dilated precapillary arterioles previously exposed to 2 mM glucose, while no significant diameter changes were observed after application of 2 mM glucose in vessels previously exposed to 20 mM glucose. No diameter changes were observed after application of 5.5 mM glucose in vessels previously exposed to both 2 mM and 20 mM glucose in either order. There was no significant difference between the diameter responses in the absence and presence of NO-synthesis blocker. Glucose induced dilatation of porcine precapillary arterioles ex vivo differs from the response in larger arterioles and capillaries, and the response is unaffected by the blocking of NO-synthesis. This may have implications for understanding the pathophysiology of diseases in the retinal microcirculation, such as diabetic retinopathy.

Retinal oxygen saturation is an independent risk factor for the severity of diabetic retinopathy

BACKGROUND

The oxygen saturation in larger retinal vessels has been shown to increase with increasing diabetic retinopathy (DR) grade and to help predict the effect of antivascular endothelial growth factor treatment in patients with diabetic maculopathy. However, it is unknown to what extent the increased oxygen saturation co-varies with other risk factors and whether it is an independent risk factor for the severity of DR.

METHODS

Seven hundred and twenty-two successive patients referred for specialist evaluation of diabetic retinopathy including retinal oximetry were studied. Multiple regression analysis was used to investigate whether oxygen saturation in the larger retinal arterioles and venules contributed to the severity of diabetic retinopathy, independently of gender, age, diabetes duration, diabetes type, body mass index, blood pressure, haemoglobin A1c, visual acuity and central retinal thickness.

RESULTS

The included parameters could explain less than 15% of the variation in retinopathy grade. Approximately, one-third of the explained variation was related to the retinal oxygen saturation.

CONCLUSIONS

Prospective studies are needed to evaluate whether retinal oxygen saturation is predictive for the development of diabetic retinopathy and how it interacts with other biomarkers and risk factors over time.

Spatial distribution of early red lesions is a risk factor for development of vision-threatening diabetic retinopathy

AIMS/HYPOTHESIS

Diabetic retinopathy is characterised by morphological lesions related to disturbances in retinal blood flow. It has previously been shown that the early development of retinal lesions temporal to the fovea may predict the development of treatment-requiring diabetic maculopathy. The aim of this study was to map accurately the area where lesions could predict progression to vision-threatening retinopathy.

METHODS

The predictive value of the location of the earliest red lesions representing haemorrhages and/or microaneurysms was studied by comparing their occurrence in a group of individuals later developing vision-threatening diabetic retinopathy with that in a group matched with respect to diabetes type, age, sex and age of onset of diabetes mellitus who did not develop vision-threatening diabetic retinopathy during a similar observation period.

RESULTS

The probability of progression to vision-threatening diabetic retinopathy was higher in a circular area temporal to the fovea, and the occurrence of the first lesions in this area was predictive of the development of vision-threatening diabetic retinopathy. The calculated peak value showed that the risk of progression was 39.5% higher than the average. There was no significant difference in the early distribution of lesions in participants later developing diabetic maculopathy or proliferative diabetic retinopathy.

CONCLUSIONS/INTERPRETATION

The location of early red lesions in diabetic retinopathy is predictive of whether or not individuals will later develop vision-threatening diabetic retinopathy. This evidence should be incorporated into risk models used to recommend control intervals in screening programmes for diabetic retinopathy.

Activation of Veratridine Sensitive Sodium Channels, But not Electrical Field Stimulation, Dilates Porcine Retinal Arterioles with Preserved Perivascular Tissue

PURPOSE

Disturbances in retinal blood flow are a prominent feature of vision threatening retinal diseases. The regulation of tone in retinal resistance vessels involves the perivascular retinal tissue, but it is unknown to what extent neurons or glial cells contribute to the effect. Therefore, the purpose of the present study was to study the contribution of neurons in the perivascular retina to vascular tone during activation of voltage-gated sodium channels with veratridine and electrical field stimulation (EFS).

METHODS

Porcine retinal arterioles with and without perivascular tissue were mounted in an isometric myograph system for studying the effects of the voltage-gated sodium channel opener veratridine and EFS on retinal vascular tone.

RESULTS

Veratridine induced concentration-dependent relaxation of retinal arterioles which was more pronounced in arterioles with preserved perivascular retinal tissue than in isolated vessels. In the presence of this tissue, veratridine-induced relaxation was inhibited by the voltage-gated sodium channel blocker tetrodotoxin and the nitric oxide synthase inhibitor, N_ω-Nitro-L-arginine methyl ester (L-NAME), but was unaffected by the inhibition of the cyclo-oxygenase inbitior ibuprofen and by blocking of adenosine receptors with 8-(p-Sulfophenyl)theophylline hydrate (8-PSPT). Electrical field stimulation induced no changes in retinal vascular tone.

CONCLUSIONS

Sodium channels of neuronal origin are likely to be involved in the regulation of retinal vascular tone. The lack of effect of EFS on retinal vascular tone may be due to the lack of autonomic nerves in the retina.

Determination of Topographic Variations in Inner Retinal Blood Flow Areas in Young Chinese Subjects Using Optical Coherence Tomography Angiography

PURPOSE

To investigate (i) topographic variations in, and (ii) establish references for, inner retinal blood flow areas of the macular region of young Chinese subjects by means of optical coherence tomography (OCT) angiography.

METHODS

To measure inner retinal blood flow areas, we scanned an 8 × 8 mm² area and centered on the fovea of 336 eyes from 170 subjects using OCT angiography. Blood flow area measurements were made from a 1-mm radius circle, centered on the fovea, and 3 mm to the fovea superiorly, inferiorly, temporally, and nasally.

RESULTS

Mean inner retinal blood flow areas in the parafoveal, superior, inferior, nasal, and temporal macular regions were 1.78 ± 0.28, 1.76 ± 0.30, 1.69 ± 0.31, 1.91 ± 0.31, and 1.71 ± 0.28 mm,² respectively. Nasal inner retinal blood flow areas were significantly larger than other macular regions (p < 0.001). Gender was significantly associated with inner retinal blood flow areas of all macular regions studied. A gender-based subgroup analysis showed that inner retinal blood flow areas in superior, inferior, nasal, and temporal macular regions were significantly larger in women than men (all p < 0.05).

CONCLUSIONS

Nasal blood flow areas were larger than parafoveal blood flow areas, which were larger than superior blood flow areas, which were larger than temporal blood flow areas, which were larger than inferior blood flow areas in the macular regions studied. Gender was significantly associated with inner retinal blood flow areas in all macular regions studied.

Diameter Changes of Retinal Vessels in Diabetic Retinopathy

PURPOSE OF REVIEW

The diameter of retinal vessels is an important source of information about retinal blood flow and metabolism. The purpose of the present study is to review how diameter changes of retinal vessels contribute to the development of diabetic retinopathy and may be a marker of the prognosis of the disease.

RECENT FINDINGS

The early stages of diabetic retinopathy are accompanied with dilatation of the diameter of retinal vessels and reduced autoregulation. Diabetic retinopathy also shows regional differences in the macular area and the retinal periphery and accompanying differences in vascular reactivity in these areas. These differences may potentially become an important source of insight into the pathophysiology of the disease in the future. Diabetic retinopathy is accompanied with changes in the diameter regulation of retinal vessels. The potential of newly developed techniques for assessing retinal blood flow and metabolism, such as Doppler techniques, adaptive optics, and retinal oximetry, is promising and may potentially contribute to significant advances in our understanding of diabetic retinopathy which remains a major cause of visual impairment.

Imaging Foveal Microvasculature: Optical Coherence Tomography Angiography Versus Adaptive Optics Scanning Light Ophthalmoscope Fluorescein Angiography

PURPOSE

To compare the use of optical coherence tomography angiography (OCTA) and adaptive optics scanning light ophthalmoscope fluorescein angiography (AOSLO FA) for characterizing the foveal microvasculature in healthy and vasculopathic eyes.

METHODS

Four healthy controls and 11 vasculopathic patients (4 diabetic retinopathy, 4 retinal vein occlusion, and 3 sickle cell retinopathy) were imaged with OCTA and AOSLO FA. Foveal perfusion maps were semiautomatically skeletonized for quantitative analysis, which included foveal avascular zone (FAZ) metrics (area, perimeter, acircularity index) and vessel density in three concentric annular regions of interest. On each set of OCTA and AOSLO FA images, matching vessel segments were used for lumen diameter measurement. Qualitative image comparisons were performed by visual identification of microaneurysms, vessel loops, leakage, and vessel segments.

RESULTS

Adaptive optics scanning light ophthalmoscope FA and OCTA showed no statistically significant differences in FAZ perimeter, acircularity index, and vessel densities. Foveal avascular zone area, however, showed a small but statistically significant difference of 1.8% (P = 0.004). Lumen diameter was significantly larger on OCTA (mean difference 5.7 μm, P < 0.001). Microaneurysms, fine structure of vessel loops, leakage, and some vessel segments were visible on AOSLO FA but not OCTA, while blood vessels obscured by leakage were visible only on OCTA.

CONCLUSIONS

Optical coherence tomography angiography is comparable to AOSLO FA at imaging the foveal microvasculature except for differences in FAZ area, lumen diameter, and some qualitative features. These results, together with its ease of use, short acquisition time, and avoidance of potentially phototoxic blue light, support OCTA as a tool for monitoring ocular pathology and detecting early disease.

The role of retinopathy distribution and other lesion types for the definition of examination intervals during screening for diabetic retinopathy

PURPOSE

It has previously been shown that the intervals between screening examinations for diabetic retinopathy can be optimized by including individual risk factors for the development of the disease in the risk assessment. However, in some cases, the risk model calculating the screening interval may recommend a different interval than an experienced clinician. The purpose of this study was to evaluate the influence of factors unrelated to diabetic retinopathy and the distribution of lesions for discrepancies between decisions made by the clinician and the risk model.

METHODS

Therefore, fundus photographs from 90 screening examinations where the recommendations of the clinician and a risk model had been discrepant were evaluated. Forty features were defined to describe the type and location of the lesions, and classification and ranking techniques were used to assess whether the features could predict the discrepancy between the grader and the risk model.

RESULTS

Suspicion of tumours, retinal degeneration and vascular diseases other than diabetic retinopathy could explain why the clinician recommended shorter examination intervals than the model. Additionally, the regional distribution of microaneurysms/dot haemorrhages was important for defining a photograph as belonging to the group where both the clinician and the risk model had recommended a short screening interval as opposed to the other decision alternatives.

CONCLUSIONS

Features unrelated to diabetic retinopathy and the regional distribution of retinal lesions may affect the recommendation of the examination interval during screening for diabetic retinopathy. The development of automated computerized algorithms for extracting information about the type and location of retinal lesions could be expected to further optimize examination intervals during screening for diabetic retinopathy.

Differential effects of nitric oxide and cyclo-oxygenase inhibition on the diameter of porcine retinal vessels with different caliber during hypoxia ex vivo

BACKGROUND

Hypoxia induced relaxation of larger retinal arterioles has been shown to be mediated by nitric oxide (NO) and cyclo-oxygenase (COX) products both in vivo and in vitro. However, the involvement of smaller retinal vessels in the response is unknown. Therefore, the purpose of the present study was to investigate the effect of blocking the synthesis of NO and COX on hypoxia induced changes in the diameter of smaller porcine retinal vessels at different branching level.

METHODS

Porcine hemiretinas were mounted in a tissue chamber and were constricted with the prostaglandin agonist U46619. Changes in the diameter of arterioles, pre-capillary arterioles and capillaries were studied during hypoxia, in the presence of the COX inhibitor ibuprofen and the NO synthase inhibitor L-NAME.

RESULTS

In the presence of L-NAME hypoxia induced dilatation was significantly smaller in arterioles and capillaries than in precapillary arterioles (p < 0.04), whereas in the presence of ibuprofen the dilatation was significantly smaller in capillaries and pre-capillary arterioles than in arterioles (p < 0.04).

CONCLUSIONS

The mechanisms underlying hypoxia induced dilatation differ among smaller porcine retinal vessels with different caliber ex vivo. This may reflect differences in the responses of retinal vessels to changes in metabolism, and may point to possible targets for pharmacological intervention on the diameter of retinal vessels with different caliber in vivo.

Non-invasive imaging of retinal blood flow in myeloproliferative neoplasms

PURPOSE

To study the circulation in the retinal vessels in patients with blood dyscrasia due to myeloproliferative neoplasms using non-invasive retinal imaging.

METHODS

Prospective consecutive case series of seven treatment-naïve patients with chronic myeloid leukaemia (n = 2), polycythemia vera (n = 4), essential thrombocytosis (n = 1) examined before and after cytoreductive treatment. We investigated retinal circulation with motion-contrast imaging, retinal oximetry and spectral-domain optical coherence tomography.

RESULTS

Retinal venous blood velocity increased by 8.14% (CI₉₅ 3.67% to 12.6%, p = 0.004) and retinal arterial oxygen saturation increased by 7.23% (CI₉₅ 2.9% to 11.6%, p = 0.010) at follow-up (mean 12 weeks, range 5-14 weeks) where complete haematological remission had been achieved by cytoreductive treatment. Abnormal optical coherence tomography reflectivity patterns were present at baseline in patients with chronic myeloid leukaemia and were replaced by normal patterns at follow-up. Retinopathy, in the form of cotton-wool spots and retinal haemorrhages, was found at presentation in the two patients with chronic myeloid leukaemia and in one patient with polycythemia vera. The retinopathy had resolved at follow-up in all patients.

CONCLUSION

With non-invasive retinal imaging, we were able to demonstrate increased retinal venous blood velocity, increased retinal arterial blood oxygenation and normalization of intravascular reflectivity patterns after successful treatment of myeloproliferative neoplasms. Larger prospective studies are needed to assess the prognostic value of these non-invasive imaging methods in predicting circulatory complications in myeloproliferative neoplasms.

Lack of differences in the regional variation of oxygen saturation in larger retinal vessels in diabetic maculopathy and proliferative diabetic retinopathy

BACKGROUND

Diabetic retinopathy is characterised by morphological lesions in the ocular fundus related to disturbances in retinal blood flow. The two vision threatening forms of retinopathy show specific patterns of distribution of retinal lesions with proliferative diabetic retinopathy (PDR) developing secondary to ischaemia and hypoxia in the retinal periphery and diabetic maculopathy (DM) developing secondary to hyperperfusion and increased vascular permeability in the macular area. These differences in the distribution of retinal lesions might be reflected in regional differences in oxygen saturation in the larger retinal vessels.

METHODS

Dual-wavelength retinal oximetry was performed in 30 normal persons, 30 patients with DM and 30 patients with PDR, and the oxygen saturation was measured in peripapillary vessels supplying the four retinal quadrants and in branches from the upper temporal arcades supplying, respectively, the macular area and the retinal periphery.

RESULTS

The overall oxygen saturation was significantly higher in diabetic patients than in normal persons and the arteriovenous (AV) saturation difference significantly lower in the patients with DM. The regional variation in oxygen saturation was similar in the three studied groups with a decreasing saturation from the upper nasal through the lower nasal, lower temporal and the upper temporal peripapillary vessels, and with a significantly higher oxygen saturation in venules draining the macular area than in venules draining the retinal periphery.

CONCLUSIONS

The regional differences in retinal lesions in vision threatening diabetic retinopathy are not reflected in regional differences in the oxygen saturation of larger retinal vessels. The development of vision threatening diabetic retinopathy depends on other factors, such as, for example, regional differences in the retinal microcirculation.

The peptidomimetic Vasotide targets two retinal VEGF receptors and reduces pathological angiogenesis in murine and nonhuman primate models of retinal disease

Blood vessel growth from preexisting vessels (angiogenesis) underlies many severe diseases including major blinding retinal diseases such as retinopathy of prematurity (ROP) and aged macular degeneration (AMD). This observation has driven development of antibody inhibitors that block a central factor in AMD, vascular endothelial growth factor (VEGF), from binding to its receptors VEGFR-1 and mainly VEGFR-2. However, some patients are insensitive to current anti-VEGF drugs or develop resistance, and the required repeated intravitreal injection of these large molecules is costly and clinically problematic. We have evaluated a small cyclic retro-inverted peptidomimetic, D(Cys-Leu-Pro-Arg-Cys) [D(CLPRC)], and hereafter named Vasotide, that inhibits retinal angiogenesis by binding selectively to the VEGF receptors VEGFR-1 and neuropilin-1 (NRP-1). Delivery of Vasotide via either eye drops or intraperitoneal injection in a laser-induced monkey model of human wet AMD, a mouse genetic knockout model of the AMD subtype called retinal angiomatous proliferation (RAP), and a mouse oxygen-induced model of ROP decreased retinal angiogenesis in all three animal models. This prototype drug candidate is a promising new dual receptor inhibitor of the VEGF ligand with potential for translation into safer, less-invasive applications to combat pathological angiogenesis in retinal disorders.

The vasodilating effects of insulin and lactate are increased in precapillary arterioles in the porcine retina ex vivo

PURPOSE

Disturbances in the diameter regulation of the larger retinal vessels are involved in the pathophysiology of a number of vision threatening retinal diseases, but the role of the smaller vessels has received less attention. Therefore, an experimental model was developed for studying diameter changes in retinal vessels at all branching levels secondary to pharmacological interventions ex vivo.

METHODS

Porcine hemiretinas were mounted in a tissue chamber that allowed the control of temperature, pH and oxygen saturation. The chamber was positioned in a fluorescence microscope, and changes in the diameter of larger arterioles, precapillary arterioles and capillaries were studied after intravascular and extravascular application of the thromboxane analogue U46619, lactate, the glutamate agonist N-Methyl-D-aspartic acid (NMDA) and insulin.

RESULTS

U46619 induced significant contraction of all vessel calibres after extravascular application, but had no significant effect on vessel diameters after intravascular application. In contracted vessels, lactate induced significant dilatation in all vessel calibres, and the effect was more pronounced after intravascular than after extravascular application where only precapillary arterioles dilated significantly. N-Methyl-D-aspartic acid (NMDA) induced similar dilatation in the three vessel types after intra- and extravascular application, whereas the dilating effect of insulin was significantly more pronounced in the precapillary arterioles than in the other vessel types.

CONCLUSION

The diameter regulation in precapillary retinal arterioles differs from that of other retinal vascular branching levels and may play a special role in the pathophysiology of retinal vascular disease.