Nitric oxide regulates retinal vascular tone in humans

Effects of hemodialysis on blood volume, macro- and microvascular function

BACKGROUND

Vascular dysfunction is considered to spur the progression of cardiovascular disease in hemodialysis (HD) patients. Whether the HD procedure itself contributes to vascular dysfunction remains incompletely investigated. The present study sought to comprehensively assess the effects of HD on arterial and venous function along with concomitant changes in blood volume (BV).

METHODS AND RESULTS

We determined BV with high-precision, automated carbon monoxide-rebreathing, arterial stiffness using applanation tonometry and intrinsic microvascular function via retinal vessel analysis prior to and after conventional 4-hour HD in fasting-controlled conditions in 10 patients. All HD patients were non-smokers and non-obese (body mass index = 22.8 ± 2.8 m·kg^-2). Hypertension (70%), coronary artery disease (40%) and diabetes mellitus (20%) were the most prevalent comorbidities. Prior to HD, all patients presented with hypervolemia (+2208 ± 1213 ml). HD decreased body weight (-1.72 ± 1.25 kg, P = 0.002) and plasma volume (-689 ± 566 ml, P = 0.004), while hematocrit (Hct) was concomitantly increased (+4.8 ± 4.5%, P = 0.009). HD did not affect large elastic artery stiffness, as determined by carotid-femoral pulse wave velocity (P = 0.448) and carotid distensibility (P = 0.562). In contrast, flicker light-induced retinal venular dilation was reduced by three-fourths after HD (-2.4 ± 1.7%, P = 0.039), in parallel to increased retinal venular diameter (+11.2 ± 4.9 μm, P = 0.002). In regression analyses, a negative association was observed between HD-induced changes in Hct and retinal venular dilation (r ≥ -0.89, P ≤ 0.045).

CONCLUSION

Conventional HD resulting in substantial plasma volume removal do not alter large artery elastic properties, whereas intrinsic microvascular venular dilator function is markedly impaired, an effect directly associated with the increase in hemoconcentration.

Retinal microvascular dysfunction in heart failure

Aims

Retinal vessel analysis (RVA) represents a novel, non-invasive, and reliable method to study the microcirculation in the eye. The goal of this study was to assess the extent of retinal microvascular dysfunction in patients with chronic heart failure (CHF) compared to controls and established measures of vascular function.

Methods and results

In this prospective, single-centre, observational study, 74 patients with compensated CHF (mean age 63.5 ± 11.2 years, 32% female, mean left-ventricular ejection fraction 37 ± 12.8%), 74 patients with cardiovascular risk factors (CVRF; 64.1 ± 12.7 years, 34% female), and 74 healthy controls (HC; 57.8 ± 14.2 years, 35% female) were included. The primary endpoint, flicker-induced dilatation of retinal arterioles (FIDart), was significantly reduced in patients with CHF compared to CVRF and HC (mean FIDart 0.9 ±  0.2 vs. 2.3 ± 0.3 and vs. 3.6 ± 0.3%, respectively, both P < 0.001 before and after propensity score-weighted analysis). Similar differences were seen for venular FID. FIDart was less impaired in patients with dilated compared to ischaemic cardiomyopathy. No significant differences were observed for arteriovenous ratio and flow-mediated dilatation. Impaired FIDven was associated with echocardiographically estimated systolic pulmonary artery pressure and left atrial volume index.

Conclusion

Retinal microvascular dilatation in response to flicker light is impaired in CHF. RVA may represent a new and useful method to non-invasively monitor microvascular abnormalities in heart failure in an easy and standardized way without the use of radiation.

Microvascular endothelial function and primary open angle glaucoma

Purpose

To determine the role of microvascular endothelial dysfunction as risk factor for primary open angle glaucoma.

Methods

A cross-sectional study was conducted involving 114 Malay patients with POAG seen at the eye clinic of Hospital Universiti Sains Malaysia. Patients aged between 40 and 80 years who were diagnosed with other types of glaucoma, previous glaucoma filtering surgery or other surgeries except uncomplicated cataract surgery and pterygium surgery were excluded. A total of 101 patients who were followed up for dry eyes, age-related cataracts or post cataracts extraction surgery were recruited as control subjects. Those with family history of glaucoma or glaucoma suspect were excluded. Microvascular endothelial function was assessed using laser Doppler fluximetry and the process of iontophoresis. Iontophoresis with acetylcholine (ACh) and sodium nitroprusside (SNP) was used to measure microvascular endothelium-dependent and endothelium-independent vasodilatations, respectively.

Results

In general, POAG patients demonstrated lower ACh% and ACh_max values compared with controls. There was significant difference in microvascular endothelial function [ACh%: mean, 95% confidence interval = 503.1 (378.0, 628.3), and ACh_max: mean, 95% confidence interval = 36.8 (30.2, 43.5)] between primary open angle glaucoma cases (p < 0.001) and controls [ACh%: mean, 95% confidence interval = 1378.4 (1245.4, 1511.3), and ACh_max: mean, 95% confidence interval = 79.2 (72.1, 86.2)]; this difference remained significant even after controlling for potential confounders. Similar difference was also found in SNP% and SNP_max between POAG and controls (p < 0.001). Age and diastolic blood pressure were inversely correlated with microvascular endothelial function.

Conclusion

There was an impairment of microvascular endothelial function and endothelial-independent vasodilatation in POAG patients. Microvascular endothelial function is a potential risk factor for POAG.

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.

Retinal Endothelial Function, Physical Fitness and Cardiovascular Risk: A Diagnostic Challenge

INTRODUCTION

Dynamic retinal vessel analysis (DVA) is a new non-invasive method to quantify microvascular endothelial dysfunction by flicker light-induced dilatation (FID). FID has been shown to be impaired in type 2 diabetes as well as heart failure. The aim of the study was to analyze FID in healthy active versus healthy sedentary and cardiovascular (CV) risk patients in addition to corresponding static vessel diameters.

METHODS

Thirty-one healthy active (HA, mean age 60 ± 8 years), 33 healthy sedentary individuals (HS, 59 ± 7 years) and 76 sedentary patients with increased CV risk (SR, 58 ± 6 years) were included in this cross-sectional study. Group differences in CV risk factors and cardiorespiratory fitness, maximal arteriolar (ADmax) and venular (VDmax) dilatation as well as the arteriolar (AFarea) and venular (VFarea) area under the flicker curve were analyzed. The central retinal arteriolar and venular diameters were used to calculate the arteriolar-to-venular diameter ratio (AVR).

RESULTS

HS [ADmax = 3.5 (2.1)%; AFarea = 48.2 (31.9)%∗s] showed higher FID compared to SR [ADmax = 2.7 (1.8)%, p = 0.021; AFarea = 34.5 (26.5)%∗s, p = 0.006] and HA [AFarea = 32.8 (23.1)%∗s, p = 0.029]. HA and SR did not significantly differ. HA had a higher AVR (0.87 ± 0.05) compared to HS (0.83 ± 0.04, p < 0.001) with further deterioration in SR (0.79 ± 0.05, p < 0.001). Interestingly, 28 participants had impaired FID but normal AVR and 43 participants had normal FID but impaired AVR.

DISCUSSION

FID can differentiate between sedentary low and high risk individuals. However, FID in healthy active persons (HA) seemed impaired with a concomitant higher AVR. We postulate that lower FID in HA may be explained by predilatated arterioles and a reduced dilatation reserve. We recommend combination of FID with analysis of retinal vessel diameters to differentiate functional non-responders from manifest microvascular endothelial dysfunction and, thereby, improve microvascular risk stratification in a personalized medicine approach.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov: NCT02796976 (https://clinicaltrials.gov/ ct2/show/NCT02796976).

Impact of Intravitreal Dexamethasone Implant on Vessel Diameters in Patients with Retinal Vein Occlusion

PURPOSE

To investigate the vasomotor responses and diameter of retinal vessels in patients with macular edema secondary to retinal vein occlusion (RVO) who were treated with intravitreal dexamethasone implant.

METHODS

We enrolled 17 eyes of 17 patients with macular edema secondary to RVO. All patients were evaluated through optical coherence tomography and dynamic and static retinal vessel analysis, using the Dynamic Vessel Analyzer (Imedos, Jena, Germany) before administration (baseline) and 1 week, 1 month, and 2 months after administration of intravitreal dexamethasone. Measurements of patients were compared to those of 17 eyes of age- and sex-matched control subjects.

RESULTS

In static analysis, arteriovenous ratio (AVR) in control subjects was 0.86 (0.80-0.88). In RVO patients, baseline AVR was 0.71 (0.54-0.84) and significantly lower than that in control subjects (p=0.016). Baseline AVR in the RVO group was significantly lower than AVR at month 1 and month 2 (p=0.001 and p < 0.001, respectively). CRVE in healthy control subjects was 183.59 ± 21.79 measurement units (MU) which was significantly different from CRVE of RVO eyes at baseline (207.00 ± 26.35 MU) (p=0.008). Static analysis showed a significant decrease of central retinal vein equivalent (CRVE) from baseline to 1 week, 1 month, and 2 months (p < 0.001, p < 0.001, and p < 0.001, respectively). CRAE in the control group was 176.24 ± 22.45 MU. CRAE in the RVO group was significantly lower at baseline, week 1, month 1, and month 2 compared to that in the control group (p=0.008, p=0.003, p=0.013, and p=0.011, respectively). Dynamic analysis showed that maximum venous and arterial dilations did not statistically differ from baseline to 1 week, 1 month, or 2 months.

CONCLUSION

Using the Dynamic Vessel Analyzer, we found that retinal veins in patients with RVO were significantly larger compared to those in the control group, and intravitreal dexamethasone treatment reduced the diameters of these veins.

Impaired Retinal Vessel Dilation Predicts Mortality in End-Stage Renal Disease

RATIONALE

Patients with end-stage renal disease (ESRD) are characterized by increased cardiovascular (CV) and all-cause mortality due to advanced remodeling of the macro- and microvascular beds.

OBJECTIVE

The aim of this study was to determine whether retinal microvascular function can predict all-cause and CV mortality in patients with ESRD.

METHODS AND RESULTS

In the multicenter prospective observational ISAR (Risk Stratification in End-Stage Renal Disease) study, data on dynamic retinal vessel analysis (DVA) was available in a sub-cohort of 214 dialysis patients (mean age 62.6{plus minus}15.0; 32% female). Microvascular dysfunction was quantified by measuring maximum arteriolar (aMax) and venular dilation (vMax) of retinal vessels in response to flicker light stimulation. During a mean follow-up of 44 months, 55 patients died, including 25 CV and 30 non-CV fatal events. vMax emerged as a strong independent predictor for all-cause mortality. In the Kaplan-Meier analysis, individuals within the lowest tertile of vMax showed significantly shorter three-year survival rates than those within the highest tertile (66.9{plus minus}5.8% vs 92.4{plus minus}3.3%). Uni- and multivariate hazard ratios for all-cause mortality per SD increase of vMax were 0.62 [0.47;0.82] and 0.65[0.47;0.91], respectively. aMax and vMax were able to significantly predict nonfatal and fatal CV events (HR 0.74[0.57;0.97] and 0.78[0.61;0.99], respectively).

CONCLUSIONS

Our results provide the first evidence that impaired retinal venular dilation is a strong and independent predictor of all-cause mortality in hemodialyzed ESRD patients. DVA provides added value for prediction of all-cause mortality and may be a novel diagnostic tool to optimize CV risk stratification in ESRD and other high-risk CV cohorts.

CLINICAL TRIAL REGISTRATION

NCT01152892.

Retinal arteriole reactivity in mice lacking the endothelial nitric oxide synthase (eNOS) gene

Dysfunctional vascular endothelial nitric oxide synthase (eNOS) has been proposed to play a main pathophysiological role in various ocular diseases. The aim of the present study was to test the hypothesis that the chronic lack of eNOS impairs endothelium-dependent vasodilation in retinal arterioles. The relevance of eNOS for mediating vascular responses was studied in retinal vascular preparations from eNOS-deficient mice (eNOS-/-) and wild-type controls in vitro. Changes in luminal diameter in response to vasoactive agents were measured by videomicroscopy. The thromboxane mimetic, U46619, induced similar concentration-dependent constriction of retinal arterioles in eNOS-/- and wild-type mice. Responses to the endothelium-independent vasodilator, nitroprusside, did not differ between both mouse genotypes, either. In contrast, responses to the endothelium-dependent vasodilator, acetylcholine, were blunted in eNOS-/- mice. Non-isoform-selective blockade of either nitric oxide synthase (NOS) or cyclooxygenase (COX) alone did not affect responses to acetylcholine. However, combined blockade of both enzyme families markedly attenuated cholinergic vasodilation. Also, combined blockade of COX and neuronal NOS (nNOS) blunted acetylcholine-induced vasodilation, while combined COX and inducible NOS (iNOS) inhibition had no effect. Simultaneous NOS and COX-1 blockade did not affect cholinergic vasodilation, whereas combined NOS and COX-2 inhibition markedly reduced vasodilation to acetylcholine. These findings are the first to demonstrate that the chronic lack of eNOS is associated with moderate endothelial dysfunction in retinal arterioles. However, eNOS-deficiency is partially compensated by nNOS and COX-2 metabolites, which are reciprocally regulated.

Retinal vascular function in asymptomatic individuals with a positive family history of cardiovascular disease

PURPOSE

To compare retinal microvascular function in healthy individuals with and without a positive family history (FH) of cardiovascular disease (CVD).

METHODS

Retinal vessel reactivity was assessed by means of dynamic retinal vessel analysis in 38 healthy subjects aged between 30 and 66 years with a positive FH of CVD and 37 age- and gender-matched control subjects. Other assessments included blood pressure (BP) profiles, blood glucose and lipid metabolism markers, Framingham risk scores (FRS), carotid intima-media thickness (c-IMT) and brachial flow-mediated dilation (FMD).

RESULTS

Family history-positive subjects showed decreased retinal arterial baseline diameter fluctuation, dilation amplitude, percent dilation, and overall constriction response slope (p = 0.001; p = 0.015; p = 0.001; and p < 0.001, respectively) and increased percent constriction (p = 0.008). On the venous side, baseline-corrected flicker response and dilation response slope were decreased in the FH-positive group (p = 0.009 and p = 0.010, respectively). There were no significant differences between groups in c-IMT scores or FMD parameters (all p > 0.05). The arterial MC% correlated negatively with decreased high-density lipoprotein cholesterol (r = -0.52, p = 0.002) in only FH-positive group.

CONCLUSION

Although macrovascular function is preserved in individuals with FH positive for CVD but with low FRS, there are, however, functional impairments at the retinal microvascular level that correlate with established plasma markers for cardiovascular risk.

Long eye relief fundus camera and fixation target with partial correction of ocular longitudinal chromatic aberration

A combined 32° full field of view refractive fundus camera and fixation target with a -20 to +10 diopter sphere correction range is described and demonstrated. The optical setup partially corrects the average longitudinal chromatic aberration and spherical aberration of the human eye, while providing a long eye relief to allow integration with reflective adaptive optics ophthalmoscopes, as a viewfinder. The fundus camera operates with 940 nm light, using a maximum 2.9 mm diameter imaging pupil at the eye. The fixation target uses a light projector capable of delivering red, green and/or blue spatially and temporally modulated stimuli to the retina. The design and performance of each sub-system are discussed, and retinal imaging at various wavelengths is demonstrated.

Retinal microvascular dysfunction in hypercholesterolemia

BACKGROUND

Hypercholesterolemia is one of the most important contributors to atherosclerosis. Whether hypercholesterolemia also affects the retinal microcirculation is unclear.

OBJECTIVE

The goal of our study was to assess the association of cholesterol levels with retinal microvascular function using dynamic and static retinal vessel analysis (RVA) in a primary prevention setting.

METHODS

This cross-sectional, observational study prospectively recruited 67 patients with hypercholesterolemia without known cardiovascular disease (mean age 64.4 ± 10.4 years; 45% female) and 78 healthy controls (mean age 61.8 ± 11.2 years; 45% female). The primary end point of the study was flicker-induced dilatation of retinal arterioles (FID_art) with secondary exploratory outcomes including venular FID (FID_ven), arteriovenous ratio, flow-mediated dilatation and arterial stiffness as measured with augmentation index and pulse wave velocity. Multiple regression analysis was performed to study the association of cholesterol levels with retinal microvascular function.

RESULTS

FID_art was significantly impaired in patients with hypercholesterolemia compared with healthy controls (mean FID_art 2.1 ± 1.8 vs 3.1 ± 1.8%, P = .001). This association remained when analysis was restricted to dyslipidemic patients without coexisting hypertension or lipid-lowering therapy. No significant differences remained for FID_ven, flow-mediated dilatation, arteriovenous ratio, or arterial stiffness between the groups. Low-density lipoprotein, but not high-density lipoprotein, cholesterol was a significant negative predictor of FID_art in multiple regression analysis.

CONCLUSION

Hypercholesterolemia is associated with significant retinal microvascular dysfunction as evidenced by a reduction in flicker-induced dilatation of retinal arterioles. Dynamic RVA may be a promising method for the study of retinal microvascular dysfunction in populations at elevated cardiovascular risk.

EFFECTS OF INTRAVITREAL RANIBIZUMAB AND BEVACIZUMAB ON THE RETINAL VESSEL SIZE IN DIABETIC MACULAR EDEMA

PURPOSE

The goal of this study was to assess the effects of a single injection of intravitreal ranibizumab (RAN) or bevacizumab (BEV) on the retinal vessel size in eyes with diabetic macular edema.

MATERIALS AND METHODS

In total, 32 patients were enrolled in the RAN group, and 30 patients were included in BEV group. Each of these groups was also subdivided into two others groups: a study group and a control group. The study groups were composed of the injected eyes, whereas the noninjected fellow eyes served as the control groups. The patients underwent complete ophthalmic examinations, including optical coherence tomography and fundus fluorescein angiography, and the primary outcome measures included the central retinal artery equivalent, central retinal vein equivalent, and artery-to-vein ratio.

RESULTS

In the RAN study group (n = 32), the preinjection mean central retinal artery equivalent (175.42 μm) decreased to 169.01 μm after 1 week, and to 167.47 μm after 1 month (P < 0.001), whereas the baseline central retinal vein equivalent (235.29 μm) decreased initially to 219.90 μm after 1 week, and to 218.36 μm after 1 month (P < 0.001). In the BEV study group (n = 30), the preinjection central retinal artery equivalent (150.21 μm) decreased to 146.25 μm after 1 week, and to 145.89 μm after 1 month (P < 0.001); whereas the baseline central retinal vein equivalent (211.87 μm) decreased initially to 204.59 μm after 1 week and was 205.24 μm after 1 month (P < 0.001). The preinjection artery-to-vein ratio values changed significantly (P = 0.001) after 1 week and after 1 month in the RAN group, but no significant alteration in the artery-to-vein ratio was observed in the BEV group (P = 0.433). In both the RAN (n = 32) and BEV (n = 30) control groups, none of the 3 parameters changed throughout the study period, when compared with the baseline.

CONCLUSION

The results of this study showed that both RAN and BEV injections significantly constricted the retinal blood vessel diameters.

Nitric oxide-donating compounds for IOP lowering in glaucoma

INTRODUCTION

An elevated intraocular pressure (IOP) remains the main risk factor for progression of glaucoma upon which we can efficiently act. Pharmacological strategies to reduce IOP are directed towards the reduction of aqueous humour (AH) production and/or the increase in AH drainage through the uveoscleral pathway. However, there are no drugs currently available as first-line treatment to increase AH outflow primarily via the conventional route. Ocular nitric oxide (NO) production takes place in AH outflow pathways and in the ciliary muscle, modulating the cellular response to elevated IOP.

METHODS

This review describes the mechanism of action of endogenous NO and NO-donating compounds that are under research. It includes information regarding pre-clinical and clinical studies previously conducted with these compounds, discussing their role and therapeutic potential in the pharmacological treatment of ocular hypertension in glaucoma.

RESULTS

The topical ocular administration of NO-donating compounds significantly lowered IOP and maintained it in animal models of glaucoma and subjects with ocular hypertension.

CONCLUSIONS

The mechanism of action of these compounds is novel and scientific evidence that shows promising results. However, there is a need for more comprehensive studies to assess long-term safety and tolerability in order to properly evaluate their use in chronic therapies.

Obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is increasing in prevalence due to rising obesity. While OSA is a disorder primarily of the upper airway during sleep, its pathophysiological impact on other body systems is increasingly recognised. There has been interest in the prevalence of OSA in different ophthalmic conditions and possible causation has been postulated. As OSA is common, it can be expected that people with co-existent OSA will be found in any ophthalmic disease population studied. To determine with confidence the significance of finding patients with OSA in a particular cohort requires a well matched control group, ideally matched for age, obesity, gender and co-morbidities. Only if one can say with certainty that the prevalence of OSA is higher in a group with a particular co-existent ophthalmic disease can we begin to speculate about possible mechanisms for the overlap in these conditions. Possible mechanisms for how OSA might affect the eye are discussed in this review. The current literature is reviewed with respect to diabetic retinopathy, glaucoma, floppy eyelid syndrome, non-arteritic ischaemic optic neuropathy, keratoconus and AMD. Associations with OSA have been found, but robust prospective studies using multi-channel sleep studies to diagnose OSA are lacking. Gaps remain in the evidence and in our knowledge. It is hoped that this review will highlight the need for ophthalmologists to consider OSA in their patients. It also makes recommendations for future research, especially to consider whether therapies for OSA can also be effective for ophthalmic disorders.

The Vasculature in Prediabetes

The frequency of prediabetes is increasing as the prevalence of obesity rises worldwide. In prediabetes, hyperglycemia, insulin resistance, and inflammation and metabolic derangements associated with concomitant obesity cause endothelial vasodilator and fibrinolytic dysfunction, leading to increased risk of cardiovascular and renal disease. Importantly, the microvasculature affects insulin sensitivity by affecting the delivery of insulin and glucose to skeletal muscle; thus, endothelial dysfunction and extracellular matrix remodeling promote the progression from prediabetes to diabetes mellitus. Weight loss is the mainstay of treatment in prediabetes, but therapies that improved endothelial function and vasodilation may not only prevent cardiovascular disease but also slow progression to diabetes mellitus.

Retinal microvascular parameters are not associated with reduced renal function in a study of individuals with type 2 diabetes

The eye provides an opportunistic "window" to view the microcirculation. There is published evidence of an association between retinal microvascular calibre and renal function measured by estimated glomerular filtration rate (eGFR) in individuals with diabetes mellitus. Beyond vascular calibre, few studies have considered other microvascular geometrical features. Here we report novel null findings for measures of vascular spread (vessel fractal dimension), tortuosity, and branching patterns and their relationship with renal function in type 2 diabetes over a mean of 3 years. We performed a nested case-control comparison of multiple retinal vascular parameters between individuals with type 2 diabetes and stable (non-progressors) versus declining (progressors) eGFR across two time points within a subset of 1072 participants from the GoDARTS study cohort. Retinal microvascular were measured using VAMPIRE 3.1 software. In unadjusted analyses and following adjustment for age, gender, systolic blood pressure, HbA1C, and diabetic retinopathy, no associations between baseline retinal vascular parameters and risk of eGFR progression were observed. Cross-sectional analysis of follow-up data showed a significant association between retinal arteriolar diameter and eGFR, but this was not maintained following adjustment. These findings are consistent with a lack of predictive capacity for progressive loss of renal function in type 2 diabetes.

Blood Pressure Effects on Retinal Vessel Diameter and Flicker Response: A 1.5-Year Follow-Up

PURPOSE

The study examined the influence of individual blood pressure changes overtime on retinal vessel diameter and the latter's response to flicker light.

METHODS

The diameter of a retinal arterial and venous segment was measured continuously on-line with a Dynamic Vessel Analyzer in 20 patients twice (mean interval between examinations of 24 months). Eleven patients had no cardiovascular disease. Nine patients had arterial hypertension and were untreated at the time of the first measurement; at the time of the second measurement they were undergoing various antihypertensive therapies. Each test consisted of a 50-s baseline plus three 20-s periods of flicker stimulation followed by an 80-s period of observation. During the examinations the blood pressure was measured at 1-minute intervals.

RESULTS

In the hypertension group changes in the mean arterial blood pressure (MAP) correlated significantly with changes in the arterial baseline diameter (y = -0.1 - 0.37x, r =0.74, p (increase) <0.03). A comparison of the two measurements showed no such relationship in the group of cardiovascularly healthy subjects. The venous baseline and the arterial and venous flicker response did not change significantly in either group between the two measurements and showed no relationship to blood pressure changes.

CONCLUSIONS

In hypertensive subjects, long-term therapy-related changes in blood pressure induced a change in the arterial baseline by approximately +3.7 microm/-10 mmHg MAP. An influence of lowering MAP to the arterial flicker response could not be detected.

Alterations of Ocular Hemodynamics Impair Ophthalmic Vascular and Neuroretinal Function

Hypertension is associated with numerous diseases, but its direct impact on the ocular circulation and neuroretinal function remains unclear. Herein, mouse eyes were challenged with different levels of hemodynamic insult via transverse aortic coarctation, which increased blood pressure and flow velocity by 50% and 40%, respectively, in the right common carotid artery, and reduced those parameters by 30% and 40%, respectively, in the left common carotid artery. Blood velocity in the right central retinal artery gradually increased up to 40% at 4 weeks of transverse aortic coarctation, and the velocity in the left central retinal artery gradually decreased by 20%. The fundus and retinal architecture were unaltered by hemodynamic changes. Endothelium-dependent vasodilations to acetylcholine and adenosine were reduced only in right (hypertensive) ophthalmic arteries. Increased cellularity in the nerve fiber/ganglion cell layers, enhanced glial fibrillary acidic protein expression, and elevated superoxide level were found only in hypertensive retinas. The electroretinogram showed decreased scotopic b-waves in the hypertensive eyes and decreased scotopic oscillatory potentials in both hypertensive and hypotensive eyes. In conclusion, hypertension sustained for 4 weeks causes ophthalmic vascular dysfunction, retinal glial cell activation, oxidative stress, and neuroretinal impairment. Although ophthalmic vasoregulation is insensitive to hypotensive insult, the ocular hypoperfusion causes neuroretinal dysfunction.

Correlation between Flicker-Induced Retinal Vessel Vasodilatation and Plasma Biomarkers of Endothelial Dysfunction in Hypertensive Patients

PURPOSE

Hypertension (HT) strongly affects the vascular endothelium, resulting in chronic inflammatory disease. Dynamic vessel analysis (DVA) is a modern methodological approach to analyze vascular function in the retinal microcirculation. The aim of this study was to examine whether a defective retinal vessels response is associated with HT-induced endothelial dysfunction.

MATERIALS AND METHODS

Retinal vessel reactions to flicker stimulation were examined by DVA in both eyes of 37 hypertensive and 41 healthy control subjects. Plasma concentrations of C-reactive protein (CRP), interleukin-6, and tumor necrosis factor alpha (TNFɑ) were measured using the enzyme-linked immunosorbent assay.

RESULTS

Both arterial and vein responses to flicker stimulation were significantly decreased in patients with HT compared with the healthy controls (dilatation of the arteries was lower in the HT group by, on average, 1.31, p = 0.001 and dilatation of the veins was lower in the HT group by, on average, 1.32, p = 0.002) after independent adjustment for age, sex, body mass index, and pressure values. In the hypertensive group, there was a negative correlation between the arterial response to flicker stimulation and the plasma CRP concentration (Spearman's Rank-order Coefficient (Rs) = -0.29, p = 0.07). Similarly, the plasma TNFα concentrations negatively correlated with the arterial response to flicker stimulation (Rs = -0.39, p = 0.02).

CONCLUSIONS

Our results indicate that DVA directly reflects the actual metabolic status of the retinal endothelium. DVA might be used as an early noninvasive screening tool to detect vascular dysregulation and pan-endothelial dysfunction in patients with HT.

Vasoconstriction and Impairment of Neurovascular Coupling after Subarachnoid Hemorrhage: a Descriptive Analysis of Retinal Changes

Impaired cerebral autoregulation and neurovascular coupling (NVC) contribute to delayed cerebral ischemia after subarachnoid hemorrhage (SAH). Retinal vessel analysis (RVA) allows non-invasive assessment of vessel dimension and NVC hereby demonstrating a predictive value in the context of various neurovascular diseases. Using RVA as a translational approach, we aimed to assess the retinal vessels in patients with SAH. RVA was performed prospectively in 24 patients with acute SAH (group A: day 5-14), in 11 patients 3 months after ictus (group B: day 90 ± 35), and in 35 age-matched healthy controls (group C). Data was acquired using a Retinal Vessel Analyzer (Imedos Systems UG, Jena) for examination of retinal vessel dimension and NVC using flicker-light excitation. Diameter of retinal vessels-central retinal arteriolar and venular equivalent-was significantly reduced in the acute phase (p < 0.001) with gradual improvement in group B (p < 0.05). Arterial NVC of group A was significantly impaired with diminished dilatation (p < 0.001) and reduced area under the curve (p < 0.01) when compared to group C. Group B showed persistent prolonged latency of arterial dilation (p < 0.05). Venous NVC was significantly delayed after SAH compared to group C (A p < 0.001; B p < 0.05). To our knowledge, this is the first clinical study to document retinal vasoconstriction and impairment of NVC in patients with SAH. Using non-invasive RVA as a translational approach, characteristic patterns of compromise were detected for the arterial and venous compartment of the neurovascular unit in a time-dependent fashion. Recruitment will continue to facilitate a correlation analysis with clinical course and outcome.

Assessing Microvascular Function in Humans from a Chronic Disease Perspective

Microvascular dysfunction (MVD) is considered a crucial pathway in the development and progression of cardiometabolic and renal disease and is associated with increased cardiovascular mortality. MVD often coexists with or even precedes macrovascular disease, possibly due to shared mechanisms of vascular damage, such as inflammatory processes and oxidative stress. One of the first events in MVD is endothelial dysfunction. With the use of different physiologic or pharmacologic stimuli, endothelium-dependent (micro)vascular reactivity can be studied. This reactivity depends on the balance between various mediators, including nitric oxide, endothelin, and prostanoids, among others. The measurement of microvascular (endothelial) function is important to understand the pathophysiologic mechanisms that contribute to MVD and the role of MVD in the development and progression of cardiometabolic/renal disease. Here, we review a selection of direct, noninvasive techniques for measuring human microcirculation, with a focus on methods, interpretation, and limitations from the perspective of chronic cardiometabolic and renal disease.

Gestational hypertensive disorders and retinal microvasculature: the Generation R Study

BACKGROUND

Changes in the microvasculature associated with pre-eclampsia and gestational hypertension have been proposed as a potential pathway in the development of cardiovascular disease. We examined whether gestational hypertensive disorders, such as pre-eclampsia and gestational hypertension, are related to the maternal retinal microvasculature status after pregnancy.

METHODS

This study is part of an ongoing population-based prospective cohort study. During pregnancy and 6.2 years after the index pregnancy (90% range 5.7-7.4 years), we examined 3391 women with available information on pre-eclampsia, gestational hypertension, and retinal vascular calibers. Retinal arteriolar and venular calibers were measured in the left eye from digitized retinal photographs.

RESULTS

Women with pre-eclampsia had smaller retinal arteriolar calibers 6 years after pregnancy than women with a normotensive pregnancy (adjusted difference: -0.40 standard deviation score [SDS]; 95% confidence interval [CI]: -0.62, -0.19). For women with previous gestational hypertension, similar trends were observed (-0.20 SDS; 95% CI: -0.34, -0.05). With respect to retinal venular calibers, we did not observe consistent trends for women with previous pre-eclampsia. However, in women with previous gestational hypertension, we observed larger venular calibers (0.22 SDS; 95% CI: 0.07-0.36) than in women with a previous normotensive pregnancy. The association of gestational hypertensive disorders with retinal vessel calibers was mediated through mean arterial pressure at the time of retinal imaging.

CONCLUSIONS

Compared to women with a previous normotensive pregnancy, women with pre-eclampsia and gestational hypertension show an altered status of the microvasculature 6 years after the index pregnancy. This is reflected by smaller retinal arteriolar calibers and wider retinal venular calibers. These microvascular changes may possibly contribute to the development of cardiovascular disease in later life.

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.

Acute and Chronic Hyperglycemia Elicit JIP1/JNK-Mediated Endothelial Vasodilator Dysfunction of Retinal Arterioles

Purpose

Hyperglycemia, a hallmark of diabetes mellitus, is associated with retinal inflammation and impairment of endothelium-dependent nitric oxide (NO)–mediated dilation of retinal arterioles. However, molecular mechanisms involved in this diminished endothelial vasodilator function remain unclear. We examined whether inflammatory stress-activated kinases, c-Jun N-terminal kinase (JNK) and p38, contribute to retinal arteriolar dysfunction during exposure to acute and chronic hyperglycemia.

Methods

Retinal arterioles were isolated from streptozocin-induced diabetic pigs (2 weeks; chronic hyperglycemia, 471 ± 23 mg/dL) or age-matched control pigs (euglycemia, 79 ± 5 mg/dL), and then cannulated and pressurized for vasoreactivity study. For acute hyperglycemia study, vessels from nondiabetic pigs were exposed intraluminally to high glucose (25 mM ≈ 450 mg/dL) for 2 hours, and normal glucose (5 mM ≈ 90 mg/dL) served as the control.

Results

Endothelium-dependent vasodilation to bradykinin was reduced in a similar manner after exposure to acute or chronic hyperglycemia. Administration of NO synthase inhibitor N^G-nitro-L-arginine methyl ester (L-NAME) nearly abolished vasodilations either in control (euglycemia and normal glucose) or hyperglycemic (acute and chronic) vessels. Treatment of either acute or chronic hyperglycemic vessels with JNK inhibitor SP600125 or JNK-interacting protein-1 (JIP1) inhibitor BI-78D3, but not p38 inhibitor SB203580, preserved bradykinin-induced dilation in an L-NAME–sensitive manner. By contrast, endothelium-independent vasodilation to sodium nitroprusside was unaffected by acute or chronic hyperglycemia.

Conclusions

Activation of JIP1/JNK signaling in retinal arterioles during exposure to acute or chronic hyperglycemia leads to selective impairment of endothelium-dependent NO-mediated dilation. Therapeutic targeting of the vascular JNK pathway may improve retinal endothelial vasodilator function during early diabetes.

Retinal vessel diameters and reactivity in diabetes mellitus and/or cardiovascular disease

Background

Retinal vessel calibre and vascular dilation/constriction in response to flicker light provocation may provide a measure distinguishing patients suffering from diabetes mellitus and/or cardiovascular disease.

Methods

One hundred and sixteen age and sex matched patients with diabetes mellitus (DM), cardiovascular disease (CVD) and both DM and CVD (DM + CVD) underwent systemic and intraocular pressure measurements. Retinal vessel calibres were assessed using a validated computer-based program to compute central retinal artery and vein equivalents (CRVE) from monochromatic retinal images. Vessel dilation and constriction responses to flicker light provocation were assessed by continuous retinal vessel diameter recordings. Plasma endothelial markers von Willebrand factor (vWf) and soluble E selectin (sEsel) were measured by ELISA.

Results

Retinal vessel calibres were comparable across groups but CRVE correlated significantly with disease duration in DM patients (r = 0.57, p < 0.001). Patients suffering DM only exhibited reduced arterial vasomotion at rest and reduced arterial constriction following flicker light induced vessel dilation compared to patients with CVD and those suffering both CVD + DM (p = 0.030). Patients suffering from CVD + DM exhibited significant differences between each flicker cycle in regards to arterial maximum constriction (p = 0.006) and time needed to reach arterial maximum dilation (p = 0.004), whereas the other two groups did not show such inconsistencies between individual flicker cycles. vWf was raised in CVD + DM compared to the other two groups (p ≤ 0.02), whilst sEsel was raised in CVD + DM compared to DM alone (p = 0.044).

Conclusions

Dynamic retinal vascular calibres as obtained by continuous diameter measurements using flicker light provocation can reveal subtle differences between groups suffering from CVD with and without DM. This difference in reaction pattern and lack of arterial constriction in DM may provide a suitable marker to monitor progression.

Retinal vascular changes after vitrectomy for idiopathic epiretinal membrane: a pilot study with dynamic vessel analysis

PURPOSE

To investigate, using the Dynamic Vessel Analyzer (DVA), the retinal vascular changes that may occur after vitrectomy for idiopathic epiretinal membrane (ERM).

METHODS

Twenty-six eyes of 13 patients affected by unilateral idiopathic ERM were prospectively analyzed. 13 fellow eyes were used as control. The static (central retinal artery and vein equivalents) and dynamic (after flicker light stimulus) DVA analysis was performed at baseline (1 day before surgery) and 6 months after vitrectomy.

RESULTS

The static DVA analysis did not highlight any significant change between investigational eyes and controls at baseline and 6 months after surgery. The DVA dynamic analysis showed similar arterial flicker response between cases and controls at baseline (p = 0.3396), but disclosed a significant reduction of the arterial flicker response after surgery in the study eyes compared to fellow eyes (p = 0.0024). No significant changes were appreciated in the venous flicker response after surgery between cases and controls, both at baseline (p = 0.3450) and at the follow-up examination (p = 0.4214).

CONCLUSIONS

The physiological flicker-induced vasoconstriction is reduced after vitrectomy in arteries. The oxygen saturation change occurring after vitrectomy might have a role in the vascular tone modification.

Retinal vascular and structural changes are associated with amyloid burden in the elderly: ophthalmic biomarkers of preclinical Alzheimer's disease

BACKGROUND

Retinal imaging may serve as an alternative approach to monitor brain pathology in Alzheimer's disease (AD). In this study, we investigated the association between retinal vascular and structural changes and cerebral amyloid-β (Aβ) plaque load in an elderly cohort.

METHODS

We studied a total of 101 participants, including 73 elderly subjects (79 ± 5 years, 22 male) with no clinical diagnosis of AD but reporting some subjective memory change and an additional 28 subjects (70 ± 9 years, 16 male) with clinically established AD. Following a complete dilated ocular examination, the amplitude of retinal vascular pulsations and dynamic response, retinal nerve fibre layer thickness and retinal ganglion cell layer (RGCL) thickness were determined in all patients. Systemic blood pressure and carotid-to-femoral pulse wave velocity were measured. The elderly cohort also underwent magnetic resonance imaging and ¹⁸F-florbetaben (FBB)-positron emission tomographic amyloid imaging to measure neocortical Aβ standardised uptake value ratio (SUVR), and this was used to characterise a 'preclinical' group (SUVR >1.4).

RESULTS

The mean FBB neocortical SUVR was 1.35 ± 0.3. The amplitude of retinal venous pulsations correlated negatively with the neocortical Aβ scores (p < 0.001), whereas the amplitude of retinal arterial pulsations correlated positively with neocortical Aβ scores (p < 0.01). RGCL thickness was significantly lower in the clinical AD group (p < 0.05).

CONCLUSIONS

The correlation between retinal vascular changes and Aβ plaque load supports the possibility of a vascular component to AD. Dynamic retinal vascular parameters may provide an additional inexpensive tool to aid in the preclinical assessment of AD.

Prediabetes and Type 2 Diabetes Are Associated With Generalized Microvascular Dysfunction

Background:

Type 2 diabetes (T2DM) is associated with an increased risk of cardiovascular disease. This can be partly explained by large-artery dysfunction, which already occurs in prediabetes (“ticking clock hypothesis”). Whether a similar phenomenon also applies to microvascular dysfunction is not known. We therefore tested the hypothesis that microvascular dysfunction is already present in prediabetes and is more severe in T2DM. To do so, we investigated the associations of prediabetes, T2DM, and measures of hyperglycemia with microvascular function measured as flicker light-induced retinal arteriolar dilation and heat-induced skin hyperemia.

Methods:

In the Maastricht Study, a T2DM-enriched population-based cohort study (n=2213, 51% men, aged [mean±standard deviation] 59.7±8.2 years), we determined flicker light-induced retinal arteriolar %-dilation (Dynamic Vessel Analyzer), heat-induced skin %-hyperemia (laser-Doppler flowmetry), and glucose metabolism status (oral glucose tolerance test; normal glucose metabolism [n=1269], prediabetes [n=335], or T2DM [n=609]). Differences were assessed with multivariable regression analyses adjusted for age, sex, body mass index, smoking, physical activity, systolic blood pressure, lipid profile, retinopathy, estimated glomerular filtration rate, (micro)albuminuria, the use of lipid-modifying and blood pressure-lowering medication, and prior cardiovascular disease.

Results:

Retinal arteriolar %-dilation was (mean±standard deviation) 3.4±2.8 in normal glucose metabolism, 3.0±2.7 in prediabetes, and 2.3±2.6 in T2DM. Adjusted analyses showed a lower arteriolar %-dilation in prediabetes (B=–0.20, 95% confidence interval –0.56 to 0.15) with further deterioration in T2DM (B=–0.61 [–0.97 to –0.25]) versus normal glucose metabolism ( P for trend=0.001). Skin %-hyperemia was (mean±standard deviation) 1235±810 in normal glucose metabolism, 1109±748 in prediabetes, and 937±683 in T2DM. Adjusted analyses showed a lower %-hyperemia in prediabetes (B=–46 [–163 to 72]) with further deterioration in T2DM (B=–184 [–297 to –71]) versus normal glucose metabolism ( P for trend=0.001). In addition, higher glycohemoglobin A1c and fasting plasma glucose were associated with lower retinal arteriolar %-dilation and skin %-hyperemia in fully adjusted models (for glycohemoglobin A1c, standardized B=–0.10 [–0.15 to –0.05], P <0.001 and standardized B=–0.13 [–0.19 to –0.07], P <0.001, respectively; for fasting plasma glucose, standardized B=–0.09 [–0.15 to –0.04], P <0.001 and standardized B=–0.10 [–0.15 to –0.04], P =0.002, respectively).

Conclusion:

Prediabetes, T2DM, and measures of hyperglycemia are independently associated with impaired microvascular function in the retina and skin. These findings support the concept that microvascular dysfunction precedes and thus may contribute to T2DM-associated cardiovascular disease and other complications, which may in part have a microvascular origin such as impaired cognition and heart failure.

Impact of combined hormonal contraceptives on vessels functionality

PURPOSE

To evaluate the dynamic and static retinal vascular functionality in young females using combined hormonal contraceptive (CHC).

METHODS

Thirty-eight consecutive young female subjects were enrolled in this study between January 2015 and December 2015. Subjects were divided in two groups: CHC group, defined as CHC use for ≥6 months, and control group, defined as no current and prior CHC use. Participants underwent a dynamic and static retinal vessel analysis using the Dynamic Vessel Analyzer (DVA, Imedos, Jena, Germany).

RESULTS

Seventeen subjects continuously took CHC for 54.6 ± 29.3 months, while 21 subjects belonged to control group. No difference was found between the CHC and control groups for age (p = 0.1), smoking status (p = 0.6), and systolic (p = 0.3) and diastolic (p = 0.1) blood pressure. With regard to dynamic analysis, women taking CHC exhibited a marked significant vasoconstriction following flicker stimulation in comparison with control group (-2.43 ± 2.5 vs 0.63 ± 2.1, respectively; p = 0.0002). No significant difference was observed between groups for mean arterial (p = 0.2) and venous dilatations (p = 0.3), arteriovenous ratio (p = 0.09), central retinal artery equivalent (p = 0.4), and central retinal venous equivalent (p = 0.5).

CONCLUSIONS

CHC may affect vessel reactivity to flicker light by increasing arteries constriction. This may reflect systemic changes in vascular functionality in subjects using CHC. Moreover, CHC should be considered as a confounding bias in studies involving DVA.

Therapeutic targeting of diabetic retinal neuropathy as a strategy in preventing diabetic retinopathy

Diabetes causes a panretinal neurodegeneration herein termed diabetic retinal neuropathy, which manifests in the retina early and progresses throughout the disease. Clinical manifestations include changes in the ERG, perimetry, dark adaptation, contrast sensitivity and colour vision which correlate with laboratory findings of thinning of the retinal neuronal layers, increased apoptosis in neurons and activation of glial cells. Possible mechanisms include oxidative stress, neuronal AGE accumulation, altered balance of neurotrophic factors and loss of mitohormesis. Retinal neural damage precedes and is a biologically plausible cause of retinal vasculopathy later in diabetes, and this review suggests that strategies to target it directly could prevent diabetes induced blindness. The efficacy of fenofibrate in reducing retinopathy progression provides a possible proof of concept for this approach. Strategies which may target diabetic retinal neuropathy include reducing retinal metabolic demand, improving mitochondrial function with AMPK and Sirt1 activators or providing neurotrophic support with neurotrophic supplementation.

The relationship of systemic markers of renal function and vascular function with retinal blood vessel responses

PURPOSE

To test the hypothesis of a significant relationship between systemic markers of renal and vascular function (processes linked to cardiovascular disease and its development) and retinal microvascular function in diabetes and/or cardiovascular disease.

METHODS

Ocular microcirculatory function was measured in 116 patients with diabetes and/or cardiovascular disease using static and continuous retinal vessel responses to three cycles of flickering light. Endothelial function was evaluated by von Willebrand factor (vWf), endothelial microparticles and soluble E selectin, renal function by serum creatinine, creatinine clearance and estimated glomerular filtration rate (eGFR). HbA1c was used as a control index.

RESULTS

Central retinal vein equivalence and venous maximum dilation to flicker were linked to HbA1c (both p < 0.05). Arterial reaction time was linked to serum creatinine (p = 0.036) and eGFR (p = 0.039); venous reaction time was linked to creatinine clearance (p = 0.018). Creatinine clearance and eGFR were linked to arterial maximum dilatation (p < 0.001 and p = 0.003, respectively) and the dilatation amplitude (p = 0.038 and p = 0.048, respectively) responses in the third flicker cycle. Of venous responses to the first flicker cycle, HbA1c was linked to the maximum dilation response (p = 0.004) and dilatation amplitude (p = 0.017), vWf was linked to the maximum constriction response (p = 0.016), and creatinine clearance to the baseline diameter fluctuation (p = 0.029). In the second flicker cycle, dilatation amplitude was linked to serum creatinine (p = 0.022).

CONCLUSIONS

Several retinal blood vessel responses to flickering light are linked to glycaemia and renal function, but only one index is linked to endothelial function. Renal function must be considered when interpreting retinal vessel responses.

The diameter response of retinal arterioles in diabetic maculopathy is reduced during hypoxia and is unaffected by the inhibition of cyclo-oxygenase and nitric oxide synthesis

PURPOSE

Diabetic retinopathy is accompanied with changes in the diameter regulation and oxygenation of retinal vessels. Previous studies have shown that in normal persons and in diabetic patients without retinopathy hypoxia-induced vasodilatation is mediated by cyclo-oxygenase (COX) products and nitric oxide (NO). The purpose of the present study was to study whether these effects can be reproduced in patients with diabetic maculopathy.

METHODS

Eighteen patients with diabetic maculopathy aged 29-57 years were examined using the Dynamic Vessel Analyzer. The resting diameter and the diameter changes of retinal arterioles during isometric exercise and flicker stimulation were studied before and during breathing a hypoxic gas mixture. The examinations were also performed before and during intravenous infusion of the NOS inhibitor L-NMMA, and were repeated on a second day after topical administration of the COX-inhibitor diclofenac.

RESULTS

The diameter of retinal arterioles showed no significant change during hypoxia or L-NMMA infusion, or after topical application of diclofenac (p > 0.25 for all comparisons). The resting diameter of the venules was significantly increased during hypoxia (p = 0.003) and decreased during L-NMMA infusion (p < 0.0001). The diameter of retinal venules during isometric exercise increased significantly during hypoxia (p = 0.01). Flicker stimulation induced significant dilatation of the venules, which was significantly reduced during hypoxia and increased during L-NMMA infusion (p < 0.0001 for all comparisons).

CONCLUSION

Hypoxia-induced dilatation of retinal arterioles is severely reduced in patients with diabetic maculopathy. Future intervention studies aimed at normalizing the diameter regulation of retinal arterioles in diabetic patients should preferentially be conducted in the early stages of the disease where the potential for changing the vessel diameter is preserved. ClinicalTrials.gov identifier: NCT01689090.

Preserved Pressure Autoregulation but Disturbed Cyclo-Oxygenase and Nitric Oxide Effects on Retinal Arterioles during Acute Hypoxia in Diabetic Patients without Retinopathy

BACKGROUND

Acute hypoxia induces retinal vasodilatation, which depends on cyclooxygenase (COX) products and nitric oxide (NO) in vitro. However, it is unknown whether these mechanisms are active in diabetic patients and may contribute to the development of diabetic retinopathy.

METHODS

The Dynamic Vessel Analyzer was used to study the diameter regulation in retinal vessels during hypoxia in type 1 diabetic patients without retinopathy. The influence of NO and COX synthesis inhibition on the diameter of larger retinal vessels was studied during hypoxia, during isometric exercise and during flicker stimulation.

RESULTS

Increased arterial blood pressure during L-NMMA infusion and isometric exercise were paralleled by constriction of the retinal arterioles suggesting normal pressure autoregulation. Hypoxia significantly reduced the diameter responses during isometric exercise and during flicker stimulation as compared to normal persons.

CONCLUSION

The findings support that changes in metabolic autoregulation develop before changes in pressure autoregulation in diabetic patients.

Lack of effect of nitroglycerin on the diameter response of larger retinal arterioles in normal persons during hypoxia

PURPOSE

Retinal hypoxia with consequent changes in blood flow play a role in a number of vision-threatening diseases, such as diabetic retinopathy. Previous studies have shown that the inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX) products are involved in the diameter regulation of the retinal vessels during hypoxia. Therefore, the aim of the present study was to examine the effects of an NO donor combined with COX inhibition on the diameter regulation of retinal vessels during hypoxia in normal persons.

METHODS

Twenty normal persons aged 21-47 years were examined. The Dynamic Vessel Analyzer (DVA) was used to measure retinal vessel diameters at rest, during isometric exercise, and during flicker stimulation. The measurements were performed during normoxia and hypoxia before and after sublingual administration of the NO donor nitroglycerin, and were repeated on a second study day after topical administration of the COX-inhibitor diclofenac.

RESULTS

The resting diameter of arterioles and venules increased significantly during hypoxia (p < 0.0001). Hypoxia also significantly reduced the arteriolar constriction during isometric exercise, and the dilatation of the arterioles and venules during flicker stimulation (p < 0.0001). Diclofenac further reduced the arteriolar constriction induced by isometric exercise during hypoxia (p = 0.005). However, the NO-donor nitroglycerin had no effect on vascular diameters.

CONCLUSION

Diameter regulation of retinal vessels during hypoxia in normal persons can be influenced by the inhibition of COX products, but not by increasing the NO concentration. The findings suggest that the vasoactive effects of NO on retinal arterioles during hypoxia are saturated in normal persons.

VEGF Receptor-2-Linked PI3K/Calpain/SIRT1 Activation Mediates Retinal Arteriolar Dilations to VEGF and Shear Stress

PURPOSE

Vasomotor responses of retinal arterioles to luminal flow/shear stress and VEGF have a critical role in governing retinal blood flow possibly via nitric oxide synthase (NOS) activation. However, the cellular mechanism for flow-sensitive vasomotor activity in relation to VEGF signaling in retinal arterioles has not been characterized. We used an isolated vessel approach to specifically address this issue.

METHODS

Porcine retinal arterioles were isolated, cannulated, and pressurized to 55 cm H2O luminal pressure by two independent reservoir systems. Luminal flow was increased stepwise by creating hydrostatic pressure gradients across two reservoirs. Diameter changes and associated signaling mechanisms corresponding to increased flow and VEGF receptor 2 (VEGFR2) activation were assessed using videomicroscopic, pharmacological, and molecular tools.

RESULTS

Retinal arterioles developed basal tone under zero-flow condition and dilated concentration-dependently to VEGF165. Stepwise increases in flow produced graded vasodilation. Vasodilations to VEGF165 and increased flow were abolished by endothelial removal, and inhibited by pharmacological blockade of VEGFR2, NOS, phosphoinositide 3-kinase (PI3K), calpains, or sirtuin-1 (SIRT1) deacetylase. A VEGF165 antibody blocked vasodilation to VEGF165 but not flow. Immunostaining indicated that VEGFR2 was expressed in the endothelial and smooth muscle layers of retinal arterioles.

CONCLUSIONS

Ligand-dependent and ligand-independent activation of VEGFR2 in the endothelium mediates NO-dependent dilations of porcine retinal arterioles in response to VEGF165 and luminal flow/shear stress, respectively. It appears that NOS stimulation via PI3K, calpain proteases, and SIRT1-dependent deacetylation downstream from VEGFR2 activation contributes to these vasodilator responses.

Insulin is a key determinant of elevated retinal arteriolar flicker response in insulin-resistant individuals

AIMS/HYPOTHESIS

Insulin may link metabolic disorders to retinal microvascular pathology. The aim of the present study was to investigate the impact of early insulin resistance on retinal microcirculation.

METHODS

Retinal diameter responses to flicker-light stimulation were investigated in 81 clinically healthy participants (32 ± 6 years [mean ± SD], 59% women) who were recruited according to their BMI. All participants underwent an OGTT and euglycaemic-hyperinsulinaemic clamp (40 mU/m(2) · min(-1) insulin dose). After stratification by low and high insulin sensitivity based on a clamp-derived glucose disposal rate of ≤ or >4.9 mg/kg body mass, respectively, baseline retinal diameters and their relative changes to flicker stimulation were compared while controlling for mean arterial pressure, BMI and sex.

RESULTS

The arterial vasodilator response at the end of flicker stimulation (p = 0.044) and the area under the arterial reaction curve during flicker stimulation (p = 0.015) were significantly higher in individuals with low vs high insulin sensitivity. Vasodilatory responses of retinal veins to flicker stimulation and baseline retinal diameters did not differ between insulin-sensitive and insulin-resistant participants (p > 0.05). In a stepwise linear regression analysis, fasting insulin remained the only predictor of the arterial vasodilator response to flicker-light (p < 0.01). Waist circumference also contributed, although to a lesser extent, to the arterial vasodilator response (p = 0.023).

CONCLUSIONS/INTERPRETATION

Insulin sensitivity is an important determinant of retinal microvascular function. We propose that the elevated arterial flicker response in insulin-resistant states is a result of higher circulating insulin levels.

Hypoxia-induced relaxation of porcine retinal arterioles in vitro depends on inducible NO synthase and EP4 receptor stimulation in the perivascular retina

PURPOSE

Hypoxia-induced relaxation of porcine retinal arterioles has been shown to be reduced during inhibition of prostaglandin synthesis and nitric oxide synthase (NOS). The purpose of this study was to identity the specific prostaglandin receptor(s) and source(s) of NO mediating this effect.

METHODS

Porcine retinal arterioles with preserved perivascular retinal tissue were mounted in a myograph and were exposed to hypoxia in the presence of one of the following: the general NO synthase inhibitor L-NAME, the selective iNOS inhibitor 1400W, the selective nNOS inhibitor 7-nitroindazole, the general cyclooxygenase (COX) inhibitor ibuprofen or an antagonist to the FP- (AL 8810), DP- (BWA868C), EP1 - (SC-19220), EP2 - (PF-044189) or EP4 receptors (GW627368X). The experiments were repeated after removal of the perivascular retinal tissue.

RESULTS

Hypoxia induced relaxation of retinal arterioles with preserved perivascular retinal tissue. This relaxation was significantly reduced in the presence of L-NAME, 1400W, ibuprofen and the EP4 receptor antagonist GW627368X. The simultaneous addition of L-NAME or 1400W in combination with ibuprofen, but not GW627368X, reduced hypoxia-induced vasorelaxation additively as compared to the effect of the compounds individually.

CONCLUSION

Hypoxia-induced vasorelaxation of porcine retinal arterioles is mediated by inducible NOS and stimulation of EP4 receptors acting through separate pathways, but mechanisms unrelated to the studied prostaglandin receptors and NOS products are also involved.

Neuronal activity-dependent regulation of retinal blood flow

Blood flow in the retina is intrinsically regulated to meet the metabolic demands of its constituent cells. Flickering light or stationary contrast reversals induce an increase in blood flow within seconds of the stimulus onset. This phenomenon is thought to compensate for an increase in ganglion cell activity and energy consumption. Ganglion cell activity is in turn dependent on signals from photoreceptors, bipolar cells, horizontal cells and amacrine cells. The physiological properties of these neurons determine how each type is affected by a particular light characteristic. Neuronal activity then triggers the release of signalling molecules that dilate local blood vessels and increase blood flow. Nitric oxide has been implicated as an important mediator, but metabolites of arachidonic acid may also be involved. Detailed elucidation of these mechanisms, together with advances in imaging technology, may facilitate the use of neurovascular tests to improve the detection of retinal damage in pathological conditions.

Systemic circulatory influences on retinal microvascular function in middle-age individuals with low to moderate cardiovascular risk

PURPOSE

To investigate the relationship between retinal microvascular reactivity, circulatory markers for CVD risk and systemic antioxidative defence capacity in healthy middle-aged individuals with low to moderate risk of CVD.

METHODS

Retinal vascular reactivity to flickering light was assessed in 102 healthy participants (46-60 years) by means of dynamic retinal vessel analysis (DVA). Other vascular assessments included carotid intima-media thickness (C-IMT) and blood pressure (BP) measurements. Total cholesterol (CHOL), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG) and blood glutathione levels in its reduced (GSH) and oxidized (GSSG) forms were also determined for each participant, along with Framingham risk scores (FRS).

RESULTS

Retinal arterial baseline diameter fluctuation (BDF) was independently, significantly and negatively influenced by LDL-C levels (β = -0.53, p = 0.027). Moreover, the arterial dilation slope (SlopeAD ) was independently, significantly and positively associated with redox index (GSH: GSSG ratio, β = 0.28, p = 0.016), while the arterial constriction slope (SlopeAC ) was significantly and negatively influenced by blood GSH levels (β = -0.20, p = 0.042), and positively associated with FRS (β = 0.25, p = 0.009). Venous BDF and dilation amplitude (DA) were also negatively influenced by plasma LDL-C levels (β = -0.83, p = 0.013; and β = -0.22, p = 0.028, respectively).

CONCLUSIONS

In otherwise healthy individuals with low to moderate cardiovascular risk, retinal microvascular dilation and constriction responses to stress levels are influenced by systemic antioxidant capacity, and circulating markers for cardiovascular risk.