Noninvasive measurement of the Bayliss effect in retinal autoregulation

Physiological influences on neurovascular coupling: A systematic review of multimodal imaging approaches and recommendations for future study designs

In this review, we have amalgamated the literature, taking a multimodal neuroimaging approach to quantify the relationship between neuronal firing and haemodynamics during a task paradigm (i.e., neurovascular coupling response), while considering confounding physiological influences. Original research articles that used concurrent neuronal and haemodynamic quantification in humans (n ≥ 10) during a task paradigm were included from PubMed, Scopus, Web of Science, EMBASE and PsychINFO. Articles published before 31 July 2023 were considered for eligibility. Rapid screening was completed by the first author. Two authors completed the title/abstract and full-text screening. Article quality was assessed using a modified version of the National Institutes of Health Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies. A total of 364 articles were included following title/abstract and full-text screening. The most common combination was EEG/functional MRI (68.7%), with cognitive (48.1%) and visual (27.5%) tasks being the most common. The majority of studies displayed an absence/minimal control of blood pressure, arterial gas concentrations and/or heart rate (92.9%), and only 1.3% monitored these factors. A minority of studies restricted or collected data pertaining to caffeine (7.4%), exercise (0.8%), food (0.5%), nicotine (2.7%), the menstrual cycle (0.3%) or cardiorespiratory fitness levels (0.5%). The cerebrovasculature is sensitive to numerous factors; thus, to understand the neurovascular coupling response fully, better control for confounding physiological influences of blood pressure and respiratory metrics is imperative during study-design formulation. Moreover, further work should continue to examine sex-based differences, the influence of sex steroid hormone concentrations and cardiorespiratory fitness.

Impaired retinal oxygen metabolism and perfusion are accompanied by plasma protein and lipid alterations in recovered COVID-19 patients

The aim of the present study was to investigate retinal microcirculatory and functional metabolic changes in patients after they had recovered from a moderate to severe acute COVID-19 infection. Retinal perfusion was quantified using laser speckle flowgraphy. Oxygen saturation and retinal calibers were assessed with a dynamic vessel analyzer. Arterio-venous ratio (AVR) was calculated based on retinal vessel diameter data. Blood plasma samples underwent mass spectrometry-based multi-omics profiling, including proteomics, metabolomics and eicosadomics. A total of 40 subjects were included in the present study, of which 29 had recovered from moderate to severe COVID-19 within 2 to 23 weeks before inclusion and 11 had never had COVID-19, as confirmed by antibody testing. Perfusion in retinal vessels was significantly lower in patients (60.6 ± 16.0 a.u.) than in control subjects (76.2 ± 12.1 a.u., p = 0.006). Arterio-venous (AV) difference in oxygen saturation and AVR was significantly lower in patients compared to healthy controls (p = 0.021 for AVR and p = 0.023 for AV difference in oxygen saturation). Molecular profiles demonstrated down-regulation of cell adhesion molecules, NOTCH3 and fatty acids, and suggested a bisphasic dysregulation of nitric oxide synthesis after COVID-19 infection. The results of this study imply that retinal perfusion and oxygen metabolism is still significantly altered in patients well beyond the acute phase of COVID-19. This is also reflected in the molecular profiling analysis of blood plasma, indicating a down-regulation of nitric oxide-related endothelial and immunological cell functions.Trial Registration: ClinicalTrials.gov ( https://clinicaltrials.gov ) NCT05650905.

Chemo-mechanical modeling of smooth muscle cell activation for the simulation of arterial walls under changing blood pressure

In this paper, a novel chemo-mechanical model is proposed for the description of the stretch-dependent chemical processes known as Bayliss effect and their impact on the active contraction in vascular smooth muscle. These processes are responsible for the adaptive reaction of arterial walls to changing blood pressure by which the blood vessels actively support the heart in providing sufficient blood supply for varying demands in the supplied tissues. The model is designed to describe two different stretch-dependent mechanisms observed in smooth muscle cells (SMCs): a calcium-dependent and a calcium-independent contraction. For the first one, stretch of the SMCs leads to an inlet of calcium ions which activates the myosin light chain kinase (MLCK). The increased activity of MLCK triggers the contractile units of the cells resulting in the contraction on a comparatively short time scale. For the calcium-independent contraction mechanism, stretch-dependent receptors of the cell membrane stimulate an intracellular reaction leading to an inhibition of the antagonist of MLCK, the myosin light chain phosphatase resulting in a contraction on a comparatively long time scale. An algorithmic framework for the implementation of the model in finite element programs is derived. Based thereon, it is shown that the proposed approach agrees well with experimental data. Furthermore, the individual aspects of the model are analyzed in numerical simulations of idealized arteries subject to internal pressure waves with changing intensities. The simulations show that the proposed model is able to describe the experimentally observed contraction of the artery as a reaction to increased internal pressure, which can be considered a crucial aspect of the regulatory mechanism of muscular arteries.

Normal tension glaucoma: A dynamic optical coherence tomography angiography study

Purpose

Vascular dysregulation seems to play a role in the pathogenesis of glaucoma, in particular normal tension glaucoma (NTG). The development of optical coherence tomography angiography (OCTA) enabled the measurement of the retinal microvasculature non-invasively and with high repeatability. Nonetheless, only a few studies transformed OCTA into a dynamic examination employing a sympathomimetic stimulus. The goal of this study was to use this dynamic OCTA exam (1) to differentiate healthy individuals from glaucoma patients and (2) to distinguish glaucoma subcategories, NTG and high-tension primary open angle glaucoma (POAG).

Methods

Retinal vessel density (VD) in NTG patients (n = 16), POAG patients (n = 12), and healthy controls (n = 14) was compared before and during a hand grip test with a hydraulic dynamometer.

Results

At baseline, mean peripapillary VD was lower in POAG and NTG (42.6 and 48.5%) compared to healthy controls (58.1%; p < 0.001) and higher in NTG compared to POAG (p = 0.024) when corrected for mean arterial pressure (MAP). Peripapillary and macular (superficial and deep) VD differences were found for gender, age, and baseline MAP. No change in VD occurred (pre-/post-stimulus) in any of the groups.

Conclusion

Retinal VD loss in glaucoma patients was confirmed and the necessity to correct for gender, age and especially MAP was established. Although replication in a larger population is necessary, OCTA might not be the most suitable method to dynamically evaluate the retinal microvasculature.

Neuro-vascular coupling and heart rate variability in patients with type II diabetes at different stages of diabetic retinopathy

Aims/Hypothesis

There is evidence that diabetes is accompanied by a break-down of functional hyperemia, an intrinsic mechanism of neural tissues to adapt blood flow to changing metabolic demands. However, to what extent functional hyperemia is altered in different stages of diabetic retinopathy (DR) in patients with type II diabetes is largely unknown. The current study set out to investigate flicker-induced retinal blood flow changes in patients with type II diabetes at different stages of DR.

Materials and methods

A total of 76 subjects were included in the present parallel-group study, of which 56 had diabetes with either no DR or different stages of non-proliferative DR (n = 29 no DR, 12 mild DR, 15 moderate to severe DR). In addition, 20 healthy subjects were included as controls. Retinal blood flow was assessed before and during visual stimulation using a combined measurement of retinal vessel calibers and blood velocity by the means of Doppler optical coherence tomography (OCT). To measure systemic autonomic nervous system function, heart rate variability (HRV) was assessed using a short-term orthostatic challenge test.

Results

In healthy controls, retinal blood flow increased by 40.4 ± 27.2% during flicker stimulation. Flicker responses in patients with DR were significantly decreased depending on the stage of the disease (no DR 37.7 ± 26.0%, mild DR 26.2 ± 28.2%, moderate to severe DR 22.3 ± 13.9%; p = 0.035, ANOVA). When assessing systemic autonomous neural function using HRV, normalized low frequency (LF) spectral power showed a significantly different response to the orthostatic maneuver in diabetic patients compared to healthy controls (p < 0.001).

Conclusion/Interpretation

Our study indicates that flicker induced hyperemia is reduced in patients with DR compared to healthy subjects. Further, this impairment is more pronounced with increasing severity of DR. Further studies are needed to elucidate mechanisms behind the reduced hyperemic response in patients with type II diabetes.

Clinical trial registration

[https://clinicaltrials.gov/], identifier [NCT03 552562].

The Influence of Coronary Heart Disease on Retinal Electrophysiological Examination (Full-field, Pattern and Multifocal Electroretinograms)

PURPOSE

To evaluate the influence of coronary heart disease (CHD) on retinal function using a battery of electrophysiological measures.

MATERIALS AND METHODS

We conducted a prospective pilot study comparing 34 patients with a confirmed diagnosis of CHD with 21 healthy participants. Further inclusion criteria were a decimal visual acuity (VA) of 0.8 or better and patient age between 40 and 80 years. All participants were divided into three groups according to the severity of CHD (1, 2 or 3 vessels involved) and one healthy control group. Testing was performed on one eye per patient, either selecting the eye with higher VA or, when equal, selecting randomly. The test procedure consisted of a pattern electroretinogram (ERG), a full-field ERG, a multifocal ERG and an ophthalmic screening examination.

RESULTS

Implicit times of the b-wave measured using scotopic full-field ERG were significantly prolonged in all CHD patient groups (p < .000). Thus, full-field ERG allowed clinicians to differentiate between healthy patients and those suffering from CHD. The multifocal ERG showed significantly different results concerning the amplitude density (p < .008) in each patient group compared with the control group. CHD had a significant impact on cone-pathway function, although the severity of CHD did not correlate with functional deficiencies of cone cells.

CONCLUSIONS

Confirmed coronary vascular diseases are correlated with macular cone and bipolar cell function, which can be detected by measuring electrophysiological retinal signals.

Changes in Choroidal Component Ratio and Circulation After Coffee Intake in Healthy Subjects

Purpose

The effects of coffee intake on the ratio of stromal and luminal components in the choroid and the underlying mechanism remain unclear. This prospective cross-sectional study aimed to explore how coffee intake affects the choroidal component ratio and circulation.

Methods

Forty-nine right eyes of healthy adult volunteers were evaluated as the coffee intake group. Thirty-two right eyes of healthy volunteers served as the control group. The participants consumed 185 mL of coffee or water, respectively, and the systemic hemodynamics, enhanced-depth imaging optical coherence tomographic (EDI-OCT) images, and foveal mean blur rate (MBR), an indicator of blood flow velocity, were recorded at baseline and after coffee or water intake. The EDI-OCT images were binarized using ImageJ software, and subfoveal choroidal thickness (SCT) and whole, luminal, and stromal choroidal areas were calculated.

Results

In the coffee intake group, significant decreases in SCT and luminal area peaked at 60 minutes after intake (both P < 0.001), whereas a significant increase in MBR peaked at 30 minutes (P < 0.001). No significant stromal area fluctuations were observed. SCT and luminal area fluctuations exhibited a significant positive correlation (r = 0.978, P < 0.001). Significant negative correlations of luminal area fluctuations with MBR fluctuations were observed by stepwise regression analysis (r = -0.220, P < 0.001). The control group exhibited no significant fluctuations.

Conclusions

Coffee-induced choroidal thinning may result mainly from a reduction in the choroidal vessel lumen, and this vessel lumen reduction correlated with an increased choroidal blood flow velocity after coffee intake. These coffee-induced changes in choroidal component ratio and circulation should be considered when evaluating choroids.

Retinal Vascular Reactivity in Type 1 Diabetes Patients Without Retinopathy Using Optical Coherence Tomography Angiography

Purpose

We hypothesize that patients with type 1 diabetes (T1D) may have abnormal retinal vascular responses before diabetic retinopathy (DR) is clinically evident. Optical coherence tomography angiography (OCTA) was used to dynamically assess the retinal microvasculature of diabetic patients with no clinically visible retinopathy.

Methods

Controlled nonrandomized interventional study. The studied population included 48 eyes of 24 T1D patients and 24 demographically similar healthy volunteers. A commercial OCTA device (AngioVue) was used, and two tests were applied: (1) the hypoxia challenge test (HCT) and (2) the handgrip test to induce a vasodilatory or vasoconstrictive response, respectively. The HCT is a standardized test that creates a mild hypoxic environment equivalent to a flight cabin. The handgrip test (i.e., isometric exercise) induces a sympathetic autonomic response. Changes in the parafoveal superficial and deep capillary plexuses in both tests were compared in each group. Systemic cardiovascular responses were also comparatively evaluated.

Results

In the control cohort, the vessel density of the median parafoveal superficial and deep plexuses increased during hypoxia (F_1,23 = 15.69, P < 0.001 and F_1,23 = 16.26, P < 0.001, respectively). In the T1D group, this physiological response was not observed in either the superficial or the deep retinal plexuses. Isometric exercise elicited a significant decrease in vessel density in both superficial and deep plexuses in the control group (F_1,23 = 27.37, P < 0.0001 and F_1,23 = 27.90, P < 0.0001, respectively). In the T1D group, this response was noted only in the deep plexus (F_1,23 = 11.04, P < 0.01).

Conclusions

Our work suggests there is an early impairment of the physiological retinal vascular response in patients with T1D without clinical diabetic retinopathy.

Anatomical and functional changes in the retina in patients with Alzheimer's disease and mild cognitive impairment

PURPOSE

There is evidence that mild cognitive impairment (MCI) or Alzheimer's disease (AD) is accompanied by alterations in the retina. The current study was performed to investigate structural and functional changes in patients with systemic neurodegenerative disease.

METHODS

A total of 47 patients with either MCI or AD and 43 healthy age- and sex-matched control subjects were included. Inclusion criteria for MCI were abnormal memory function and a mini-mental state examination (MMSE) score >26 points, for patients with AD a diagnosis of probable AD of mild to moderate degree and an MMSE score in the range of 20-26. Retinal blood flow was measured using a Doppler optical coherence tomography (OCT) system. Retinal vessel diameter, oxygen saturation and flicker-induced vasodilatation were measured using a Vessel Analyzer. Retinal nerve fibre layer thickness (RNFLT) was assessed using an OCT system.

RESULTS

Global RNFLT was lower in patients compared to healthy controls (93.7 ± 12.8 µm versus 99.1 ± 9.0 µm, p = 0.02). The same was found in regards to retinal arterial blood flow, which was 9.3 ± 2.4 and 12.3 ± 3.2 μl/min in the patient and control groups, respectively (p < 0.001). Mean retinal arterial diameter was reduced in patients (76.0 ± 8.9 µm versus 80.6 ± 8.0 µm, p = 0.03). Arteriovenous difference in oxygen saturation was lower in patients (20.4 ± 5.1% versus 23.5 ± 4.0%, p < 0.01). No difference in the flicker response was observed.

CONCLUSION

In patients with MCI and AD, arteriovenous difference in oxygen saturation, retinal blood flow and arterial vessel diameter was reduced. No difference was found in flicker response between groups. This indicates alterations in retinal oxygen metabolism in patients with neurodegenerative disease.

Caffeine Delays Retinal Neurovascular Coupling during Dark to Light Adaptation in Healthy Eyes Revealed by Optical Coherence Tomography Angiography

Purpose

The purpose of this study was to investigate the acute effects of caffeine on retinal hemodynamics during dark to light adaptation using optical coherence tomography angiography (OCTA).

Methods

Thirteen healthy individuals (13 eyes) underwent OCTA imaging after dark adaptation and at repeated intervals during the transition to ambient light in two imaging sessions: control and after ingesting 200 mg of caffeine. We analyzed the parafoveal vessel density (VD) and adjusted flow index (AFI) of the superficial capillary plexus (SCP), middle capillary plexus (MCP), and deep capillary plexus (DCP), as well as the vessel length density (VLD) of the SCP. After adjusting for age, refractive error, and scan quality, we compared parameters between control and caffeine conditions.

Results

In the dark, MCP VD decreased significantly after caffeine (−2.63 ± 1.28%). During the transition to light, initially, DCP VD increased (12.55 ± 2.52%), whereas SCP VD decreased (−2.09 ± 0.91%) significantly with caffeine compared to control. By 15 minutes in light, DCP VD reversed and was significantly decreased (−5.45 ± 2.62%), whereas MCP VD increased (4.65 ± 1.74%). There were no differences in AFI or VLD.

Conclusions

We show that, overall, caffeine causes a trend of delayed vascular response in all three macular capillary plexuses in response to ambient light. Whereas the MCP is constricted in the dark, during the transition from dark to light, there is initially delay followed by prolonged constriction of the DCP and constriction followed by slow dilation of the SCP. We posit that these delayed vascular responses may present potential risk of capillary ischemia.

Regulation of blood flow in diabetic retinopathy

Blood flow in the retina increases in response to light-evoked neuronal activity, ensuring that retinal neurons receive an adequate supply of oxygen and nutrients as metabolic demands vary. This response, termed "functional hyperemia," is disrupted in diabetic retinopathy. The reduction in functional hyperemia may result in retinal hypoxia and contribute to the development of retinopathy. This review will discuss the neurovascular coupling signaling mechanisms that generate the functional hyperemia response in the retina, the changes to neurovascular coupling that occur in diabetic retinopathy, possible treatments for restoring functional hyperemia and retinal oxygen levels, and changes to functional hyperemia that occur in the diabetic brain.

Immediate Changes in Peripapillary Retinal Vasculature after Intraocular Pressure Elevation -an Optical Coherence Tomography Angiography Study

Purpose: To investigate changes in peripapillary retinal vessel density after acute intraocular pressure (IOP) elevation caused by laser peripheral iridotomy (LPI) in primary angle-closure suspects (PACS) by optical coherence tomography angiography (OCTA).Materials and Methods: Ninety-seven participants with PACS were included in this cross-sectional observational study. OCTA and IOP measurement were performed at baseline and 1 h after LPI. PACS eyes were further divided into three groups according to IOP increase 1 h after LPI (group 1 = IOP elevation <5 mmHg, 42eyes; group 2 = IOP elevation ≥5 mmHg and <10 mmHg, 34 eyes; group 3 = IOP elevation ≥10 mmHg, 21eyes). The changes of vessel density in radial peripapillary capillary (RPC) and entire retina were compared among groups.Results: When all eyes were included, the vessel density of RPC and entire retina 1 h after LPI were significantly decreased compared to the baseline (RPC: 64.5 ± 7.9 vs.67.8 ± 6.8, P < .001; retina: 86.3 ± 4.6 vs.88.3 ± 3.8, P < .001). There were significant differences among the three groups in the RPC and retinal vessel density at 1 h after LPI (RPC: 67.4 ± 7.3 vs. 63.2 ± 7.6 vs. 60.9 ± 7.5, P = .003; retinal: 87.7 ± 4.0 vs. 85.8 ± 4.5 vs. 84.3 ± 5.2, P = .015). In group 2 with an increased IOP from 5 mmHg to 10 mmHg, the reduction of vessel density in the RPC was more significant than that of the entire retina (RPC vs. retina: 7.1 ± 10.0% vs. 3.0 ± 4.4%, P = .006).Conclusions: LPI-induced IOP spikes resulted in a decrease in retina vessel density with PACS eyes by OCTA. The reduction of RPC vessel density was more significant than that of the entire retina in the subgroup of IOP increase from 5 to 10 mmHg. This suggests that vessel density in RPC was more sensitive to IOP increase than that of the entire retina in the peripapillary area.

Pulsed Electrical Stimulation of the Human Eye Enhances Retinal Vessel Reaction to Flickering Light

Recent studies indicate therapeutic benefits of electrical stimulation in cases of specific ophthalmic diseases that are associated with dysfunctional ocular microcirculation. This suggests effects of electrical stimulation on vascular functions. In the present study, we investigated the effects of electrical stimulation on retinal vessel reactions using dynamic vessel analysis (DVA). Eighty healthy subjects were randomly assigned to one of three groups receiving electrical stimulation with different current intensities: 400 μA (n = 26); 800 μA (n = 27); 1200 μA (n = 27). The electrode montage for electrical stimulation consisted of a ring-shaped active electrode surrounding one eye and a square return electrode at the occiput. Rectangular, monophasic, positive current pulses were applied at 10 Hz for a duration of 60 s per stimulation period. DVA was used to observe the stimulation-induced reactions of retinal vessel diameters in response to different provocations. In three DVA measurements, three stimulus conditions were investigated: flicker light stimulation (FLS); electrical stimulation (ES); simultaneous electrical and flicker light stimulation (ES+FLS). Retinal vasodilation caused by these stimuli was compared using paired t-test. The subjects receiving electrical stimulation with 800 μA showed significantly increased retinal vasodilation for ES+FLS compared to FLS (p < 0.05). No significant differences in retinal vessel reactions were found between ES+FLS and FLS in the 400 and 1200 μA groups. No retinal vasodilation was observed for ES for all investigated current intensities. The results indicate that positive pulsed electrical stimulation of an adequate intensity enhances the flicker light-induced retinal vasodilation.

Hemoglobin A1c and retinal arteriolar narrowing in children with type 1 diabetes: the diagnostics of early atherosclerosis risk in kids study

BACKGROUND/OBJECTIVE

Microvascular alterations play a key role in the development of diabetes complications. Retinal vessel analysis is a unique method to examine microvascular changes in brain-derived vessels.

METHODS

Sixty-seven pediatric and adolescent type 1 diabetes patients and 58 healthy control persons (mean age 12.4 ± 2.9 years) underwent non-mydriatic retinal photography of both eyes. Central retinal arteriolar and central retinal venular (CRVE) diameter equivalents as well as the arteriolar-to-venular ratio were calculated using a semiautomated software. All anthropometric and laboratory parameters were measured according to standardized procedures for children.

RESULTS

Retinal vessel diameter did not differ between type 1 diabetic children and healthy controls. However, there was an independent association of higher hemoglobin A1c (HbA1c) levels with arteriolar narrowing. Arteriolar narrowing of 5.4 μm was observed with each percent increase in HbA1c. Longer duration of diabetes was associated with wider retinal arterioles. CRVE was not associated with diabetes duration or HbA1c.

CONCLUSIONS

Microvascular arteriolar alterations are already present in childhood and may indicate subclinical atherosclerosis and increased risk of diabetes complications later in life. Future research will have to investigate the potential use of retinal vessel diameters for treatment monitoring and guidance of therapy in children.

A Protocol to Evaluate Retinal Vascular Response Using Optical Coherence Tomography Angiography

Introduction

Optical coherence tomography angiography (OCT-A) is a novel diagnostic tool with increasing applications in ophthalmology clinics that provides non-invasive high-resolution imaging of the retinal microvasculature. Our aim is to report in detail an experimental protocol for analyzing both vasodilatory and vasoconstriction retinal vascular responses with the available OCT-A technology.

Methods

A commercial OCT-A device was used (AngioVue^®, Optovue, CA, United States), and all examinations were performed by an experienced technician using the standard protocol for macular examination. Two standardized tests were applied: (i) the hypoxia challenge test (HCT) and (ii) the handgrip test, in order to induce a vasodilatory and vasoconstriction response, respectively. OCT-A was performed at baseline conditions and during the stress test. Macular parafoveal vessel density of the superficial and deep plexuses was assessed from the en face angiograms. Statistical analysis was performed using STATA v14.1 and p < 0.05 was considered for statistical significance.

Results

Twenty-four eyes of 24 healthy subjects (10 male) were studied. Mean age was 31.8 ± 8.2 years (range, 18–57 years). Mean parafoveal vessel density in the superficial plexus increased from 54.7 ± 2.6 in baseline conditions to 56.0 ± 2.0 in hypoxia (p < 0.01). Mean parafoveal vessel density in the deep plexuses also increased, from 60.4 ± 2.2 at baseline to 61.5 ± 2.1 during hypoxia (p < 0.01). The OCT-A during the handgrip test revealed a decrease in vessel density in both superficial (55.5 ± 2.6 to 53.7 ± 2.9, p < 0.001) and deep (60.2 ± 1.8 to 56.7 ± 2.8, p < 0.001) parafoveal plexuses.

Discussion

In this work, we detail a simple, non-invasive, safe, and non-costly protocol to assess a central nervous system vascular response (i.e., the retinal circulation) using OCT-A technology. A vasodilatory response and a vasoconstriction response were observed in two physiologic conditions—mild hypoxia and isometric exercise, respectively. This protocol constitutes a new way of studying retinal vascular changes that may be applied in health and disease of multiple medical fields.

Caffeine and the eye

Caffeine, a popular psychostimulant that acts as an adenosine receptor antagonist, is the most widely used drug in history, consumed daily by people worldwide. Knowledge of the physiological and pathological effects of caffeine is crucial in improving public health because of its widespread use. We provide a summary of the current evidence on the effect of caffeine on the eye. Most of the research conducted to date is in relation to cataract and glaucoma, two of the most common eye diseases among the elderly.

The relationship of systemic markers of haemostasis with retinal blood vessel responses in cardiovascular disease and/or diabetes

Background Hypercoagulability is a leading factor in diabetes and cardiovascular disease. Retinal vessel responses to flickering light are an important tool for assessing ocular function. We hypothesised a significant relationship between systemic markers of haemostasis and retinal vessel function. Methods Intra-ocular pressure and retinal microcirculation function were measured in 116 patients with diabetes and/or cardiovascular disease using unstimulated and stimulated arterial and venous retinal vessel responses to flickering light. Haemostasis was evaluated by platelet microparticles, soluble P selectin, and five functional markers of fibrin clot formation and lysis, hyperglycaemia by HbA1c. Results Intra-ocular pressure was linked to the rates of clot formation (p = 0.006) and clot dissolution (p = 0.013) whilst central retinal vein equivalent was linked to HbA1c (p = 0.017). In the first of three flickering light cycles only, arterial baseline diameter fluctuation was linked to the lag time to clot formation (p = 0.017), whilst maximum venous dilatation was linked to HbA1c (p = 0.001) and clot density (p = 0.011). HbA1c was linked to venous dilatation amplitude (p = 0.003). There were no significant links between any ocular index and any platelet index. Conclusions In addition to glycaemia, several haemostasis measures, but no measures of platelet activity, are linked to ocular and retinal blood vessel indices in patients with diabetes and/or cardiovascular disease. These associations may have pathophysiological significance.

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.

Myogenic Response Reduction by High Blood Glucose Levels in Human Retinal Arterioles

PURPOSE

The effect of elevated blood glucose level on the myogenic response of human retinal arterioles to acute increases in blood pressure is investigated.

METHODS

The vascular response to raised blood pressure (Bayliss effect) was measured in 12 healthy volunteers by use of the retinal vessel analyzer (RVA). For a 9-minute period an on-line measurement of the diameter of a retinal branch arteriole was performed. After the first 3 minutes (baseline measurement) a second phase with 3 minutes of isometric exercise caused an acute rise in blood pressure, followed by 3 minutes of recovery (phase III). After the first session 100 g glucose were administered per os. After 30 minutes blood glucose was measured again and an identical second session was performed with higher blood glucose levels. The Wilcoxon test was used for statistical analysis.

RESULTS

During the first session a rise in mean arterial pressure of 22.8 (+/-8.4) mmHg was followed by an arterial vasoconstriction of -6.6 (+/-1.7) %. The administration of 100 g glucose resulted in a significant rise in blood glucose levels within 30 minutes between the two sessions (4.35 mmol/L vs 7.46 mmol/L) (p=0.002). The blood pressure rise of 25.7 (+/-7.3) mm Hg in the second session was associated with a significant loss in arterial vasoconstriction of -2.3 (+/-1.4) % (session I vs session II p=0.002).

CONCLUSIONS

The myogenic response of the arterial wall in human retinal arterioles was significantly reduced during acute rise of blood glucose levels.

Retinal Vessel Reaction to Short-Term IOP Elevation in Ocular Hypertensive and Glaucoma Patients

PURPOSE

Regulation of ocular blood flow might be impaired in glaucoma patients. We compared the reaction of retinal vessels to a short-term increase of intraocular pressure (IOP), using a retinal vessel analyzer (RVA), in normal volunteers, ocular hypertensive patients (OH) and primary open angle glaucoma patients (POAG).

METHODS

Ten healthy subjects (56+/-8 years, IOP 13.7+/-1.6 mmHg), 10 OH patients (55+/-12 years, IOD 23.4+/-4.1 mmHg) and 11 POAG patients (60+/-11 years, IOP 23.3+/-1.95 mmHg) were evaluated. Arterial and venous retinal vessel diameter was measured continuously before, during and after raising IOP to suprasystolic values by the suction cup method, described as ocular oscillo-dynamography.

RESULTS

The change in vessel diameter after the IOP rise differed in its temporal sequence and in absolute values depending on the group examined. In the retinal branch veins the reduction of vessel diameter during the IOP rise was significantly different in POAG (0%+/-6.7) and volunteers (-6.7%+/-8.5; p = 0.06) and in POAG and OH (-6.7%+/-7.0; p = 0.04). At 70-130 sec after IOP increase a dilatation occurred, again differing significantly in POAG (+5.8%+/-3.9) and volunteers (+9.7%+/-4.3; p = 0.03). Systemic blood pressure did not show any significant differences between groups or during the course of the examination.

DISCUSSION

At short-term rise in IOP leads to less retinal vessel reaction in POAG patients than in volunteers and OH. This might be due to impaired autoregulation to ocular perfusion changes in POAG patients.

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.

Microvascular endothelial function and severity of primary open angle glaucoma

PurposeThe role of microvascular endothelial dysfunction on severity of primary open angle glaucoma (POAG) was investigated in this study.Patients and methodsA prospective cohort study was conducted. One hundred and fourteen ethnically Malay patients (114 eyes) with POAG treated at the eye clinic of Hospital University Sains Malaysia between April 2012 and December 2014 were recruited. Patients aged between 40 and 80 years with two consecutive reliable and reproducible Humphrey visual field 24-2 analyses were selected. Patients who were diagnosed with any other type of glaucoma, previous glaucoma-filtering surgery, or other surgeries except uncomplicated cataract and pterygium surgery were excluded. Humphrey visual field analysis 24-2 was used to stratify the severity of glaucoma using Advanced Glaucoma Intervention Study (AGIS) score at the time of recruitment. Microvascular endothelial function was assessed using Laser Doppler fluximetry and iontophoresis. Iontophoresis process with acetylcholine (ACh) and sodium nitroprusside (SNP) was used to measure microvascular endothelium-dependent and -independent vasodilatation, respectively.ResultsBased on the AGIS score, 55 patients showed mild glaucoma, with 29 moderate and 30 severe. There was statistically significant difference in microvascular endothelial function (ACh% and ACh_max) between mild and moderate POAG cases (P=0.023) and between mild and severe POAG cases (P<0.001). There was negative correlation between microvascular endothelial function and severity of POAG (r=-0.457, P<0.001).ConclusionMicrovascular endothelial dysfunction may have a role in influencing the severity of POAG in Malay patients.

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.

Detecting Blood Flow Response to Stimulation of the Human Eye

Retinal blood supply is tightly regulated under a variety of hemodynamic considerations in order to satisfy a high metabolic need and maintain both vessel structure and function. Simulation of the human eye can induce hemodynamics alterations, and attempt to assess the vascular reactivity response has been well documented in the scientific literature. Advancements in noninvasive imaging technologies have led to the characterization of magnitude and time course in retinal blood flow response to stimuli. This allowed for a better understanding of the mechanism in which blood flow is regulated, as well as identifying functional impairments in the diseased eye. Clinically, the ability to detect retinal blood flow reactivity during stimulation of the eye offers potential for the detection, differentiation, and diagnosis of diseases.

Subsequent ischemic events associated with lower extremity amputations in patients with type 2 diabetes: a population-based cohort study

AIMS

To explore the risk of subsequent ischemic events in type 2 diabetes mellitus (DM) patients who had lower extremity amputations (LEAs) were compared with DM patients without LEAs.

METHODS

A population-based cohort study was conducted utilizing the data of 2011 patients with newly diagnosed DM with and without LEAs sourced from the Longitudinal Health Insurance Database 2000 (LHID 2000) of the Taiwan National Health Insurance (NHI) program between 1996 and 2008.

MAIN OUTCOME MEASURES

Relative risks (RRs), hazard ratios (HRs), and disease-free rates for various ischemic events.

RESULTS

In contrast with the comparison group, subjects with LEAs were more likely to reside in less urbanized areas, be white collar workers, and have higher DM-related costs (p<0.05). Subjects with LEAs also had significantly higher risks of developing ischemic diseases, except intestinal ischemia. In the multivariate Cox proportional hazards regression model analysis, the HR of end-stage renal disease (ESRD) was highest (HR=3.91, 95% CI=2.38-6.42), followed by embolism and thrombosis (HR=3.47, 95% CI=2.12-5.67), other peripheral vascular diseases (HR=3.11, 95% CI=2.11-4.57), atherosclerosis (HR=2.64, 95% CI=1.60-4.35), retinopathy (HR=2.24, 95% CI=1.79-2.80), cerebral ischemia (HR=1.61, 95% CI=1.25-2.06), and coronary artery disease (HR=1.44, 95% CI=1.18-1.74).

CONCLUSIONS

DM patients with LEAs had significantly higher risks for subsequent ischemic events, particularly among men. The greatest risk detected among DM patients with LEA's was for end-stage renal disease. Disease free survival rates also indicated that the course of generalized DM ischemia proceeded despite treatment.

Retinal vascular calibre and response to light exposure and serial imaging

Purpose

To investigate whether retinal vessel calibre measurements on optical retinal photography are affected by light and dark exposure prior to photography and whether the vessel calibre changes during an imaging sequence of several images.

Methods

Digital optical retinal photographs were obtained from 32 healthy adults in two separate image sequences of six images during 1 min; one sequence with 10 min of dark exposure and one with 10 min of light exposure prior to imaging. Retinal arteriolar and venular calibres were measured computer-assisted and summarized as central retinal artery and vein equivalents (CRAE and CRVE). Outcome measures were difference in calibres after prior light versus prior dark exposure and difference in calibre during each of the two imaging sequences.

Results

CRVE was wider with prior light exposure (2.7%, p = 0.0001), comparing the first image in each image sequence. Within each sequence, there was a venular dilatation from first to last image, both with prior light exposure (1.7%, p = 0.0003) and prior dark exposure (3.1%, p < 0.0001), with the change less pronounced with prior light exposure (p = 0.0164). CRAE showed no significant change in either outcome.

Conclusions

Retinal venular calibre was wider with light exposure prior to imaging and increased slightly during the imaging sequences, less pronounced after prior light than dark exposure. Measurement error due to these effects will probably be reduced by avoiding dark prior to imaging, and a possible bias effect of endothelial dysfunction may possibly be reduced by measuring calibre on an image taken early in the image sequence.

Interaction between leukocytes and erythrocytes in the human retina: effects of pentoxifylline on hyperoxia-induced vasoconstriction during increased neutrophil counts

PURPOSE

Pentoxifylline, a nonselective phosphodiesterase inhibitor, shows vasodilator effects in certain vascular beds and reduces blood viscosity. We have previously shown that under states of vasoconstriction an interaction between circulating erythrocytes and leukocytes may play a role in the control of blood flow. The reason for this observation is not entirely clear but may be related to a mechanical interaction between red and white blood cells. In the present study we hypothesized that pentoxifylline may alter this interaction during oxygen-induced vasoconstriction.

METHODS

24 healthy male subjects participated in this double masked, randomized, placebo-controlled 2 way cross over trial. In order to increase white blood cell count (WBC) count, 300 μg of G-CSF was administered intravenously. Vasoconstriction of retinal vessels was induced by oxygen inhalation. 400mg of pentoxifylline or placebo was infused at two different study days. White blood cell flux was assessed with the blue-field entoptic technique. Vessel calibers were measured with a dynamic vessel analyzer (DVA) and red blood cell velocity (RBCV) was determined with laser Doppler velocimetry (LDV). Retinal blood flow was calculated based on retinal vessel diameters and RBCV.

RESULTS

Administration of G-CSF induced a significant increase in WBC, both in the placebo and the pentoxifylline group (p<0.01 for both groups). Retinal vessel diameter, RBCV, calculated retinal blood flow and white blood cell flow were not altered by administration of pentoxifylline. Hyperoxia induced a pronounced decrease in retinal blood flow parameters. No difference was observed between groups during oxygen breathing in vessel diameters (p=0.54), RBCV (p=0.34), calculated retinal blood flow (p=0.3) and white blood cell flow (p=0.26).

CONCLUSION

Our data indicate that short time administration of pentoxifylline does not alter the oxygen-induced effect on ocular blood flow parameters during leukocytosis. Whether long-term treatment could improve retinal blood flow under states of vasoconstriction remains to be investigated.

The influence of posture change on ocular blood flow in normal subjects, measured by laser speckle flowgraphy

PURPOSE

To investigate, using laser speckle flowgraphy (LSFG), the autoregulation of ocular blood flow (BF) in response to posture change.

METHODS

This study comprised 20 healthy volunteers (mean age 30.0 ± 8.5). The mean blur rate (MBR) of the ocular circulation in the subjects was assessed in both a sitting and a supine position every 2 min over the course of 10 min. Baseline measurements of the MBR at the optic nerve head (ONH) and the choroid were taken in a sitting position. Increases in the MBR ratio in a supine position were calculated with reference to this baseline. Intraocular pressure (IOP), systemic blood pressure and heart rate in the brachial artery were also recorded.

RESULTS

In the ONH, the MBR ratio increased significantly over the baseline after 2 min (104.8 ± 5.0%, p = 0.001) and 4 min (104.4 ± 5.6%, p = 0.005), in a supine position, but decreased to the initial level after only 6 min. In the choroid, on the other hand, while the MBR ratio also increased significantly after 2 min in a supine position (113.7 ± 8.1%, p < 0.001), it kept this significant increase over the time course of 10 min. After 10 min in a supine position, IOP increased significantly (p < 0.001), systolic blood pressure decreased significantly (p < 0.001), but diastolic blood pressure did not change significantly compared to the baseline. (p = 0.07) CONCLUSIONS: ONH and choroidal circulation have significantly different hemodynamics in response to posture change in healthy volunteers. This finding suggests that LSFG enables us to assess the autoregulation of BF in the ONH.

The effect of caffeine on retinal vessel diameter in young healthy subjects

PURPOSE

To investigate the effect of caffeine on retinal vessel diameter before and during flicker light stimulation in young healthy subjects.

METHODS

Seventeen healthy subjects (mean age: 29.6 ± 3.73 years, range: 22-35 years) were included in this study. The diameter of retinal vessels was measured continuously with the retinal vessel analyzer (RVA) before and 1 hr after 200 mg oral caffeine intake. After baseline assessment, a green luminance flicker of 20-second duration was applied to stimulate retinal activity. The diameter of a segment of an arteriole and of a venule were measured during stimulation and 80 second after cessation of the stimulus. Flicker stimulation and 80-second measurement interval were carried out three times. Blood pressure parameters, systemic mean arterial pressure (MAP), ocular perfusion pressure (OPP) and intraocular pressure (IOP) were obtained before and after oral caffeine intake.

RESULTS

The mean diameter of the arterioles at baseline before caffeine intake was 123.30 ± 14.0 μm (arithmetic mean standard deviation) and after caffeine 117.30 ± 13.0 μm which was significantly different (p=0.004). The mean diameter of the venules at baseline before caffeine intake was 147.60 ± 19.5 μm and after caffeine 137.73 ± 19.9 μm which was significantly different (p = 0.005). The mean diameter of the arterioles during flicker light stimulation before caffeine intake was 126.65 ± 13.24 μm and after caffeine intake 121.59 ± 12.12 μm (p = 0.012). The mean diameter of the venules during flicker light stimulation before caffeine intake was 151.87 ± 18.63 μm and after caffeine intake was 145.14 ± 19.82 μm (p = 0.027). The flicker response of the arterioles increased from 2.8% before caffeine to 3.8% after caffeine intake (p = 0.010). The flicker response of the venules increased from 3.4% before caffeine to 5.5% after caffeine intake (p = 0.0001). Baseline diameters and diameters during flicker light stimulation after caffeine intake showed a significant negative correlation to the MAP for the arterioles (baseline: r = -0.338, p = 0.049 and flicker: r = -0.345, p = 0.046) and the venules (baseline: r = -0.496, p = 0.003 and flicker: r =-0.479, p = 0.004).

CONCLUSIONS

The present study showed a significant vasoconstrictory response of the retinal vessels 1 hr after caffeine intake in young healthy subjects. Retinal vessel diameter changes were negatively correlated with MAP after caffeine consumption. These effects seem to be elicited by an autoregulatory response of the retinal vessels to the increased blood pressure changes after caffeine.

Effect of NO synthase inhibition on retinal vessel reaction to isometric exercise in healthy humans

PURPOSE

It has been shown that retinal blood flow is autoregulated, meaning that flow is independent of perfusion pressure within a certain range. We tested the hypothesis that nitric oxide (NO) synthase inhibition alters the response of retinal arterial and venous vessels during isometric exercise.

METHODS

In this study, nine healthy subjects were included. Each subject received the NO synthase inhibitor Ng-monomethyl-l-Arginine (l-NMMA, the α-receptor agonist phenylephrine or placebo intravenously on three study days. Retinal vessel diameter was assessed with the retinal vessel analyser (RVA), at baseline and during a squatting period of 6-7 min in absence or presence of l-NMMA, phenylephrine or placebo.

RESULTS

Mean arterial pressure (MAP) and pulse rate (PR) increased significantly during all pretreatment squatting periods (p < 0.001) Retinal venous and arterial diameters showed a continuous decrease during squatting (p < 0.001). Phenylephrine increased MAP and PR but did not alter the retinal vessel diameter response to squatting. Administration of l-NMMA lead to a significant decrease in venous diameter before isometric exercise (p = 0.004). In addition, the retinal venous diameter response during administration of the NO synthase inhibitor was less pronounced than during phenylephrine or placebo (p < 0.001).

CONCLUSION

Our study confirms that NO plays an important role in the control of retinal vascular tone at rest. In addition, the present data indicate a role of NO in retinal autoregulation, because the response of retinal venous diameters was altered after NO synthase inhibition. The nature of involvement, however, appears to be complex and requires further studies.

Retinal vessel diameter can reliably be determined in minipigs using Retinal Vessel Analyser with a microscope-mounted fundus camera

PURPOSE

Retinal Vessel Analyser (RVA) is a validated instrument to measure retinal vessel diameter in humans. The purpose of this study was to assess the reproducibility (inter-observer reliability) and the repeatability (test-retest reliability) of RVA with a microscope-mounted fundus camera to determine retinal vessel diameter in minipigs.

METHODS

Ocular fundus image from five anaesthetized minipigs was recorded in a digital videotape for approximately 5 min, under stable systemic arterial pressure and gas conditions. To evaluate the reproducibility, each one of two investigators used RVA to measure the diameter of the superior temporal retinal artery on five separate 30-second video sequences from each minipig, which were the same video sequences for both investigators. To evaluate the repeatability, one investigator performed five measurements on a single, randomly selected, 30-second video sequence from each minipig. The reproducibility was determined using the intra-class correlation coefficient (ICC), and the repeatability was assessed using the coefficient of variation (COV). Bland-Altman plots were also used to assess agreement between the two investigators.

RESULTS

Retinal arteriolar diameter measurements with RVA in minipigs were highly reproducible. Differences between the two investigators were lower than 0.7%. The ICC was 1.00, indicating perfect reproducibility, and the mean COV was 0.18%, reflecting excellent repeatability of the measurements with RVA.

CONCLUSION

Retinal vessel diameter can reliably be determined not only in humans, but also in minipigs, using the commercially available RVA apparatus and a microscope-mounted fundus camera.

Cellular and physiological mechanisms underlying blood flow regulation in the retina and choroid in health and disease

We review the cellular and physiological mechanisms responsible for the regulation of blood flow in the retina and choroid in health and disease. Due to the intrinsic light sensitivity of the retina and the direct visual accessibility of fundus blood vessels, the eye offers unique opportunities for the non-invasive investigation of mechanisms of blood flow regulation. The ability of the retinal vasculature to regulate its blood flow is contrasted with the far more restricted ability of the choroidal circulation to regulate its blood flow by virtue of the absence of glial cells, the markedly reduced pericyte ensheathment of the choroidal vasculature, and the lack of intermediate filaments in choroidal pericytes. We review the cellular and molecular components of the neurovascular unit in the retina and choroid, techniques for monitoring retinal and choroidal blood flow, responses of the retinal and choroidal circulation to light stimulation, the role of capillaries, astrocytes and pericytes in regulating blood flow, putative signaling mechanisms mediating neurovascular coupling in the retina, and changes that occur in the retinal and choroidal circulation during diabetic retinopathy, age-related macular degeneration, glaucoma, and Alzheimer's disease. We close by discussing issues that remain to be explored.

Effects of increased white blood cell count on endothelin-induced vasoconstriction in healthy subjects

It is known that administration of granulocyte-colony stimulating factor is followed by an increase of white blood cell count. There is evidence from other vascular beds that an increase in white blood cell count impairs blood flow regulation especially in the microcirculation. Whether this also holds true for the ocular circulation is unknown. In the following study we investigated whether an increase in white blood cell count alters the endothelin-1 induced vasoconstriction in humans. Neither granulocyte-colony stimulating factor nor endothelin-1 had any consistent effect on blood pressure, pulse rate or intraocular pressure. Administration of granulocyte-colony stimulating factor induced a pronounced increase in retinal white blood cell density (p < 0.01). Administration of endothelin-1 decreased choroidal (p < 0.01) and retinal blood flow (p < 0.01). The change in choroidal blood flow in response to endothelin-1 was not altered by pre-treatment with granulocyte-colony stimulating factor. By contrast, the decrease in retinal blood flow was more pronounced during an increase in white blood cell count (p = 0.02) when compared to placebo. Our data indicates that during pronounced vasoconstriction, as induced by administration of endothelin-1, vascular regulation can be altered by the number of circulating white blood cells. Whether this effect is caused by an interaction of red and white blood cells in the microcirculation or a yet unknown mechanism needs further investigation.

Lack of effect of short-term treatment with amlodipine and lisinopril on retinal autoregulation in normotensive patients with type 1 diabetes and mild diabetic retinopathy

PURPOSE

Diabetic retinopathy is characterized by morphological changes in the retina secondary to disturbances in retinal blood flow. It has been shown that antihypertensive treatment has a protective effect on the development of diabetic retinopathy, and evidence suggests that inhibitors of the renin-angiotensin system have a protective effect beyond the antihypertensive effect. The background for this additional effect is unknown but might be related to an effect on retinal autoregulation.

METHODS

In a double-blinded, two-way cross-over study, 25 normotensive patients with type 1 diabetes (T1D) aged 20.6-33.9 (mean 27.9) with mild retinopathy were randomized to receive either 5 mg of the calcium channel blocker (CCB) amlodipine for 14 days followed by a washout period and treatment with 10 mg of the angiotensin converting enzyme (ACE) inhibitor lisinopril for another 14 days or the two treatments in the reverse order. Using a Dynamic Vessel Analyzer (DVA), the diameter response of retinal arterioles during an acute increase in the blood pressure induced by isometric exercise, during flicker stimulation and during both stimulus conditions simultaneously was studied before and during the two treatments periods.

RESULTS

Amlodipine and lisinopril induced a similar non-significant decrease in the arterial blood pressure. At baseline, the arterial diameter decreased by 2.4 ± 0.9% (p = 0.004) during isometric exercise, increased by 2.2 ± 0.9% (p = 0.019) during flicker stimulation and increased by 1.8 ± 0.9% (p = 0.03) during the combined stimulus conditions. Neither of the antihypertensive drugs amlodipine (p = 0.76) or lisinopril (p = 0.11) changed the diameter response of retinal vessels significantly; however, the two treatments induced a different response in the veins during combined exercise and flicker (p = 0.021).

CONCLUSIONS

Short-term treatment with amlodipine and lisinopril had no significant effect on retinal autoregulation in young normotensive patients with T1D and mild retinopathy, and this lack of effect was similar for the two drugs. A possible normalizing effect of antihypertensive treatment on retinal autoregulation was not observed; however, it might take longer time to improve autoregulation than to reduce the arterial blood pressure.

Retinal vessel analysis in hypercholesterolemic patients before and after LDL apheresis

OBJECTIVE

Changes in the microcirculation per se may precede macrovascular changes, hence, may present a good surrogate for monitoring vascular changes during treatment. Using retinal vascular imaging techniques, we attempted to investigate the microvascular benefit of a single LDL apheresis in hypercholesterolemia.

METHODS

Twenty-one hypercholesterolemic patients (57+/-15 years) on regular LDL apheresis treatment, seven women and 14 men, were examined for retinal endothelial function before and after a single LDL apheresis. The Dynamic Vessel Analyzer was applied for static and dynamic retinal vessel analysis. Retinal vessel diameter was measured at rest and during flicker light stimulation. Changes in vasodilation are expressed as percent changes over baseline.

RESULTS

Cholesterol fractions and triglycerides were reduced by 21-74 % by the LDL apheresis procedure. In hypercholesterolemic patients, flicker-induced vasodilation was diminished in both retinal arterioles and venules, but had significantly improved in retinal venules after a single LDL apheresis (p = 0.013). This increase in vasodilatory capacity of retinal venules was linked to a wider basal retinal venular caliber (p = 0.031), but was unrelated to changes in serum lipids (p > 0.05).

CONCLUSION

In hypercholesterolemia, abnormal retinal autoregulation is improved by a single LDL apheresis. Our findings suggest that venules represent a dynamic component, which is highly responsive to metabolic changes in the microcirculation.

The complex interaction between ocular perfusion pressure and ocular blood flow - relevance for glaucoma

Glaucoma is an optic neuropathy of unknown origin. The most important risk factor for the disease is an increased intraocular pressure (IOP). Reducing IOP is associated with reduced progression in glaucoma. Several recent large scale trials have indicated that low ocular perfusion pressure (OPP) is a risk factor for the incidence, prevalence and progression of the disease. This is a strong indicator that vascular factors are involved in the pathogenesis of the disease, a hypothesis that was formulated 150 years ago. The relation between OPP and blood flow to the posterior pole of the eye is, however, complex, because of a phenomenon called autoregulation. Autoregulatory processes attempt to keep blood flow constant despite changes in OPP. Although autoregulation has been observed in many experiments in the ocular vasculature the mechanisms underlying the vasodilator and vasoconstrictor responses in face of changes in OPP remain largely unknown. There is, however, recent evidence that the human choroid regulates its blood flow better during changes in blood pressure induced by isometric exercise than during changes in IOP induced by a suction cup. This may have consequences for our understanding of glaucoma, because it indicates that blood flow regulation is strongly dependent not only on OPP, but also on the level of IOP itself. Indeed there is data indicating that reduction of IOP by pharmacological intervention improves optic nerve head blood flow regulation independently of an ocular vasodilator effect.

Effect of systemic moxaverine on ocular blood flow in humans

PURPOSE

A number of common eye diseases are associated with ocular perfusion abnormalities. The present study aimed to investigate whether systemically administered moxaverine improves ocular blood flow.

METHODS

Sixteen healthy volunteers were studied in this randomized, double-masked, placebo-controlled, two-way crossover study. Moxaverine in a dose of 150 mg was administered i.v. Ocular haemodynamic parameters were measured before and after drug administration. Retinal arterial and venous diameters were measured with a retinal vessel analyser. Retinal blood velocity was assessed using laser Doppler velocimetry and choroidal and optic nerve head blood flow was measured with laser Doppler flowmetry.

RESULTS

Moxaverine increased choroidal blood flow (22.6 ± 27.9%), an effect which was significant versus placebo (p = 0.015). Red blood cell velocity in retinal veins tended to increase by 13.6 ± 13.3% after infusion of moxaverine, but this effect was not significant compared with placebo (p = 0.25). In the optic nerve head moxaverine also tended to increase blood flow (11.8 ± 12.7%), but, again, this effect was not significant versus placebo (p = 0.12). Neither moxaverine nor placebo had an effect on retinal arterial diameters. In retinal veins moxaverine tended to induce vasodilation (2.6 ± 2.8%) and to increase blood flow (19.6 ± 16.5%), but these effects were not significant (both p = 0.12).

CONCLUSIONS

 The present study indicates an increase in choroidal blood flow after systemic infusion of a single dose of moxaverine in healthy subjects. Further studies are warranted to investigate whether these effects are also seen after longterm treatment in patients with ocular vascular disease.

Abnormal retinal autoregulation is detected by provoked stimulation with flicker light in well-controlled patients with type 1 diabetes without retinopathy

AIMS

Investigation of retinal vasodilation under flickering light is considered a dynamic analysis in contrast to the static analysis of retinal vessel equivalents (mean retinal vessel diameter). We investigated whether dynamic analysis apart from the static one in type 1 diabetic patients without diabetic retinopathy with well-controlled diabetes could lead to additional information regarding retinal autoregulation.

METHODS

18 normotensive type 1 diabetic patients without retinopathy and 19 healthy subjects were included. Diameter of retinal vessels was measured with Dynamic Vessel Analyzer. Changes in vasodilation are expressed as percent change over baseline values.

RESULTS

HbA(1c) was 7.5+/-1.0% in diabetic patients. In arteries, the response to flicker was diminished in diabetic patients compared to healthy volunteers (p<0.023). In patients flicker stimulation increased arterial diameter by +2.7% in contrast to +4.4% in controls. Venous vessel diameter increased by +3.1% in diabetic individuals and by +5.3% in the control group (p<0.002). There were no differences in static analysis between both groups.

CONCLUSIONS

Diabetic patients without retinopathy with relatively good glycemic control show reduced retinal vasodilation after flicker indicating dysfunction in retinal autoregulation. The use of provocation test in conjunction with static analysis could lead to additional information regarding abnormal retinal autoregulation.

Functional imaging using the retinal function imager: direct imaging of blood velocity, achieving fluorescein angiography-like images without any contrast agent, qualitative oximetry, and functional metabolic signals

The Retinal Function Imager (RFI; Optical Imaging, Rehovot, Israel) is a unique, noninvasive multiparameter functional imaging instrument that directly measures hemodynamic parameters such as retinal blood-flow velocity, oximetric state, and metabolic responses to photic activation. In addition, it allows capillary perfusion mapping without any contrast agent. These parameters of retinal function are degraded by retinal abnormalities. This review delineates the development of these parameters and demonstrates their clinical applicability for noninvasive detection of retinal function in several modalities. The results suggest multiple clinical applications for early diagnosis of retinal diseases and possible critical guidance of their treatment.

Dorzolamide Influences the Autoregulation of Major Retinal Vessels Caused by Artificial Intraocular Pressure Elevation in Patients with POAG: A Clinical Study

PURPOSE

The study investigated whether dorzolamide influences the autoregulatory behavior of major retinal arterioles in glaucoma patients via a moderate perfusion pressure reduction.

METHODS

The study included one eye each of 12 untreated patients with a primary open-angle glaucoma (POAG) (age 60.8 +/- 8.3, IOP 22.3 +/- 6.5 mmHg). Changes in the diameter of a retinal artery segment before (120 s), during (100 s), and after (380 s) artificial IOP elevation to 38 mmHg for 100 s were recorded continuously by means of a Retinal Vessel Analyzer. The measurement was repeated after 4-week treatment with dorzolamide eye drops t.i.d.

RESULTS

Ocular perfusion pressure (mmHg) was reduced by the intraocular pressure (IOP) elevation from 58 (+/- 10) to 41 (+/- 11) in the pretreatment examination and from 60 (+/- 8) to 40 (+/- 8) posttreatment (differences between the examinations n.s.). Before IOP elevation, the arterial diameter was found to be +1.7 +/- 3.5% greater in the posttreated eyes than in the pretreated eyes (p < 0.02). During IOP elevation, the arterial diameter decreased by -1.8% +/- 3.8 in the pretreated eyes, whereas dilatation by +1.4% +/- 2.5 was observed in the posttreated eyes (p = 0.02). At the end of the observation period following IOP elevation, the vessel diameter in the pretreated eyes had increased by +1.8% +/- 4.2, whereas in the posttreated eyes it had decreased by -1.7% +/- 3.0. On average, dorzolamide reduced IOP by -5.6 mmHg (p = 0.001).

CONCLUSIONS

The arterial diameter dilatation during IOP elevation in dorzolamide-treated eyes could be an accelerated counter-regulation on the induced elevated IOP and could constitute an additional therapeutic effect.

Influence of exercise induced hyperlactatemia on retinal blood flow during normo- and hyperglycemia

PURPOSE

Short term hyperglycemia has previously been shown to induce a blood flow increase in the retina. The mechanism behind this effect is poorly understood. We set out to investigate whether exercise-induced hyperlactatemia may alter the response of retinal blood flow to hyperglycemia.

METHODS

We performed a randomized, controlled two-way cross over study comprising 12 healthy subjects, performed a 6-minutes period of dynamic exercise during an euglcaemic or hyperglycaemic insulin clamp. Retinal blood flow was assessed by combined vessel size measurement with the Zeiss retinal vessel analyzer and measurement of red blood cell velocities using bi-directional laser Doppler velocimetry. Retinal and systemic hemodynamic parameters were measured before, immediately after and 10 and 20 minutes after isometric exercise.

RESULTS

On the euglycemic study day retinal blood flow increased after dynamic exercise. The maximum increase in retinal blood flow was observed 10 minutes after the end of exercise when lactate plasma concentration peaked. Hyperglycemia increased retinal blood flow under basal conditions, but had no incremental effect during exercise induced hyperlactatemia.

CONCLUSIONS

Our results indicate that both lactate and glucose induce an increase in retinal blood flow in healthy humans. This may indicate a common pathway between glucose and lactate induced blood flow changes in the human retina.

Retinal vessel response to flicker light in children and adolescents with type 1 diabetes mellitus and overweight or obesity

It was the goal of the trial to assess cardiovascular risk factors in children and adolescents with type 1 diabetes mellitus or overweight/obesity. All children and adolescents (n=77 [n=45 patients with diabetes mellitus, n=32 patients with overweight/obesity]) admitted to our hospital during the period from 01/07 to 31/08/2006 were included in the trial. Socio-demographic and laboratory data (age, sex, diabetes duration, BMI, BMI-SDS, HbA1c, fasting blood glucose, oGTT in patients with overweight/obesity, lipids, CRP, TSH, creatinine, and microalbuminuria) were assessed. The diameter of a retinal arterial and a venous segment was measured continuously on-line with a Dynamic Vessel Analyzer, carotid intima-media thickness (IMT) was measured, and 24-h-blood pressure monitoring was applied.

RESULTS

Flicker light stimulation induces a comparable arterial dilatation in patients with type 1 diabetes and overweight/obesity. Univariate ANOVA in patients with type 1 diabetes shows an influence of diastolic blood pressure on arterial dilatation. Other factors such as BMI, age, diabetes duration, smoking, sex, HbA1c and insulin dose/kg had no effect.

CONCLUSIONS

In children and adolescents with diabetes or overweight/obesity retinal vascular alterations seem to be more sensitive and already present before the occurrence of classic cardiovascular markers.

Functional in vivo assessment of retinal artery microirregularities in glaucoma

PURPOSE

We investigated whether retinal branch arteries in healthy subjects, and non-treated and treated primary open-angle glaucoma (POAG) patients show irregular local patterns during dynamic reaction to acute increases of different magnitudes in intraocular pressure (IOP).

METHODS

Nine POAG patients and nine age-matched normal volunteers were examined with the retinal vessel analyser (RVA) using a suprasystolic IOP increase (Study 1). Fourteen POAG patients and 13 age-matched controls were examined using a moderate IOP increase for 100 seconds (Study 2). Longitudinal arterial profiles were obtained for the chosen time intervals. The high-frequency waviness (HFW) of these profiles was analysed quantitatively.

RESULTS

No significant changes in HFW were found in controls in different phases of the arterial reaction. Significant increases in HFW from baseline to dilation (Study 1, P < 0.03) and from dilation to constriction (Study 2, P < 0.05) were found in POAG patients. High-frequency waviness was higher in POAG patients than in controls during dilation (P < 0.05) in both studies.

CONCLUSIONS

Our results indicate a local vessel wall difference in glaucoma patients compared with age-matched controls. Increasing HFW might worsen hydraulic resistance of the vessel segment to blood flow. Significant increase of arterial microirregularities in the POAG retina during vascular dilation might be an indication for vascular endothelial alterations in glaucoma, leading to impaired perfusion in response to IOP increase.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.