Novel methodology to comprehensively assess retinal arteriolar vascular reactivity to hypercapnia

pH in the vertebrate retina and its naturally occurring and pathological changes

This review summarizes the existing information on the concentration of H+ (pH) in vertebrate retinae and its changes due to various reasons. Special features of H+ homeostasis that make it different from other ions will be discussed, particularly metabolic production of H+ and buffering. The transretinal distribution of extracellular H+ concentration ([H+]o) and its changes under illumination and other conditions will be described in detail, since [H+]o is more intensively investigated than intracellular pH. In vertebrate retinae, the highest [H+]o occurs in the inner part of the outer nuclear layer, and decreases toward the RPE, reaching the blood level on the apical side of the RPE. [H+]o falls toward the vitreous as well, but less, so that the inner retina is acidic to the vitreous. Light leads to complex changes with both electrogenic and metabolic origins, culminating in alkalinization. There is a rhythm of [H+]o with H+ being higher during circadian night. Extracellular pH can potentially be used as a signal in intercellular volume transmission, but evidence is against pH as a normal controller of fluid transport across the RPE or as a horizontal cell feedback signal. Pathological and experimentally created conditions (systemic metabolic acidosis, hypoxia and ischemia, vascular occlusion, excess glucose and diabetes, genetic disorders, and blockade of carbonic anhydrase) disturb H+ homeostasis, mostly producing retinal acidosis, with consequences for retinal blood flow, metabolism and function.

Retinal blood vessel diameters in children and adults exposed to a simulated altitude of 3,000 m

Introduction: Technological advances have made high-altitude ski slopes easily accessible to skiers of all ages. However, research on the effects of hypoxia experienced during excursions to such altitudes on physiological systems, including the ocular system, in children is scarce. Retinal vessels are embryologically of the same origin as vessels in the brain, and have similar anatomical and physiological characteristics. Thus, any hypoxia-related changes in the morphology of the former may reflect the status of the latter. Objective: To compare the effect of one-day hypoxic exposure, equivalent to the elevation of high-altitude ski resorts in North America and Europe (∼3,000 m), on retinal vessel diameter between adults and children. Methods: 11 adults (age: 40.1 ± 4.1 years) and 8 children (age: 9.3 ± 1.3 years) took part in the study. They spent 3 days at the Olympic Sports Centre Planica (Slovenia; altitude: 940 m). During days 1 and 2 they were exposed to normoxia (F_iO₂ = 0.209), and day 3 to normobaric hypoxia (F_iO₂ = 0.162 ± 0.03). Digital high-resolution retinal fundus photographs were obtained in normoxia (Day 2) and hypoxia (Day 3). Central retinal arteriolar equivalent (CRAE) and venular equivalents (CRVE) were determined using an Automated Retinal Image Analyser. Results: Central retinal arteriolar and venular equivalents increased with hypoxia in children (central retinal arteriolar equivalent: 105.32 ± 7.72 µm, hypoxia: 110.13 ± 7.16 µm, central retinal venular equivalent: normoxia: 123.39 ± 8.34 µm, hypoxia: 130.11 ± 8.54 µm) and adults (central retinal arteriolar equivalent: normoxia: 105.35 ± 10.67 µm, hypoxia: 110.77 ± 8.36 µm; central retinal venular equivalent: normoxia: 126.89 ± 7.24 µm, hypoxia: 132.03 ± 9.72 µm), with no main effect of group or group*condition interaction. A main effect of condition on central retinal arteriolar and venular equivalents was observed (central retinal arteriolar equivalent:normoxia: 105.34 ± 9.30 µm, hypoxia: 110.50 ± 7.67 µm, p < 0.001; central retinal venular equivalent: normoxia: 125.41 ± 7.70 µm, hypoxia: 131.22 ± 9.05 µm, p < 0.001). Conclusion: A 20-hour hypoxic exposure significantly increased central retinal arteriolar and venular equivalents in adults and children. These hypoxia-induced increases were not significantly different between the age groups, confirming that vasomotor sensitivity of the retinal vessels to acute hypoxia is comparable between adults and prepubertal children.

Retinal Physiology and Circulation: Effect of Diabetes

In this article, we present a discussion of diabetes and its complications, including the macrovascular and microvascular effects, with the latter of consequence to the retina. We will discuss the anatomy and physiology of the retina, including aspects of metabolism and mechanisms of oxygenation, with the latter accomplished via a combination of the retinal and choroidal blood circulations. Both of these vasculatures are altered in diabetes, with the retinal circulation intimately involved in the pathology of diabetic retinopathy. The later stages of diabetic retinopathy involve poorly controlled angiogenesis that is of great concern, but in our discussion, we will focus more on several alterations in the retinal circulation occurring earlier in the progression of disease, including reductions in blood flow and a possible redistribution of perfusion that may leave some areas of the retina ischemic and hypoxic. Finally, we include in this article a more recent area of investigation regarding the diabetic retinal vasculature, that is, the alterations to the endothelial surface layer that normally plays a vital role in maintaining physiological functions. © 2020 American Physiological Society. Compr Physiol 10:933-974, 2020.

Hypercapnia Impairs Vasoreactivity to Changes in Blood Pressure and Intraocular Pressure in Rat Retina

SIGNIFICANCE

The balance between oxygen and carbon dioxide sets the resting tone (or diameter) of retinal blood vessels. Eyes that are hypercapnic use up their "vasodilatory reserve" and therefore fail to respond adequately to changes in intraocular or blood pressure.

PURPOSE

Retinal vessels are regulated by both myogenic and metabolic mechanisms. We considered whether alteration of metabolic status would modify the vascular response to ocular perfusion pressure (OPP) lowering in rat retina.

METHODS

In pentobarbital anesthetized adult Brown-Norway rats, normocapnia or hypercapnia was achieved by artificially ventilating animals with air or 5% carbon dioxide in ~30% oxygen, respectively. Ocular perfusion pressure was gradually reduced to ~20 mmHg by either lowering blood pressure (slowly drawing blood from a femoral artery/vein) or manometrically increasing intraocular pressure under normocapnic or hypercapnic conditions. In all four groups (n = 7 eyes for each), a confocal scanning laser ophthalmoscope was used to acquire image sequences centered on the optic nerve throughout pressure modification. The diameter of arterioles and venules at various OPP levels was measured and expressed as percentage relative to their own baseline. The response of arterioles and venules to OPP lowering was compared between normocapnic and hypercapnic groups.

RESULTS

Average arterial carbon dioxide partial pressures were 36.9 ± 2.6 mmHg in normocapnic and 64.1 ± 5.9 mmHg in hypercapnic (P < .001) animals. In the normocapnic groups, blood pressure lowering and intraocular pressure elevation resulted in significant vasodilation of both arterioles and venules (P < .0001). In the hypercapnic groups, OPP lowering-induced vasodilation was significantly attenuated compared with the corresponding normocapnic groups (P < .0001 for both, two-way analysis of variance).

CONCLUSION

Hypercapnia significantly modified myogenic vascular autoregulation in response to OPP reduction.

Reactivity in the human retinal microvasculature measured during acute gas breathing provocations

Although changes in vessel diameter following gas perturbation have been documented in retinal arterioles and venules, these responses have yet to be quantified in the smallest vessels of the human retina. Here, using in vivo adaptive optics, we imaged 3–25 µm diameter vessels of the human inner retinal circulation and monitored the effects of altered gas-breathing conditions. During isocapnic hyperoxia, definite constrictions were seen in 51% of vessel segments (mean ± SD for pre-capillary arterioles −9.5 ± 3.0%; capillaries −11.8 ± 3.3%; post-capillary venules −6.3 ± 2.8%); these are comparable with responses previously reported in larger vessels. During isoxic hypercapnia, definite dilations were seen in 47% of vessel segments (mean ± SD for pre-capillary arterioles +9.8 ± 1.5%; capillaries +13.7 ± 3.8%; post-capillary venules +7.5 ± 4.2%); these are proportionally greater than responses previously reported in larger vessels. The magnitude of these proportional changes implies that the capillary beds themselves play an important role in the retinal response to changes in carbon dioxide levels. Interestingly, the distribution of microvascular responses shown here differs from our previously reported responses to flicker stimulation, suggesting differences in the way blood supply is coordinated following gas perturbation and altered neural activity.

Effect of acute hypercapnia during 10-day hypoxic bed rest on posterior eye structures

To gain insights into microgravity-induced ophthalmic changes (microgravity ocular syndrome), and as part of a project investigating effects of future planetary habitats, we investigated the effect of acute hypercapnia following 10-day bed rest and hypoxia on posterior eye structures. Female subjects ( N = 7) completed three 10-day experimental interventions: 1) normoxic bed rest [NBR; partial pressure of inspired O2 (PiO2) = 132.9 ± 0.3 Torr]; 2) hypoxic ambulatory confinement (HAMB; PiO2 = 90.4 ± 0.3 Torr); and 3) hypoxic bed rest (HBR; n = 12; PiO2 = 90.4 ± 0.3 Torr). Before and on the last day of each intervention, optical coherence tomography (OCT) of the optic disk was performed, and the thicknesses of the retinal nerve fiber layer (RNFL), retina, and choroid were measured. OCT examinations were conducted with the subjects breathing the prevailing normocapnic breathing mixture (either normoxic or hypoxic) and then following a 10-min period of breathing the same gas mixture, but with the addition of 1% CO2. Choroidal thickness was greater during both bed-rest conditions (NBR and HBR) compared with the ambulatory (HAMB) condition (ANOVA, P < 0.001). Increases in RNFL thickness compared with baseline were observed in the hypoxic trials (HBR, P < 0.001; and HAMB, P = 0.021), but not the normoxic trial (NBR). A further increase in RNFL thickness ( P = 0.019) was observed after the 10-min hypercapnic trial in the NBR condition only. The fact that choroidal thickness was not affected by Po2 or Pco2, but increased by bed rest, suggests a hydrostatic rather than a vasoactive effect. The increments in RNFL thickness were most likely associated with local hypoxia and hypercapnia-induced dilatation of the retinal blood vessels.

Evaluation of the Effect of Hypercapnia on Vascular Function in Normal Tension Glaucoma

INTRODUCTION

Altered ocular perfusion and vascular dysregulation have been reported in glaucoma. The aim of this paper was to evaluate the vascular response to a hypercapnic stimulus.

METHODS

Twenty normal tension glaucoma (NTG) patients and eighteen age- and gender-matched controls had pulsatile ocular blood flow (POBF) measurements, systemic cardiovascular assessment, and laser Doppler digital blood flow (DBF) assessed. Measurements were taken at baseline, after 10-minutes rest, in the stable sitting and supine positions and following induction and stabilization of hypercapnia, which induced a 15% increase in end-tidal pCO2. The POBF response to hypercapnia was divided into high (>20%) and low responders (<20%).

RESULTS

65% of NTG patients had a greater than 41% increase in POBF following CO2 rebreathing (high responders). These high responders had a lower baseline POBF, lower baseline DBF, and a greater DBF response to thermal stimulus.

CONCLUSION

NTG patients that have a greater than 20% increase in POBF after a hypercapnic stimulus have lower baseline POBF and DBF values. This suggests that there is impaired regulation of blood flow in a significant subgroup of NTG patients. This observation may reflect a generalised dysfunction of the vascular endothelium.

Measurement of Retinal Blood Flow Using Fluorescently Labeled Red Blood Cells

Blood flow is a useful indicator of the metabolic state of the retina. However, accurate measurement of retinal blood flow is difficult to achieve in practice. Most existing optical techniques used for measuring blood flow require complex assumptions and calculations. We describe here a simple and direct method for calculating absolute blood flow in vessels of all sizes in the rat retina. The method relies on ultrafast confocal line scans to track the passage of fluorescently labeled red blood cells (fRBCs). The accuracy of the blood flow measurements was verified by (1) comparing blood flow calculated independently using either flux or velocity combined with diameter measurements, (2) measuring total retinal blood flow in arterioles and venules, (3) measuring blood flow at vessel branch points, and (4) measuring changes in blood flow in response to hyperoxic and hypercapnic challenge. Confocal line scans oriented parallel and diagonal to vessels were used to compute fRBC velocity and to examine velocity profiles across the width of vessels. We demonstrate that these methods provide accurate measures of absolute blood flow and velocity in retinal vessels of all sizes.

Assessment of total retinal blood flow using Doppler Fourier Domain Optical Coherence Tomography during systemic hypercapnia and hypocapnia

The purpose of this study was to investigate changes in total retinal blood flow (RBF) using Doppler Fourier Domain Optical Coherence Tomography (Doppler FD‐OCT) in response to the manipulation of systemic partial pressure of CO₂ (P_ETCO₂). Double circular Doppler blood flow scans were captured in nine healthy individuals (mean age ± standard deviation: 27.1 ± 4.1, six males) using the RTVue^™ FD‐OCT (Optovue). P_ETCO₂ was manipulated using a custom‐designed computer‐controlled gas blender (RespirAct^™) connected to a sequential gas delivery rebreathing circuit. Doppler FD‐OCT measurements were captured at baseline, during stages of hypercapnia (+5/+10/+15 mmHg P_ETCO₂), return to baseline and during stages of hypocapnia (−5/−10/−15 mmHg P_ETCO₂). Repeated measures analysis of variance (reANOVA) and Tukey's post hoc analysis were used to compare Doppler FD‐OCT measurements between the various P_ETCO₂ levels relative to baseline. The effect of P_ETCO₂ on TRBF was also investigated using linear regression models. The average RBF significantly increased by 15% (P < 0.0001) with an increase in P_ETCO₂ and decreased significantly by 10% with a decrease in P_ETCO₂ (P = 0.001). Venous velocity significantly increased by 3.11% from baseline to extreme hypercapnia (P < 0.001) and reduced significantly by 2.01% at extreme hypocapnia (P = 0.012). No significant changes were found in the average venous area measurements under hypercapnia (P = 0.36) or hypocapnia (P = 0.40). Overall, increased and decreased P_ETCO₂ values had a significant effect on RBF outcomes (P < 0.002). In healthy individuals, altered end‐tidal CO₂ levels significantly changed RBF as measured by Doppler FD‐OCT.

Total retinal blood flow changes in response to the manipulation of systemic partial pressure of CO₂ was measured in healthy individuals using Doppler Fourier Domain Optical Coherence Tomography. Increased total retinal blood flow and decreased blood flow were found in response to hypercapnia and hypocapnia, respectively.

The impact of topical mydriatic ophthalmic solutions on retinal vascular reactivity and blood flow

The impact of mydriatic agents on the standardized provocation of retinal vascular reactivity has not been systematically investigated. Our aim was to investigate the effect of commonly used mydriatic agents on the provoked vascular response of retinal arterioles. One eye was randomly selected for mydriasis from 10 healthy volunteers (age 23.3 ± 4.9 years). A single drop of: 1% tropicamide (T), or a combination of 0.8% tropicamide and 5% phenylephrine (TP), or 1% cyclopentolate (C) were instilled into the volunteers lower fornix at each of three visits. Volunteers underwent a standardized isocapnic hyperoxic provocation. Four retinal hemodynamic measurements were acquired with the Canon Laser Blood Flowmeter at equivalent positions on the superior temporal arteriole (STA) and inferior temporal arteriole (ITA) at baseline, during provocation and after recovery. Statistical analysis was performed using linear mixed-effect models. Pre- and post-dilation measurements indicated that pupil diameter increased with use of T, TP, C (all <0.001), while systolic blood pressure, diastolic blood pressure and intraocular pressure did not change significantly (all >0.05). In response to a standardized isocapnic hyperoxic challenge, blood vessel diameter, blood velocity and flow decreased in both the STA and ITA relative to baseline. Comparison between the change elicited by isocapnic hyperoxic gas stimuli with respect to blood vessel diameter, blood velocity, blood flow, were equivalent for each mydriatic agent in the STA (p = 0.66, p = 0.99, p = 0.99, respectively) and the ITA (p = 0.85, p = 0.80, p = 0.66, respectively). Furthermore, comparison between the change in the STA and ITA with respect to the above parameters showed equivalent responses in both vessels for each mydriatic agent: T (p = 0.92, p = 0.99, p = 0.35; respectively), TP (p = 0.89, p = 0.96, p = 0.62; respectively), and C (p = 0.87, p = 0.35, p = 0.56; respectively). The provoked retinal vascular reactivity response to standardized isocapnic hyperoxia was equivalent irrespective of the agent used to achieve mydriasis.

Assessment of retinal and choroidal blood flow changes using laser Doppler flowmetry in rats

PURPOSE

A new noninvasive laser Doppler flowmetry (LDF) probe (one emitting fiber surrounded by a ring of eight collecting fibers, 1-mm interaxis distance) was tested for its sensitivity to assess the retinal/choroidal blood flow variations in response to hypercapnia, hyperoxia, diverse vasoactive agents and following retinal arteries photocoagulation in the rat.

MATERIALS AND METHODS

After pupil dilation, a LDF probe was placed in contact to the cornea of anesthetized rats in the optic axis. Hypercapnia and hyperoxia were induced by inhalation of CO(2) (8% in medical air) and O(2) (100%) while pharmacological agents were injected intravitreously. The relative contribution of the choroidal circulation to the LDF signal was estimated after retinal artery occlusion by photocoagulation.

RESULTS

Blood flow was significantly increased by hypercapnia (18%), adenosine (14%) and sodium nitroprusside (16%) as compared to baseline values while it was decreased by hyperoxia (-8%) and endothelin-1 (-11%). Photocoagulation of retinal arteries significantly decreased blood flow level (-45%).

CONCLUSIONS

Although choroidal circulation most likely contributes to the LDF signal in this setting, the results demonstrate that LDF represents a suitable in vivo noninvasive technique to monitor online relative reactivity of retinal perfusion to metabolic or pharmacological challenge. This technique could be used for repeatedly assessing blood flow reactivity in rodent models of ocular diseases.

The impact of cataract on the quantitative, non-invasive assessment of retinal blood Flow

PURPOSE

The aim of the study was to determine the impact of cataract on the quantitative, non-invasive assessment of retinal blood flow assessed by bidirectional laser Doppler flowmetry and simultaneous vessel densitometry.

METHODOLOGY

Ten patients scheduled for extracapsular cataract extraction using phacoemulsification and intraocular lens implantation between the ages of 61 and 84 (mean age 73 years, SD ± 8) were prospectively recruited. Two visits were required to complete the study; one visit prior to extracapsular cataract extraction and one at least 6 weeks after the surgery to allow for sufficient postoperative recovery. The severity of cataract was documented using the Lens Opacity Classification System (LOCS, III) at the first visit. Retinal arteriolar hemodynamics were measured at both visits using the high-intensity setting of the Canon Laser Blood Flowmeter.

RESULTS

All eyes showed no clinical signs of postoperative intraocular inflammation. The quantitative assessment of retinal arteriolar diameter and blood flow were reduced following extracapsular cataract extraction (Wilcoxon signed-rank test, p = 0.022 and p=0.028, respectively); however, centreline blood velocity was not significantly changed (Wilcoxon signed-rank test, p=0.074). Intraocular pressure was unchanged pre- and postcataract extraction.

CONCLUSIONS

Retinal vessel densitometry assessment in the presence of cataract results in the erroneous elevation of the diameter measurement and thereby the calculation of blood flow. The bidirectional Doppler assessment of blood velocity appears to be more robust to light scatter induced by cataract. Care needs to be exercised in the interpretation of studies of retinal vessel diameter or blood flow that utilize similar densitometry techniques.

Relative magnitude of vascular reactivity in the major arterioles of the retina

The relative magnitude of vascular reactivity to inhaled gas stimuli in the major retinal arterioles has not been systematically investigated. The purpose of this study was to compare the magnitude of retinal vascular reactivity in response to inhaled gas provocation at equivalent measurement sites in the superior-, and inferior-, temporal retinal arterioles (STA, ITA). One randomly selected eye of each of 17 healthy volunteers (age 24.4 ± 4.7) was prospectively enrolled. Volunteers were connected to a sequential gas delivery circuit and a computer-controlled gas blender (RespirAct™, Thornhill Research Inc., Canada) and underwent an isocapnic hyperoxic challenge i.e. P(ET)O(2) of 500 mm Hg with P(ET)CO(2) maintained at 38 mm Hg during baseline and hyperoxia. Four retinal hemodynamic measurements were acquired using bi-directional laser Doppler velocimetry and simultaneous vessel densitometry (Canon Laser Blood Flowmeter, CLBF-100, Japan) at equivalent positions on the STA and ITA. Statistical analysis was performed using linear mixed-effect models. During the hyperoxic phase, the vessel diameter (STA p=0.004; ITA p=0.003), blood velocity (STA p<0.001; ITA p<0.001) and flow (STA p<0.001; ITA p<0.001) decreased in both the STA and the ITA relative to baseline. The diameter, velocity and flow were equivalent between STA and ITA at baseline and during hyperoxia; and their magnitude of change remained comparable with hyperoxia (p>0.05). The magnitude of retinal arteriolar vascular reactivity in response to isocapnic hyperoxic inhaled gas challenge was not significantly different between the STA and ITA. However, the correlation analysis did not reveal a significant relationship between the percentage changes in diameter, velocity and flow of the STA and ITA and did not demonstrate equal responses from the STA and ITA to gas provocation.

Retinal arteriolar and capillary vascular reactivity in response to isoxic hypercapnia

The aim of the study was to compare the magnitude of vascular reactivity of the retinal arterioles in terms of percentage change to that of the retinal capillaries using a novel, standardized methodology to provoke isoxic hypercapnia. Ten healthy subjects (mean age 25 years, range 21-31) were recruited. Subjects attended a single visit comprising two study sessions separated by 30 min. Subjects were fitted with a sequential re-breathing circuit connected to a computer-controlled gas blender. Each session consisted of breathing at rest for 10 min (baseline), increase of P(ET)CO(2) (maximum partial pressure of CO(2) during expiration) by 15% above baseline whilst maintaining isoxia for 20 min, and returning to baseline conditions for 10 min. Retinal hemodynamic measurements were performed using the Canon Laser Blood Flowmeter and the Heidelberg Retina Flowmeter in random order across sessions. Retinal arteriolar diameter, blood velocity and flow increased by 3.3%, 16.9% and 24.9% (p<0.001), respectively, during isoxic hypercapnia. There was also an increase of capillary blood flow of 34.8%, 21.6%, 24.9% (p< or =0.006) at the optic nerve head neuroretinal rim, nasal macula and fovea, respectively. The coefficient of repeatability (COR) was 5% of the average P(ET)CO(2) both at baseline and during isoxic hypercapnia and was 10% and 7% of the average P(ET)O(2) (minimum partial pressure of oxygen at end exhalation), respectively. The overall magnitude of retinal capillary vascular reactivity was equivalent to the arteriolar vascular reactivity with respect to percentage change of flow. The magnitude of isoxic hypercapnia was repeatable.

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.

Impact of simulated light scatter on the quantitative, noninvasive assessment of retinal arteriolar hemodynamics

We determine the impact of artificial light scatter on quantitative, noninvasive assessment of retinal arteriolar hemodynamics. One eye from each of 10 healthy young subjects between the ages of 18 and 30 (23.6+/-3.4) is randomly selected. To simulate light scatter, cells comprising a plastic collar and two plano lenses are filled with solutions of differing concentration of polystyrene microspheres (Polysciences Inc., USA). We prepare 0.002, 0.004, 0.006, and 0.008% microsphere concentrations as well as distilled water only. The Canon laser blood flowmeter (CLBF) is used to noninvasively assess retinal arteriolar blood flow. After a preliminary screening to confirm subject eligibility, seven arteriolar blood flow measurements are taken by randomly placing the cells between the instrument objective lens and the subjects' cornea. To achieve a baseline, subjects are first imaged with no cell in place. Both low- and high-intensity CLBF laser settings are assessed. Our light scatter model results in an artifactual increase of retinal arteriolar diameter (p<0.0001) and thereby increased retinal blood flow (p<0.0001). The 0.006 and 0.008% microsphere concentrations produce significantly higher diameter and flow values than baseline. Centerline blood velocity, however, is not affected by light scatter. Retinal arteriolar diameter values are significantly less with the high-intensity laser than with the low-intensity laser (p=0.0007). Densitometry assessment of vessel diameter is increasingly impacted as the magnitude of artificial light scatter increases; this effect can be partially negated by increasing laser intensity. A cataract is an inevitable consequence of aging and, therefore, care must be exercised in the interpretation of studies of retinal vessel diameter that use similar densitometry techniques.