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OBJECTIVE: To investigate retinal lesions in children with severe falciparum malaria. METHODS: Color photography and fluorescein angiography were performed in consecutive children admitted to a pediatric high-dependency unit in Kenya during 1 malaria season. The presence and category of retinal lesion was compared with disease severity, clinical outcome, anemia, lactic acidosis, and parasite count. RESULTS: Twenty-six patients with cerebral malaria and 14 patients who were prostrate were studied. Thirty-one of the patients had clinical features of ocular disease, including round, flame-shaped, and white-centered hemorrhages; peripheral and foveal retinal opacification; peripheral vascular occlusion; venous dilation; disc edema with hyperemia; and arterial pulsatility. Of 8 patients with retinal opacification, only 2 showed small, infrequent zones of capillary nonperfusion on fluorescein angiography; the leakage of dye at sites of opacification was not seen. Retinal opacification was significantly associated with a higher parasite count (P < .02). White-centered hemorrhages were significantly associated with a higher parasite count (P < .05), severe disease (p < .05), and severe anemia (P < .02). CONCLUSIONS: The blood-retina barrier and retinal vascular flow remain substantially normal despite widespread pathological features. Retinal features in children with severe malaria are consistent with cellular hypoxia, nutritional deficiency, or both rather than with vascular occlusion; they support the concept of metabolic steal by parasites.

Much of our basic knowledge of retinal blood flow regulation is based on data obtained from animal experiments through the use of invasive techniques. However, during the last decades, major developments in the field of optics and lasers have led to a variety of noninvasive techniques, which have been applied to the human eye for the investigation of retinal hemodynamics, and more specifically the regulation of retinal blood flow in response to a number of physiological and pharmacological stimuli. The Retinal Vessel Analyzer has markedly simplified the measurement of the diameter of retinal vessels, as well as the change in this diameter evoked by various physiological stimuli (dynamic measurements). Bidirectional laser Doppler velocimetry allows the measurement of absolute red blood cell centerline velocity, which, when combined with the diameter allows the calculation of retinal blood flow in the main retinal vessels. Laser Doppler flowmetry and laser speckle flowgraphy are techniques that measure the velocities of blood in discrete areas of the retinal tissue microcirculation. Adding a scanning capability, a spatial map of velocities across the retinal tissue is obtained. The blue-field simulation technique allows the quantification of the velocity, number and velocity pulsatility of leukocytes moving in the retinal capillaries of the macular region. With color Doppler imaging, the peak systolic and end-diastolic values of blood velocity in the ophthalmic and central retinal artery are measured, from which a resistivity index is obtained. These techniques may help better understand the role of altered retinal blood flow and its regulation in the pathogenesis of retinal diseases of vascular origin.

Purpose: To investigate the change in subfoveal choroidal blood flow in patients with glaucoma and to assess the effect of myopia, as one of the vascular risk factors for glaucoma on this flow. Methods: Subfoveal choroidal blood flow in groups of 12 myopic and glaucomatous eyes has been investigated by means of the laser Doppler flowmetry (LDF), comparing the results with those of 17 myopic eyes without glaucoma, 34 non-myopic glaucomatous eyes and of 50 control eyes. The subfoveal choroidal LDF parameters, that is, blood velocity (ChBVel), volume (ChBVol), and flow (ChBF), as well as the vascular resistance were studied in each group. Statistical analysis was performed by means of anova and t-test according to the Bonferroni procedure for multiple comparisons. Pearson correlation was used to establish the correlations between the hemodynamic parameters and the degree of myopia in dioptres. Results: All LDF parameters (ChBVel, ChBVol and ChBF) were significantly reduced in glaucomatous patients (1.3 ± 0.4, 0.14 ± 0.06 and 4 ± 2 respectively) and myopic patients with primary open-angle glaucoma (POAG) (1.3 ± 0.4, 0.08 ± 0.04 and 2 ± 0.7 respectively) and without POAG (1.2 ± 0.3, 0.11 ± 0.08 and 2 ± 1 respectively) in comparison with age-matched controls (1.5 ± 0.4, 0.27 ± 0.1 and 8 ± 2 respectively). On the other hand, the choroidal vascular resistance (Rm) was increased in the previously described studied patients groups (16 ± 7, 26 ± 9 and 24 ± 9 respectively) compared with controls (7 ± 2). The LDF parameters did not differ significantly between myopic subjects without and with POAG (p = 0.09, p = 0.09, p = 0.2, p = 0.08 and p = 0.9 respectively). Compared to patients with emmetropic glaucomatous, significant reduction in the ChBVol and ChBF and increased Rm were recorded in patients with glaucomatous myopia (p = 0.05, p = 0.04 and p = 0.04 respectively). Pearson correlation demonstrated a significant correlation between the degree of myopia in dioptres and the ChBF (p = 0.012). Conclusions: The subfoveal choroidal LDF parameters were reduced in patients with POAG and myopia. Theses alterations are more in glaucomatous patients with myopia in comparison with age-matched glaucomatous patients without myopia. These data suggest that the impaired choroidal circulation caused by myopia might be an important additional risk factor involved in the glaucomatous damaging process.

This observational study is conducted to demonstrate optic nerve head (ONH) blood flow alterations during extracorporeal circulation (ECC) in routine on-pump cardiovascular operations in order to evaluate the perfusion status of important autoregulatory tissue vascular beds during moderate hypothermia. Twenty-one patients free from eye disease were prospectively enrolled in our database. Perioperative ONH blood flow measurements were performed using a hand-held portable ocular laser Doppler flowmeter just after administration of general anesthesia and during cardiopulmonary bypass (CPB) upon the lowest temperature point of moderate hypothermia. Important operative flow variables were correlated to optic nerve blood flow during surgical phases. Statistical analysis showed significant reduction of 32.1 ± 14.5% of mean ONH blood flow in phase 2 (P < 0.0001) compared to the reference flow values of phase 1. A negative univariate association between ECC time and ONH blood flow in phase 2 (P = 0.031) is noted. This angiokinetic approach can detect changes of flow within autoregulatory vascular tissue beds like ONH, thus creating a 'window' on cerebral microvasculature. ONH blood flow is reduced during CPB. Our data suggest that it is of paramount importance to avoid extracorporeal prolongation even in moderate hypothermic cardiovascular operations.

Cerebral malaria (CM) is a disease of high mortality worldwide. It can be associated with malarial retinopathy (MR) resulting from impaired perfusion within the retinal microvasculature. Areas of capillary non-perfusion (CNP) appear white (retinal whitening) on ophthalmoloscopy. In this study, electrophysiological investigations were performed to investigate the physiological consequences of these hypoxic and ischaemic changes.Children admitted with CM were assessed for inclusion in the study. Those with MR underwent further detailed fundus assessment to quantify retinal whitening and were then designated a severity score. Electrophysiological recordings were performed using a miniganzfeldt stimulator with calibration to the International Society for Clinical Electrophysiology of Visual (ISCEV) standards. ERG data were then analysed with respect to presence of MR and also graded disease severity.Thirty-one children were recruited with a diagnosis of CM, 20 had MR (group 1), and 11 had absent MR (group 2). Statistical analyses of these two groups showed a significant relationship between reduced single flash cone b wave amplitude (CBWA) and increased severity of retinal whitening/CNP (P0.05). Cone and maximal response b : a wave ratios remained1 in all subjects.Retinal whitening/CNP in MR is associated with significant changes in ERG cone b wave function. The relatively high b : a ratio is compatible with the high frequency of MR resolution without sequelae.

Publisher Summary The eye fundus in higher mammals is supplied by two distinct vascular systems, namely the retinal and the uveal systems. Both systems are derived from the ophthalmic artery (OA), a branch of the internal carotid. The uveal system includes the anterior uvea, which comprises the iris and the ciliary body, and the choroid. The retina has a dual blood supply, the inner two-thirds being nourished by branches from the central retinal artery (CRA), whilst the outer third is nourished by the choroidal circulation. Quantitative information on blood velocity and flow in the normal human eye under physiological conditions has been obtained from the various ocular vascular systems using a number of different techniques. In the human eye, two vascular systems are present which differ anatomically and physiologically: the retinal vessels, which supply the neural region of the retina and the prelaminar portion of the ON and the uveal or ciliary blood vessels, which supply the rest of the eye. The rate of blood flow in the intraocular tissues is determined by the ocular perfusion pressure (OPP) and the vascular resistance (R). R depends upon the blood viscosity, length, and diameter of the blood vessels. The diameter of the resistance vessels is modulated by the interaction of multiple control mechanisms affecting the smooth muscle and vascular pericytes: myogenic, metabolic, neurogenic, humoral, and endothelium-derived factors. The determinants of ocular blood flow include systemic, local, neural, endothelial, endocrine, and paracrine factors.

The retina and optic nerve are both optically accessible parts of the central nervous system. They represent, therefore, highly valuable tissues for studies of the intrinsic physiological mechanism postulated more than 100 years ago by Roy and Sherrington, by which neural activity is coupled to blood flow and metabolism. This article describes a series of animal and human studies that explored the changes in hemodynamics and oxygenation in the retina and optic nerve in response to increased neural activity, as well as the mechanisms underlying these changes. It starts with a brief review of techniques used to assess changes in neural activity, hemodynamics, metabolism and tissue concentration of various potential mediators and modulators of the coupling. We then review: (a) the characteristics of the flicker-induced hemodynamical response in different regions of the eye, starting with the optic nerve, the region predominantly studied; (b) the effect of varying the stimulus parameters, such as modulation depth, frequency, luminance, color ratio, area of stimulation, site of measurement and others, on this response; (c) data on activity-induced intrinsic reflectance and functional magnetic resonance imaging signals from the optic nerve and retina. The data undeniably demonstrate that visual stimulation is a powerful modulator of retinal and optic nerve blood flow. Exploring the relationship between vasoactivity and metabolic changes on one side and corresponding neural activity changes on the other confirms the existence of a neurovascular/neurometabolic coupling in the neural tissue of the eye fundus and reveals that the mechanism underlying this coupling is complex and multi-factorial. The importance of fully exploiting the potential of the activity-induced vascular changes in the assessment of the pathophysiology of ocular diseases motivated studies aimed at identifying potential mediators and modulators of the functional hyperemia, as well as conditions susceptible to alter this physiological response. Altered hemodynamical responses to flicker were indeed observed during a number of physiological and pharmacological interventions and in a number of clinical conditions, such as essential systemic hypertension, diabetes, ocular hypertension and early open-angle glaucoma. The article concludes with a discussion of key questions that remain to be elucidated to increase our understanding of the physiology of ocular functional hyperemia and establish the importance of assessing the neurovascular coupling in the diagnosis and management of optic nerve and retinal diseases.

An ocular fundus reflectometer to measure non-invasively the light reflectance changes in response to diffuse flicker stimulation at two wavelengths (569 and 600 nm) simultaneously is presented. Image registration to minimize the effect of involuntary eye motion is described. Two-dimensional maps of reflectance changes of the optic disc and peripapillary retinal tissues in healthy volunteers are presented. These functional maps suggest increases in blood volume during stimulation, with changes in the optic disc as large as 10.7% at 569 nm and 3.5% at 600 nm.

A fundus camera-based instrument to investigate increased retinal neuronal activity effect on optic nerve blood flow is described. It incorporates (a) near infrared fundus illumination and observation, (b) near infrared laser Doppler flowmeter (LDF), (c) pupil position monitoring and (d) delivery of visual stimuli. Two types of stimulation are currently used: diffuse heterochromatic and chromatic luminance flicker generated by light emitting diodes, and contrast reversal pattern displayed on a video monitor. Recordings of the changes in the LDF parameters (blood velocity, volume and flow) from the optic nerve in response to these stimuli illustrate the potential of the technique.

A non-invasive functional imaging technique of the nervous tissue of the human eye fundus based on two wavelength light reflectance changes (DR %) is presented. The effect of 308 diffuse luminance flicker field made of two spectral components (569 and 600 nm) on DR was studied in six normal volunteers. Group averaged DR was significant and similar in all quadrants of the optic disc at 569 nm with a mean equal to 210.7% for the tissue of the optic disc and 2 3.6% for the peripapillary regions. At 600 nm, the DR were also significant but smaller, i.e. 2 3.5% for the disc tissue and 21.7% for the peripapillary region. The changes at 569 and 600 nm represent most probably blood volume changes. The 600 nm reflectance did not show evident features suggesting the presence of flicker-induced oxygenation changes. q 2004 Elsevier Ireland Ltd. All rights reserved.

Near-infrared laser Doppler flowmetry was applied in 15 normal volunteers to record the time course and magnitude of changes in the velocity (Vel), volume (Vol) and flow (F) of blood and tissue reflectance (R) at the optic disk in response to 40 and 50 s of increased retinal neural activity. This activity was evoked by diffuse luminance flicker of the retinal posterior pole. After 20 s of flicker, the group averages of Vel, Vol, and F were significantly higher than at baseline (pre-flicker) by 12, 24 and 38%. Time constants of the increases in Vel, Vol, and F were 3.4, 12.7 and 9.1 s, respectively. The group average change in R of 1% was not significant. However, in one subject, 15 recordings from the same site of the optic disk showed a significant increase in R of 8%, with a time course similar to that of Vol. Our findings show that, in the human optic nerve, a white matter tissue, the temporal dynamics and magnitude of the response of blood flow to an increase in retinal neural activity are similar to those reported for brain gray matter. Furthermore, although the R-response could be due, in part, to changes in blood volume, other factors, such as activity-evoked tissue scattering changes, may also affect this response.

The purpose of the present study was to investigate the contribution of basal nitric oxide (NO) on retinal vascular tone in humans. In addition, we set out to elucidate the role of NO in flicker-induced retinal vasodilation in humans. Twelve healthy young subjects were studied in a three-way crossover design. Subjects received an intravenous infusion of either placebo or NG-monomethyl-l-arginine (l-NMMA; 3 or 6 mg/kg over 5 min), an inhibitor of NO synthase. Thereafter, diffuse luminance flicker was consecutively performed for 16, 32, and 64 s at a frequency of 8 Hz. The effect of l-NMMA on retinal arterial and venous diameter was assessed under resting conditions and during the hyperemic flicker response. Retinal vessel diameter was measured with a Zeiss retinal vessel analyzer. l-NMMA significantly reduced arterial diameter (3 mg/kg: –2%; 6 mg/kg: –4%, P < 0.001) and venous diameter (3 mg/kg: –5%; 6 mg/kg: –8%, P < 0.001). After placebo infusion, flicker induced a significant increase in retinal vessel diameter ( P < 0.001). At a flicker duration of 64 s, arterial diameter increased by 4% and venous diameter increased by 3%. l-NMMA did not abolish these hyperemic responses but blunted venous vasodilation ( P = 0.017) and arterial vasodilation ( P = 0.02) in response to flicker stimulation. Our data indicate that NO contributes to basal retinal vascular tone in humans. In addition, NO appears to play a role in flicker-induced vasodilation of the human retinal vasculature.

To investigate the effects of quinine on the electroretinograms (ERGs) of children with cerebral malaria (CM), we recruited subjects during a single malaria season in Blantyre, Malawi. Seventy ERG investigations were performed, on 34 children with CM. Time recorded from completion of the most recent quinine infusion was termed quinine elapsed time (QET). In a subgroup of 16 children, whole-blood quinine concentrations were estimated in a sample of capillary blood, for validation. A significant positive association was found between QET and both maximal-response A-wave amplitude (MRAWA; P = .03) and cone A-wave amplitude (P = .04). Longitudinal analysis demonstrated a significant trend of increasing MRAWA with increasing QET (P = .03). Parenteral quinine administered in therapeutic doses to a pediatric population appears to cause a transient depression in photoreceptor function. No evidence of ocular quinine toxicity was found at the therapeutic doses used.

The pathogenesis of cerebral malaria (CM), a significant cause of death in the tropics, is still not understood. Cerebral blood flow measurements would be important but are difficult under the conditions prevailing in CM clinics in the tropics. With the goal of using optic nerve head (ONH) blood flow (F(onh)) instead of cerebral blood flow to help outcome prediction, we have tested the feasibility of performing F(onh) measurements in comatose CM children, using a portable ocular laser Doppler flowmeter (LDF). Measurements were performed in one eye of each of 13 children (2.7 +/- 1.1 years) during a period of about 8 min. The Vel(onh) (the relative blood velocity), Vol(onh) (the relative blood volume), and F(onh) values were determined for each child from three to five measurements of 2 to 20 s. Average Vel(onh), Vol(onh), and F(onh) values were 0.49 +/- 0.08 kHz, 2.4 +/- 1.06 arbitrary units (a.u.), and 89 +/- 16.8 a.u., respectively. The average coefficients of variation of the flow parameters based on all segments in each child were 13 +/- 9% (range 1-29%) for Vel(onh), 27 +/- 13% (range 3-65%) for Vol(onh), and 23 +/- 12% (range 5-42%) for F(onh). This study demonstrates the feasibility of LDF measurements in CM children. The large range of these coefficients of variations could be due to the presence of fluctuations of ONH blood flow on a short time scale in these neurologically unstable children.

BACKGROUND: To investigate the effect of isovolumic hemodilution on the tissue oxygenation of the optic nerve head (ONH). MATERIAL AND METHODS: In 9 miniature pigs (6 - 12 kg), hemodilution was performed by replacing 100 - 140 ml of blood by an equivalent volume of 6 % hydroxyethyl starch in saline. The blood flow parameters in the ONH microcirculation, namely velocity (BVel), volume (BVol) and flow (BF), were measured by laser Doppler flowmetry. Microelectrodes placed at approximately 50 micrometer from the disk surface and the phosphorescence quenching technique were used to measure pO 2 in the vitreous (pO 2 (vitr)) and in the ONH capillary blood (pO 2 (blood)), respectively. Arterial blood pressure, hematocrit (Hct) and gas content were documented. RESULTS: In all animals, hemodilution reduced Hct by 30 +/- 6 %. All the flow parameters increased markedly. Simultaneously, pO 2 (blood) (baseline: 31 +/- 3 mm Hg) dropped to approximately 75 % of its initial value, while pO 2 (vitr) (baseline: 26 +/- 8 mm Hg) increased by 15 +/- 8 %. CONCLUSIONS: Hemodilution leads to an increase of pO 2 (vitr) reflecting a similar rise of the ONH tissue pO 2. This effect results from an enhanced blood perfusion of the ONH, which over-compensates the decrease of the blood O 2 content.

Purpose To investigate the impact of different gas mixtures (hyperoxia, hypoxia, and hypercapnia) on the optic nerve head (ONH) and choroidal (Ch) hemodynamics. Methods Twenty-three healthy subjects (28 ± 6 years) took part in the study. Variations in inspired oxygen and carbon dioxide fraction were produced by a gas mixing device. Arterial oxygen saturation (SpO2) was measured continuously using a transcutaneous sensor and end-tidal carbon dioxide partial pressure by capnography. The experiment comprised three successive periods: 3-minute baseline (room air breathing), 15-minute gas mixture inhalation (normocapnic hypoxia, hypercapnia, or hyperoxia), and 15-minute recovery (room air breathing). Laser Doppler flowmeter parameters-velocity (VEL), volume (VOL), and flow (BF) of red blood cells-were measured. Two-way ANOVAs were performed for statistical analysis. Results In response to hyperoxia, ONHBF significantly decreased by -18% ± 6% (P = 0.04) from baseline, due to significant changes in VEL (-12% ± 3% P = 0.0002). During hypoxia at 85% SpO2, ONH VEL increased by +12% ± 3% (P = 0.0009), whereas VOL and BF did not change significantly. ChBF significantly increased by +7% ± 2% (P = 0.004) in response to hypoxia, due to significant changes in VEL +5% ± 2% (P = 0.03). Both Ch and ONHBFs did not vary significantly in response to hypercapnia. Conclusions The magnitude of the blood flow response is the most significant during hyperoxia for ONH and hypoxia for ChBF. For ONHBF, a 37% difference between hyperoxia and hypoxia can be useful when vasoreactivity to O2 will be tested in patients.

The study of the hemodynamics in the human eye requires the measurement of blood flow in the various tissues of this organ. Ideally, the measurement technique should be reproducible, accurate, sensitive enough to be able to reveal early pathological alterations, and clinically applicable. Furthermore, its spatial resolution should permit flow measurement in the microcirculation without interference from the larger vessels and its temporal response should be fast enough to allow the investigation of the process of flow regulation evoked by various physiological stimulations.

Background: The precise and rapid measurement ofeye length and eye shape isessential for investigating eye growth regulation andmyopia. For this purpose, we developedan optical low coherence reflectometer (OLCR) andpresent preliminary measurements.Methods: The OLCR includes a super luminescentdiode (wavelength: 845 nm,coherence length: ∼30 μm) and rotatingglass cube to produce longitudinal scansat a velocity of 0.42 m/s and a repetition rateof ∼13 scans/s. Heterodyne detection oflight reflected from the anterior cornea andthe posterior retina permits to measure axial eyelength and eye shape (off-axis eye length).Each measurement consists of five consecutivescans. Reproducibility and precision weredetermined in one volunteer by measuring axialeye length five consecutive times, each timerepositioning the eye. Eye shapes weredetermined in right eyes of four volunteers bymeasuring eye length every 3.3° from10° nasally to 10° temporally.Results: Axial eye length measured repeatedlyin one volunteer did not differ between orwithin the measurements (one-factor ANOVA). Theaverage standard deviation was11 μm. Eye shapes (a) varied substantiallyamong subjects and (b) differed considerablyfrom the corresponding shapes of sphericalmodel eyes with identical axial eye lengths.Conclusion: The newly developed OLCRpermits the precise and rapid measurement ofeye length and eye shape. Such measurements,especially in children, may provide importantinformation about mechanisms of eye growthregulation and the development of myopia.

Purpose. Arterial carbon dioxide tension and arterial oxygen tension are important determinants of retinal and cerebral blood flow. In the present study the hypothesis that changes in arterial blood gases also influence choroidal blood flow was tested. Methods. The effect of breathing different mixtures of oxygen (O 2) and carbon dioxide (CO 2) on choroidal blood flow in the foveal region was investigated in healthy subjects. The study was performed in a randomized, double-masked four way cross-over design in 16 subjects. Using a compact laser Doppler flowmeter, red blood cell velocity (ChBVel), volume (ChBVol), and flow (ChBF) in the choroidal vasculature were measured during the breathing of various mixtures of O 2 and CO 2 (hyperoxia-hypercapnia): 100% O 2, 97% O 2 + 3% CO 2, 95% O 2 + 5% CO 2 (carbogen) and 92% O 2 + 8% CO 2. Arterial oxygen tension (pO 2) and carbon dioxide tension (pCO 2) were measured from arterialized blood samples from the earlobe. Results. Breathing 100% O 2 had no significant e...

BACKGROUND Previous studies have demonstrated, in the cat, a vasodilatation of retinal vessels in response to neuronal activity induced by diffuse luminance flicker. The aim of this study was to determine whether a similar diameter variation is detectable in humans. MATERIALS AND METHODS Nine normal subjects were exposed to 1 min of sinusoidally varying diffuse luminance flicker (10 Hz, 30 degrees around optic nerve head). Monochromatic fundus pictures before and after the stimulation were taken. The diameter of retinal arteries and veins was measured on the digitised photographs with the NIH-Image software and an own algorythm. RESULTS The diameter immediately after flicker was significantly larger than the pre-stimulus diameter by 4.2 +/- 2.2% (p < 0.014) (mean +/- SD) for the retinal arteries and 2.7 +/- 1.7% (p < 0.001) for the retinal veins. Six seconds after cessation of the flicker, arterial diameter was not significantly different from that of pre-flicker value. CONCLUSIONS Diffuse luminance flicker induces an increase in retinal vessel diameter. This suggest that retinal blood flow is coupled with neuronal activity as previously evidenced by the blue field simulation technique in the macula.

Increased neuronal activity induced by diffuse flickering illumination of the ocular fundus generates increases in blood flow and decreases in tissue pO 2 in the optic nerve head (ONH). This suggests that in the ONH, as in the brain, a coupling exists between function, perfusion, and metabolism, in accordance with the hypothesis of Roy and Sherrington. Following a review of the characteristics of the activity-induced blood flow response in the cat ONH, the relationship between blood flow and metabolism is discussed. We then examine possible mediators of the neurovascular coupling, some of the factors which may interfere with it, and point out various problems which remain to be addressed in order to better understand this coupling.

Objective: To investigate capillary blood flow in the optic nerve head (ONH) of children with cerebral malaria. Methods: Malawian children with cerebral malaria admitted to a paediatric research ward were examined by direct and indirect ophthalmoscopy. ONH blood flow was measured using laser Doppler flowmetry (LDF) in suitable patients. Mean blood volume and velocity were obtained from 30 to 60 s recordings from the temporal ONH and used to calculate blood flow. These were compared with admission variables, funduscopic findings and disease outcomes. Results: 45 children with cerebral malaria had LDF recordings; 6 subsequently died and 5 survivors had neurological sequelae. 12 (27%) had papilloedema. The mean microvascular blood volume was higher in patients with papilloedema (3.28 v 2.54 arbitrary units, p = 0.002). The blood velocity correlated directly with haematocrit (r = 0.46, p = 0.001) and inversely with blood glucose (r = −0.49, p = 0.001). Conclusion: The increase in ONH microvascular blood volume in papilloedema measured by LDF is consistent with current theories of pathogenesis of papilloedema. LDF has potential as a tool to distinguish papilloedema from pseudopapilloedematous disc swellings. The relationship between blood velocity and haematocrit may relate to levels of sequestration in cerebral malaria.

• Background: Studies in animals have demonstrated that optic nerve head (ONH) blood flow (Fonh) is autoregulated, but there is a lack of evidence for such a process in humans. Therefore, we investigated the relationship between Fonh and mean ocular perfusion pressure (PPm) in normal volunteers when PPm is decreased through elevation of the intraocular pressure (IOP). • Methods: Laser Doppler flowmetry (LDF) was used to measure relative mean velocity (Velohn), volume (Volonh) and Fonh of blood at sites of the ONH away from visible vessels, while PPm was decreased in two ways: (1) rapidly, by IOP increments of 15 s duration, and (2) slowly, by IOP increments of 2 min duration, both by scleral suction cup in one eye of each of nine subjects. • Results: A rapid and large decrease of PPm of more than 100% induced a decrease of more than 80% in Fonh. With the slower decrease in PPm Fonh remained constant down to a PPm of ≃22 mm Hg (IOP=40 mm Hg) and then decreased, predominatly due to a decrease in Velohn. Immediately after removal of the suction cup, Fonh increased transiently by 44% above baseline. • Conclusions: This study demonstrates efficient blood flow autoregulation in the OHN, which is probably brought about by an increase in vascular capacitance. The magnitude of the reactive hyperaemia agrees with the compensatory decrease in ONH vascular resistance during IOP elevation. The time scale of the autoregulatory process and the dependence of the hyperaemia upon duration of IOP elevation suggest a metabolic mechanism of autoregulation.

Purpose This study investigates the effect of increased ocular perfusion pressure (OPP) on optic nerve head (ONH) hemodynamics. Methods In 21 healthy subjects, the increase in arterial blood pressure (BP), measured continuously using a pneumatic transcutaneous sensor, was produced by isometric exercise consisting of 2 minutes of hand-gripping. ONH blood flow parameters-namely the velocity (Vel), number (Vol), and flux (F) of red blood cells-were measured using the laser Doppler flowmeter (LDF). Results In those 14 healthy subjects who exhibited a similar increase in BP to handgrip superior to 30% of baseline BP, group average increases of BP and OPP amounted to 34% ± 3% (SEM) and 43% ± 3%, respectively. The increase in F of 19% ± 8%, resulting from an increase in Vel (17% ± 7%) and Vol (6% ± 7%), was significantly less than predicted for a passive autoregulatory response, as revealed also by the increase in vascular resistance (R = OPP/F). Spearman test of linear correlations between F and time during handgrip led to the identification of one group of eight subjects (with a stable F) and one group of six subjects (with an increase in F). A closed-loop gain (G) of the regulatory process, defined as G = 1 - {(F - Fbl)/Fbl}/{(OPP - OPPbl)/OPPbl}, was found to be rather independent from the OPP, with an average value 0.7 ± 0.07. G was 0.83 ± 0.06 for the group of eight subjects with stable F and 0.3 ± 0.15 for the group of six subjects with F increasing with the OPP. Conclusions The continuous recording of both BP and LDFs represents a novel and more precise approach to the characterization of ONH hemodynamics during isometric exercise, especially useful in the future for patients with ocular diseases. The efficiency of the ONH blood flow autoregulation appears to vary significantly between healthy subjects. (ClinicalTrials.gov number, NCT00874913.).

Purpose: The aim of this study was to assess (i) whether pupil dilation with tropicamide influences subfoveal choroidal blood flow, as assessed by continuous laser Doppler flowmetry (LDF) and (ii) if this is the case, whether the effect is due to a haemodynamic response of the drug-induced dilation. Methods: Following the instillation of one drop of 1% tropicamide in one eye of 18 healthy, nonsmoking volunteers (age 20–25 years), the subfoveal choroidal LDF parameters (Vel, Vol and ChBF) were recorded during 30 min, at 3-min intervals under two paradigms: through an artificial pupil (4 mm diameter) placed in front of the cornea (P1) and without this artificial pupil (P2). Results: Tropicamide increased the pupil diameter from 3.3 ± 0.4 mm (mean ± SD) to 8.3 ± 0.4 mm. Full dilation was reached at ∼24 min. During this period of time, linear regression analysis demonstrated that none of the LDF parameters varied significantly (p > 0.05), either under P1 or P2. Based on a group of 12 subjects, the smallest (%) change in the mean value of ChBF (ChBFm) that would be detectable (sensitivity of the method, S) was found to be 2% for P1 and 6% for P2. The average coefficient of variation of ChBFm based on eight measurements during dilation was greater for P2 than for P1 by a factor of approximately 2. Conclusion: Tropicamide had no significant influence on the subfoveal choroidal LDF parameters measured by continuous LDF during pupil dilation. Furthermore, pupil dilation did not affect ChBFm by more than the calculated minimum percentage change of 6% detectable with our method.

The effect of decreased ocular mean perfusion pressure (PPm), defined as mean arterial blood pressure minus intraocular pressure (IOP), on optic nerve head blood flow (Fonh) and on the response of this flow (RFonh) to diffuse luminance flicker was investigated in 19 anesthetized cats using laser Doppler flowmetry. PPm was decreased by increasing the IOP. The flicker stimulus consisted of 20-msec flashes delivered at 10 Hz for 30-60 sec. It illuminated a 30 degrees diameter area of the fundus, centered at the optic disk. Decreasing PPm by 10-35% from its resting value resulted in a 23% increase in RFonh (supranormal RFonh). With further decreases in PPm, RFonh decreased, reaching zero at a PPm below 20 mmHg. Fonh remained constant until PPm was < 40 mmHg and then decreased thereafter. When PPm was brought back to resting value after having been decreased for approximately 45 min, Fonh first increased by approximately 380% and then returned to its value at rest within approximately 4.5 min. At low PPm, hyperoxia decreased Fonh by 23% and restored the attenuated RFonh back to the value at resting PPm and hypoxia did not increase Fonh, as it did at normal PPm. This study confirms that the optic nerve head circulation is autoregulated over a wide range of PPm and reveals, for the first time, a hyperemic response to a prolonged decrease in PPm. It suggests that hypoxia plays a role in abolishing RFonh at low PPm and that the supranormal RFonh at moderately decreased PPm is due to an increase in the flicker-induced ganglion cell activity.

The role of nitric oxide (NO) in the regulation of optic nerve head (ONH) blood flow was investigated from direct electrochemical measurements of NO and simultaneous measurements of ONH blood flow by laser Doppler flowmetry during increased neuronal activity with diffuse luminance flickering light stimuli. For normal physiological conditions, there was a consistent increase in ONH blood flow with each flicker stimulus (n = 10 cats). Simultaneous NO measurements were made in the vitreous humor immediately in front of the ONH (n = 6 cats). During flicker, NO increased by 88 +/- 23 nM (P < 0.05) above basal NO levels. Measurements were repeated after inhibiting NO synthase by intravenous infusion of either NG-nitro-L-arginine (n = 5 cats) or NG-nitro-L-arginine methyl ester (n = 5 cats). After inhibition, NO levels were significantly reduced and both ONH blood flow and NO responses to flicker were significantly attenuated. We conclude that NO plays a key vasodilatory role in the ONH, modulating blood flow during greater neuronal activity with flicker stimuli.

AIMS/BACKGROUND: The retinal circulation was investigated in a group of 19 patients with insulin dependent diabetes mellitus with less than 4 years of disease duration and no evidence of diabetic retinopathy. Results of these patients were compared with those of 16 age-matched normal controls. METHODS: Venous diameter (D) was measured from monochromatic fundus photographs. Maximum erythrocyte velocity (Vmax) was assessed by bidirectional laser Doppler velocimetry in the major retinal veins of one eye of each subject. Total volumetric blood flow rate (QT) was calculated by adding the flow rates of the major retinal veins. RESULTS: Average QT was 12% larger than normal in diabetic patients (one tailed, non-paired Student's t test, p < 0.05). A statistically significant correlation was observed between QT and disease duration (r = 0.35, p < 0.04). Patients with longer disease duration tended to have somewhat larger QT. The average retinal vascular regulatory responses to hyperoxia were not significantly different from normal in diabetic patients. In these patients, however, higher blood glucose levels were associated with decreased regulatory responses to hyperoxia. CONCLUSIONS: Patients with diabetes mellitus of relatively short duration have mildly increased QT, suggesting that increased blood flow may play an early role in the development of diabetic retinal microangiopathy.

Purpose. Laser Doppler interferometry (LDI) permits the measurement of intraocular distances to a precision of better than 20 microm. The signal complex from the posterior segment of the eye consists of four peaks in the chick, an animal frequently used in ocular development studies. The present study sought to identify anatomical landmarks corresponding to these LDI peaks. Methods. Distances obtained with LDI at the posterior pole were compared to axial length components measured with three independent methods: vernier calipers, tissue sections and high frequency A-scan ultrasound. Results. LDI reflections appear to originate from the retinal inner limiting membrane, Bruch's membrane and the inner and outer scleral surfaces. Conclusions. The non-invasive and highly precise nature of LDI measurements enables repetitive and accurate assessment of intraocular distances. Such measurements should prove particularly useful for the assessment of short-term cyclic variations in intraocular distances as well as post-natal eye growth.

We determined the effects of endothelin-1 on optic nerve head (ONH) blood flow in anesthetized cats. Endothelin-1 (50-2500 pmol) injected into the vitreous produced a dose-related and sustained decrease (22 +/- 6% by 500 pmol and 36 +/- 11% by 2500 pmol) in the ONH blood flow. Blood pressure (BP) and heart rate (HR) remained, however, unchanged. In contrast, intravenous (i.v.) injected endothelin-1 (0.004-0.4 nmol kg-1) produced a dose-related and short-lasting increase in the ONH blood flow; its maximum increase by 0.4 nmol kg-1 was 135 +/- 34%. Endothelin-1, at 0.4 nmol kg-1, i.v., produced a transient decrease followed by a more sustained increase in BP, but had no remarkable effect on HR. The increase in ONH blood flow by i.v. injection of endothelin-1 was abolished by N(G)-nitro-L-arginine methyl ester (L-NAME, 5 mg kg-1 min-1, i.v.). The suppression of blood flow by L-NAME was reversed by the addition of L-arginine (50 mg kg-1 min-1). Pressor responses to endothelin-1 (0.4 nmol kg-1, i.v.) were augmented in the presence of L-NAME, but reversed in combination with L-arginine. These findings suggest that i.v. injection of endothelin-1 causes NO release from endothelial cells which increases ONH blood flow, whereas intravitreal injection of endothelin-1 decreases ONH blood flow by its vasoconstrictive effect. Different populations of ET receptors on vascular smooth muscles or endothelial cells would reflect the opposing effects of endothelin-1.

The present study was conducted to assess the feasibility of laser Doppler velocimetry in young infants, as a prelude to ultimately undertaking such measurements in premature infants. A portable, unidirectional laser Doppler velocimeter was developed based on a Kowa RC-2 hand-held fundus camera. Six infants between 1 and 21 weeks of age were studied. Relative red blood cell velocity (δfmax) at the centre of retinal arteries was measured over approximately 10 heart cycles. A pulsatility parameter (P=1−δfmax.dia/δfmax.sys), a summary index of vascular status, was determined from the average diastolic and systolic values of δfmax. Velocity waveforms were obtained in four of the six infants. Arterial pulsatility for the group was 0.63±0.13. Precise non-invasive measurement of arterial red blood cell velocity waveforms in young infants was achieved. The high signal-to-noise ratio and temporal resolution of this data suggest that relative measurements of retinal blood flow may permit assessment of haemodynamic changes in premature infants.

AIMS--The effect of rebreathing into a bag (RB) on retinal macular blood velocity was evaluated in healthy volunteers. METHODS--Ten normal volunteers, whose ages ranged from 17 to 34 years, performed RB over 135 to 260 seconds (mean (SD) 193 (38) seconds) while retinal macular blood velocity was determined non-invasively using the blue field simulation technique. RESULTS--Leucocyte velocity significantly increased (p < 0.05) at 2 minutes and at the end of RB by 53% (42%) and 92% (65%), respectively (95% confidence interval of the mean (CIM)). All subjects observed an increase in the density of leucocytes. At the end of RB, mean systolic brachial arterial pressure and heart rate were significantly increased by 24% (11%) and 37% (15%) respectively (p < 0.01). At 2 minutes, end tidal oxygen concentration in the exhaled air was 47% (8%) (95% CIM) below and carbon dioxide was 41% (16%) above baseline (p < 0.001). The RB produces a large increase in macular leucocyte velocity, suggesting an increase in blood flow. CONCLUSION--Although RB has some systemic risk due to hypoxia and hypercapnia, RB for a short period of 1 or 2 minutes might be of help in the treatment of retinal arterial obstructive diseases in young patients without cardiovascular disorders if other treatments do not show any beneficial effects.

The measurement of blood flow in the ocular fundus is of scientific and clinical interest. Investigating ocular blood flow in the choroid may be important to understand the pathogenesis of numerous ocular diseases, such as glaucoma or agerelated macular degeneration (AMD). Laser Doppler flowmetry (LDF) was applied to measure relative velocity, volume and flux of red blood cells in the tissues of human eye. Its main application lies in the possibility of assessing alterations in blood flow early in the course of diseases. The purpose of the present study was to investigate the effect of pupil dilatation with one drop of 1% tropicamide on blood flow in the foveal region of the choroid of the human fundus. The blood flow parameters were measured in 24 eyes during 30 minutes (one measurement in every 3 minutes) after the application of the drop. Since the Doppler parameters depend on the scattering geometry, which may change as the pupil dilates; an artificial pupil of 4mm in diameter was placed directly in front the eye. Following the administration of tropicamide the mean pupil diameter was increased from 3.29 mm to 8.25 mm (P

PURPOSE: To assess subfoveal choroidal blood flow (ChBF) in patients with early manifest glaucoma (EMG) and to compare blood flow with functional measures of central retinal integrity, standard automated perimetry (SAP) and pattern electroretinogram (PERG). METHODS: Subfoveal ChBF was determined by confocal, real-time laser Doppler flowmetry in 24 patients with EMG [>-6 dB mean deviation (MD), age range: 29-77 years, visual acuity: 20/25-20/20] and 23 age-matched control subjects. All patients had a therapeutically (topical beta-blockers with or without prostaglandin analogues) controlled intraocular pressure (IOP < 20 mmHg). Subfoveal choroidal blood volume (ChBVol), velocity (ChBVel) and ChBF were determined as the average of three 60 second recordings. In all patients and controls, the PERG and SAP (Humphrey 30-2), following standardized protocols, were also recorded. RESULTS: In patients with EMG, reductions in average ChBVel and ChBF were roughly equal, respectively, by 30% and 33.4% (p < 0.01), when compared to control subjects, so that there was no significant difference in ChBVol between the two groups. Pattern electroretinogram amplitudes were reduced by 46% (p < 0.01) in patients compared to controls. No correlation was found between any of the ChBF parameters and PERG amplitude, or Humphrey 30-2 MD and pattern standard deviation. CONCLUSION: The results suggest a significant alteration of both ChBVel and ChBF in EMG, which does not appear to be associated with the severity of central retinal dysfunction. These findings may contribute to a better understanding of the pathophysiology of early glaucomatous damage in EMG and have implications for the treatment of this pathologic condition.

The effects of strict diabetic control on retinal haemodynamics were studied to elucidate whether such effects are associated with retinopathy changes. In 28 patients with poorly controlled insulin dependent diabetes mellitus and non-proliferative retinopathy, retinal haemodynamics were investigated at baseline, 5 days, 2 months, and 6 months after the institution of strict diabetic control using the bidirectional laser Doppler velocimetry technique and monochromatic fundus photography. Changes in retinal blood flow measured in a major retinal vein (Q) on the fifth day of strict diabetic control correlated significantly with changes in retinopathy level observed at the end of the 6 months of this study (rank correlation 0.65, p < 0.01). On the fifth day of strict diabetic control, 16 out of 20 eyes that showed no progression (NP) of retinopathy at the end of the study had decreases in Q, whereas six out of eight eyes that showed progression (P) had increases in Q. The difference in these changes in Q between P and NP eyes was statistically significant (one way analysis of variance, p = 0.001). No significant changes in Q were detected at 2 months or 6 months. Following the institution of strict diabetic control, no significant changes in time were detected in the regulatory response to 100% oxygen breathing characterised as the percentage decrease in Q at 4-6 minutes of oxygen breathing (analysis of variance, p = 0.36). Changes in Q following institution of strict diabetic control are associated with progression of retinopathy. Measurements described in this study may help identify diabetic patients at risk of progression when their metabolic control is improved.

Background: The relationship between the flicker-evoked changes in optic nerve blood flow (F onh ) and neural retinal activity was investigated by laser Doppler flowmetry and electroretinogram (ERG), respectively. Material and methods: In five normal subjects F onh was continuously recorded before, during and after exposure to green flicker modulation (30° field at the posterior pole), at different levels of mean illuminance between 0.9 and 13.5 lux. During flicker stimulation, ERGs were simultaneously recorded with F onh . The flicker-evoked changes in F onh (RF onh ) and the amplitudes of the first (1F) and second (2F) harmonic component of the ERG were measured. Results: By increasing mean flicker illuminance, RF onh and ERG 2F, but not 1F amplitude first increased and then saturated beyond 10 lux. RF onh and the corresponding 2F amplitudes, recorded at the various mean illuminances, showed a significant positive correlation (r = 0.79, p < 0.01). Conclusions: Under specific experimental conditions, changes in human RF onh are quantitatively correlated with those of the flicker ERG 2F amplitudes. Since the 2F component reflects, unlike 1F, a strong contribution from inner retina, the present findings support the presence of an association between vasoactivity and inner retinal activity changes in the human eye. This finding may provide a new approach to the study of neuro-ophthalmic disorders.

Laser Doppler flowmetry (LDF) is a technique used to measure relative average velocity, number and flux (number times velocity) of red blood cells in vessels or capillaries. In this study, the effect of topical timolol on the choroidal circulation was investigated in 12 healthy subjects. Maximum velocity of red blood cells and volumetric blood flow rate in sub-foveal choroids are determined in each eye just before instillation of drops and then every 30 min upto 2 hours. Average intraocular pressure (IOP) decreased significantly in the timolol-treated eyes compared to that of placebo-treated eyes. Nevertheless no significant differences in choroidal blood hemodynamic between timolol and placebo-treated eyes were observed.

PURPOSE: To determine whether subfoveal choroidal blood flow is altered in retinitis pigmentosa (RP) and whether this alteration is associated with central cone-mediated dysfunction. METHODS: In 31 RP patients (age range, 15-72 years) with preserved visual acuity (range: 20/30-20/20), subfoveal choroidal blood flow was measured by real-time, confocal laser Doppler flowmetry, and focal macular (18°) electroretinograms (FERGs) were elicited by 41 Hz flickering stimuli. Twenty normal subjects served as controls. The following average blood flow parameters were determined based on three 60-second recordings: volume (ChBVol), velocity (ChBVel), and flow (ChBF), the last being proportional to blood flow if the hematocrit remains constant. The amplitude and phase of the FERG first harmonic component were measured. RESULTS: On average, ChBF and ChBVel were reduced by 26% (P ≤ 0.02) in RP patients compared to controls, whereas ChBVol was similar in the two groups. FERG amplitudes were reduced by 60% (P < 0.01) in patients compared with controls. FERG phases of patients tended to be delayed (P < 0.08) compared with their values in the controls. In patients, FERG phase delays were correlated (r = 0.50, P < 0.01) with ChBF and ChBVel values. FERG amplitudes were correlated (r = 0.49, P < 0.01) with ChBVol values. CONCLUSIONS: These data indicate significant alterations of subfoveal choroidal hemodynamic in RP and suggest a link between the alteration of ChBF and the RP-associated central cone-mediated dysfunction as assessed by the FERG

Laser Doppler flowmetry (LDF) was performed on a simulated blood vessel in a model eye through optic nerve tissue sections in order to ascertain the ability to detect flow through them. LDF was performed using either near-infra-red or green laser light. Tissue section thickness ranged from 50 μm to 1000 μm. As expected, we found that our ability to detect flow with LDF decreased as we increased the thickness of optic nerve sections interposed between the LDF apparatus and the simulated blood vessel. We also found that the sampled depth of LDF increased with increasing separation of the optical detection fibre from the centre of the illuminated tissue volume. With adequate separation, we were able to detect flow with LDF through tissue sections of 1000 μm thickness using either near-infra-red or green laser light.

Recessed cathode O2 microelectrodes were used to measure spatially detailed oxygen tension (PO2) gradients in the vitreous humor near the cat retina. Measurement sites (n = 41 in 8 cats) included single arterioles and venules and parallel vessel pairs. Mean vitreous PO2 was 37.9 +/- 1.5 (SE) Torr. Close to the retinal surface (approximately 200 microns), PO2 was found to be both higher and lower than the vitreous PO2, depending on the proximity of the microelectrode tip to retinal vessels. Both positive (inward) and negative (outward) O2 fluxes (JO2) were measured, consistent with the anatomy and expected boundary conditions in the eye. The PO2 at the closest approach above arterioles was 55.2 +/- 2.3 Torr, significantly higher than in the vitreous (P < 0.0001). All arterioles had outward JO2 with an overall mean of -2.58.10(-6) ml O2/sec/cm2. Some of the venules were also losing O2, but at much lower rates than arterioles. Several venules were gaining O2. Countercurrent transport (A-V shunting) was also seen between vessel pairs. Our experimental results allow theoretical predictions to be made for the axial drop in blood PO2 along an arteriole as a function of blood flow.

Purpose: To determine the response of human optic nerve head (ONH) blood flow (ΔF) to heterochromatic equiluminant flicker modulation (eql-fl) and compare it to the response induced by pure luminance flicker (I-fl). Methods: In 5 normal volunteers the ONH blood flow was measured by conventional laser Doppler flowmetry. Stimuli were generated by green and red light emitting diodes and delivered to the eye through a fundus camera illumination optic. Both green and red illuminances were square wave modulated in counter phase at different frequencies between 2 and 40 Hz. ΔF was defined as the ratio between the ONH blood flow after 1 min stimulation and a baseline blood flow measured prior to the stimulation. Results: In response to a 2 Hz eql-fl, ONH blood flow increases by 36% in average. AF versus flicker frequency displayed the characteristics of a low-pass function with a cutoff frequency of 10 Hz for an eql-fl and a band-pass function with broad maximum around 10 Hz for the I-fl. Conclusions: AF in human ONH can be evoked by heterochromatic equiluminant flicker modulation. The blood flow frequency response to eql-fl and I-fl are similar to the neural activity dominated by the Parvo- and Magno-cellular activity, respectively. These findings offer a new approach to study the neurovascular coupling at the ONH in both physiological and diseased conditions involving predominantly or selectively the Magno- and Parvo-pathways.

A laser Doppler interferometer was built for the precise measurement of intraocular optical distances in humans and chicks. A technique using Purkinje images was developed to position the chick's eye reproducibly. A computer algorithm for the objective analysis of the interference signal and determination of the optical distances is presented. The precision of this noncontact interferometric method for measuring the cornea-retina distance is better than ±20 µm.

Clinical use of laser Doppler velocimetry of absolute red blood cell velocity V(max) in the center of major retinal vessels becomes possible with recent advances in microcomputer technology- Speed and automation of Doppler photocurrent analysis and identification of proper laser beam positioning are the major requirements in making direct quantitative assessment of retinal hemodynamics a routine diagnostic tool. We discuss our efforts toward achieving this goal and illustrate our current capabilities with examples of changes in retinal blood flow in response to physiologic maneuvers. In veins, V(max) can now be determined on-line. In arteries, current computing speed only supports intermittent on-line data acquisition.