Effect of visual stimulation on blood oxygenation in the optic nerve head of miniature pigs: a pilot study.

BACKGROUND: Visual stimulation is increasingly used to investigate the coupling between neuronal activity, blood flow and metabolism in the neural tissue of the ocular fundus. In an attempt to clarify whether the oxygen metabolism is involved in this coupling, we investigated the changes in the partial pressure of oxygen of venous blood (pO (2,blood)) in the optic nerve head of pigs in response to two different visual stimuli. MATERIALS AND METHODS: In 3 miniature pigs, the pO (2,blood) was measured in the optic disk rim using the technique of phosphorescence quenching by oxygen. This parameter was recorded every 8 seconds during a dark-to-light transition and during diffuse luminance flicker (field of 30 degrees centered at the optic disk, temporal frequencies of 2 to 80 Hz). RESULTS: The venous pO (2,blood) level (mean +/- standard deviation) did not change between dark- and light-adapted conditions (26.2 +/- 5.3 and 26.0 +/- 6.2 mm Hg, respectively), nor did we observe any transient change of pO (2,blood) during the light adaptation phase. On the other hand, the venous pO (2,blood) increased, on average, relative to its level during continuous light conditions (24.5 +/- 1.9 mm Hg) by at least 6 % for all flickering frequencies, with a maximum response of 14 % at 15 Hz. CONCLUSIONS: The phosphorescence quenching technique can reveal changes in venous pO (2,blood) induced by visual stimulation. Our results show that the pO (2,blood) in the optic nerve head of miniature pigs does not change with the light adaptation state of the retina, but increases during flicker stimulation with a band-pass type response. The previously reported increase of the ONH blood flow in response to flicker stimulation could lead to this increase of pO (2,blood).

Measurement of eye length and eye shape by optical low coherence reflectometry.

BACKGROUND: The precise and rapid measurement of eye length and eye shape is essential for investigating eye growth regulation and myopia. For this purpose, we developed an optical low coherence reflectometer (OLCR) and present preliminary measurements. METHODS: The OLCR includes a super luminescent diode (wavelength: 845 nm, coherence length: approximately 30 microm) and rotating glass cube to produce longitudinal scans at a velocity of 0.42 m/s and a repetition rate of approximately 13 scans/s. Heterodyne detection of light reflected from the anterior cornea and the posterior retina permits to measure axial eye length and eye shape (off-axis eye length). Each measurement consists of five consecutive scans. Reproducibility and precision were determined in one volunteer by measuring axial eye length five consecutive times, each time repositioning the eye. Eye shapes were determined in right eyes of four volunteers by measuring eye length every 3.3 degrees from 10 degrees nasally to 10 degrees temporally. RESULTS: Axial eye length measured repeatedly in one volunteer did not differ between or within the measurements (one-factor ANOVA). The average standard deviation was 11 microm. Eye shapes (a) varied substantially among subjects and (b) differed considerably from the corresponding shapes of spherical model eyes with identical axial eye lengths. CONCLUSION: The newly developed OLCR permits the precise and rapid measurement of eye length and eye shape. Such measurements, especially in children, may provide important information about mechanisms of eye growth regulation and the development of myopia.

Estimation of projection errors in human ocular fundus imaging.

Photogrammetric analysis of features in human ocular fundus images is affected by various sources of errors, for example aberrations of the camera and eye optics. Another--usually disregarded--type of distortion arises from projecting the near spherical shape of the fundus onto a planar imaging device. In this paper we quantify such projection errors based on geometrical analysis of the reduced model eye imaged by a pinhole camera. The projection error found near the edge of a 50 degrees fundus image is 24%. In addition, the influence of axial ametropia is investigated for both myopia and hyperopia. The projection errors found in hyperopia are similar to those in emmetropia, but decrease in myopia. Spherical as well as ellipsoidal eye shapes were used in the above calculation and their effect was compared. Our results suggest that the simple spherical eye shape is sufficient for correcting projection distortions within a range of ametropia from -5 to +5 diopters.

[Effect of dark adaptation on retinal blood flow]. / Effet de l'adaptation à l'obscurité sur le flux rétinien.

PURPOSE: Laser Doppler measurements performed immediately after the transition from dark adaptation (DA) to light led to the hypothesis that retinal blood flow, Fret, is increased during DA, but the use of visible lasers had prevented measurements during DA. Our aim was to test this hypothesis by measuring Fret during and after DA. MATERIAL AND METHODS: Fret in retinal vessels at the optic disk surface was recorded quasi-continuously in one eye of 6 normal subjects (age 27 to 60 years) using a laser Doppler flowmeter in the near-infrared (810 nm). Measurements were performed during light (baseline), various periods of DA and again during light. DA lasted between 2 and 32 min. RESULTS: Average Fret for the 6 subjects did not change significantly (-2.7 +/- 8% sd, p > 0.05) during the various periods of DA, as determined from linear regressions of the flux versus time. Following the transition from DA to light, there was, in most cases, a rapid transient increase of the flux, which reached an average value of 37 +/- 10% above the pre-transition value and peaked at 30-60 sec after the transition. CONCLUSIONS: These results do not support the hypothesis that Fret in normal volunteers is increased during DA. Rather, they strongly suggest that the transient increase in flux observed after DA is induced by the transition from dark to light (FNSRS #3200-043157 et CNR, It. #95.01715.CT04).

Iris blood flow response to acute decreases in ocular perfusion pressure: a laser Doppler flowmetry study in humans.

The relationship between blood flow and ocular perfusion pressure in the iris vascular bed of the human eye has not been established yet. Consequently, it is not known whether the iris circulation has some autoregulatory capacity. The aim of the present study was to investigate this relationship in the particular case where the perfusion pressure was decreased by increasing the intraocular pressure. Using laser Doppler flowmetry, mean velocity, volume and flow of blood through the iris were measured in normal subjects during acute decreases of the mean ocular perfusion pressure induced by raising the intraocular pressure with a scleral suction cup. Two experimental paradigms were applied. In the first, the suction pressure was rapidly increased from baseline, in steps of 50-100 mmHg, to a level where the intraocular pressure was above the mean ophthalmic artery blood pressure. In the second, the suction pressure was increased from baseline in four successive steps of 50 mmHg each. The suction pressure was kept constant for 2 min at each step. With the first paradigm (nine eyes), a 72% decrease in perfusion pressure induced a 63% decrease of iris blood flow due mainly to a drop in blood velocity. Immediately after suction release, blood flow increased by 62% above baseline and then returned to its baseline value within 2 min. With the second paradigm (six eyes), a 28% decrease in perfusion pressure resulted in a 30% flow reduction, which was due to significant decreases (P<0. 001) of both blood volume and velocity. Combining the results of both paradigms, we observed a significant linear correlation between iris blood flow and perfusion pressure (R =0.964, P<0.001). These results demonstrate that a decrease of the perfusion pressure due to an increase of the intraocular pressure induces a decrease of the iris blood flow. No evidence of an autoregulatory process in the iris vascular bed could be demonstrated. A reactive hyperemia was observed in response to a sudden increase in perfusion pressure occurring after a period of decreased blood flow.

[Effect of decreased ocular perfusion pressure on iris blood flow measured by laser Doppler flowmetry]. / Effet d'une diminution de la pression de perfusion oculaire sur le flux sanguin dans l'iris mesuré par fluxmétrie laser Doppler.

PURPOSE: To determine whether iris blood flow (IBF) is regulated in response to an acute decrease in mean ocular perfusion pressure (PPm = MOAP-IOP, MOAP = mean ophthalmic arterial pressure) induced by increasing the intraocular pressure (IOP). METHODS: Iris blood flow was measured using a slit lamp incorporating a laser Doppler flowmetry (LDF) module. The study was conducted on 12 normal volunteers (14 to 59 years old). IOP was raised using a scleral suction cup. In Exp. #1, the suction pressure was successively raised in steps of 50 to 100 mm Hg, each lasting about 10 sec, until IOP reached the MOAP level. In Exp. #2, the suction was raised to 200 mm Hg in 4 successive steps of 2 min duration. RESULTS: In Exp. #1, no significant change of IBF was observed for small decreases of PPm (< 23%); greater decreases of PPm resulted in a linear IBF decrease (p < 0.01). In Exp. #2, such a IBF versus PPm decrease was also observed (p < 0.001). Immediately after release of suction, a significant, transient IBF increase of 79% above baseline level was observed. CONCLUSION: These results suggest that some IBF regulation occurs for small PPm decreases (< 23%); no IBF compensatory mechanism appears to operate for further decreases of PPm (> 23%).

Laser Doppler flowmetry and optic nerve head blood flow.

PURPOSE: Ischemic disorders of the optic nerve head constitute an important cause of visual loss. The optic nerve head is supplied by two main sources of blood flow: the superficial layers by the central retinal artery and the deeper layers by the posterior ciliary arteries. This study was conducted in rhesus monkey eyes to obtain a better understanding of which part of the optic nerve head circulation is measured by laser Doppler flowmetry. METHODS: By means of a fundus camera-based laser Doppler flowmetry technique to measure blood flow in the optic nerve head tissue, laser Doppler flowmetry measurements were taken at baseline and then after experimental occlusion of central retinal artery (12 eyes), posterior ciliary arteries (nine eyes), and combined occlusion of central retinal artery and posterior ciliary arteries (nine eyes). Optic nerve head, choroidal, and retinal circulations were investigated by fluorescein fundus angiography after the various arterial occlusions. RESULTS: Average laser Doppler flowmetry flow during central retinal artery occlusion alone was significantly decreased (P<.001) by 39%+/-21% (mean +/- 95% confidence interval) compared with normal baseline. Combined occlusion of central retinal artery and posterior ciliary arteries reduced laser Doppler flowmetry flow even more markedly by 57%+/-27% (P<.0005), but the difference between this flow reduction and that with central retinal artery occlusion alone was not significant (P>.20). After posterior ciliary artery occlusion alone, however, measurements showed a nonsignificant increase in laser Doppler flowmetry flow of 17%+/-37%. CONCLUSIONS: The findings of this study suggest that the standard laser Doppler flowmetry technique is predominantly sensitive to blood flow changes in the superficial layers of the optic nerve head and less sensitive to those in the prelaminar and deeper regions, and their relative proportions are not known. In this laser Doppler flowmetry technique, the weaker Doppler signal from the deep layers cannot be separated from the dominant signal from the superficial layers to exclusively study the circulation in the deep layers; the latter circulation is of interest in optic nerve head ischemic disorders, including glaucoma. Emerging new optical modalities of the laser Doppler flowmetry technique may help in selectively measuring blood flow in the deeper layers.

Blood flow in the human iris measured by laser Doppler flowmetry.

A new instrument based on laser Doppler flowmetry (LDF) has been developed to determine noninvasively the relative flux of red blood cells (RBCs) through the microcirculatory network of the iris of the human eye. The probing laser, photodetector, and target fixation devices required by this method were adapted to a slit lamp. Electronic processing and computer analysis of the Doppler signal allow determination of relative velocity, number, and flux of RBCs in the iris, as well as the pulsatility of these flow parameters during the heart cycle. Based on measurements in one eye of eight normal volunteers, the sensitivity of the technique, i.e., the minimum change detectable at the P < 0.05 level, was 4% for the flux. The decrease in blood flow in response to decreases of the ocular perfusion pressure demonstrates the capability of the technique to detect flow changes and its suitability for investigating the physiology and the pharmacology of iris circulation.

[Flicker stimulation induces retinal vasodilation in man]. / Le papillotement induit une vasodilatation rétinienne chez l'homme.

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.

[Blood flow measurement in the optic nerve head during isometric exercise]. / Mesure du flux sanguin au niveau de la tête du nerf optique lors de l'exercise isométrique.

PURPOSE: Autoregulation of optic nerve head blood flow (Fonh) in response to decreases in perfusion pressure has been demonstrated in animals and humans. The aim of this study was to determine change in Fonh when systemic blood pressure is increased. METHODS: Blood flow parameters, i.e. relative mean velocity, number, and flux of red blood cells in the ONH tissue (Velonh, Volonh and Fonh, respectively) were measured by laser Doppler flowmetry in one eye of 13 normal subjects (aged 16 to 58 years), at baseline, during, and after isometric exercises consisting of squatting. Brachial artery blood pressure was measured by sphygmomanometry. IOP was measured at baseline and at the end of squatting. RESULTS: During squatting mean arterial pressure increased from 103 +/- 6 mm Hg to 139 +/- 58 mm Hg (average +/- 95% confidence interval), IOP increased from 13 +/- 0.5 to 17 +/- 1 mm Hg. An average increase in PPm from 56 +/- 4 to 80 +/- 7 mm Hg induced no significant (p > 0.05) change in the blood flow parameters. The sensitivity (detection threshold) of the blood flow changes was 8%. CONCLUSION: This study shows for the first time in human autoregulation of Fonh when PPm is increased by increasing the systemic blood pressure.

Blood flow in the human optic nerve head during isometric exercise.

Investigating blood flow autoregulation in the optic nerve is important to understand the physiopathology of various ocular diseases such as glaucoma. This investigation requires that one establishes the relationship between optic nerve blood flow and perfusion pressure. Previous work has documented the effect of lowering the perfusion pressure on optic nerve blood flow. The purpose of the present study was to investigate the effect of elevated perfusion pressure on blood flow in this tissue. Laser Doppler flowmetry was applied to measure relative mean velocity, volume and flux of red blood cells in the tissue of the optic nerve head. These parameters were measured in 13 subjects during isometric exercise consisting of squatting. In the range of perfusion pressures from 56+/-4 to 80+/-5 mmHg (30+/-8%), there was no significant variation of mean velocity, volume and flux of red blood cells, but vascular resistance increased by about 50%. Intraocular pressure was increased significantly above baseline at the end of squatting and decreased during recovery. The results suggest that the maintenance of constant blood flow is achieved by an increase in vascular resistance taking place either at the arterioles feeding or at the veins draining the blood from the ONH or at the ophthalmic artery and/or vessels between this artery and the site of LDF measurements. Combining the results of this study with those of a previous one where perfusion pressure was decreased by increasing the intraocular pressure, we show the entire relationship between perfusion pressure and optic nerve blood flow in man.

Choroidal blood flow during isometric exercises.

PURPOSE: To investigate the response of choroidal blood flow in the foveal region of the human eye to increases in mean perfusion pressure (PPm = mean ophthalmic artery pressure - intraocular pressure; IOP) induced by isometric exercises. METHODS: Using laser-Doppler flowmetry, changes in velocity (ChBVel), number (ChBVol), and flux (ChBF) of red blood cells in the choroidal vascular system in the foveal region of the fundus were measured in both eyes of 11 normal subjects (ages 18 to 57 years) during isometric exercises. RESULTS: During 90 seconds of squatting, PPm increased by an average of 67%, from 46 to 77 mm Hg. This resulted in a significant increase of 12% in ChBFm (the mean of ChBF during the heart cycle), mainly caused by an increase in ChBVelm. A further increase in PPm to a value approximately 85% above baseline resulted in a 40% increase in ChBFm. A significant negative correlation was found between the changes in ChBVelm and ChBVolm, during squatting. CONCLUSIONS: Previous studies have demonstrated that during isometric exercise, blood pressures in the ophthalmic and brachial arteries rise in parallel. These observations and the current results indicate that an increase in PPm up to 67% induces an increase in choroidal vascular resistance that limits the increase in choroidal blood flow to approximately 12%. This regulatory process fails when PPm is further increased.

Effect of acute decreases of perfusion pressure on choroidal blood flow in humans.

PURPOSE: To investigate the relationship between choroidal blood velocity (ChBVel), blood volume (ChBVol) and blood flow (ChBF) in the foveal region of the human ocular fundus and ocular perfusion pressure and to determine whether the choroidal circulation has some autoregulatory capacity. METHODS: Measurements of ChBVel, ChBVol and ChBF were obtained by laser Doppler flowmetry in healthy subjects (age range, 21 to 57 years) with normal eye examination results. Measurements were performed at normal intraocular pressure (IOP) and during successive step increases in IOP induced by scleral suction. In experiment 1, in six eyes (five subjects), the IOP was increased rapidly, in steps of 50 to 100 mm Hg of suction pressure, which each lasted approximately 10 seconds to a level above diastolic ophthalmic artery blood pressure (IOP = approximately 72 mm Hg). In experiment 2, in 14 eyes (seven subjects), the IOP was increased slowly in four successive steps at 2-minute intervals to a level of approximately 42 mm Hg. We also determined the pulsatility of the flow parameters during the heart cycle, pulsatility = 1 - diast value/syst value. RESULTS: For both rates of suction cup increase, the relationship between ChBFm (mean ChBF over the heart cycle) and mean perfusion pressure was not linear. At high pressure, ChBFm was less affected by decreases in the pressure than expected from a passive vascular system. In some cases, no change in ChBFm was detectable, although significant changes in PChBF occurred. Further decreases in perfusion pressure resulted in a proportional decrease in ChBFm. On release of suction, a significant increase in ChBFm over baseline value was detectable in experiment 1. CONCLUSIONS: The relationship between ChBFm and ocular mean perfusion pressure appears to be bilinear and reveals some autoregulation for moderate step decreases in perfusion pressure. The temporal characteristics of the ChBFm-response suggest a neural or passive hemodynamical process rather than a myogenic or metabolic compensatory mechanism.

[Measuring leukocyte velocity in macular capillaries using a miniaturized blue field simulator: effect of aperture of the pupil]. / Messung der Leukozytengeschwindigkeit in makulären Kapillaren mittels eines miniaturisierten Blaufeldsimulators: Effekt der Blendengrösse in der Pupille.

BACKGROUND: In this paper we present a miniaturized blue field simulator (BFS) which allows the measurement of the velocity (V) pulsatility (P) and number (D) of leukocytes in the macular retinal capillaries. A study on the effect of the aperture size of the blue light source at the subject's pupil on the measured flow parameters was performed. METHODS: A blue field entoptoscope with a small halogen lamp was used to induce the perception of the "flying corpuscles" and a flat color screen was selected to display a computer simulation of this entoptic phenomenon. The aperture of the blue light source at the pupil was varied by a diaphragm placed at a conjugate pupil plane while the size and retinal irradiance of the blue field stimulus at the fundus was held constant. RESULTS: The results show significant correlations between log pupil area and both V (-18% per log unit) and D (+42% per log unit). When the retinal illuminance is expressed in Log Trolands, V shows no dependency, but D is strongly correlated (+40% per log Troland). CONCLUSIONS: When applying the BFS technique, the size of the blue light stimulus at the pupil, if undilated, must be well controlled to minimize instrument-related variations of the blood flow measurements.

[Autoregulation in ischemia of the optic nerve in the human]. / Autoregulation du débit sanguin du nerf optique chez l'homme.

BACKGROUND: Autoregulation is defined as the maintenance of constant blood flow in a vascular system in spite of changes in perfusion pressure (PPm). MATERIALS AND METHODS: PPm was decreased by increasing the intraocular pressure (IOP) with a suction cup and optic nerve blood flow was measured with the laser Doppler flowmetry technique (LDF) in 9 normal volunteers. RESULTS: The blood flow was autoregulated down to a PPm of 13 mm Hg (IOP = 47 mm Hg). CONCLUSIONS: These results confirm previous studies in cats and monkeys. The mechanism of autoregulation is probably a decrease in resistance due to capillary recruitment.

[Effect of changes in ocular perfusion pressure on choroid ischemia in man]. / Effet des changements de la pression de perfusion oculaire sur le débit sanguin choroïdien chez l'homme.

BACKGROUND: The effect of changes in ocular perfusion pressure (PPm) on the choroidal blood flow (ChBF) in man was studied with the laser Doppler flowmetry (LDF) technique. MATERIALS AND METHODS: We changed the PPm by increasing the intraocular pressure (IOP) or by increasing the blood pressure (BP) with isometric exercises. RESULTS: We observed that a) ChBF was not significantly different from baseline up to an IOP of 27 mm Hg and b) ChBF remained constant even if PPm increased by as much as 72%. CONCLUSION: Our results suggest that ChBF is autoregulated in response to an increase in IOP up to about 27 mm Hg. ChBF remains also constant in spite of an increase in systemic BP, probably due to a vasoconstriction induced by increased sympathetic activity.

Effect of decreased ocular perfusion pressure on blood flow and the flicker-induced flow response in the cat optic nerve head.

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.

Measurement by laser Doppler interferometry of intraocular distances in humans and chicks with a precision of better than ±20 µm.

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.

Strict control of glycaemia: effects on blood flow in the large retinal vessels and in the macular microcirculation.

AIMS: The purpose of this study was to investigate the effect of instituting strict diabetic glycaemic control on the retinal macular microcirculation and to compare this effect with that observed in the main retinal veins. METHODS: In 28 insulin dependent diabetic patients with poor glycaemic control a regimen of strict diabetic control, consisting of four daily insulin injections was instituted and maintained for 6 months. Retinal haemodynamics were investigated in the macular microcirculation by the blue field simulation technique and in the major retinal veins by a combination of bidirectional laser Doppler velocimetry and monochromatic fundus photography. Progression of diabetic retinopathy was assessed from fundus photographs taken at baseline and at the end of the study. RESULTS: Institution of strict diabetic control resulted in a significant increase in leucocyte velocity in the macular circulation (p = 0.013). No significant difference in this increase was observed between eyes that showed progression (n = 8) and no progression (n = 20) of retinopathy during the study. Significant correlations were found between relative changes over time of blood flow measured in the main retinal veins and relative changes of leucocyte velocity determined in the macular microcirculation at 2 months (p = 0.008) and 6 months (p = 0.001) but not at 5 days (p = 0.49). In the eight eyes that showed progression of retinopathy, the product of leucocyte velocity and density at baseline was significantly higher than normal (p < 0.05). During the length of this study, this product was also significantly higher in the eight eyes that showed retinopathy progression than in the 20 eyes that did not show progression (p = 0.005). CONCLUSION: Our results suggest that increased flow in the macular microcirculation may be associated with progression of retinopathy, thus supporting the hypothesis that increased blood flow may play a role in the development of diabetic microangiopathy. Although there are correlations between the changes detected in the macular microcirculation and those measured in the main retinal vessels, there are also differences which need to be further investigated in order to better understand pathogenetic mechanisms.

Effects of adenosine on ocular blood flow.

PURPOSE: To determine the effect of intravascular adenosine on blood flow in the ocular fundus and to examine indirectly whether the blood-brain barrier to adenosine, which exists in the cerebrovasculature of the cat, is present in the eye of this animal. METHODS: The noninvasive techniques of laser Doppler flowmetry and velocimetry along with fundus photography were used to measure the change in optic nerve head and choroidal and retinal blood flow during intravenous infusions of 0.18 and 0.6 mg/kg per minute of adenosine. RESULTS: Infusions of adenosine induced significant increases in choroidal blood flow (60% with 0.6 mg/kg per minute) but not in optic nerve head or retinal blood flows. CONCLUSIONS: The lack of effect of intravenously infused adenosine on the optic nerve and retinal circulations is most likely caused by the tight junctions in the vessels of these vascular beds, which prevent adenosine from reaching its receptors. Perivascular adenosine in the choroid most likely accounts for the increase in blood flow in this tissue.