Major Retinal Arteries Research Articles

Oxygen tension and gradient measurements in the retinal microvasculature of rats

Oxygen delivery from the retinal vasculature plays a crucial role in maintaining normal retinal metabolic function. Therefore, measurements of retinal vascular oxygen tension (PO(2)) and PO(2) longitudinal gradients (gPO(2)) along retinal blood vessels may help gain fundamental knowledge of retinal physiology and pathological processes. Three-dimensional retinal vascular PO(2) maps were generated in rats by optical section phosphorescence lifetime imaging. A major retinal artery and vein pair, and a smaller blood vessel (microvessel) between them were segmented, and PO(2) along each blood vessel was measured. In each blood vessel, an average PO(2) (mPO(2)) was calculated, and gPO(2) was determined by linear regression analysis. Reproducibility of measurements was assessed by calculating intraclass correlation coefficient (ICC) of repeated measurements. The correlations of mPO(2) and gPO(2) measurements with systemic arterial oxygen tension (P(a)O(2)) and carbon dioxide tension (P(a)CO(2)) was determined. Measurements of mPO(2) and gPO(2) in retinal arteries, microvessels and veins were reproducible (ICC > 0.86; p < 0.01; N = 8), except for retinal arterial gPO(2). Retinal arterial, microvessel and venous mPO(2) were 41 ± 8, 32 ± 8 and 25 ± 7mmHg, respectively (mean ± SD; N = 27). Retinal arterial mPO(2) was correlated with P(a)O(2) and P(a)CO(2) (R > 0.44; p < 0.03), while retinal microvessel and venous mPO(2) were only correlated with P(a)CO(2) (R > 0.68; p < 0.01). Retinal microvessel gPO(2) (-3.8 ± 1.5mmHg/100μm) was significantly steeper (more negative) than venous gPO(2) (0.02 ± 0.43mmHg/100μm) (p < 0.01; N = 27), and neither were significantly correlated with P(a)O(2) or P(a)CO(2). Quantitative measurement of mPO(2) and gPO(2) in the retinal microvasculature was demonstrated. A significant decrease in PO(2) was observed along most retinal microvessels, indicative of substantial oxygen extraction by the retinal tissue. This method has the potential to help elucidate retinal microvascular oxygen transport in health and disease.

Computer-assisted image analysis of temporal retinal vessel width and tortuosity in retinopathy of prematurity for the assessment of disease severity and treatment outcome

To quantify the severity of retinopathy of prematurity (ROP) through temporal retinal vessel diameter and tortuosity by the use semiautomated analysis and to evaluate the effects of laser treatment on retinal vessel measurements. A total of 176 RetCam digital fundus images from 63 infants diagnosed with ROP between January 2005 and December 2008 were retrospectively analyzed with Retinal Vessel Measurement V5.8 software. Patients were classified into 1 of 3 groups: stage 1 and 2 (Group 1), stage 3 not requiring treatment (Group 2), and stage 3 requiring treatment (Group 3). Diameter and tortuosity of the 4 major temporal retinal arteries and veins close to the optic disk were quantified. Tortuosity was measured as central (within 2 disk diameters from the center of the optic disk) and paracentral (from 2 to 4 disk diameters). For Group 3, measurements of vessels immediately before treatment and all follow-up post-treatment measurements of vessels were analyzed. Differences in central and paracentral arteriolar tortuosity were pronounced. The average central arteriolar tortuosities were 1.12 ± 0.018 for Group 1, 1.13 ± 0.018 for Group 2, and 1.32 ± for Group 3; the paracentral arteriolar tortuosities were 1.11 ± 0.019 for Group 1, 1.12 ± 0.019 for Group 2, and 1.31 ± 0.02 for Group 3. In the post-treatment analysis of Group 3 patients, significant reductions in all vascular measurements were observed compared to pre-treatment, except for central and paracentral vein tortuosity. Computerized systems can be used to objectively assess differences in vascular parameters between ROP stages.

Effect of regular smoking on flicker induced retinal vasodilatation in healthy subjects

BackgroundHabitual smoking is a risk factor for a variety of vascular diseases, including ocular pathologies. In the current study, we set out to investigate whether the regulation of retinal vascular tone is impaired in habitual smokers. For this purpose, vascular reactivity was tested during flicker light induced vasodilatation in smokers and in a non-smoking control group. MethodsIn this prospective, balanced, parallel group study 24 chronic smokers (28.1±3.3years) and 24 age-matched never-smoking volunteers (28.2±4.0years) were included. Flicker induced vasodilatation was measured in major retinal arteries and veins using a retinal vessel analyzer and flicker induced changes in retinal blood velocities were assessed in retinal veins by laser Doppler velocimetry. Three flicker periods of 60s were scheduled. Blood cotinine concentration was determined and a Fagerstrom questionnaire was performed to evaluate nicotine dependency. ResultsIn non-smoking subjects, stimulation with flicker light increased retinal venous diameter by +7.7±3.1%, +6.9±2.9% and +7.1±2.8% during the three flicker periods, respectively. Flicker induced vasodilatation in veins was significantly diminished in chronic smokers (+4.9±2.4%, +6.3±3.1% and +5.7±3.4%, ANOVA between groups, p=0.032) as compared to the non-smoking control group. Calculated retinal blood flow in the measured veins increased by a maximum of +54±21%, +43±18% and +46±19% during the three stimulation periods in the non-smoking subjects, respectively. The flicker induced increase in retinal blood flow as assessed in the veins was significantly reduced in chronic smokers as compared to the non-smoking control group (+19±16%, +26±14%, +24±13%, ANOVA between groups, p=0.013). In retinal arteries, flicker stimulation increased retinal arterial diameters by 5.2±3.8%, 5.8±4.8% and 5.5±5.6% during the three flicker periods in the non-smoking group. In smokers, the flicker induced arterial vasodilatation was not significantly different compared to non-smokers (4.6±4.1%, 3.8±3.7% and 4.8±3.4%, ANOVA between groups, p=0.4). ConclusionOur data indicate that the flicker induced hemodynamic response of retinal veins is reduced in chronic smokers as compared to age matched healthy volunteers. This supports the hypothesis that chronic smoking leads to vascular dysfunction in the eye.

Retinal Oxygen Saturation in Patients with Systemic Hypoxemia

To assess spectrophotometric oximetry across a broad range of arterial saturation levels and to study the effect of chronic systemic hypoxemia on retinal oxygen extraction. The study included 16 patients with Eisenmenger syndrome, a cyanotic cardiac defect, and 17 healthy volunteers. Oxygen saturation in selected major retinal arteries and veins was assessed using noninvasive spectrophotometric oximetry. Arterial blood gases were determined within 1 day of the ophthalmic examination in blood samples from the femoral artery. The retinal arterial oxygen saturation of 81% ± 9% (mean ± SD) in patients with Eisenmenger syndrome was subnormal and demonstrated more interindividual variation than the 93% ± 3% observed in healthy subjects (P < 0.001). A comparable difference was found for the respective retinal venous oxygen saturations of 44% ± 12% and 59% ± 5% (P < 0.001). Fractional arteriovenous oxygen extraction was comparable between the two groups (37% ± 6% and 34% ± 5%, respectively; P = 0.29). Retinal and femoral artery oxygen saturation were correlated (ρ = 0.82; P < 0.001), the former approximating the latter at least as well as fingertip oximetry. When compared to arterial blood gas analysis of blood samples drawn by arterial puncture, the gold standard in the field, fundus oximetry was found to be in good overall agreement with the arterial blood samples. Blood flow measurements will be needed to determine whether the systemic hypoxia is completely compensated, as suggested by oxygen extraction being comparable between the two groups.

Ranibizumab for retinal arterial macroaneurysms

Dear Editor, We read with great interest the case report entitled “Intravitreal ranibizumab in retinal macroaneurysm” by Wenkstern et al. [1] regarding the effects of intravitreal ranibizumab (Lucentis®; Genentech, Inc, South San Francisco, CA, USA) as monotherapy for retinal arterial macroaneurysms (RAM). RAM are acquired retinal vascular abnormalities, typically found as a solitary, round or fusiform aneurysm arising in one of the four major retinal arteries within the posterior pole of the eye. RAM predominantly affect middle-aged female patients with cardiovascular diseases, mainly systemic hypertension and arteriosclerosis [2, 3]. A focal embolic damage to arterial walls is thought to cause RAM, leading to localized ischemia with vascular endothelial growth factor (VEGF)-induced increased permeability and dilatation of the retinal artery. These cause visual impairment due to hemorrhage, exudation and macular thickening. Ranibizumab has been shown to reduce vascular permeability in choroidal neovascularization due to neovascular age-related macular degeneration [4], as well as in macular edema due to retinal vein occlusion [5]. The intravitreal administration of VEGF inhibitors may reduce the VEGF-induced vascular hyperpermeability present in the retinal changes due to RAM. Bevacizumab (Avastin®; Genentech, Inc, South San Francisco, CA, USA) has been reported to reduce macular edema in patients suffering from RAM [6]. As we share Dr Wenkstern's opinion about the limited visual improvement achieved with current therapeutic options, we have prospectively included five female patients (mean age 68.7 years) with visual loss and RAM in a pilot study to evaluate the efficacy of intravitreal ranibizumab injection for this disease. The inclusion criteria were: best-corrected visual acuity (BCVA) ≥0.5 logMAR, macular hemorrhage with foveal involvement; and macular edema with more than 350 μm as measured with spectral-domain optical coherence tomography (SD-OCT). Baseline examination included BCVA, SD-OCT and fluorescein angiography. We immediately applied a first intravitreal ranibizumab injection, and thereafter scheduled patients into a quarterly followup (according to the current follow-up periods for ranibizumab therapy in choroidal neovascularization) in order to assess BCVA improvement and SD-OCT changes. Re-treatments were performed whenever we documented a The authors have no financial interest to disclose.

Pitavastatin Attenuates Leukocyte-Endothelial Interactions Induced by Ischemia-Reperfusion Injury in the Rat Retina

Purpose: Statins (3-hydroxy-methylglutaryl coenzyme A reductase inhibitors) have been shown to lower serum cholesterol levels in clinical use. Moreover, it has been reported that statins exert pleiotropic and beneficial effects on vascular endothelium. Therefore, we investigated the effects of pitavastatin, a new statin, on leukocyte accumulation during ischemia-reperfusion injury. Materials and Methods: Transient retinal ischemia was induced in Long-Evans rats for 60 min by temporal ligation of the optic nerve. Pitavastatin (0.12, 0.35, or 1.1 mg/kg) was administered 5 min prior to the induction of retinal ischemia. Leukocyte-endothelial interactions in the post-ischemic retina were evaluated in vivo with acridine orange digital fluorography. The number of rolling leukocytes, number of accumulated leukocytes, and diameters of the major retinal artery and vein were evaluated. Intercellular adhesion molecule-1 (ICAM-1) mRNA expression in the retina was semiquantitatively studied using the RT-PCR method. Results: Pitavastatin-treated rats at doses of 0.35 and 1.1 mg/kg showed mild arterial narrowing (p < 0.01) and venous dilation (p < 0.01) compared with vehicle-treated (ischemic) rats. In rats treated with 0.35 mg/kg pitavastatin, the number of rolling leukocytes was significantly reduced by 35.5% (p < 0.01) 12 hr after reperfusion compared with that of vehicle-treated rats. With treatment at a dose of 0.35 mg/kg pitavastatin, the number of accumulated leukocytes was reduced to 68.7% (p < 0.01) 24 hr after reperfusion. Moreover, pitavastatin treatment significantly reduced ICAM-1 mRNA expression in the retina during ischemia-reperfusion injury. Conclusions: Pitavastatin effectively attenuated ischemia-induced leukocyte-endothelial interactions in the rat retina.

Intravitreal injection of endothelin-1 caused optic nerve damage following to ocular hypoperfusion in rabbits

The purpose of this study was to investigate the time course of the ocular hypoperfusion, retinal damage, and optic nerve damage induced by intravitreal injection of endothelin-1 (ET-1) in rabbits. ET-1, at 5 pmol (20 μL, twice a week for 2 or 4 weeks), was injected from the pars plana into the posterior vitreous of the right eye. Optic nerve head (ONH) blood flow and retinal artery diameter, together with the neurofilament light chain (NF-L) content, retinal morphology, and axon density of the optic nerve, were evaluated at 2, 4, and 8 weeks after the first injection of ET-1 ( n = 7 or 8). Tissue blood velocity in ONH was measured using a laser speckle method, and the diameter of major retinal arteries on the rim of the ONH was calculated from fundus photographs by a masked observer. Histological analysis and immunoblot evaluation of NF-L in the optic nerve were performed to evaluate optic nerve damage. At 2 weeks after the first ET-1 injection, tissue blood velocity was decreased by approximately 20% (versus the contralateral eye), and the diameter of retinal arteries had decreased by approximately 40%. These changes were sustained at the same level until 8 weeks after the first ET-1 injection. At 4 and 8 weeks after the first ET-1 injection, the amount of NF-L in the optic nerve was significantly less in the ET-1 treated eyes than in the contralateral eyes. At 8 weeks after the first ET-1 injection, a loss of myelinated axons and increases in gliosis and connective tissue were noted in the optic nerve of the treated eye, and the optic nerve-axon number had decreased significantly (each, versus the untreated eye). Retinal ganglion cells in the retina were not observed any damage at 2, 4, and 8 weeks after ET-1 injection. In conclusion, intravitreal injection of ET-1 induced chronic hypoperfusion in the ONH and retina, which presumably caused decreases in NF-L content and axon number in the optic nerve noted in the later part of the observation period.

Fundus and Histopathological Study of Radial Optic Neurotomy in the Normal Miniature Pig Eye

To demonstrate the fundus and histopathological changes in the normal miniature pig eye after radial optic neurotomy. Ophthalmoscopic examinations, fundus photography, and fluorescein angiography were performed on 12 eyes that underwent radial optic neurotomy, 5 normal eyes, and 7 eyes that underwent vitrectomy (from 12 pigs in total) preoperatively and 1, 7, 14, 30, and 90 days postoperatively. These eyes were enucleated 120 days postoperatively. Sixteen eyes (of 8 pigs in total) that underwent radial optic neurotomy were enucleated 1, 3, 7, and 48 days (2 pigs each time) postoperatively. The enucleated eye sections were stained with hematoxylin-eosin, Masson trichrome, and Luxol fast blue. The retina radial to the site of radial optic neurotomy darkened gradually with the increasing curvature of the major retinal arteries. The filling time intervals from the retinal artery to the retinal vein were prolonged. At the incision site, there was a loss of nerve fibers, which were subsequently replaced by collagenous tissue. No anastomotic vessels formed by the end of the study. Postoperatively, the retinal circulation seemed somewhat sluggish compared with that seen preoperatively. Segmental retinal nerve atrophy eventually formed. The procedure itself may not be the sole factor for the formation of shunt vessels. To date, radial optic neurotomy is controversial and persuasive animal studies are lacking. In combining fluorescein angiography with histopathological examination, this study may be somewhat helpful.

Improved Interpretation of Flow Maps Obtained by Scanning Laser Doppler Flowmetry Using a Rat Model of Retinal Artery Occlusion

To improve the interpretation of Heidelberg Retina Flowmeter (HRF; Heidelberg Engineering GmBH, Dosselheim, Germany) flow maps by examining the influence of specific vascular structures and focus depth in the presence and absence of retinal blood flow. HRF flow maps were recorded from the inferior retina of anesthetized Brown Norway rats over a wide range of focus levels, before and after laser occlusion of the retinal circulation. Analysis of the resultant flow maps showed that the sample window was positioned on a retinal artery, arteriole, or vein, or in a retinal capillary area, with or without a visible underlying choroidal vessel. The relationship between HRF-measured flow (arbitrary units) and focus depth was determined for each location. At the conclusion of each experiment, the effect of reduction of systemic blood pressure on the choroidal circulation and the level of background signal in the HRF flow map with no ocular blood flow were assessed. The strongest flow signals came from the retinal arteries, veins, and arterioles and were reduced to choroidal background level after occlusion of the central retinal artery. Larger choroidal vessels also contributed strong flow signals. In contrast, the flow signal from the retinal capillary area was weak and unaffected by retinal artery occlusion. Changing the depth of focus significantly altered the contribution from the major retinal arteries, arterioles, and veins, but no significant depth effect was seen for retinal capillaries or choroidal vessels. The HRF flow signal remaining when systemic blood pressure was reduced to zero was not significantly different from the capillary sampling location when the eye was normally perfused. In the pigmented rat eye, the HRF signal from retinal capillaries is not significantly different from the background noise unrelated to blood flow. Strong flow signals can be obtained from the retinal arteries, retinal arterioles, retinal veins, and choroidal vessels. Current HRF flow maps in the rat therefore reflect blood flow in the larger elements of the microvasculature rather than the capillary network.

Effects of Topically Instilled Bunazosin, an α1-Adrenoceptor Antagonist, on Constrictions Induced by Phenylephrine and ET-1 in Rabbit Retinal Arteries

To examine the inhibitory effects of topically instilled bunazosin hydrochloride (bunazosin), a selective alpha1-adrenoceptor antagonist, on the retinal artery constrictions induced by intravitreous phenylephrine hydrochloride (phenylephrine) and endothelin (ET)-1 in rabbits. Phenylephrine or ET-1 (20 microL) was injected into the central part of the vitreous in both eyes in pigmented rabbits. Color fundus photographs were taken at 5 minutes before and 60 minutes after the injection. The average diameter of the major retinal arteries at the rim of the optic nerve head (ONH) was normalized with respect to ONH diameter. Bunazosin was instilled into one eye (chosen randomly) and vehicle into the fellow eye at 60 minutes before the intravitreous injection. To examine any interaction between the alpha1-adrenoceptor and ET receptor, phenylephrine and ET-1 were co-injected at individually ineffective doses. In addition, ET-1-induced vasoconstriction was examined after unilateral superior cervical ganglionectomy. The binding affinities of bunazosin for ETA and ETB receptors were also evaluated. The series of experiments was performed as masked tests. Retinal arteries were dose-dependently constricted by both intravitreous phenylephrine and intravitreous ET-1. Topically instilled bunazosin at 0.01% partly inhibited both of these vasoconstrictions on the ipsilateral side, but not on the contralateral side. Bunazosin did not bind to ET receptors. Co-injection of phenylephrine and ET-1 at individually ineffective doses constricted retinal arteries significantly. An adrenergic supersensitivity in retinal arteries was observed after superior cervical ganglionectomy only on the ganglionectomized eye. The ET-1-induced vasoconstriction was significantly weaker in cervical ganglionectomized eyes than in sham-surgery eyes. The present findings suggest that topically instilled bunazosin reaches the posterior retina by local penetration at concentrations sufficient to attenuate the phenylephrine- or ET-1-induced constriction of retinal arteries in normal rabbit eyes, and that the inhibitory effect of bunazosin on the ET-1-induced vasoconstriction in this tissue may be partly attributable to an interaction between the alpha1-adrenoceptor and ET receptor.

Consequences of an increase in the ocular perfusion pressure on the pulsatile ocular blood flow.

In previous studies we have demonstrated that physical exertion constricted the major retinal arteries, increased the redness of the optic nerve head, and attenuated scotopic white flash oscillatory potentials. These anatomic and functional changes in the human retina suggested that blood flow to the innermost retinal layers was modified in some way to cause these changes. The objective of this study was to determine whether physical exertion also affected blood flow in the choroid, the exclusive source of blood to the outermost layers of the retina. Eighteen healthy adults volunteered for this study. An OBF Tonograph system (OBF Laboratories UK Ltd., Malmesbury, Wiltshire, U.K.) was used to derive the pulsatile ocular blood flow, which reflects the pulsatile component of blood flow in the choroid, at rest and 20 minutes after biking at a heart rate of 140 beats/min. At the systemic level, biking increased the blood pressure and the heart rate. At the ocular level, the duration of the systolic and diastolic phases of the intraocular pulse was shortened, and the pulse amplitude and volume were reduced. Despite the attenuation of the intraocular pulse parameters, the pulsatile ocular blood flow increased by some 18% after exercise largely because of the much larger increase in heart rate. The ocular perfusion pressure increased, whereas the intraocular pressure decreased. Physical exertion in the form of aerobic exercise increased the pulsatile component of blood flow in the choroid. Because the choroid is the sole blood supply to the outer retina, it was concluded that the degree of perfusion of the photoreceptors necessary for vision is increased by physical exertion. This increased choroidal blood flow is presumably to sustain vision as blood is directed to the large muscle groups involved in the physical activity.

Diffuse luminance flicker increases blood flow in major retinal arteries and veins

It has been shown that diffuse luminance flicker increases optic nerve head blood flow. The current study has been performed to quantify changes in retinal blood flow during flicker stimulation. In a group of 11 healthy volunteers, red blood cell velocity and retinal vessel diameters were assessed with bi-directional laser Doppler velocimetry and the Zeiss retinal vessel analyzer before, during and after stimulation with diffuse luminance flicker. Retinal blood flow was calculated for each condition. Flicker stimulation increased retinal blood flow by +59 ± 20% ( p<0.01) in arteries and by +53 ± 25% ( p<0.01) in retinal veins. These results demonstrate that diffuse luminance flicker increases retinal blood flow in the human retina.

Multiple Occlusive Retinal Arteritis in Both Eyes of a Patient with Rheumatoid Arthritis

Purpose: To report multiple occlusive retinal arteritis as a complication of rheumatoid arthritis. Case: A 67-year-old woman developed superotemporal branch retinal artery occlusion in both eyes, together with arterial sheathing and large cotton wool patches around the optic disc, in the course of rheumatoid arthritis with moderate activity. Observations: Fluorescein angiography disclosed delayed filling of the superotemporal retinal artery in the right eye and no filling of the superotemporal artery in the left eye. In addition, segmental absence of filling was found in peripheral branches of the other major retinal arteries in both eyes. After hyperbaric oxygen therapy and intravenous administration of prostaglandin E1 and urokinase for 2 weeks, there was improvement in her vision. Conclusion: Multiple occlusive retinal arteritis in rheumatoid arthritis can manifest as retinal artery occlusion. Rheumatoid arthritis should be included in the differential diagnosis of bilateral retinal artery occlusion.

Retinal blood flow alterations associated with scleral buckling and encircling procedures.

The bidirectional laser Doppler technique and monochromatic photography were used to measure the absolute blood flow rate in the major temporal retinal arteries in seven patients following unilateral scleral buckling and encircling procedures, and in two patients before and after removal of scleral buckling elements. In the seven patients who had undergone uncomplicated scleral buckling procedures the arterial flow rates were on average 50% lower (p = 0.01) in the surgically treated eyes than in the contralateral eyes. Removal of scleral buckling elements in two patients produced increases of 73% and 44% in arterial blood flow rates.

Fundus Lesions in Malignant Hypertension: IV. Focal Intraretinal Periarteriolar Transudates

Experimental renovascular malignant arterial hypertension was produced in 57 rhesus monkeys by a modified Goldblatt's procedure and their eyes were studied by serial ophthalmoscopy, by stereoscopic color fundus photography, and by fluorescein fundus angiography over a period of months or years. A very common, and one of the earliest, lesions in hypertensive retinopathy was focal intraretinal periarteriolar transudates (FIPTs). In the past, FIPTs have been described erroneously as “cotton-wool spots” The two types of lesions differ very much in shape, size, color, location, fluorescein fundus angiographic pattern, resolution pattern, life cycle, and pathogenesis. FIPTs, on ophthalmoscopy, usually are pinpoint to pinhead size, round or oval, dull white in color, and situated in deeper layers of the retina and beside the major retinal arteries and their main branches. On fluorescein angiography, FIPTs show multiple punctate foci of fluorescein leakage from dilated precapillary retinal arterioles, and there is no focal retinal capillary obliteration. They usually last for two to three weeks, and on resolution leave no ophthalmoscopic, angiographic, or microvascular abnormality. Cotton-wool spots are seen in a variety of retinopathies; FIPTs, however, are a specific retinal lesion of malignant arterial hypertension only. They develop due to breakdown of blood-retinal barrier in precapillary retinal arterioles, due to dilatation of the arterioles from failure of autoregulation (caused by severe rise of blood pressure)

Laser Doppler Measurements of the Effect of Panretinal Photocoagulation on Retinal Blood Flow

The laser Doppler technique was used to measure the pulsatile characteristics of the retinal arterial blood flow in diabetic patients with severe retinopathy immediately before argon laser panretinal photocoagulation (PRP) and again one to two months after PRP. In each measurement the relative variation of the maximum red blood cell velocity, Vmax, in a major branch retinal artery was determined throughout the cardiac cycle. Flow pulsatility was defined as the ratio of Vmax at maximum systole to Vmax at minimum diastole. Following PRP, each eye showed a decrease in flow pulsatility as well as a decrease in retinal arterial and venous diameters. The measurements are consistent with an autoregulatory response of the retinal circulation to increased inner retinal oxygen concentration following PRP. Laser Doppler measurements may be a useful means of assessing the effectiveness of a particular PRP treatment.

Arterial vascular anomalies of the retina.

The incidence of nonaneurysmal congenital anomalies of the retinal arteries was determined by ophthalmoscopic examination of the eyes of 2,100 consecutive healthy individuals whose ages ranged from 6 to 68 years. Unusual anomalies were triple branching, anomalous course, arteriolar-arterial crossing, unusual tortuosity, prepapillary loops, aberrant macular arteries, unusual supply of the optic disc, presumed total ciliary arterial supply of the retina, anomalous relationship with the central retinal vein at the optic disc, and pseudoaneurysm of a major retinal artery. Cilioretinal arteries are the commonest of the congenital vascular anomalies of the retina.

Retinal vasculature of the pig: Light and electron microscope studies

The retinal vasculature of the pig was examined in histological sections, in whole mounts, and by electron microscopy. The pig retina usually has four major retinal arteries and veins and a capillary network in multiple layers similar to those of the human. There is no macula, fovea or tapetum in the pig. The arteries and veins are composed of endothelial cells separated from several layers of smooth muscle fibers by basement membranes. The capillaries consist of endothelial cells which surround a small lumen and are, themselves, surrounded by a complete basement membrane which occasionally splits to surround a pericyte. The structure of the retinal vasculature of the pig is compared with that of the human.