Vascular Retinopathies Research Articles

Classification of Human Retinal Microaneurysms Using Adaptive Optics Scanning Light Ophthalmoscope Fluorescein Angiography

Microaneurysms (MAs) are considered a hallmark of retinal vascular disease, yet what little is known about them is mostly based upon histology, not clinical observation. Here, we use the recently developed adaptive optics scanning light ophthalmoscope (AOSLO) fluorescein angiography (FA) to image human MAs in vivo and to expand on previously described MA morphologic classification schemes. Patients with vascular retinopathies (diabetic, hypertensive, and branch and central retinal vein occlusion) were imaged with reflectance AOSLO and AOSLO FA. Ninety-three MAs, from 14 eyes, were imaged and classified according to appearance into six morphologic groups: focal bulge, saccular, fusiform, mixed, pedunculated, and irregular. The MA perimeter, area, and feret maximum and minimum were correlated to morphology and retinal pathology. Select MAs were imaged longitudinally in two eyes. Adaptive optics scanning light ophthalmoscope fluorescein angiography imaging revealed microscopic features of MAs not appreciated on conventional images. Saccular MAs were most prevalent (47%). No association was found between the type of retinal pathology and MA morphology (P = 0.44). Pedunculated and irregular MAs were among the largest MAs with average areas of 4188 and 4116 μm(2), respectively. Focal hypofluorescent regions were noted in 30% of MAs and were more likely to be associated with larger MAs (3086 vs. 1448 μm(2), P = 0.0001). Retinal MAs can be classified in vivo into six different morphologic types, according to the geometry of their two-dimensional (2D) en face view. Adaptive optics scanning light ophthalmoscope fluorescein angiography imaging of MAs offers the possibility of studying microvascular change on a histologic scale, which may help our understanding of disease progression and treatment response.

Visual impairment and blindness in rural central India: the Central India Eye and Medical Study

The aim of the study was to investigate prevalence of visual impairment in rural central India. The population-based Central India Eye and Medical Study included 4711 subjects with an age of 30+ years. Presenting visual acuity (PRVA) and best-corrected visual acuity (BCVA) were recorded. Visual impairment and blindness were defined using the World Health Organization (WHO) standard and United States (US) standard. On the basis of PRVA and using WHO and US standards, 1049 [22%; 95% confidence interval (CI): 21.1, 23.5] subjects and 1290 (27%; 95% CI: 26.1, 28.7) subjects, respectively, were visually impaired, and 35 (0.7%; 95% CI: 0.5, 1.0) subjects and 116 (2.5%; 95% CI: 2.0, 2.9) subjects, respectively, were blind. The corresponding age-standardized prevalence figures were 17%, 21%, 0.5% and 2%, respectively. Using best-correcting glasses could eliminate PRVA-visual impairment/blindness in 729 subjects (67% of all subjects with visual impairment/blindness). On the basis of BCVA and using WHO and US standards, 333 (7%; 95% CI: 6.3, 7.8) subjects and 473 (10%; 95% CI: 9.2, 10.9) subjects, respectively, had visual impairment, and 22 (0.5%; 95% CI: 0.3, 0.7) and 31 (0.7%; 95% CI: 0.4, 0.9) subjects, respectively, were blind. Corresponding age-standardized prevalence figures were 5%, 8%, 0.4% and 0.5%, respectively. Causes for BCVA-visual impairment/blindness were cataract (75%), postoperative posterior capsular opacification (4%), surgical complications (2%), corneal opacifications (2%), age-related macular degeneration (2%), other macular diseases (1%), and glaucoma (1%). Age-standardized prevalence of PRVA-visual impairment/blindness (WHO definition) in the adult population of rural central India was 17%. Most frequent cause was undercorrected refractive error. Supply of correct glasses is the most efficient way to improve vision in the rural central India.

VEGF Signaling in Health and Disease

Vascular Endothelial Growth Factor (VEGF), also known as vascular permeability factor, is a homodimeric 46kDa protein that signals through two main tyrosine kinase receptors, VEGFR1 (flt) and VEGFR2 (flk, KDR). In addition to these receptors, the non‐tyrosine kinase receptors: Neuropilin1 and Neuropilin2 can bind to VEGF and contribute to signaling. Activation downstream VEGF receptors have been recognized to initiate a series of events resulting in the morphogenesis of blood vessels. However, amongst all these receptors, VEGFR2 is known to be the major mediator of VEGF‐responses associated with angiogenesis. In fact, pharmacological blockade of either the VEGF or VEGFR2 results in the suppression of angiogenesis, as shown in several pathological models. These findings have resulted in the generation of antibodies and small molecule inhibitors. Many of these molecules are currently in the clinic to suppress the angiogenic cascade in cancer, vascular retinopathies and other afflictions. Interestingly, the interventions have worked well in some settings, but are not as effective in other situations. The basis for this disconnect is unclear at this point. Our laboratory has recently uncovered a novel mean for VEGFR2 activation that does require ligand and is also independent of the kinase activity of the receptor. This ligand‐independent, kinase‐independent mode of VEGR2 activation results in downstream signaling but is not affected by current modes of pharmacological blockade of the pathway. In this symposia, we will present our findings, disclose the pathway and discuss its potential impact to several pathological conditions.

Persistent Inhibition of Oxygen-Induced Retinal Neovascularization by Anthrax Lethal Toxin

To evaluate the role of mitogen-activated protein kinase kinase (MKK) signaling in a mouse model of oxygen-induced retinopathy (OIR) that mimics retinopathy of prematurity (ROP). Postnatal day 7 mice were exposed to elevated oxygen for 5 days to induce retinopathy. Anthrax lethal toxin (LeTx), an MKK inhibitor, was injected into the vitreous after restoration to normoxia, and its effects on vascular growth were analyzed by whole mount immunofluorescence and confocal microscopy. Pericyte coverage was determined by PDGFR-β and α-SMA staining. Macrophage presence was determined by F4/80 staining. Vitreal cytokine secretion was measured by ELISA and multianalyte profiling. Intravitreal injection of LeTx over a restricted time interval after return to normoxic conditions blocked the progression of OIR. This block was independent of vascular endothelial growth factor (VEGF) release and did not alter the release of cytokines and growth factors associated with OIR. VEGFR2 expression and activation were similarly unaffected. LeTx had no statistically significant effect on macrophage recruitment. LeTx sensitivity correlated with vessel maturity, extent of hypoxia, and growth of the deep vascular plexus network. Correlation among pericyte coverage, deep vascular plexus growth, and hypoxia after LeTx treatment indicate immature vessels in a hypoxic environment are preferentially sensitive to LeTx-mediated MKK inhibition. The persistence of VEGF without concomitant induction of neovascular growth or revascularization of vaso-obliterated zones suggests MKK inhibition causes an inability of the cells that are present, or a failure to recruit cells able, to respond to proangiogenic stimuli. These results indicate the inhibition of MKK signaling presents a novel strategy for the inhibition of vascular retinopathies such as OIR and ROP.

Von Hippel-Lindau tumor suppressor mutants faithfully model pathological hypoxia-driven angiogenesis and vascular retinopathies in zebrafish

Biallelic inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene predisposes human patients to the development of highly vascularized neoplasms in multiple organ systems. We show that zebrafish vhl mutants display a marked increase in blood vessel formation throughout the embryo, starting at 2 days post-fertilization. The most severe neovascularization is observed in distinct areas that overlap with high vegfa mRNA expression, including the vhl mutant brain and eye. Real-time quantitative PCR revealed increased expression of the duplicated VEGFA orthologs vegfaa and vegfab, and of vegfb and its receptors flt1, kdr and kdr-like, indicating increased vascular endothelial growth factor (Vegf) signaling in vhl mutants. Similar to VHL-associated retinal neoplasms, diabetic retinopathy and age-related macular degeneration, we show, by tetramethyl rhodamine-dextran angiography, that vascular abnormalities in the vhl(-/-) retina lead to vascular leakage, severe macular edema and retinal detachment. Significantly, vessels in the brain and eye express cxcr4a, a marker gene expressed by tumor and vascular cells in VHL-associated hemangioblastomas and renal cell carcinomas. VEGF receptor (VEGFR) tyrosine kinase inhibition (through exposure to sunitinib and 676475) blocked vhl(-/-)-induced angiogenesis in all affected tissues, demonstrating that Vegfaa, Vegfab and Vegfb are key effectors of the vhl(-/-) angiogenic phenotype through Flt1, Kdr and Kdr-like signaling. Since we show that the vhl(-/-) angiogenic phenotype shares distinct characteristics with VHL-associated vascular neoplasms, zebrafish vhl mutants provide a valuable in vivo vertebrate model to elucidate underlying mechanisms contributing to the development of these lesions. Furthermore, vhl mutant zebrafish embryos carrying blood vessel-specific transgenes represent a unique and clinically relevant model for tissue-specific, hypoxia-induced pathological angiogenesis and vascular retinopathies. Importantly, they will allow for a cost-effective, non-invasive and efficient way to screen for novel pharmacological agents and combinatorial treatments.

TRANSLATIONAL IMPACT

von Hippel-Lindau disease (VHL) is a rare, genetic multisystem disorder characterized by the abnormal growth of blood vessels and tumors. Between 60% and 80% of all patients have benign hemangioblastomas of the retina and central nervous system (CNS) that frequently cause neurological morbidity and mortality. VHL patients can also develop retinal neovascularization (also termed angiogenesis; the formation of new blood vessels), similar to diabetic retinopathy and age-related macular degeneration. Angiogenic retinopathies exhibit vascular leakage, macular edema (fluid accumulation) and detachment of the retina, and are the biggest cause of blindness in the Western world, with very limited treatment options. Unfortunately, the current models for angiogenesis do not fully recapitulate the complexity of angiogenic-related disorders. For instance, hemangioblastomas of the retina and CNS have not been found in any of the VHL mouse models.The VHL tumor suppressor gene, VHL, is crucial for oxygen homeostasis. Its loss causes upregulation of HIF (hypoxia-inducible factor), a transcription factor with a central role in oxygen-regulated gene expression, which in turn increases the expression of hypoxia-inducible mRNAs, including those encoding vascular endothelial growth factor A (VEGFA), VEGFA receptors, and proteins involved in the HIF signaling pathwayIn this study, the authors show that zebrafish vhl mutants provide an in vivo model for pathological angiogenesis and vascular retinopathies. Using blood vessel-specific transgenes, they show that vhl mutants develop severe neovascularization in the brain, eye and trunk. The retinal vasculature of vhl−/− zebrafish is leaky, with severe macular edema and retinal detachment, mimicking human angiogenic retinopathies. As in humans, the zebrafish VEGFA orthologs, Vegfaa/Vegfab and Vegfb, are upregulated by loss of vhl, and are key effectors of the vhl−/− angiogenic phenotype; treatment with VEGF receptor inhibitors can block pathological angiogenesis in mutant fish.These zebrafish vhl mutants can be used to understand the mechanisms underlying the development of vascular lesions in VHL disease, and are a unique and clinically relevant angiogenesis model, as pathological angiogenesis and vascular retinopathies can be studied non-invasively under normoxic conditions. Zebrafish vhl mutant embryos with fluorescently-labeled blood vessels are also a cost-effective and efficient model for whole organism screening of anti-angiogenic pharmacological agents and combinatorial therapies.

Von Hippel-Lindau tumor suppressor mutants faithfully model pathological hypoxia-driven angiogenesis and vascular retinopathies in zebrafish

Biallelic inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene predisposes human patients to the development of highly vascularized neoplasms in multiple organ systems. We show that zebrafish vhl mutants display a marked increase in blood vessel formation throughout the embryo, starting at 2 days post-fertilization. The most severe neovascularization is observed in distinct areas that overlap with high vegfa mRNA expression, including the vhl mutant brain and eye. Real-time quantitative PCR revealed increased expression of the duplicated VEGFA orthologs vegfaa and vegfab, and of vegfb and its receptors flt1, kdr and kdr-like, indicating increased vascular endothelial growth factor (Vegf) signaling in vhl mutants. Similar to VHL-associated retinal neoplasms, diabetic retinopathy and age-related macular degeneration, we show, by tetramethyl rhodamine-dextran angiography, that vascular abnormalities in the vhl(-/-) retina lead to vascular leakage, severe macular edema and retinal detachment. Significantly, vessels in the brain and eye express cxcr4a, a marker gene expressed by tumor and vascular cells in VHL-associated hemangioblastomas and renal cell carcinomas. VEGF receptor (VEGFR) tyrosine kinase inhibition (through exposure to sunitinib and 676475) blocked vhl(-/-)-induced angiogenesis in all affected tissues, demonstrating that Vegfaa, Vegfab and Vegfb are key effectors of the vhl(-/-) angiogenic phenotype through Flt1, Kdr and Kdr-like signaling. Since we show that the vhl(-/-) angiogenic phenotype shares distinct characteristics with VHL-associated vascular neoplasms, zebrafish vhl mutants provide a valuable in vivo vertebrate model to elucidate underlying mechanisms contributing to the development of these lesions. Furthermore, vhl mutant zebrafish embryos carrying blood vessel-specific transgenes represent a unique and clinically relevant model for tissue-specific, hypoxia-induced pathological angiogenesis and vascular retinopathies. Importantly, they will allow for a cost-effective, non-invasive and efficient way to screen for novel pharmacological agents and combinatorial treatments.

VEGF therapy: a timely retreat

The formation of new blood vessels is required for the onset and progression of many biological processes during embryonic development and into adulthood. Excessive neovascularization contributes to the pathogenesis of disorders such as tumour growth, chronic inflammation, and vascular retinopathies. On the other hand, an inadequate blood supply is a determining factor in the progression of tissue damage in ischaemic processes.1 In spite of the continuous efforts at prevention, brain and heart ischaemia as an end consequence of atherosclerosis is still a major cause of morbidity and mortality in western countries. A major challenge in cardiovascular research today is to improve revascularization of ischaemic tissues through the development of new treatments that can either substitute high-risk surgical procedures or be used as an adjuvant treatment. The increasing knowledge on the intricate mechanisms that regulate neovascularization has allowed the design of new therapies based on the administration of proangiogenic factors such as VEGF-A, FGF-1, -2 and -4, HGF, and Del-1 (therapeutic angiogenesis).2 A variety of approaches have been used to deliver proangiogenic factors to ischaemic tissues. Direct administration of recombinant factors (protein therapy) allows strict dose control; however, the maintenance of effective concentrations over prolonged periods is hard to achieve. Gene therapy with non-viral vectors, such as naked plasmid DNA, has a low transduction efficiency and the duration of transgene expression is limited. An alternative option is gene therapy with viral vectors, which provide higher levels of transduction and protein expression. The choice of viral vector will depend on the particular application; nevertheless, vector-specific characteristics related to transduction efficiency and biological safety … *Corresponding author. Tel: +34 91 4531200; fax: +34 91 453 1265. E-mail address : aalfranca{at}cnic.es

The kinase inhibitor PKC412 suppresses epiretinal membrane formation and retinal detachment in mice with proliferative retinopathies.

Platelet-derived growth factor (PDGF) is an important stimulatory factor for proliferative retinopathies. Expression of PDGF-B in the retinas of transgenic mice (hemizygous rho/PDGF-B mice) results in rapid-onset retinal detachment caused by proliferation of glial cells, endothelial cells, and pericytes, whereas expression of PDGF-AA (homozygous rho/PDGF-A or PDGF-AA mice) causes slowly progressive retinal detachment from proliferation of glial cells. In this study, we investigated the effect in rho/PDGF-B and rho/PDGF-AA mice of several different receptor kinase inhibitors. Hemizygous rhoPDGF-B or homozygous rho/PDGF-A mice were treated orally with PKC412 (an inhibitor of PDGF, VEGF, and c-kit receptor kinases and several isoforms of PKC), PTK787 (an inhibitor of PDGF, VEGF, and c-kit receptor kinases), SU1498 (an inhibitor of VEGF receptor kinases), imatinib mesylate (an inhibitor of PDGF, c-kit, and v-abl receptor kinases), or vehicle, and at appropriate time points epiretinal membrane (ERM) formation and retinal detachment were quantified. In either rho/PDGF-B or rho/PDGF-A mice, oral administration of PKC412 or PTK787, but not SU1498 or imatinib mesylate, significantly reduced ERM formation. PKC412 reduced the incidence of severe retinal detachments in both models and PTK787 did so in homozygous rho/PDGF-A mice. These data indicate that PKC412 (and possibly PTK787) has appropriate activity and sufficient intraocular bioavailability after oral administration to prevent retinal detachment in models of proliferative retinopathy. PKC412 should be considered for treatment of vascular and nonvascular proliferative retinopathies in humans.

Classification par division/fusion pour la détection des rétinopathies vasculaires en angiographie rétinienne en fluorescence

Dans cet article, nous proposons une nouvelle approche d'analyse de forme pour la détection des rétinopathies vasculaires. La méthode procède à la classification des vaisseaux et quantifie la modification de leur tortuosité sur les images d'angiographie rétinienne en fluorescence. La classification est basée sur une technique de division/fusion. Deux classes sont définies: artère/veine et artériole/veinule. Dans une séquence d'images, les vaisseaux classifiés sont mis en correspondance. La quantification consiste à comparer la tortuosité des vaisseaux classifiés correspondants. La tortuosité est définie par la valeur de l'excentricité déterminée avec les moments invariants 2D d'ordre 2. La méthode est appliquée aux images d'angiographie rétinienne en fluorescence de patients diabétiques et drépanocytaires. In this paper, we present a novel approach of quantification of retinal vascular shape changes for detecting vascular retinopathies. The method classifies vessels and quantifies their tortuosity changes in fluorescein angiographic images. The classification approach is based on a split and merge technique. Two classes of vessel are defined: artery/vein and arteriole/veinlet. In a sequence of images, we form correspondence of classified vessels. Quantifying classified vessels consists of comparing the correspondent tortuosity. We use eccentricity to define tortuosity. Eccentricity computation is based on 2D moment invariant techniques. The method is applied to images of patients suffering of diabetes and sickle cell hemoglobin C disease.

Segmentation of macular fluorescein angiographies. A statistical approach

This paper is concerned with the use of Bayesian methods in the segmentation of macular fluorescein angiographies. Fluorescein angiography is used in ophthalmic practice to evaluate vascular retinopathies and choroidopathies: Sodium fluorescein is injected in the arm's cubital vein of the patient and its distribution is observed along retinal vessels at certain times. In this task a previous and essential step is the segmentation of the image into its relevant components. In order to obtain this segmentation Bayesian methods can be used because a previous knowledge about the spatial structure of the scene to be segmented is available in this kind of images. The stochastic model assumed for the observed intensities is a simple model with a Gaussian noise process which is statiscally independent between pixels. The process of labels x is modelled as a Markov random field with a space-dependent external field expressing the anatomy of the ocular fundus and higher order interactions encouraging blood vessels to be thin and large. This procedure is applied to different cases of diabetic retinophaty and vein occlusions. Two algorithms have been used to estimate x, simulated annealing and iterated conditional modes. In order to evaluate the accuracy of the estimation several error measures have been calculated.

Bayesian detection of the fovea in eye fundus angiographies

This paper is concerned with the automatic detection of the avascular foveal zone (fovea), in eye fundus angiographies. Fluorescein angiography is used in ophthalmic practice to evaluate vascular retinopathies and choroidopathies: sodium fluorescein is injected in the arm's cubital vein of the patient and its distribution along retinal vessels at certain times is observed. The proposed methodology is based on Bayesian statistical methods that allow to incorporate the previous knowledge about the eye fundus in the model. The contour of the fovea is modeled by means of a unidimensional Markov chain and the observed intensities are assumed Gaussian and statistically independent between pixels. Two approximations are used in order to estimate the model's parameters: Empirical and Fully Bayesian. Two algorithms have been used in order to estimate the fovea contour: Simulated Annealing (SA) and Iterated Conditional Modes (ICM). This procedure is applied to different cases of diabetic retinopathy and vein occlusions.

Oscillatory potentials in the retina: what do they reveal.

This chapter is an overview of current knowledge on the oscillatory potentials (OPs) of the retina. The first section describes the characteristics of the OPs. The basic, adaptational, pharmacological and developmental characteristics of the OPs are different from the a- and b-waves, the major components of the electroretinogram (ERG). The OPs are most easily recorded in mesopic adaptational conditions and reflect rapid changes of adaptation. They represent photopic and scotopic processes, probably an interaction between cone and rod activity in the retina. The OPs are sensitive to disruption of inhibitory (dopamine, GABA-, and glycine-mediated) neuronal pathways and are not selectively affected by excitatory amino acids. The earlier OPs are associated with the on-components and the late OPs with the off-components in response to a brief stimulus of light. The postnatal appearance of the first oscillatory activity is preceded by the a- and b-waves. The earlier OPs appear postnatally prior to, and mature differently from, the later ones. The second section deals with present views on the origin of the OPs. These views are developed from experimental studies with the vertebrate retina including the primate retina and clinical studies. Findings favor the conclusion that the OPs reflect neuronal synaptic activity in inhibitory feedback pathways initiated by the amacrines in the inner retina. The bipolar (or the interplexiform) cells are the probable generators of the OPs. Dopaminergic neurons, probably amacrines (or interplexiform cells), are involved in the generation of the OPs. The earlier OPs are generated in neurons related to the on-pathway of the retina and the later ones to the off-channel system. Peptidergic neurons may be indirectly involved as modulators. The individual OPs seem to represent the activation of several retinal generators. The earlier OPs are more dependent on an intact rod function and the later ones on an intact cone system. Thus, the OPs are good indicators of neuronal adaptive mechanisms in the retina and are probably the only post-synaptic neuronal components that can be recorded in the ERG except when structured stimuli are used. The last section describes the usefulness of the oscillatory response as an instrument to study the postnatal development of neuronal adaptation of the retina. In this section clinical examples of of the sensitivity of the OPs for revealing early disturbance in neuronal function in different retinal diseases such as pediatric, vascular and degenerative retinopathies are also given.

Irregular motion recovery in fluorescein angiograms

Fluorescein angiography is a common procedure in ophthalmic practice, mainly to evaluate vascular retinopathies and choroidopathies from sequences of ocular fundus images. In order to compare the images, a reliable overlying is essential. This paper proposes some methods for the recovery of irregular motion in fluorescein angiograms (FA). The overlying is done by a three step procedure: detection of relevant points, matching points from different images and estimation of the assumed linear geometric transformation. A stochastic model (closely related to the general linear model) allows to fuse the second and third steps. Two different estimators of the geometric transformation are proposed and tested with real FAs. Images from choroido-retinal diseases have been analysed: diabetic retinopathy, vein occlusions and choroidal neovascular membrane. Results have been evaluated using a different number of relevant points with different spatial arrangements. Registration accuracy is evaluated as the mean squared error between real and transformed relevant point locations for those points not used to estimate the transformation.

Retinopathy of Prematurity

ROP is a challenging disease of the decade of the 1980s. Answers, even partial answers, to many of its questions may provide information bearing on those same questions in other blinding vascular retinopathies, such as diabetes and sickle cell disease. Answers more clearly defining the role of oxygen, ventilation, antioxidants, blood transfusions, and a host of diseases of the premature infant will lead to better care of that infant. I have tried in this article to present the boundaries of the problem, a theory of its genesis and progression, and a review of the major issues to be confronted by the pediatric, ophthalmologic, and basic science communities through its recurrence today. I have tried to make it clear to the reader when I was so doing. I have used information liberally from studies both under way and in the planning stages to make the reader aware of what is being done, even if these have not yet reached fruition, for the field is a rapidly growing one. Finally, I have tried to point out directions that I believe clinical and experimental work should take on certain critical issues.

VEP pattern after photostress: an index of macular function.

Eighty-seven subjects were examined by means of visual evoked potentials (VEPs) to evaluate the recovery time of the amplitude of the pattern VEP after photostress. Twenty-eight subjects were normal and 59 patients were affected by carotid occlusive disease, vascular retinopathies with and without macular involvement, and connective tissue diseases treated with antimalarial drugs. This examination method emphasizes visual dysfunction in a stage where ophthalmological changes are absent or minimal and therefore may be a useful test for the investigation of macular function.

An Examination of the Possibilities of Preventive Therapy in Arteriolar Retinopathies

The identification of certain chronic vascular retinopathies has been marked by a search for their classification. Such proper classification has therapeutic implications. The similarities of these retinopathies are based on a shared pathogenic process, initiated by diminished arteriolar perfusion pressure, as the result of varied causes but leading to somewhat nonspecific capillary and reactive stages. On the basis of such a pathogenesis, various possible prophylactic and therapeutic measures and their organizational requirements are significant. These measures are directed to the correction of a blood perfusion pressure defect and to leaking capillaries and new vessels. Prophylactic and therapeutic exercises will be required for various chronic arteriolar capillaropathies, although the experience with one disease may be applicable to the others. Senile macular disciform degeneration provides the factors necessary for a therapeutic trial.

The blood-retinal barriers.

The Blood-Retinal Barrier (BRB) is a situation of restricted permeability which is present between the blood and the retina. This barrier has a well defined anatomic substrate, particular permeability characteristics and appears to play a role of major importance in the pathophysiology and therapeutics of retinal disease. The BRB phenomenon operates fundamentally at two levels, retinal vessels and chorioepithelial interface, forming which may be better called an inner BRB and an outer BRB. The main structures involved are, for the inner BRB, the endothelial membrane of the retinal vessels, and for the outer BRB, the retinal pigment epithelium. 'Zonulae occludentes' are present in these membranes forming complete belts around the cells, sealing off the spaces between them. Other structures appear to play an accessory role. Both barriers show an apparent predominance of processes of active transport over mechanisms of passive transfer, these being extremely restricted. Much information on the pathophysiology of the BRB mechanism has been obtained from studies of its experimental breakdown. In this way, a breakdown of the inner BRB may be induced by acute distension of the vessel walls, ischaemia, chemical influences, defects in the endothelial cells and failure of the active transport system, whereas experimental ischaemia, mechanical distension of the pigment epithelial membrane, defects in the pigment epithelium and failure of the active transport systems can cause a breakdown of the outer BRB. The increased permeability of the inner BRB, and of the outer BRB, appears to be related to changes in the vascular endothelial membrane and retinal pigment epithelium, respectively. In clinical ophthalmology there are two methods for the diagnosis of breakdown of the BRB, fundus fluorescein angiography and vitreous fluorophotometry. Vitreous fluorophotometry being capable of detecting functional alterations of the barrier before any pathological changes are apparent. There is evidence of an intimate relationship between breakdown of the BRB and almost every retinal disease, particularly the vascular retinopathies and the pigment epitheliopathies. Diabetic retinopathy, hypertensive retinopathy, retinal vein obstruction, blood diseases, trauma or surgery to the eye, temporary arterial obstruction, perivasculitis, Behçet's and Coats' diseases, retinoblastoma, hemangioblastoma and retinal neovascularization are examples of situations where a breakdown of the inner BRB has been demonstrated. On the other hand, examples of breakdown of the outer BRB include situations of choroidal ischaemia, detachment of the pigment epithelium, choroidal neovascularization, photocoagulation, retinal detachment, Koyanagi's disease, central serous choroidopathy, multifocal inner choroiditis and acute placoid pigment epitheliopathy.

Clinical photocoagulation with the krypton laser.

The first clinical application of the krypton laser has been reported. Ten patients with vascular retinopathies were photocoagulated with the monochromatic yellow krypton laser emission at λ568.2 nm λ5,682 Angstroms) and coagulations ranging between 150μ to 250μ in diameter were produced after one-fourth to onehalf second exposures with power levels of 100 to 250 milliwatts. The krypton laser can effectively treat ophthalmic vascular and chorioretinal abnormalities with certain advantages over other lasers, but, presently, its relatively weak output power limits its effectiveness in many cases.