Anatomy of the choroidal vasculature

Aging in Ocular Blood Vessels: Molecular Insights and the Role of Oxidative Stress

Acknowledged as a significant pathogenetic driver for numerous diseases, aging has become a focal point in addressing the profound changes associated with increasing human life expectancy, posing a critical concern for global public health. Emerging evidence suggests that factors influencing vascular aging extend their impact to choroidal and retinal blood vessels. The objective of this work is to provide a comprehensive overview of the impact of vascular aging on ocular blood vessels and related diseases. Additionally, this study aims to illuminate molecular insights contributing to vascular cell aging, with a particular emphasis on the choroid and retina. Moreover, innovative molecular targets operating within the domain of ocular vascular aging are presented and discussed.

The vascular geometry of the choriocapillaris is associated with spatially heterogeneous molecular exchange with the outer retina

Vision relies on the continuous exchange of material between the photoreceptors, retinal pigment epithelium and choriocapillaris, a dense microvascular bed located underneath the outer retina. The anatomy and physiology of the choriocapillaris and their association with retinal homeostasis have proven difficult to characterize, mainly because of the unusual geometry of this vascular bed. By analysing tissue dissected from 81 human eyes, we show that the thickness of the choriocapillaris does not vary significantly over large portions of the macula or with age. Assessments of spatial variations in the anatomy of the choriocapillaris in three additional human eyes indicate that the location of arteriolar and venular vessels connected to the plane of the choriocapillaris is non-random, and that venular insertions cluster around arteriolar ones. Mathematical models built upon these anatomical analyses reveal that the choriocapillaris contains regions where the transport of passive elements is dominated by diffusion, and that these diffusion-limited regions represent areas of reduced exchange with the outer retina. The width of diffusion-limited regions is determined by arterial flow rate and the relative arrangement of arteriolar and venular insertions. These analyses demonstrate that the apparent complexity of the choriocapillaris conceals a fine balance between several anatomical and functional parameters to effectively support homeostasis of the outer retina. KEY POINTS: The choriocapillaris is the capillary bed supporting the metabolism of photoreceptors and retinal pigment epithelium, two critical components of the visual system located in the outer part of the retina. The choriocapillaris has evolved a planar multipolar vascular geometry that differs markedly from the branched topology of most vasculatures in the human body. Here, we report that this planar multipolar vascular geometry is associated with spatially heterogenous molecular exchange between choriocapillaris and outer retina. Our data and analyses highlight a necessary balance between choriocapillaris anatomical and functional parameters to effectively support homeostasis of the outer retina.

Bruch's Membrane Calcification in Pseudoxanthoma Elasticum: Comparing Histopathology and Clinical Imaging

Purpose

To investigate the histology of Bruch's membrane (BM) calcification in pseudoxanthoma elasticum (PXE) and correlate this to clinical retinal imaging.

Design

Experimental study with clinicopathological correlation.

Subjects and Controls

Six postmortem eyes from 4 PXE patients and 1 comparison eye from an anonymous donor without PXE. One of the eyes had a multimodal clinical image set for comparison.

Methods

Calcification was labeled with OsteSense 680RD, a fluorescent dye specific for hydroxyapatite, and visualized with confocal microscopy. Scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy (SEM-EDX) and time-of-flight secondary ion mass spectrometry (TOF-SIMs) were used to analyze the elemental and ionic composition of different anatomical locations. Findings on cadaver tissues were compared with clinical imaging of 1 PXE patient.

Main Outcome Measures

The characteristics and topographical distribution of hydroxyapatite in BM in eyes with PXE were compared with the clinical manifestations of the disease.

Results

Analyses of whole-mount and sectioned PXE eyes revealed an extensive, confluent OsteoSense labeling in the central and midperipheral BM, transitioning to a speckled labeling in the midperiphery. These areas corresponded to hyperreflective and isoreflective zones on clinical imaging. Scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy and TOF-SIMs analyses identified these calcifications as hydroxyapatite in BM of PXE eyes. The confluent fluorescent appearance originates from heavily calcified fibrous structures of both the collagen and the elastic layers of BM. Calcification was also detected in an aged comparison eye, but this was markedly different from PXE eyes and presented as small snowflake-like deposits in the posterior pole.

Conclusions

Pseudoxanthoma elasticum eyes show extensive hydroxyapatite deposition in the inner and outer collagenous and elastic BM layers in the macula with a gradual change toward the midperiphery, which seems to correlate with the clinical phenotype. The snowflake-like calcification in BM of an aged comparison eye differed markedly from the extensive calcification in PXE.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Visualization of choroidal vasculature in pigmented mouse eyes from experimental models of AMD

A variety of techniques exist to investigate retinal and choroidal vascular changes in experimental mouse models of human ocular diseases. While all have specific advantages, a method for evaluating the choroidal vasculature in pigmented mouse eyes has been more challenging especially for whole mount visualization and morphometric analysis. Here we report a simple, reliable technique involving bleaching pigment prior to immunostaining the vasculature in whole mounts of pigmented mouse choroids. Eyes from healthy adult pigmented C57BL/6J mice were used to establish the methodology. The retina and anterior segment were separated from the choroid. The choroid with retinal pigment epithelial cells (RPE) and sclera was soaked in 1% ethylenediaminetetraacetic acid (EDTA) to remove the RPE. Tissues were fixed in 2% paraformaldehyde (PFA) in phosphate-buffered saline (PBS). Choroids were subjected to melanin bleaching with 10% hydrogen peroxide (H2O2) at 55 °C for 90 min, washed in PBS and then immunostained with anti-podocalyxin antibody to label vascular endothelium followed by Cy3-AffiniPure donkey anti-goat IgG at 4 °C overnight. Images of immunostained bleached choroids were captured using a Zeiss 710 confocal microscope. In addition to control eyes, this method was used to analyze the choroids from subretinal sodium iodate (NaIO3) RPE atrophy and laser-induced choroidal neovascularization (CNV) mouse models. The H2O2 pretreatment effectively bleached the melanin, resulting in a transparent choroid. Immunolabeling with podocalyxin antibody following bleaching provided excellent visualization of choroidal vasculature in the flat perspective. In control choroids, the choriocapillaris (CC) displayed different anatomical patterns in peripapillary (PP), mid peripheral (MP) and far peripheral (FP) choroid. Morphometric analysis of the vascular area (VA) revealed that the CC was most dense in the PP region (87.4 ± 4.3% VA) and least dense in FP (79.9 ± 6.7% VA). CC diameters also varied depending on location from 11.4 ± 1.97 mm in PP to 15.1 ± 3.15 mm in FP. In the NaIO3-injected eyes, CC density was significantly reduced in the RPE atrophic regions (50.7 ± 5.8% VA in PP and 45.8 ± 6.17% VA in MP) compared to the far peripheral non-atrophic regions (82.8 ± 3.8% VA). CC diameters were significantly reduced in atrophic regions (6.35 ± 1.02 mm in PP and 6.5 ± 1.2 mm in MP) compared to non-atrophic regions (14.16 ± 2.12 mm). In the laser-induced CNV model, CNV area was 0.26 ± 0.09 mm2 and luminal diameters of CNV vessels were 4.7 ± 0.9 mm. Immunostaining on bleached choroids with anti-podocalyxin antibody provides a simple and reliable tool for visualizing normal and pathologic choroidal vasculature in pigmented mouse eyes for quantitative morphometric analysis. This method will be beneficial for examining and evaluating the effects of various treatment modalities on the choroidal vasculature in mouse models of ocular diseases such as age-related macular degeneration, and degenerative genetic diseases.

Uveal vascular bed in health and disease: lesions produced by occlusion of the uveal vascular bed and acute uveal ischaemic lesions seen clinically. Paper 2 of 2

From studies on postmortem anatomical descriptions of the uveal vascular bed, it was generally concluded that occlusion of PCA or its branches should not produce an ischemic lesion. However, in vivo studies have recorded that the PCAs and their branches, right down to the terminal choroidal arterioles, and the choriocapillaris, have a segmental distribution in the choroid, and that PCAs and choroidal arteries function as end-arteries. This explains the basis of the occurrence of isolated inflammatory, ischemic, metastatic, and degenerative choroidal lesions, which are usually localized. Thus, in vivo studies have completely revolutionized our concept of the uveal vascular bed in disease.

Studies of the retinal microcirculation using human donor eyes and high-resolution clinical imaging: Insights gained to guide future research in diabetic retinopathy

The microcirculation plays a key role in delivering oxygen to and removing metabolic wastes from energy-intensive retinal neurons. Microvascular changes are a hallmark feature of diabetic retinopathy (DR), a major cause of irreversible vision loss globally. Early investigators have performed landmark studies characterising the pathologic manifestations of DR. Previous works have collectively informed us of the clinical stages of DR and the retinal manifestations associated with devastating vision loss. Since these reports, major advancements in histologic techniques coupled with three-dimensional image processing has facilitated a deeper understanding of the structural characteristics in the healthy and diseased retinal circulation. Furthermore, breakthroughs in high-resolution retinal imaging have facilitated clinical translation of histologic knowledge to detect and monitor progression of microcirculatory disturbances with greater precision. Isolated perfusion techniques have been applied to human donor eyes to further our understanding of the cytoarchitectural characteristics of the normal human retinal circulation as well as provide novel insights into the pathophysiology of DR. Histology has been used to validate emerging in vivo retinal imaging techniques such as optical coherence tomography angiography. This report provides an overview of our research on the human retinal microcirculation in the context of the current ophthalmic literature. We commence by proposing a standardised histologic lexicon for characterising the human retinal microcirculation and subsequently discuss the pathophysiologic mechanisms underlying key manifestations of DR, with a focus on microaneurysms and retinal ischaemia. The advantages and limitations of current retinal imaging modalities as determined using histologic validation are also presented. We conclude with an overview of the implications of our research and provide a perspective on future directions in DR research.

Characterization of Choroidal Morphology and Vasculature in the Phenotype of Pachychoroid Diseases by Swept-Source OCT and OCTA

The objective of this study was to characterize the choroidal morphology and vasculature in pachychoroid diseases (PCD). A total of 49 eyes with polypoidal choroidal vasculopathy (PCV), 43 eyes with neovascular age-related macular degeneration (nAMD), and 50 eyes with central serous chorioretinopathy (CSC), along with 80 healthy eyes, were enrolled in this nested case-control study. The swept-source optical coherent tomography (OCT), OCT angiography, and En face images were quantitatively analyzed. Multivariate logistic regression models showed that older age and increased vessel density (VD) in the choriocapillaris (CC) layer were independent risk factors for both PCV (page < 0.001, pVD = 0.004), and nAMD (page < 0.001, pVD = 0.005). Decreased VD in the Sattler’s layer was an independent risk factor for PCV (p = 0.014). Increased VD in the Haller’s layer was an independent risk factor for CSC (p = 0.001). The proportion of the diffuse type of collateral circulation in the Sattler’ layer in CSC group was significantly higher than in the other three groups (p < 0.001). We concluded that the involvement of the blood flow in the CC, Haller’s, and Sattler’s layers are differently affected in CSC, nAMD, and PCV eyes, indicating the different pathological mechanism underlying the phenotype of PCD. The age-dependent establishment of collateral circulation in the Sattler’s layer may play a compensatory role regarding ischemic injury in the development of PCD.

Correlation of choriocapillaris hemodynamic data from dynamic indocyanine green and optical coherence tomography angiography

To investigate the correlation between posterior pole choroidal blood flow evaluated with digital subtraction indocyanine green angiography and enface optical coherence tomography angiography (OCTA). Imaging in animal study. The anatomy of 2 cynomogulus monkeys was studied. Each monkey was given a 0.75 mg/kg injection of indocyanine green in the saphenous vein. The dynamic angiographic filling sequence was recorded at 15 frames per second using the Heidelberg Spectralis. After image registration, sequential frame subtraction was used to image the dye front moving through the choroid. The OCTA was obtained by frame averaging nine separate choriocapillaris slab flow images obtained from the Zeiss Plex Elite 9000. Posterior pole choriocapillaris filling pattern in relation to the choriocapillaris anatomy as imaged by OCTA. In the posterior pole, the choriocapillaris fills in the pattern of discrete units with variable sizes and shapes. The cycle of dye filling begins in the peripapillary area and progresses toward the periphery in a wavelike manner. This filling pattern repeats in a cyclical manner, consistent with the cardiac cycle. OCTA shows a uniform mesh of vessels. While OCTA shows a uniform meshwork appearance of the choriocapillaris, the dynamic dye angiography suggests an irregular configuration of functional units partitioned by pressure gradients as opposed to structural boundaries. Disturbance of local perfusion pressure within choroidal vasculature may result in abnormal flow patterns, which could be evaluated in the clinic using commercially available equipment.

Choroidal Cavitary Disorders

The structure and functions of the choroid have been long acknowledged but the pathophysiology behind various anomalies has been difficult to understand until the advent of optical coherence tomography (OCT). With OCT imaging, choroidal cavitations appear as optically empty spaces between the outer retinal and choroidal layers with attenuation or loss of outer retinal layers. Choroidal cavitations are found in the posterior pole and seen in conditions such as pathologic myopia, north carolina macular dystrophy (NCMD), focal choroidal excavation (FCE), and torpedo maculopathy (TM). To date, these disorders have not been linked. A commonality they all share is malformation of the RPE-photoreceptor-choroid complex. The following report describes the differences and similarities of choroidal cavitation amongst the different retinal disorders and emphasizes the importance of multimodal imaging in the detection and management of potential complications.

Blindness Following Carotid Artery Stenting Due to Ocular Hyperperfusion - Report and Review of Literature

Cerebral hyperperfusion syndrome is a well-recognised phenomenon following carotid revascularisation. It is defined as cerebral blood flow increase of more than 100% of the baseline. A similar phenomenon can occur in the eye and maybe termed as ocular hyperperfusion syndrome. We present a 65-year-old male who developed an ipsilateral red eye with visual loss following carotid artery stenting. There was a past history of recurrent right middle cereberal artery (MCA) territory embolic infarcts and recurrent trasient episodes of vision loss in the right eye. Flow reversal was noted in the ophthalmic artery on Transcranial doppler (TCD). Digital subtraction angiography (DSA) showed more than 95% stenosis in right internal carotid artery (ICA) ostium and completely occluded left ICA. Following carotid artery, stenting patient developed severe headache and right eye pain along with vision loss despite intensive blood pressure monitoring and control. NCCT head showed mild right cortical SAH and the intra-ocular pressure (IOP) in the right eye was high. It was hypopthesised that aqueous over production due to neovascularity secondary to chronic ocular ischemia, lack of outflow and sudden change in ocular hemodynamics post stenting was the pathogenic mechanism. The patient was commenced on measures to reduce aqueous production along with strict blood pressure control. Prestenting evalvation for chronic ocular ischemia with tanscranial dopplar and angiographic flow reversal in ophthalmic artery, fluorescein angiography to look for watershed zones and slit lamp for neovascularity and angle closure can help in identifing high-risk patients, particularly in patients with bilateral carotid artery disease.

A Model to Study Thermal Energy Delivery to the Choroid: A Comparison of Surgical Devices

Purpose

We measure and compare surgical devices using an ex vivo, temperature-controlled, choroidal incision model during thermal energy transfer with a high-resolution infrared camera.

Methods

Ex vivo porcine choroidal tissue specimens (n = 516) were isolated and placed on a temperature-regulated (37°C) perfusion platform. We tested the pulsed electron avalanche knife (PEAK), micropulse laser (MpL), continuous laser (CL), and bipolar cautery (BpC) at three energy settings (11 [low], 45 [medium], and 134 [high] mJ/mm). Each device was clamped to a stationary mechanical arm. Movement of tissue specimens beneath the surgical device was achieved using a stepping motor-driven x-y table. An infrared video camera measured orthogonal temperature variation in the surrounding tissue.

Results

Increased power resulted in greater lateral thermal spread using all modalities (P < 0.001). Mean (standard deviation) lateral thermal spread at low energy was smallest for the MpL at 0.0 (0.01) mm (P < 0.001), whereas BpC had the least collateral tissue damage at medium and high energies (0.02 [0.08] and 0.34 [0.22] mm, respectively; P < 0.001). Fluidics of the ex vivo system may limit thermal spread. The PEAK had the greatest thermal spread across all energy groups (P < 0.001), with clinically relevant variation between disposable blades.

Conclusions

Our ex vivo model enabled direct comparison of threshold thermal tissue injury across four devices. MpL and BpC showed the least thermal damage. PEAK had a higher variation in energy delivery, but also has the advantage of more effective tissue cutting.

Translational Relevance

Our ex vivo surgical device analysis provides thermal tissue injury predictions for choroidal surgery.

Creating a Full-thickness Choroidal Incision: An Ex Vivo Analysis of Human and Porcine Tissue Contraction Dynamics

Purpose

We hypothesized that the elastic nature of the choroid leads to tissue contraction following a full-thickness, sharp incision. Furthermore, we sought to quantify, measure, and compare tissue contraction in ex vivo porcine globes and human globes of various ages using predetermined variables.

Method

A full-thickness, ex vivo choroidal incision was performed in either pig (n = 97) or human (n = 30) specimens. Variables included trephine diameter (1.5, 2.0, or 2.5 mm) versus a straight surgical blade, and temperature (1.7 °–4.4° vs. 36.6°F). Central centripetal and surround centrifugal tissue contractions were measured. Mean percentage tissue contraction was assessed as a ratio of trephine diameter to final tissue contraction measured immediately following each incision using a standardized device.

Results

For trephination in pig specimens, centripetal contraction ranged from 38% to 50% with a mean of 44%. Centrifugal contraction was approximately 15%. Human choroidal contraction was 39% and 15%, respectively, with a statistically significant inverse relationship to age (R² = 0.35, P ≤ 0.01). Asymmetric contraction was noted when incisions were closer to choroidal attachment sites to the sclera, such as near vortex ampullae. Linear incisions resulted in contraction that correlated with incision length (R² = 0.35, P ≤ 0.001).

Conclusions

A full-thickness choroidal incision results in significant tissue contraction. For circular incisions, the centripetal contraction approaches 50% of the original incision size. For linear incisions, the contraction corresponds directly with incision length. In human specimens, there is less contraction with advancing age.

Translational Relevance

Our findings have clinical relevance for choroidal biopsy, traumatic injury, and choroidal translocation surgery.

Peripheral Reticular Pigmentary Degeneration and Choroidal Vascular Insufficiency, Studied by Ultra Wide-Field Fluorescein Angiography

Purpose

To explore the pathogenesis of peripheral reticular pigmentary degeneration (PRPD) and its clinical significance.

Methods

This cross-sectional, observational study (conducted between January 2010 and May 2015) enrolled 441 eyes of 229 subjects, including 35 eyes with PRPD and 406 eyes without PRPD, which was identified by ultra-wide-field fluorescein angiography (UWFA). The distribution and angiographic circulation time of PRPD were assessed by UWFA. The frequencies of systemic and ophthalmologic comorbidities were compared between groups. Univariate and multivariate generalized estimation equation methods were used to determine the risk factors for PRPD.

Results

The patients with PRPD had a mean age of 75.7 ± 8.5 years (range, 59–93 years), whereas the patients without PRPD had a mean age of 60.1 ± 14.9 years (range, 9–92 years). All eyes with PRPD manifested the lesion in the superior nasal periphery with or without circumferential extension. Among those, only 16 eyes (45.7%) in the PRPD group showed distinctive features in the same location on fundus photographs. There was significant choroidal filling delay in the PRPD group when compared with the control group (1.42±1.22 vs. -0.02±1.05 seconds, P < 0.001). Multivariate regression analysis revealed that older age (P < 0.001), stroke (P = 0.018), ischemic optic neuropathy (P < 0.001), and age-related macular degeneration (P = 0.022) were significantly associated with PRPD.

Conclusions

UWFA may enhance the diagnostic sensitivity of PRPD. Choroidal vascular insufficiency with compromised systemic circulation in the elderly was related to the manifestation of PRPD. These results help to better understand the pathophysiology of PRPD. Co-existence of systemic and ophthalmic circulatory disorders should be considered in patients with PRPD.

Structural changes of the choroid in sarcoid- and tuberculosis-related granulomatous uveitis

AIM

The aim of this study is to characterise the choroidal features of patients diagnosed with sarcoid- and tuberculosis (TB)-associated granulomatous uveitis using spectral domain optical coherence tomography (OCT).

METHODS

Twenty-seven patients (27 eyes) diagnosed with sarcoid- (13 eyes) and TB (14 eyes)-related uveitis were included in this retrospective, cross-sectional study. Over a six-month period, patients diagnosed with sarcoid and TB granulomatous uveitis were scanned using enhanced depth imaging OCT. Clinical and demographical characteristics were recorded, including the method of diagnosis, disease activity, site of inflammation (anterior or posterior), treatments, and visual acuity (VA). Manual segmentation of the choroidal layers was performed using custom image analysis software.

RESULTS

The main outcome measure was OCT-derived thickness measurements of the choroid and choroidal sublayers (Haller's large vessel and Sattler's medium vessel layers) at the macula region. The ratio of Haller's large vessel to Sattler's medium vessel layer was significantly different at the total macula circle in eyes diagnosed with TB uveitis (1.47 (=140.71/95.72 μm)) compared with sarcoid uveitis (1.07 (=137.70/128.69 μm)) (P=0.001). A thinner choroid was observed in eyes with a VA ≥0.3 LogMAR (Snellen 6/12; 198.1 μm (interquartile range (IQR)=147.0-253.4 μm) compared with those with VA <0.3 LogMAR (292.4 μm (IQR=240.1-347.6 μm)) at the total macula circle (P=0.004). At the foveal central subfield, the median choroidal thickness was 336.8 μm (IQR=272.3-375.4 μm) in active compared with 239.3 μm (IQR=195.3-330.9 μm) in quiescent disease (P=0.04).

CONCLUSION

A disproportionately enlarged Sattler's layer may indicate a diagnosis of sarcoid-related uveitis, and choroidal thickening may be a feature of active granulomatous uveitis.

Lipids, lipoproteins, and age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness among the elderly. While excellent treatment has emerged for neovascular disease, treatment for early AMD is lacking due to an incomplete understanding of the early molecular events. A prominent age-related change is the accumulation of neutral lipid in normal Bruch's membrane (BrM) throughout adulthood and also disease-related BrM accumulations called basal deposits and drusen. AMD lesion formation has thus been conceptualized as sharing mechanisms with atherosclerotic plaque formation, where low-density lipoprotein (LDL) retention within the arterial wall initiates a cascade of pathologic events. However, we do not yet understand how lipoproteins contribute to AMD. This paper explores how systemic and local production of lipoproteins might contribute to the pathogenesis of AMD.

The choroidal circulation assessed by laser-targeted angiography

The choroid plays an important role in supplying nutrients to and removing waste products from the outer region of the retina. Abnormal choroidal blood flow can disrupt normal retinal function and lead to alterations in visual function. Visualization of the choriocapillaris in vivo is a great challenge to understanding its normal physiology and involvement in the disease process. Laser-targeted angiography (LTA) is a relatively new method used to visualize and analyze the choroidal circulation. Carboxyfluorescein (CF), encapsulated in heat-sensitive liposomes, is released locally in the choroid through the application of a heat beam provided by an infrared laser. Video angiograms are generated with excitation illumination provided by an argon laser. Obtained images are highly selective to the choriocapillaris and are sharply contrasted against underlying and overlying structures. The images can be obtained repetitively, during which period the circulating liposome concentration is sufficient to generate adequate angiograms. These high-quality images have revealed three distinct phases (filling, plateau, and draining) of the choriocapillaris. In the plateau phase, a cluster of lobules fed by a common arteriole has been uniformly illuminated. This defined cluster area does not change in size while an infrared laser is continuously applied to the same spot, which demonstrates that each cluster is functionally independent and no physiological communication exists between them. Only in posterior regions do the angiograms demonstrate during the filling and draining phases that each lobule is filled from a central spot and drained along a peripheral ring, showing honeycomb flow patterns. The regional differences in choriocapillaris flow patterns revealed by LTA suggests that the choriocapillaris provides a more highly efficient system of outflow in posterior regions than in peripheral regions. LTA is useful in analyzing choroidal circulation in vivo and has the potential for clinical application in the future. Additionally, LTA has a unique capability to image choroidal neovascularization in animal models and it promises potential application in age-related macular degeneration (AMD).

Microvascular architecture of the rat choroid: corrosion cast study

This study presents the details of the microvasculature of the rat choroid visualized by scanning electron microscopy of vascular corrosion casts. Wistar Kyoto rats were anesthetized with intraperitoneal sodium pentobarbital. The vascular system was perfused with heparinized saline, and Mercox resin was injected into the cannulated carotid arteries. After polymerization of the resin, the vascular casts were macerated with potassium hydroxide, washed with water, and freeze dried. The casts were examined with a Hitachi S-2360N scanning electron microscope (SEM). Corrosion casts of the entire choroidal vasculature revealed that the two long posterior ciliary arteries supplied the entire uveal vasculature. In the posterior choroid, these arteries formed five to seven branches on each side supplying the adjacent choriocapillaris. No interarterial or arteriovenous anastomoses were seen. The choriocapillaris appeared as a nonhomogeneous and nonlobular monolayer capillary network, consisting of dense honeycomb and irregular patterns. There are two distinct venous systems in the rat choroid. The venous blood from the central region, peripapillary choroid, and sometimes the optic nerve head drain into the posterior ciliary vein. The venous blood from the iris, ciliary body, anterior choroid, and half of the posterior choroid drain into the vortex veins. Corrosion casts and the SEM have shown details of the choroidal vascular architecture. These three-dimensional observations indicate that the rat choroidal vasculature has different features from those of humans and other primates. Despite these interspecies differences, the establishment of a thorough baseline concept of choroidal vasculature should permit additional studies of the choroidal pathology and enable the proper interpretation of results from rat experimental models for extrapolation to humans.

Watershed zone in the human peripheral choroid

PURPOSE

To evaluate the angioarchitecture and the hemodynamics in the peripheral choroid in humans.

METHODS

Choroidal circulation was examined in 23 normal eyes and in 7 with equatorial lattice degeneration by wide-angle indocyanine green (ICG) angiography using a scanning laser ophthalmoscope. A hand-held 30-diopter lens was placed in front of the eye to expand the observation angle from the original 40 degrees to 70 degrees. The peripheral temporal choroid and the ora serrata thus could be seen in one frame.

RESULTS

An arterial watershed zone was identified in the temporal peripheral choroid in 21 of 30 eyes. This watershed zone was located in a sectorial region posterior to the ora serrata. It did not form a circular belt along the equator. The area anterior to the watershed zone was supplied by peripheral retrograde choroidal arteries. Inflow of dye into the peripheral retrograde choroidal arteries was consistently later than into the more posterior choroid via short posterior ciliary arteries. The terminal branches of peripheral and short posterior ciliary arteries did not form functional anastomoses. Peripheral retrograde choroidal arteries showed wide variations in number and distribution. These arteries originated from recurrent branches of long posterior ciliary arteries in 18 of 21 eyes and from anterior ciliary arteries in 3 eyes. There was no correlation between the watershed zone and equatorial lattice degeneration in location and extent.

CONCLUSION

A peripheral watershed zone is a regular feature in the far temporal sector of the choroid. The far peripheral choroid anterior to the watershed zone was perfused by peripheral retrograde choroidal arteries which originated from the long posterior ciliary arteries and/or the anterior ciliary arteries.

Anatomical and functional choroidal lobuli

The purpose of this paper is to clarify the controversy between anatomists and clinicians regarding the choroidal angioarchitecture. Vascular casts from 36 human and 10 Rhesus monkey eyes were studied using scanning electron microscopy (SEM). Both the human and monkey choriocapillaris (CC) are non-homogenous structures. They have patterns which change from the peripapillary to peripheral areas. Anatomically, the 'lobular' appearance of the CC exists only in part of the posterior pole. One or more collecting venules were found in the center of 86% of the anatomical lobules, while a central feeding arteriole was observed in 14%. Both major and minor feeding arterioles supply the CC areas which may be recognized as the choroidal functional vascular unit (CFVU) or functional lobule described in the past by Hayreh. Our vascular casts and SEM study show that the choroidal anatomical lobuli are not identical with those observed by angiographical study. Thus, two distinct models of choroidal lobuli, anatomical and functional, should be recognized. The CFVU seen on fluorescein (FA) and indocyanine green (ICG) angiographies as a lobular appearance is most likely caused by the pressure gradient of the blood flow.

Choroidal vascular ischaemia

The term 'choroidal vascular ischaemia' refers to all the choroido-retinal changes which follow an acute or chronic circulatory disorder in the arterial capillary or venous network of the choroid. However, different clinical syndromes can be observed according to the type of vessel which is occluded, the origin and rate of development of the process. Experimentation on animals, especially on monkeys and observations on those with limited retinal pigmentation has enabled us to study the anatomy of the choroidal circulation and the changes found in choroidal vascular ischaemia following capillary embolism or thrombosis after laser. These experiments have been accompanied by histochemical, bioelectrical and genetic examination. For the past 20 years, angiography has been used to study the choroidal circulation, although only cardiogreen and fluorescein have proved useful.

Observations on the lobular organization of the human choriocapillaris

Vascular casts (Batson mixture) of ocular angioarchitecture were prepared from ten human orbits removed at autopsy. The age of donor specimens ranged from seven months to 68 years. The time interval between death and injection of the orbits averaged seven hours with a range of five hours. The resulting vascular casts were examined using light microscopy and scanning electron microscopy (SEM). We present photomicrographs of our vascular casts that demonstrate predictable variations of the human choriocapillaris angioarchitecture according to the region of the choriocapillaris under examination (i.e. posterior pole, equatorial, peripheral, peripapillary, or submacular areas). Our discussion represents an effort to help clarify some persistent controversies concerning the human choroidal angioarchitecture.

Functional anatomy of the choroidal circulation: methyl methacrylate casting of human choroid

The functional anatomy of the normal choroidal circulation in man is described from scanning electron microscopic examination of methyl methacrylate microvascular casts. Distal and para-optic short posterior ciliary arteries supply wedge-shaped areas of choroid. Regional variations in choriocapillary morphology are well recognised; the lobules are densely packed at the posterior pole with a high capillary to inter-capillary ratio. Choroidal capillaries are flattened providing a large surface area for metabolic exchange with the retinal pigment epithelium. In acute choroidal ischaemia (from a variety of systemic disease), wedge shaped choroidal lesions may correspond to occlusion of short posterior ciliary arteries and geographic or focal lesions to occlusion of choroidal arterioles or choriocapillary lobules.

Bilateral retinal detachments in thrombotic thrombocytopenic purpura

Bilateral serous retinal detachments are rare complications of thrombotic thrombocytopenic purpura (TTP). We report the case of a 28-year-old woman with chronic relapsing TTP with bilateral serous retinal detachments, which resolved after prompt aggressive medical therapy. Unlike several previous reports, the retinal detachments in our patient were unrelated to systemic hypertension.

Oxygen saturation of blood in the anterior ciliary veins in patients with primary glaucoma

Results of the investigation of the oxyhaemoglobin content in the blood of the anterior ciliary veins in 23 patients with primary glaucoma and in 4 patients with senile cataract are presented. An extremely high content of oxyhaemoglobin in the anterior ciliary veins was revealed in all patients irrespective of the level of the intraocular pressure and the stage of the disease.

The choriocapillaris in the macular area. Clinical and angiographic study

Fluorescein fundus angiographies performed with the Cannon wide-angle fundus camera enable a good vascular definition, which improves our knowledge of the choroidal circulation. Owing to the better quality of filters, the choroidal arteriolar network can be seen more precisely. Our recent studies have enabled us to confirm the existence of a special avascular area in the central part of the submacular choroid, surrounded by a perimacular choriocapillaris ring. In the macular area, the choriocapillaris corresponds to a thin layer which is most probably supplied by several arterioles from the short posterior ciliary arteries near their site of entry into the eyeball. As stated by Hayreh, there may be a central area where the capillary pressure, while being well balanced, is very low.

PIgment epithelial diseases with abnormal choroidal perfusion

Sixteen patients, seen during the first week of their disease, had swelling of the retinal pigment epithelium and angiographic evidence of a widespread abnormality of choroidal filling. The patients fell into three groups: In Group 1 there were multifocal pigment epithelial lesions identical to those of acute posterior multifocal placoid pigment epitheliopathy. In Group 2 the pigment epithelial lesions were confluent. The retina was not detached in either of the these groups. In Group 3, multifocal lesions of the pigment epithelium were associated with retinal detachment. We postulated that all patients had diffuse choroidal hypoperfusion and focal pigment epithelial infarct which, in turn, may have been caused by a variety of basic disorders. We detected no relevant systemic abnormalities in any of the patients.

Congenital bilateral arterial anastomosis between the choroid and peripheral retina

A healthy 26-year-old woman, had a decrease of vision in her left eye caused by a macular pucker, showed deformed retinal vessels, small exudative detachments of the neuroretina, and retinal exudates and hemorrhages in both fundi peripheral to the equator at the 6-o'clock position. Fluorescein angiography and stereoscopic photographs clearly showed an anastomosis of a recurrent ciliary artery with periphral retinal vessels and localized absence of the choriocapillaris under the detached retina in both eyes. The anastomosis probably developed in the last month of fetal life as the result of a localized absence of Bruch's membrane caused by an incomplete coloboma of the choroid that originated during the second month of gestation.

Choroidal vascular changes in toxemia of pregnancy

Twenty-seven patients with toxemia of pregnancy were examined during a postpartum period of two days to eight months. Five of these manifested abnormal photographic and angiographic findings including disk and retinal edema, retinal striae, deep retinal yellow-white focal lesions, choroidal nonfilling, leakage of dye from the optic disk and deep retinal lesions, and retinal pigment epithelial window defects. The location and configuration of the leakage correlated with the defects seen on color photography. The normal retinal, and predominantly abnormal choroidal vascular patterns provide evidence implicating choroidal vascular insufficiency as the primary basis for secondary retinal detachments seen in toxemia of pregnancy.

The effect of systolic hypertension on rhesus monkey eyes after ocular sympathectomy

Systolic hypertension was induced in rhesus monkeys by obstructing the thoracic aorta and by ocular sympathectomy obtained with the retrobulbar injection of 6-hydroxydopamine. During the late phase of fluorescein angiography, fluorescein leaked from the choroidal circulation into the retina. The retinal vessels did not leak fluorescein. The effect was not permanent since fluorescein did not leak into the retina after the blood pressure returned to normal.

Fluorescein pattern of the choriocapillaris in the neonatal rhesus monkey

Seven neonatal rhesus monkeys were studied by serial fundus photography and fluorescein angiography at weekly intervals during the first 12 weeks of life. The neonatal rhesus showed little pigmentation of the fundus during the first six weeks of life. The fluorescein angiograms showed a definite sequential as well as segmental filling pattern at the level of the choriocapillaris. This filling pattern was based on individual units called lobules. In addition there was a segmental flow in the macula; that is, the nasal macula filled before the temporal macula.

Submacular choroidal circulation

A neoprene latex and pigment mixture was injected into the choroidal blood vessels of human cadaver eyes. When peripheral resistence was normal, a single arteriole filled a single lobule of the choriocapillaris drained by mulitple surrounding venules. The choridal veins interconnected and an increase in the peripheral resistance caused by elevation of the venous pressure resulted in filling of contiguous lobules in a retrograde manner through the venules that normally drain them.

Segmental nature of the choroidal vasculature

The various in vivo studies on posterior ciliary arteries and choroidal vasculature reported piecemeal by the author over the past few years have now been collated. A coherent picture of the vasculature emerges which is of considerable clinical significance. The observations show that the posterior ciliary arteries and their branches right down to the terminal choroidal arterioles, the choriocapillaris, and the vortex veins have a segmental distribution in the choroid, and that the posterior ciliary arteries and choroidal arteries function as end-arteries. No special mascular artery supplying the submacular choroid has been found. The posterior ciliary arteries have only supply the choroid but are also the main source of blood supply to the anterior part of the optic nerve, and have an important role in the blood supply of the retina. The clinical significance of this segmental distribution of the uveal vasculature is discussed.

Segmental vascular patterns of the choriocapillaris

I investigated the vascular architecture of the peripheral choriocapillaris in the eyes at autopsy, utilizing the histochemical alkaline phosphatase reaction. There was a segmental distribution of the capillaries, with the feeding arterioles at the periphery and the draining venule in the center of capillary lobules. These findings support the clinical postulation that the choriocapillaris is segmentally arranged.