Vascular supply to the optic nerve of primates

Oxidative Stress: A Suitable Therapeutic Target for Optic Nerve Diseases?

Optic nerve disorders encompass a wide spectrum of conditions characterized by the loss of retinal ganglion cells (RGCs) and subsequent degeneration of the optic nerve. The etiology of these disorders can vary significantly, but emerging research highlights the crucial role of oxidative stress, an imbalance in the redox status characterized by an excess of reactive oxygen species (ROS), in driving cell death through apoptosis, autophagy, and inflammation. This review provides an overview of ROS-related processes underlying four extensively studied optic nerve diseases: glaucoma, Leber's hereditary optic neuropathy (LHON), anterior ischemic optic neuropathy (AION), and optic neuritis (ON). Furthermore, we present preclinical findings on antioxidants, with the objective of evaluating the potential therapeutic benefits of targeting oxidative stress in the treatment of optic neuropathies.

Unraveling the Enigma of Nonarteritic Anterior Ischemic Optic Neuropathy

Non-arteritic anterior ischemic optic neuropathy (NAON) is the second most common optic neuropathy in adults. Despite extensive study, the etiology of NAION is not definitively known. The best evidence suggests that NAION is caused by an infarction in the region of the optic nerve head (ONH), which is perfused by paraoptic short posterior ciliary arteries (sPCAs) and their branches. To examine the gaps in knowledge that defies our understanding of NAION, a historical review was performed both of anatomical investigations of the ONH and its relevant blood vessels and the evolution of clinical understanding of NAION. Notably, almost all of the in vitro vascular research was performed prior our current understanding of NAION, which has largely precluded a hypothesis-based laboratory approach to study the etiological conundrum of NAION. More recent investigative techniques, like fluorescein angiography, have provided valuable insight into vascular physiology, but such light-based techniques have not been able to image blood vessels located within or behind the dense connective tissue of the sclera and laminar cribrosa, sites that are likely culpable in NAION. The lingering gaps in knowledge clarify investigative paths that might be taken to uncover the pathogenesis of NAION and possibly glaucoma, the most common optic neuropathy for which evidence of a vascular pathology also exists.

Effects of antiglaucoma drugs on blood flow of optic nerve heads and related structures

An association between glaucoma development or progression and compromised ocular blood flow has been postulated as a result of population-based studies and prospective cohort studies. Blood flow in the optic nerve head (ONH) is of primary importance in the pathogenesis of glaucoma. The potential to modify the blood flow in the ONH and its related structures has been reported in various agents, including topical antiglaucoma drugs and systemic drugs such as calcium channel antagonists, which are reviewed in this manuscript. Clinical implications of the improvement in ocular blood flow on the treatment of glaucomatous optic neuropathy require further investigation.

Anatomy and physiology of the afferent visual system

The efficient organization of the human afferent visual system meets enormous computational challenges. Once visual information is received by the eye, the signal is relayed by the retina, optic nerve, chiasm, tracts, lateral geniculate nucleus, and optic radiations to the striate cortex and extrastriate association cortices for final visual processing. At each stage, the functional organization of these circuits is derived from their anatomical and structural relationships. In the retina, photoreceptors convert photons of light to an electrochemical signal that is relayed to retinal ganglion cells. Ganglion cell axons course through the optic nerve, and their partial decussation in the chiasm brings together corresponding inputs from each eye. Some inputs follow pathways to mediate pupil light reflexes and circadian rhythms. However, the majority of inputs arrive at the lateral geniculate nucleus, which relays visual information via second-order neurons that course through the optic radiations to arrive in striate cortex. Feedback mechanisms from higher cortical areas shape the neuronal responses in early visual areas, supporting coherent visual perception. Detailed knowledge of the anatomy of the afferent visual system, in combination with skilled examination, allows precise localization of neuropathological processes and guides effective diagnosis and management of neuro-ophthalmic disorders.

Histomorphometry of the circular peripapillary arterial ring of Zinn-Haller in normal eyes and eyes with secondary angle-closure glaucoma

PURPOSE

To examine the location and size of the peripapillary arterial circle of Zinn-Haller (PACZH) and its associations with other eye measures in normal eyes and eyes with secondary angle-closure glaucoma.

METHODS

  The study included 29 human globes enucleated because of malignant choroidal melanoma (n = 19) (control group) or because of secondary angle-closure glaucoma (n=10). Anterior-posterior histological sections were morphometrically evaluated.

RESULTS

The PACZH was present in all eyes. The glaucoma group and the control group did not vary significantly in the distance from the PACZH to Bruch's membrane (297 ± 67 versus 270 ± 67 μm; p = 0.29), optic disc border (281 ± 103 versus 391 ± 170 μm; p=0.07), optic disc centre (1059 ± 191 versus 978 ± 205μm; p = 0.30) and retrobulbar cerebrospinal fluid space (173 ± 58 versus 172 ± 81 μm; p = 0.97) nor in the minimal PACZH diameter (39± 18 versus 36 ± 18 μm; p = 0.74) and maximal PACZH diameter (78 ±37 versus 65 ± 25 μm; p = 0.36). The PACZH location, measured as distance from Bruch's membrane, disc border, disc centre and cerebrospinal fluid space, was not significantly associated with axial length (p > 0.39), horizontal globe diameter (p > 0.17) and vertical globe diameter (p > 0.22). Both diameters were statistically independent of axial length (p = 0.72 and p = 0.58, respectively), horizontal globe diameter (p = 0.60 and p = 0.41, respectively) and vertical globe diameter (p = 0.64 and p = 0.52, respectively). All parameters were statistically independent of age (p > 0.10) and gender (p > 0.10).

CONCLUSIONS

The PACZH was present in all human eyes examined and did not vary significantly in location and diameter between eyes with secondary angle-closure glaucoma and nonglaucomatous eyes, nor between myopic versus hyperopic eyes.

Morphometric changes of the choriocapillaris and the choroidal vasculature in eyes with advanced glaucomatous changes

PURPOSE

In addition to an elevated intraocular pressure a compromise of the ocular blood supply have been implicated in the pathogenesis of primary open-angle glaucoma. The purpose of this study was to quantify morphologic changes in the choroid including choriocapillaris thickness and density and diameter of large choroidal vessels in post mortem eyes with advanced primary open-angle glaucoma.

METHODS

We analyzed 20 eye bank eyes (provided from the Georgia Eye Bank in Atlanta) with end stage primary open-angle glaucoma and compared them with 20 age-matched control eye bank eyes. The eyes were processed for light microscopy and following variables were measured with a digital filar micrometer: density and diameter of large choroidal vessels in the macular and equatorial choroid; thickness of the choroid in the macular and equatorial region; density and thickness of choriocapillaris in the macular, peripapillary, and equatorial choroid; and peripapillary capillary-free area nasal and temporal to the optic disk.

RESULTS

Eyes with glaucoma displayed a lower density of the capillaries of the choriocapillaris as compared to control eyes in the macular, temporal peripapillary, and equatorial choroid with 0.50-0.55 (p=0.018), 0.46-0.51 (p=0.016), and 0.50-0.55 (p=0.038), respectively. There was no significant difference for the choriocapillaris density in the nasal peripapillary choroid, the thickness of the capillaries of the choriocapillaris in all assessed locations, and the nasal and temporal peripapillary capillary-free zone of the choriocapillaris between eyes with glaucomatous damage and controls. Assessment of large choroidal vessels in the macular choroid showed that eyes with glaucoma had a decreased density of veins (11.7-38.9 mm(-2); p<0.001) and arteries (7.7-12.4 mm(-2); p=0.005) and arteries with a higher diameter (45.6-28.2 microm; p<0.001) as compared to control eyes. The large vessels in the equatorial choroid displayed no significant difference in diameter but a lower density (21.2-44.1 mm(-2); p=0.017) in eyes with glaucomatous damage as compared to controls.

CONCLUSION

Eyes with advanced glaucomatous damage after long standing primary open-angle glaucoma exhibit several changes including decreased density of capillaries of the choriocapillaris and decreased density of large choroidal vessels. We cannot conclude from our study whether the observed vascular changes in the choroid are primary pathogenic factors or secondary phenomena.

The blood supply of the optic nerve head and the evaluation of it - myth and reality

Evidence has gradually emerged that there is vascular insufficiency in the optic nerve head (ONH) in both anterior ischemic optic neuropathy (AION) and glaucomatous optic neuropathy (GON); thus both represent ischemic disorders of the ONH. Together these diseases constitute a major cause of blindness or seriously impaired vision in man. Consequently there has recently been great interest in the ONH circulation in health and disease and in how to evaluate it. Many studies of the subject have been published, with conflicting interpretations and claims. The basis of the inconsistent information seems to be confusion on some fundamental issues concerning the ONH circulation itself. The objective of this paper is to differentiate myths and misconceptions from reality about the ONH blood supply; to elucidate the reasons for disagreement on the blood supply of the ONH; and to evaluate the reliability and validity of various methods currently used to measure ONH blood flow.

Morphological variations of the peripapillary circle of Zinn-Haller by flat section

AIMS

To evaluate the morphometric and morphological variations of the circle of Zinn-Haller (CZH) in the human eye.

METHODS

42 human enucleated eyes were used in this study. After transverse flat thick sections were cut through the optic nerve and adjacent sclera, tissue sections were stained with haematoxylin and eosin or examined immediately by wet preparation under a light microscope. The average vessel diameter of the arterial circle and the average distance between the optic nerve head (ONH) and the arterial circle were determined. Various branching patterns of the CZH were also evaluated.

RESULTS

The vessel diameter of the arterial circle was 123 (SD 75) microm (range 20-230 microm). The distance of the CZH from the ONH margin was 403 (352) microm (0-1050 microm). The CZH gave off branches to the optic nerve and to the peripapillary choroid (PPC) with various branching patterns especially at the entry point of paraoptic short posterior ciliary artery.

CONCLUSIONS

The CZH exists within a variable distance from the ONH and its average diameter is similar to that of the central retinal vessels though it shows marked variation even in the same circle. The CZH also shows variable configurations in branching patterns. These variations may act as contributing factors that are responsible for the individual susceptibility of the anterior optic nerve and the PPC to circulatory disturbances.

Assessment of human ocular hemodynamics

Vascular abnormality and altered hemodynamics play important roles in many ophthalmic pathologies. Much of our knowledge of ocular hemodynamics was gained from invasive animal research, although a number of noninvasive methods suitable for in vivo use in humans have been developed. Data from these methods now produce a significant literature of their own. Understanding the origins of the data and appreciating their limitations can be difficult. Modern hemodynamic assessment techniques each examine a unique facet of the ocular circulation. No single facet provides a complete description of the hemodynamic state of the eye. These methods have contributed a great deal to our understanding of normal hemodynamics. More importantly, they continue to add to our understanding of altered hemodynamics found in disease. Some have found their way into limited clinical practice. The predominant ocular hemodynamic assessment techniques are reviewed with the aims of introducing the fundamental principles behind each, highlighting their inherent advantages and limitations, highlighting their contributions to understanding ocular physiology, and considering their potential to provide signs for diagnosis.

Peripapillary circle of Zinn-Haller revealed by fundus fluorescein angiography

AIMS

To observe the vascular pattern of the peripapillary circle of Zinn-Haller in humans by fundus fluorescein angiography.

METHODS

307 cases (from 212 patients) of fundus fluorescein angiograms performed in patients with myopic degeneration were evaluated to find the circle of Zinn-Haller and to observe its fundus fluorescein angiographic features.

RESULTS

15 cases (from 13 patients) with the circle of Zinn-Haller were found. It appeared as concentric or zigzag-shaped vascular fillings within the temporal crescent region. All cases were observed in pathological myopia with peripapillary atrophy and a tilted disc. Each arterial circle showed variations in location and shape.

CONCLUSIONS

The temporal part of the circle of Zinn-Haller can be revealed by fundus fluorescein angiography particularly in pathological eyes with prominent peripapillary atrophy and a tilted disc. The morphological variation of this arterial circle should be considered.

Effect of oxygen on relaxation of retinal pericytes by sodium nitroprusside

BACKGROUND

This study addresses whether oxygen modulates the relaxation induced in retinal pericytes by sodium nitroprusside (SNP), a nitric oxide (NO) donor that stimulates the NO/guanylate cyclase pathway.

METHODS

Bovine retinal pericytes were cultured on silicone. On the silicone surface, basal pericyte contractile tone induces wrinkles. Drug-induced changes in pericyte contractile tone were assessed by changes in the number of wrinkles. The effects of 100% nitrogen (hypoxia) and 100% oxygen (hyperoxia) were studied on: (a) the basal tone of quiescent pericytes, (b) the relaxation to 3 and 10 microM SNP or 1 microM forskolin, and (c) the recontraction that followed the washout of 3 microM SNP or 1 microM forskolin.

RESULTS

Neither hypoxia nor hyperoxia had any apparent influence on pericyte basal tone, on forskolin-induced relaxation, or on pericyte recontraction after a forskolin-induced relaxation. In hypoxia, relaxations to SNP 3 microM (P < 0.05) and 10 microM (P < 0.01) were significantly more pronounced than in hyperoxia. Hypoxia also reduced the recontraction after an SNP-induced relaxation (P < 0.001).

CONCLUSION

Oxygen modulates the relaxation of bovine retinal pericytes evoked by SNP (guanylate cyclase-mediated), but not the relaxation induced by forskolin (adenylate cyclase-mediated). These results suggest that in the retinal capillary circulation an interaction between oxygen and the NO/guanylate cyclase pathway modulates pericyte tone, and thus potentially blood flow.

Peripapillary venous drainage from the choroid: a variable feature in human eyes

AIMS/BACKGROUND

Drainage of blood from the choroid is thought to occur exclusively through the vortex veins in the absence of a venous equivalent of the posterior ciliary arteries. A chance observation in the peripapillary region of the choroid, inconsistent with this concept, suggested that the subject required review.

METHODS

Ten nerve heads from 10 individuals were examined histologically using interrupted serial resin sections. They were obtained from eyes free of posterior segment pathology.

RESULTS

Peripapillary veins varying in number and size were present in seven preparations and none in the other three. All veins penetrated the sclera from the choroid close to the optic nerve head and entered the pia mater directly, receiving small veins from the laminar and postlaminar nerve head. No other locations of posterior venous penetrations of the sclera were found. This is the first description of these vessels in normal eyes; they are named 'choroidopial veins'.

CONCLUSIONS

Choroidopial veins represent a minor and inconstant route for blood drainage from the choroid, with a role in optic nerve head circulation.

The 1994 Von Sallman Lecture. The optic nerve head circulation in health and disease

This is a brief overview of multifaceted anatomical, experimental and clinical studies conducted by the author since 1955 on the optic nerve head circulation in health and disease. Conclusions, based on the accumulated information provided by these studies, are summarized. The studies on the pattern of blood supply of the optic nerve head have shown that: (a) its main source of blood supply is the posterior ciliary artery circulation, with retinal circulation supplying only the surface nerve fiber layer, (b) there is marked interindividual variation in the blood supply pattern, and (c) the blood supply in the optic nerve head has a sectorial distribution. The various factors which produce interindividual variation in the blood supply of the optic nerve head are discussed, particularly those in the posterior ciliary artery circulation; this is because all available evidence indicates that it is derangement in the posterior ciliary circulation in the optic nerve head that is primarily responsible for the common ischemic disorders of the optic nerve head, e.g. anterior ischemic optic neuropathy and glaucomatous optic neuropathy. Factors that may derange the blood flow in the optic nerve head include defective autoregulation of blood flow in it, vascular changes in its feeding arteries, hematologic abnormalities, systemic arterial hypertension and hypotension, and intraocular pressure; their roles are discussed. For better understanding and management of optic nerve head ischemic disorders, there is an urgent need for an accurate clinical method of assessment of blood flow in the posterior ciliary circulation in optic nerve head, since no satisfactory method is currently available. Redness or pallor of the optic disk on ophthalmoscopy is not a true guide to the optic nerve head vascularity as it gives no information about the state of the posterior ciliary circulation. Fluorescein fundus angiography, though far superior to the optic disk color for evaluation of optic nerve head vascularity, has a number of limitations. All these topics and various controversies about them are discussed briefly.

Retinal nerve fiber layer changes and visual field loss in idiopathic intracranial hypertension

PURPOSE

The authors retrospectively analyzed changes in the retinal nerve fiber layer in patients with idiopathic intracranial hypertension and studied their relation to visual field loss to determine the clinical usefulness of retinal nerve fiber analysis in the clinical management of patients with papilledema.

METHODS

Retinal nerve fiber layer photographs and visual fields from 36 eyes of 21 patients with papilledema due to idiopathic intracranial hypertension were analyzed for abnormalities in a masked fashion.

RESULTS

Nerve fiber layer changes were found in 67% of eyes studied. Superior areas within the nerve fiber layer were affected 5.4 times more frequently than inferior regions. Visual field loss was more prevalent in eyes with diffuse nerve fiber layer loss (89%) than in eyes with slit defects (29%). The location of the nerve fiber layer changes correlated with corresponding areas of visual field loss. Nerve fiber layer changes were as common in mild to moderate as in atrophic papilledema; however, slit defects predominated in patients with mild to moderate papilledema, and diffuse loss predominated in atrophic papilledema.

CONCLUSIONS

Changes in the retinal nerve fiber layer observed in patients with idiopathic intracranial hypertension provide objective information regarding the status of their optic nerve and may improve their clinical management.

Microvasculature of the human optic nerve

PURPOSE

Methyl methacrylate vascular corrosion casting techniques were used to examine the normal anterior optic nerve microvasculature in 18 human eye bank eyes.

METHODS

Selective cannulation of the central retinal artery, the short posterior ciliary arteries, or both, allowed the methyl methacrylate to be injected into the anterior optic nerve circulation. Preflushing with tissue plasminogen activator greatly enhanced the filling of the fine microvasculature by dissolving the intraluminal clots.

RESULTS

The superficial nerve fiber layer of the optic nerve received its primary blood supply from the central retinal artery. In 11 of 13 eyes injected with methyl methacrylate through the short posterior ciliary arteries, there was a perineural, circular arterial anastomosis (circle of Zinn-Haller) at the scleral level. Branches from this circle penetrated the optic nerve to supply the prelaminar and laminar regions and the peripapillary choroid. In the two eyes without this arterial circle, direct branches from the short posterior ciliary arteries supplied the anterior optic nerve. The venous drainage of the anterior optic nerve was almost entirely through the central retinal vein and its tributaries.

CONCLUSIONS

This study demonstrates that the main arterial vascular supply to the anterior optic nerve is from the short posterior ciliary arteries. The contribution of the peripapillary choroid to the anterior optic nerve is minimal in comparison to the direct contribution from the short posterior ciliary arteries.

Mechanisms of optic nerve damage in primary open angle glaucoma

Several mechanisms have been postulated to explain the optic nerve damage that occurs in primary open angle glaucoma (POAG). No single mechanism can adequately explain the great variations in susceptibility to damage and the patterns of damage seen in this syndrome. The etiology of POAG is likely to be multifactorial. Mechanical, vascular and other factors may influence individual susceptibility to optic nerve damage. An enhanced understanding of the nature of the optic nerve damage in POAG and improved methods of study may result in earlier diagnosis or may allow us to distinguish among different pathological processes all currently grouped under the diagnosis of POAG. As we gain a better understanding of the neuropharmacology and cellular biology of injury and repair of the visual system we will undoubtedly refine the concepts of glaucomatous optic neuropathy.

Microvasculature of the anterior optic nerve

The traditional definition of glaucoma as a disorder of increased intraocular pressure (IOP) oversimplifies the clinical situation. Some glaucoma patients never have higher than normal IOP and others continue to develop optic nerve damage despite maximal lowering of IOP. Another possible factor in the etiology of glaucoma may be regulation of the regional microvasculature of the anterior optic nerve. One reason to believe that microvascular factors are important is that many microvascular diseases are associated with glaucomatous optic neuropathy. In this study, endothelin-1 was used to produce vasoconstriction of the ocular vasculature following both intravitreal and perineural delivery. Intravitreal injections (1.25 and 2.50 micrograms) were given to six rabbits to assess the vascular response of the posterior segment. In addition, endothelin (0.0-4.2 micrograms) was delivered with an osmotically driven minipump system to the perineural region of six rabbits. Vasoconstriction of the anterior optic nerve vasculature was observed following both methods of endothelin delivery. A better understanding of the microvasculature of the anterior optic nerve is needed to evaluate its relationship to glaucomatous damage. Modulation of this vascular supply, with agents that mimic autoregulatory controls, may allow the sites of vasomotor control to be established and, in the future, their status to be manipulated.

Retinochoroidal (optociliary) shunt veins, blindness and optic atrophy: a non-specific sign of chronic optic nerve compression

Fifteen patients are described in whom the triad of blindness, optic disc swelling followed by optic atrophy, and optociliary shunt veins occurred. The causes of the syndrome included spheno-orbital meningioma, optic nerve glioma, meningocele of the optic nerve, and chronic papilloedema. It is postulated that chronic compression of the intraorbital portion of the optic nerve produces gradual obstruction of the central retinal vein, thus preventing the normal passage of venous blood from the retina through the central retinal vein to the cavernous sinus. Optociliary veins are a pre-existing shunt system that allows retinal venous blood to bypass the central retinal vein and exit from the orbit via the choroidal circulation and its anastomoses.

Microvascular study of the retrolaminar optic nerve in man: the possible significance in anterior ischaemic optic neuropathy

The morphology of the circle of Haller and Zinn and its variations were examined using methyl-methacrylate microvascular corrosion casting of human orbits obtained at post-mortem. It was found to be an elliptical microvascular anastomosis formed by branches of the medial and lateral para-optic short posterior ciliary arteries. The ellipse was divided into superior and inferior parts by the entry points of these branches into the eye, providing an altitudinal blood supply to the retrolaminar optic nerve. Morphological variations in terms of form, position and branches existed between subjects and between eyes from the same subject. The clinical implications of an elliptical 'circle' of Haller and Zinn providing an altitudinal blood supply to the retrolaminar optic nerve are relevant to the pathogenesis of altitudinal visual field defects in anterior ischaemic optic neuropathy.

Optic disc swelling in young diabetic patients: a diagnostic dilemma

We describe four young diabetic patients presenting with swelling of one or both optica discs ('papiloedema'). Two were asymptomatic while the others complained of blurred vision. Spontaneous resolution occurred in all within 6 months although three had developed optic atrophy within 5 years. These four patients fit into a spectrum of acute involvement of the optic nerve in young diabetic patients, of which over 50 cases have now been reported. Most have florid ophthalmoscopic appearances which resolve within weeks or months and usually carry a good visual prognosis. The condition appears to be a variant of anterior ischaemic optic neuropathy though its possible pathophysiology in diabetes is uncertain. Knowledge of the condition by diabetologists and ophthalmologists is important as patients and doctors may be reassured after fairly simple investigation.

Retinal ganglion cells and axons survive optic nerve transection

The effects of optic nerve transection on optic axon and retinal cell survival was studied in C57BL/6J mice. The optic nerve was transected either intracranially or intraorbitally . Data are presented which show that an intracranial transection of the optic nerve may be achieved while maintaining the blood supply to the retina. Intraorbital optic nerve transection, however, necessitates destruction of the retinal blood supply. Cell survival in the retinal ganglion cell layer is compared for both transection methods. In addition, electron microscopic evidence is presented which indicates axonal survival in the optic nerve up to at least 90 days following intracranial transection.

Infarction of the optic nerve head in children with accelerated hypertension

Four cases of anterior ischaemic optic neuropathy occurred in children with accelerated hypertension. The cause may have been a sudden relative fall in arterial pressure which reduced the perfusion of the optic disc, whose circulation was compromised by long-standing hypertensive vascular disease.

Short posterior ciliary artery insufficiency with hyperthermia (Uhthoff's symptom)

Transient visual blurring with heart or exercise (Uhthoff's symptom) is associated with multiple sclerosis. To our knowledge, this is the first report of its occurrence in cases of documented vascular disease. Two patients had insufficiency of the short posterior ciliary circulation. In one it was caused by a carotid occlusion and in the other by cranial (giant-cell, temporal) arteritis. The monocular blurring may have occurred during periods of relative arterial hypotension.

Role of axoplasmic transport in the pathophysiology of ischaemic disc swelling

Pathological changes in the optic disc and anterior part of the optic nerve of monkeys were studied 5 hours and several days after occlusion of the temporal short posterior ciliary arteries. Ischaemic vacuolation of neural tissue was observed in some of the animals studied and was restricted to the lamina cribrosa and the immediately retrolaminar portion of the optic nerve. Axonal swelling and organelle aggregation were demonstrated in the prelaminar region, and autoradiography after intravitreal injection of tritiated leucine revealed an associated obstruction of rapid and slow orthograde axoplasmic transport. The experimental findings correlate with those seen clinically in acute ischaemic optic neuropathy. The pale swelling of the optic disc in this condition represents an accumulation of axoplasmic debris in retinal ganglion-cell axons owing to obstruction of axoplasmic transport at the lamina cribrosa; it is thus equivalent to a 'cotton-wool spot' of the disc.

Arteriovenous malformation in the retina of a monkey

A vascular abnormality of the retina of a rhesus monkey was studied with fluorescein angiography, microvascular examination after silicone rubber injection, and histological examination. Fluorescein angiography revealed that this abnormality was an arteriovenous shunt. Microvascular examination showed a vascular abnormality on the sclera and an end-to-end communication of the arteriovenous malformation that was a continuation of abnormally large central retinal vessels observed just before their insertion into the optic nerve in the orbit. Histologic examination proved that the arteriovenous shunt originated from the central retinal vessels in the orbit; that degeneration of the retina and the choroid was extensive near the abnormal vessels; that the abnormal vessels had normal endothelium and adventitia but remarkably widened media; and that the cavernous hamangioma-like structure in the optic disk was clearly distinguishable from a cavernous hamangioma.

Histologic studies of the vasculature of the anterior optic nerve

Thirty-one normal human anterior optic nerves were studied in thin serial sections and their vessels stained by standard histologic stains and a new modification of the silver reticulin stain. The retrolaminar cribrosa was profusely supplied by centripetal and longitudinal vessels of pial origin; small branches of the central retinal artery were also seen. Longitudinal continuity of small vessels extended from the retrolaminar nerve to the retinal surface. This system freely anastomosed with three transverse systems; a significant anteriolar input from short posterior ciliary arteries in the sclera to the lamina cribrosa; smaller short posterior ciliary branches and occasional choroidal capillaries to the prelaminar portion; and branches derived from the central retinal artery in and around the disk. Vessels entering the prelaminar region at the level of the choroid were mainly derived from scleral branches of short posterior ciliary vessels entering through the border tissue of Elschnig, rather than from choroidal branches of ciliary arteries. Venous drainage for the anterior optic nerve was primarily through the central retinal vein, although alternate smaller paths to surrounding tissues were also identified.

Reevaluation of the optic disk vasculature

The optic disk vessels of owl monkeys and humans were studied in whole mounts injected with silicone rubber and by serial sections of paraffin-embedded tissue. The capillary bed of the optic nerve head in continuous on one side with the retinal vessels and on the other side with the optic nerve behind the globe. The choriocapillaris is a separate capillary bed, even though it and the disk capillaries are both supplied by branches of the posterior ciliary artery. At the junction between choroid and optic disk is a cuff of vessels that seems equivalent to an anterior extension of the pial vessels. This cuff includes capillaries of the central nervous system, was well as arterial branches of the posterior ciliary artery that pass retrograde from the peripapillary choroid and sclera into the pial plexus. While not contradicting previous descriptions of the vascular anatomy in this region, we emphasized that the microvascular bed of the disk is anatomically an integral part of the retina-optic nerve vascular system and deemphasized its relationship to the choroidal vasculature.