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Pathophysiology of the Distal Portion of the Optic Nerve
380
AMERICAN JOURNAL OF OPHTHALMOLOGY
the lamina cribrosa is independent of acute changes in the intraocular pressure. The optic nerve pressure was found, however, to be directly affected by the CSF pressure. The latter is in agreement with studies indicating that there is direct communication between the intravaginal compartment of the optic nerve and the cranial subarachnoid space. The elevation of the CSF pressure which had an effect on the optic nerve was great, however, and may not be a factor under normal circumstances. REFERENCES 1. Becker, B. : In discussion of Zimmerman, L. E., de Venecia, G. and Hamasaki, D. I. : Pathology of the optic nerve in experimental acute glaucoma. Invest. Ophth. 6:109, 1967. 2. Neetleship, E. : Comparative anatomy. Notes on the blood vessels of the optic disk in some of the lower animals. Tr. Ophth. Soc U.K. 25:338, 1905.
PATHOPHYSIOLOGY
S E P T E M B E R , 1968
3. Michaelson, I. D . : Retinal Circulation in Man and Animals. Springfield, 111., Thomas, 1954, p. 40. 4. Tansley, K . : Comparison of the lamina cribrosa in mammalian species with good and with indifferent vision. Brit J. Ophth. 40:178, 1956. 5. Swann, H. G., Montgomery, A . V., Davis, J. C , Jr. and Mickle, E. R. : A method for rapid measurement of intrarenal and other tissue pressure. J. Exper. Med. 92:625,1950. 6. Flieringa, H . J. : Procedure to prevent vitreous loss. Am. J. Ophth. 36:1618, 1953. 7. Parsons, J. H . : The Ocular Circulation. London, John Bale, Sons & Danielsson, L t d , 1903, p. 40. 8. McMaster, P. D. : The pressure and interstitial resistance prevailing in the normal and edematous skin of animals and man. J. Exper. Med. 84:473, 1946. 9. Bush, W . L., Coffman, G. M., Montgomery, A . V. and Swann, H . G. : A study of intrarenal pressure. Texas Rep. Biol. Med. 7:492, 1949. 10. Wolff, E. and Davies, F. : A contribution to the pathology of papilloedema. Brit J. Ophth. 15:609, 1931. 11. Miller, P. M . : The pressure in the orbit. Acta Ophth. Suppl. 4 3 : 1 , 1 9 5 5 .
OF THE DISTAL
PORTION
OF THE OPTIC
NERVE
I I . VASCULAR RELATIONSHIPS J TERRY ERNEST, M . D . * AND ALBERT M . POTTS, M . D . Chicago, Illinois In the first of a series of reports, it was shown that the intraocular pressure is not transmitted posterior to the lamina cribrosa. In the study herein reported, the effect of elevated intraocular pressures on the small vessel structure of the distal segment of the optic nerve is evaluated. Both latex injections with clearing and fluorescein angiography techniques were employed. 1
From the Eye Research Laboratories, the University of Chicago. This investigation was supported in part by U S P H S Grants 1 F L O N B 170601 V S N , N B 05079 and N B 02521 from the National Institute of Neurological Diseases and Blindness. Presented in part at the Glaucoma Research Conference, Williamsburg, Virginia, September, 1967. * Present address: Department of Experimental Psychophysiology, Forest Glen Section, Building 101, Walter Reed Army Institute of Research, Washington, D.C. 20012.
METHODS AND RESULTS Six rhesus monkeys weighing between 2.64 and 3.22 kg were used in this study. The animals were anesthetized with pentobarbital sodium 27.5 mg/kg intravenously. One eye was used as a control but the anterior chamber of the other was cannulated and the intraocular pressure was regulated as previously described. Bárány, using the Mackay-Marg tonometer, measured the intraocular pressure in vervet monkeys. The mean intraocular pressure of 208 eyes under phencyclidine was 17.7 mm Hg. No such measurements were made in this study but ophthalmodynamometry was performed with the anterior chamber cannulated. The ophthalmic artery pressure varied between animals, depending on the systemic blood pressure. The representative animal demonstrated 1
2
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in Figure 2 and 3 had a systemic blood pressure of 130/90 mm Hg and an ophthalmic artery pressure of 100/55 mm Hg in the cannulated eye. Potts has recently used a neoprene-pigment mixture, plus clearing of the tissues, to study the blood vessels of human and animal eyes. His methods were employed in the studies herein reported. At the conclusion of fluorescein angiography studies (to be described) the intraocular pressure in one eye was elevated and maintained. The thoracic cavity was opened and a blunt 19-gage needle was inserted into the left ventricle and clamped in place. The descending aorta was clamped and the right ventricle incised. Five hundred milliliters of 0.9% saline solution at 40°C were then infused. This was immediately followed by 30 ml of a mixture of six parts of neoprene latex,* two parts of pigment and two parts of saline. Particular attention was paid to the latex infusion pressure. Using a syringe, the latex was pulsed in at mean pressures approximating those of the living animal's systemic blood pressure. Following the injections, a Stryker bone saw was used to remove the lateral walls of the orbits, and bilateral enucleations were performed. The specimens were placed in 10% formaldehyde solution containing one part glacial acetic acid for 24 hours. The tissues were then dehydrated and cleared as described previously. Figures 1 to 3 are photographs of three of the resulting cleared eyes. 8
4
8
Study of six injected normal eyes (normal intraocular pressure) revealed that the major source of vessels to the papilla and lamina cribrosa was from the choroid and branches from the posterior ciliary arteries, and from the pia. Figure 1 is a photograph of one of the cleared latex-injected monkey * Neoprene latex compound A423A-1. Average particle diameter 120 millimicrons. t Red RW-635P water-dispersed color with an average particle diameter of 2-4 microns. Both from E. I. DuPont, DeNemours nad C o , Inc., Elastomer Chemicals Department, Akron, Ohio, U.S.A.
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eyes. The intraocular pressure was normal. The central retinal artery (a) is seen within the optic nerve and can be followed through the lamina cribrosa and into the papilla where it branches into the retinal arteries. The posterior ciliary arteries are seen on both sides of the optic nerve. Some of the posterior ciliary artery branches anastomose in the sclera around the optic nerve but not to the extent of forming a definite circle comparable to that of the circle of ZinnHaller seen in man. No branches of the central retinal artery were observed in the optic nerve in these normal eyes. Further, in all eyes studied, there was at least one branch, and sometimes as many as three branches, of a posterior ciliary artery which penetrated the optic nerve approximately 2 mm posterior to the lamina cribrosa and then ran directly forward to the lamina cribrosa and papilla. One such vessel is visible in Figure 1 (b). Six eyes were infused while the intraocular pressure was maintained between 70 and 100 mm Hg. In these eyes, the pronounced vasculature of the papilla, lamina cribrosa and distal segment of the optic nerve originating from the posterior ciliary arteries was gone. Figure 2 is to be compared with Figure 3. The intraocular pressure of the eye shown in Figure 3 was maintained at 90 mm Hg during the latex infusion. A photograph of the opposite eye of the same monkey is shown in Figure 2. The intraocular pressure in this eye was normal. Comparison of the two eyes revealed that the heavily anastomotic papilla and lamina cribrosa vasculature did not fill as well in the eye with an increase in the intraocular pressure as it did in the eye with a normal intraocular pressure. In the injected eye with an elevated intraocular pressure, the capillary-size vessels of the lamina cribrosa and in the area posterior to the lamina cribrosa also were not filled. The latex did not pool within the optic nerve posterior to the lamina cribrosa. In fact, in all the experiments the latex filling of the vessels for 1-2 mm posterior to the lamina cribrosa was di-
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Fig. 1 (Ernest and Potts). Latex-injected monkey eye has been cleared and partially sectioned. The central retinal artery (a) is seen insi'de the optic nerve. Small branch of one of the ciliary arteries penetrates optic nerve and runs forward to the papilla (b).
Fig. 2 (Ernest and Potts). Latex-injected monkey eye photographed from vitreous side. Intraocular pressure was normal at time of injection.
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Fig. 3 (Ernest and Potts). Intraocular pressure was 90 mm H g at time of the latex injection. There is a decrease in papillary and peripapillary vascular filling.
minished relative to the opposite control eye. In the eyes perfused during elevation of the intraocular pressure, the central retinal artery showed several branches into the substance of the optic nerve posterior to the lamina cribrosa. These branches appeared to be end arteries and averaged only approximately 1 mm in length. Fluorescein angiograms were done on the monkeys before they were injected with latex and sacrificed. The method employed for fluorescein angiography was the same as the one first suggested by Novotny and Alvis. A control fluorescein angiogram was made by injecting 5 ml of 5% fluorescein in the leg vein of the monkey under general anesthesia. The animal was then returned to his cage and allowed to recover completely. The clearing time of fluorescein is approximately 36 hours. Usually the final experiment was performed three days after the first experiment. The animal was again anesthetized and the anterior chamber of the 6
6
same eye was cannulated. The intraocular pressure was elevated and maintained while the fluorescein angiogram was repeated. Figure 4-A is a fluorescein photograph of the fundus of an eye with normal tension. The angiogram is in the arterial phase. The small disc arterioles are filled but the retinal veins do not contain the dye at this stage. Many small vessels can be seen radiating on the disc surface and into the peripapillary retina. Figure 4-B is the same eye three days later but this time the angiogram was made while the intraocular pressure was maintained at 80 mm Hg. Comparison of the two arterial phase angiograms reveal that the majority of small disc vessels are absent from the eye subjected to the raised intraocular pressure. In Figure 4-B, small retinal vessel branches are seen filled but the small disc vessels of comparable size are not. The fact that small disc vessels fill poorly while small retinal vessel branches fill well under conditions of elevated intraocular pressure was empha-
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Fîg. 4 (Ernest and Potts). Arterial phase fluoresi :in angiograms of monkey eye done three days apart. ( A ) Intraocular pressure normal ( B ) Intraocular p: :ssure was maintained at 80 mm H g during angiogram. Small disc vessels are not filled.
sized by another experiment. In this experiment the intraocular pressure was maintained at 100 mm Hg during the fluorescein angiogram. Figure 5-A is a photograph of the arterial phase under conditions of a normal intraocular pressure. Figure S-B is a photograph of the arterial phase angiogram of the same eye obtained under conditions of an intraocular pressure of 100 mm Hg.
Small vessels are seen branching from the retinal arterioles even though the elevated intraocular pressure has prevented both small disc vessels as well as the choroid from filling. DISCUSSION Hayreh and Vrabec have done histologic studies on 24 rhesus monkey eyes. A trans7
Fig. S (Ernest and Potts). Arterial phase fluorescein angiograms of monkey eye done three days apart. ( A ) Intraocular pressure normal. ( B ) The intraocular pressure was maintained at 100 mm H g during the angiogram. Small branches of the central retinal artery are filled but neither the small disc vessels nor the choroid contain dye.
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verse section through the lamina cribrosa showed heavy connective tissue septa similar to those seen in man and the domestic cat. The monkey is similar to man and differs most markedly from the cat in having a central retinal artery. Study of the six latex-injected normal eyes (normal intraocular pressure) revealed findings similar to those of Hayreh. Posterior ciliary artery branches anastomosed in the sclera around the optic nerve but a definite circle comparable to that of the circle of Zinn-Haller seen in man was not evident. The considerable vascular network of the distal segment of the optic nerve of the monkey depends on the posterior ciliary arteries and the pial vessels and not on the central retinal artery. 8
Hamasaki and Fujino have recently published a study on the effect of the intraocular pressure on the ocular vessels of the owl monkey. Their technique was similar to the latex injection method used in this study. The authors employed a gelatin-India ink solution injected retrogradely through the descending aorta. The intraocular pressure was elevated in one eye and not in the fellow eye. Their India ink injection specimens were not dissimilar from the latex injections herein reported. The capillaries of the distal segment of the optic nerve filled poorly. They did note, however, that the deficiency in optic nerve capillary filling was greater in the temporal one-half than in the nasal onehalf of the optic nerve. This finding was not evident in our latex-injected specimens. The eyes injected under conditions of an elevation in the intraocular pressure herein reported showed a marked decrease in papillary and peripapillary capillary filling. Toussaint and associates have demonstrated peripapillary vessels in man, coming from the disc. Comparable vessels are compromised by increased intraocular pressure in the monkey eye. Both the study by Hamasaki and Fujino and the experiments herein reported demonstrate that injection of the distal optic nerve segment vasculature of ciliary artery origin is diminished by elevated intra9
10
ocular pressure. Furthermore, the intraocular pressures required are below the levels which close the central retinal artery. Two fluorescein angiogram studies this past year are pertinent to this discussion. David performed fluorescein angiography in patients with central retinal artery occlusions. In one of the patients there was capillary filling of the nerve head 10 seconds before the dye appeared in the central retinal artery. This is further evidence that the central retinal artery contributes little if at all to the nutrition of the papilla and distal optic nerve. Hayreh and Walker investigated 33 glaucoma patients with fluorescein angiography. They concluded that there was a decrease in fluorescence of the optic disc in some of the glaucoma patients. They assumed that the arterial phase fluorescein angiogram is an index of disc vascularity. If this is true, and since at the time of the angiograms the patients all had normal intraocular pressures, then it may be evidence that there is a decrease in disc vascularity in glaucoma. The fluorescein angiography herein reported adds information about the vascular effects of elevated intraocular pressure in the intact living animal. The technique of using a delay between the two angiograms was employed because if the intraocular pressure was varied after the fluorescein injection, there was little demonstrable effect on disc fluorescence. This may be due to a loculation of the dye in small vessels between cellular components. There also may be an extravasation of dye into the disc which would not be affected by changes in the intraocular pressure. There are problems in comparing two angiograms of the same eye done at different times. The speed of injection, the state of the circulation and the concentration of the bolus reaching the eye may not be the same in both instances. Nonetheless, the results were consistent in the animals in which the technique was used. It was not possible to evaluate the relative fluorescence of the discs as Hayreh and 11
12
Walker have done in their glaucoma patients. It is evident, however, that the small vessels which are resolved* in the papilla are obliterated by an increase of the intraocular pressure. Further, levels of the intraocular pressure which compromise the papilla vasculature has less effect on the central retinal arterial circulation. The implication is that the intravascular pressure in the vessels which supply the disc is lower than the intravascular pressure in the retinal vessels of central artery origin. Figure 6 is a drawing of the arterial supply of the optic nerve and retina in man. It is adapted from the studies of Hayreh, François, Wybar and others. The source of blood to the globe in man is the ophthalmic artery. Both the central retinal artery and the ciliary arteries branch from the ophthalmic artery. At their origin, the central retinal artery and the ciliary arteries are of equal caliber and it is assumed that their intravascular pressures are approximately the same ( P ) . The central retinal artery has a few intraneural branches but there are none 0
* The resolving power of the Zeiss fundus camera under the conditions of fluorescein photography has not been measured. Microaneurysms known to be approximately 75 microns in diameter or smaller are visible, however."
in the region of the lamina cribrosa and papilla. The central retinal artery branches spread out on the retina and supply its inner one-half. The intravascular pressure of the retinal arterioles is designated in Figure 6 as Pj. A s the short posterior ciliary arteries penetrate the sclera to supply the choroid and thereby the posterior half of the retina, they give off small branches to the circle of Zinn-Haller. The pressure designated P» in Figure 6 is the pressure in the circle of Zinn-Haller which surrounds the optic nerve. The vasculature of the distal segment of the optic nerve in the monkey is similar to that of man. The central retinal artery does not branch at the level of the lamina cribrosa and the papilla. The branches to the lamina cribrosa and papilla originate from intrascleral ciliary artery anastomoses surrounding the optic nerve. Anatomic studies have demonstrated that the branches from the ciliary arteries supplying the optic nerve must course through the sclera and are of small caliber. This is in contrast to the larger arterioles found in the retina which branch from the central retinal artery. The results of both the latex injections and fluorescein angiography indicated that elevated levels of intraocular pressure decrease the filling of vessels
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of the papilla and lamina cribrosa. The latex injection studies further demonstrated a decrease in optic nerve capillary filling posterior to the lamina cribrosa. Since it has been demonstrated that the intraocular pressure is not transmitted posterior to the lamina cribrosa, another explanation for the poor capillary filling in this area is necessary. It may be that the capillaries of the distal optic nerve segment posterior to the lamina cribrosa receive the majority of their blood from anastomoses with papillary and lamina cribrosa vessels. The latter are directly subjected to elevated intraocular pressure and thus may secondarily decrease the blood flow to the posterior vessels. These studies suggest that the blood pressure in the disc vessels of ciliary artery origin is at a lower level than the blood pressure in the retinal vessels from the central retinal artery. Furthermore, the tissue pressure of the papilla is many times greater than tissue pressures found in other parts of the body, since the papilla is subjected to the intraocular pressure. It might be expected, then, that slight increases in the intraocular pressure found in glaucomatous eyes could compromise the vasculature. The result may be atrophy of tissue with cupping of the disc and visual field loss. SUMMARY This study investigated the effect of elevated intraocular pressure on the vascular conductivity in the distal segment of the optic nerve in the monkey. Both injection, followed by fixation and clearing, and fluorescein angiography in the living animal were used. The anatomic findings agreed with those previously reported, indicating that the papilla and distal optic nerve receive the majority of their blood supply from the
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posterior ciliary circulation and not the central retinal artery. Both latex injections and fluorescein angiography were done with the intraocular pressure elevated. It was demonstrated that elevated intraocular pressure compromises the small vessel structure of the distal optic nerve segment and it is hypothesized that this accounts for the glaucomatous disc changes seen in man. REFERENCES 1. Ernest, J. T. and Potts, A. M . : Pathophysiology of the distal portion of the optic nerve. I. Tissue pressure relationships. Am. J. Ophth. 66:373, 1968. 2. Bárány, E. : Applanation tonometry and ophthalmoscopy of the vervet monkey (Cercopithecus ethiops) in phencyclidine catalepsia. Invest. Ophth. 2:322, 1963. 3. Potts, A. M . : Anatomic methods for study of the bulbus oculi. Am. J. Ophth. 65:155, 1968. 4. Spalteholz, W . : Uber das Durchsichtigmachen von Menschlichen und Tierischen Präparaten. Leipzig, Hirzel, 1914, ed. 2. 5. Novotny, H . R. and Alvis, D. L. : A method of photographing fluorescence in circulating blood in the human retina. Circulation 24:82, 1961. 6. Dollery, C. T., Hodge, J. V . and Engel, M . : Studies of the retinal circulation with fluorescein. Brit. Med. J. 2:1210, 1962. 7. Hayreh, S. S. and Vrabec, F. : The structure of the head of the optic nerve in rhesus monkey. Am. J. Ophth. 61:136, 1966. 8. Hayreh, S. S. : The orbital vessels of rhesus monkeys. Exper. Eye Res. 3:16, 1964. 9. Hamasaki, D. I. and Fujino, T. : Effect of intraocular pressure on ocular vessels: Filling with India ink. Arch. Ophth. 78:369, 1967. 10. Toussaint, D., Kuwabara, T. and Cogan, D. G. : Retinal vascular patterns. Arch. Ophth. 65:575, 1961. 11. David, N. J., Norton, E.W.D., Gass, J. D. and Beauchamp, J. : Fluorescein angiography in central retinal artery occlusion. Arch. Ophth. 77:619, 1967. 12. Hayreh, S. S. and Walker, W . M . : FluoresOphth. 63 :982, 1967. cent fundus photography in glaucoma. Am. J. 13. Scott, D. J., Dollery, C. T., Hill, D. W . , Hodge, J. V . and Fraser, R. : Fluorescein studies of diabetic retinopathy. Brit. Med. J. 1:811, 1964.
AMERICAN JOURNAL OF OPHTHALMOLOGY
the lamina cribrosa is independent of acute changes in the intraocular pressure. The optic nerve pressure was found, however, to be directly affected by the CSF pressure. The latter is in agreement with studies indicating that there is direct communication between the intravaginal compartment of the optic nerve and the cranial subarachnoid space. The elevation of the CSF pressure which had an effect on the optic nerve was great, however, and may not be a factor under normal circumstances. REFERENCES 1. Becker, B. : In discussion of Zimmerman, L. E., de Venecia, G. and Hamasaki, D. I. : Pathology of the optic nerve in experimental acute glaucoma. Invest. Ophth. 6:109, 1967. 2. Neetleship, E. : Comparative anatomy. Notes on the blood vessels of the optic disk in some of the lower animals. Tr. Ophth. Soc U.K. 25:338, 1905.
PATHOPHYSIOLOGY
S E P T E M B E R , 1968
3. Michaelson, I. D . : Retinal Circulation in Man and Animals. Springfield, 111., Thomas, 1954, p. 40. 4. Tansley, K . : Comparison of the lamina cribrosa in mammalian species with good and with indifferent vision. Brit J. Ophth. 40:178, 1956. 5. Swann, H. G., Montgomery, A . V., Davis, J. C , Jr. and Mickle, E. R. : A method for rapid measurement of intrarenal and other tissue pressure. J. Exper. Med. 92:625,1950. 6. Flieringa, H . J. : Procedure to prevent vitreous loss. Am. J. Ophth. 36:1618, 1953. 7. Parsons, J. H . : The Ocular Circulation. London, John Bale, Sons & Danielsson, L t d , 1903, p. 40. 8. McMaster, P. D. : The pressure and interstitial resistance prevailing in the normal and edematous skin of animals and man. J. Exper. Med. 84:473, 1946. 9. Bush, W . L., Coffman, G. M., Montgomery, A . V. and Swann, H . G. : A study of intrarenal pressure. Texas Rep. Biol. Med. 7:492, 1949. 10. Wolff, E. and Davies, F. : A contribution to the pathology of papilloedema. Brit J. Ophth. 15:609, 1931. 11. Miller, P. M . : The pressure in the orbit. Acta Ophth. Suppl. 4 3 : 1 , 1 9 5 5 .
OF THE DISTAL
PORTION
OF THE OPTIC
NERVE
I I . VASCULAR RELATIONSHIPS J TERRY ERNEST, M . D . * AND ALBERT M . POTTS, M . D . Chicago, Illinois In the first of a series of reports, it was shown that the intraocular pressure is not transmitted posterior to the lamina cribrosa. In the study herein reported, the effect of elevated intraocular pressures on the small vessel structure of the distal segment of the optic nerve is evaluated. Both latex injections with clearing and fluorescein angiography techniques were employed. 1
From the Eye Research Laboratories, the University of Chicago. This investigation was supported in part by U S P H S Grants 1 F L O N B 170601 V S N , N B 05079 and N B 02521 from the National Institute of Neurological Diseases and Blindness. Presented in part at the Glaucoma Research Conference, Williamsburg, Virginia, September, 1967. * Present address: Department of Experimental Psychophysiology, Forest Glen Section, Building 101, Walter Reed Army Institute of Research, Washington, D.C. 20012.
METHODS AND RESULTS Six rhesus monkeys weighing between 2.64 and 3.22 kg were used in this study. The animals were anesthetized with pentobarbital sodium 27.5 mg/kg intravenously. One eye was used as a control but the anterior chamber of the other was cannulated and the intraocular pressure was regulated as previously described. Bárány, using the Mackay-Marg tonometer, measured the intraocular pressure in vervet monkeys. The mean intraocular pressure of 208 eyes under phencyclidine was 17.7 mm Hg. No such measurements were made in this study but ophthalmodynamometry was performed with the anterior chamber cannulated. The ophthalmic artery pressure varied between animals, depending on the systemic blood pressure. The representative animal demonstrated 1
2
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in Figure 2 and 3 had a systemic blood pressure of 130/90 mm Hg and an ophthalmic artery pressure of 100/55 mm Hg in the cannulated eye. Potts has recently used a neoprene-pigment mixture, plus clearing of the tissues, to study the blood vessels of human and animal eyes. His methods were employed in the studies herein reported. At the conclusion of fluorescein angiography studies (to be described) the intraocular pressure in one eye was elevated and maintained. The thoracic cavity was opened and a blunt 19-gage needle was inserted into the left ventricle and clamped in place. The descending aorta was clamped and the right ventricle incised. Five hundred milliliters of 0.9% saline solution at 40°C were then infused. This was immediately followed by 30 ml of a mixture of six parts of neoprene latex,* two parts of pigment and two parts of saline. Particular attention was paid to the latex infusion pressure. Using a syringe, the latex was pulsed in at mean pressures approximating those of the living animal's systemic blood pressure. Following the injections, a Stryker bone saw was used to remove the lateral walls of the orbits, and bilateral enucleations were performed. The specimens were placed in 10% formaldehyde solution containing one part glacial acetic acid for 24 hours. The tissues were then dehydrated and cleared as described previously. Figures 1 to 3 are photographs of three of the resulting cleared eyes. 8
4
8
Study of six injected normal eyes (normal intraocular pressure) revealed that the major source of vessels to the papilla and lamina cribrosa was from the choroid and branches from the posterior ciliary arteries, and from the pia. Figure 1 is a photograph of one of the cleared latex-injected monkey * Neoprene latex compound A423A-1. Average particle diameter 120 millimicrons. t Red RW-635P water-dispersed color with an average particle diameter of 2-4 microns. Both from E. I. DuPont, DeNemours nad C o , Inc., Elastomer Chemicals Department, Akron, Ohio, U.S.A.
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eyes. The intraocular pressure was normal. The central retinal artery (a) is seen within the optic nerve and can be followed through the lamina cribrosa and into the papilla where it branches into the retinal arteries. The posterior ciliary arteries are seen on both sides of the optic nerve. Some of the posterior ciliary artery branches anastomose in the sclera around the optic nerve but not to the extent of forming a definite circle comparable to that of the circle of ZinnHaller seen in man. No branches of the central retinal artery were observed in the optic nerve in these normal eyes. Further, in all eyes studied, there was at least one branch, and sometimes as many as three branches, of a posterior ciliary artery which penetrated the optic nerve approximately 2 mm posterior to the lamina cribrosa and then ran directly forward to the lamina cribrosa and papilla. One such vessel is visible in Figure 1 (b). Six eyes were infused while the intraocular pressure was maintained between 70 and 100 mm Hg. In these eyes, the pronounced vasculature of the papilla, lamina cribrosa and distal segment of the optic nerve originating from the posterior ciliary arteries was gone. Figure 2 is to be compared with Figure 3. The intraocular pressure of the eye shown in Figure 3 was maintained at 90 mm Hg during the latex infusion. A photograph of the opposite eye of the same monkey is shown in Figure 2. The intraocular pressure in this eye was normal. Comparison of the two eyes revealed that the heavily anastomotic papilla and lamina cribrosa vasculature did not fill as well in the eye with an increase in the intraocular pressure as it did in the eye with a normal intraocular pressure. In the injected eye with an elevated intraocular pressure, the capillary-size vessels of the lamina cribrosa and in the area posterior to the lamina cribrosa also were not filled. The latex did not pool within the optic nerve posterior to the lamina cribrosa. In fact, in all the experiments the latex filling of the vessels for 1-2 mm posterior to the lamina cribrosa was di-
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Fig. 1 (Ernest and Potts). Latex-injected monkey eye has been cleared and partially sectioned. The central retinal artery (a) is seen insi'de the optic nerve. Small branch of one of the ciliary arteries penetrates optic nerve and runs forward to the papilla (b).
Fig. 2 (Ernest and Potts). Latex-injected monkey eye photographed from vitreous side. Intraocular pressure was normal at time of injection.
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Fig. 3 (Ernest and Potts). Intraocular pressure was 90 mm H g at time of the latex injection. There is a decrease in papillary and peripapillary vascular filling.
minished relative to the opposite control eye. In the eyes perfused during elevation of the intraocular pressure, the central retinal artery showed several branches into the substance of the optic nerve posterior to the lamina cribrosa. These branches appeared to be end arteries and averaged only approximately 1 mm in length. Fluorescein angiograms were done on the monkeys before they were injected with latex and sacrificed. The method employed for fluorescein angiography was the same as the one first suggested by Novotny and Alvis. A control fluorescein angiogram was made by injecting 5 ml of 5% fluorescein in the leg vein of the monkey under general anesthesia. The animal was then returned to his cage and allowed to recover completely. The clearing time of fluorescein is approximately 36 hours. Usually the final experiment was performed three days after the first experiment. The animal was again anesthetized and the anterior chamber of the 6
6
same eye was cannulated. The intraocular pressure was elevated and maintained while the fluorescein angiogram was repeated. Figure 4-A is a fluorescein photograph of the fundus of an eye with normal tension. The angiogram is in the arterial phase. The small disc arterioles are filled but the retinal veins do not contain the dye at this stage. Many small vessels can be seen radiating on the disc surface and into the peripapillary retina. Figure 4-B is the same eye three days later but this time the angiogram was made while the intraocular pressure was maintained at 80 mm Hg. Comparison of the two arterial phase angiograms reveal that the majority of small disc vessels are absent from the eye subjected to the raised intraocular pressure. In Figure 4-B, small retinal vessel branches are seen filled but the small disc vessels of comparable size are not. The fact that small disc vessels fill poorly while small retinal vessel branches fill well under conditions of elevated intraocular pressure was empha-
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Fîg. 4 (Ernest and Potts). Arterial phase fluoresi :in angiograms of monkey eye done three days apart. ( A ) Intraocular pressure normal ( B ) Intraocular p: :ssure was maintained at 80 mm H g during angiogram. Small disc vessels are not filled.
sized by another experiment. In this experiment the intraocular pressure was maintained at 100 mm Hg during the fluorescein angiogram. Figure 5-A is a photograph of the arterial phase under conditions of a normal intraocular pressure. Figure S-B is a photograph of the arterial phase angiogram of the same eye obtained under conditions of an intraocular pressure of 100 mm Hg.
Small vessels are seen branching from the retinal arterioles even though the elevated intraocular pressure has prevented both small disc vessels as well as the choroid from filling. DISCUSSION Hayreh and Vrabec have done histologic studies on 24 rhesus monkey eyes. A trans7
Fig. S (Ernest and Potts). Arterial phase fluorescein angiograms of monkey eye done three days apart. ( A ) Intraocular pressure normal. ( B ) The intraocular pressure was maintained at 100 mm H g during the angiogram. Small branches of the central retinal artery are filled but neither the small disc vessels nor the choroid contain dye.
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verse section through the lamina cribrosa showed heavy connective tissue septa similar to those seen in man and the domestic cat. The monkey is similar to man and differs most markedly from the cat in having a central retinal artery. Study of the six latex-injected normal eyes (normal intraocular pressure) revealed findings similar to those of Hayreh. Posterior ciliary artery branches anastomosed in the sclera around the optic nerve but a definite circle comparable to that of the circle of Zinn-Haller seen in man was not evident. The considerable vascular network of the distal segment of the optic nerve of the monkey depends on the posterior ciliary arteries and the pial vessels and not on the central retinal artery. 8
Hamasaki and Fujino have recently published a study on the effect of the intraocular pressure on the ocular vessels of the owl monkey. Their technique was similar to the latex injection method used in this study. The authors employed a gelatin-India ink solution injected retrogradely through the descending aorta. The intraocular pressure was elevated in one eye and not in the fellow eye. Their India ink injection specimens were not dissimilar from the latex injections herein reported. The capillaries of the distal segment of the optic nerve filled poorly. They did note, however, that the deficiency in optic nerve capillary filling was greater in the temporal one-half than in the nasal onehalf of the optic nerve. This finding was not evident in our latex-injected specimens. The eyes injected under conditions of an elevation in the intraocular pressure herein reported showed a marked decrease in papillary and peripapillary capillary filling. Toussaint and associates have demonstrated peripapillary vessels in man, coming from the disc. Comparable vessels are compromised by increased intraocular pressure in the monkey eye. Both the study by Hamasaki and Fujino and the experiments herein reported demonstrate that injection of the distal optic nerve segment vasculature of ciliary artery origin is diminished by elevated intra9
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ocular pressure. Furthermore, the intraocular pressures required are below the levels which close the central retinal artery. Two fluorescein angiogram studies this past year are pertinent to this discussion. David performed fluorescein angiography in patients with central retinal artery occlusions. In one of the patients there was capillary filling of the nerve head 10 seconds before the dye appeared in the central retinal artery. This is further evidence that the central retinal artery contributes little if at all to the nutrition of the papilla and distal optic nerve. Hayreh and Walker investigated 33 glaucoma patients with fluorescein angiography. They concluded that there was a decrease in fluorescence of the optic disc in some of the glaucoma patients. They assumed that the arterial phase fluorescein angiogram is an index of disc vascularity. If this is true, and since at the time of the angiograms the patients all had normal intraocular pressures, then it may be evidence that there is a decrease in disc vascularity in glaucoma. The fluorescein angiography herein reported adds information about the vascular effects of elevated intraocular pressure in the intact living animal. The technique of using a delay between the two angiograms was employed because if the intraocular pressure was varied after the fluorescein injection, there was little demonstrable effect on disc fluorescence. This may be due to a loculation of the dye in small vessels between cellular components. There also may be an extravasation of dye into the disc which would not be affected by changes in the intraocular pressure. There are problems in comparing two angiograms of the same eye done at different times. The speed of injection, the state of the circulation and the concentration of the bolus reaching the eye may not be the same in both instances. Nonetheless, the results were consistent in the animals in which the technique was used. It was not possible to evaluate the relative fluorescence of the discs as Hayreh and 11
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Walker have done in their glaucoma patients. It is evident, however, that the small vessels which are resolved* in the papilla are obliterated by an increase of the intraocular pressure. Further, levels of the intraocular pressure which compromise the papilla vasculature has less effect on the central retinal arterial circulation. The implication is that the intravascular pressure in the vessels which supply the disc is lower than the intravascular pressure in the retinal vessels of central artery origin. Figure 6 is a drawing of the arterial supply of the optic nerve and retina in man. It is adapted from the studies of Hayreh, François, Wybar and others. The source of blood to the globe in man is the ophthalmic artery. Both the central retinal artery and the ciliary arteries branch from the ophthalmic artery. At their origin, the central retinal artery and the ciliary arteries are of equal caliber and it is assumed that their intravascular pressures are approximately the same ( P ) . The central retinal artery has a few intraneural branches but there are none 0
* The resolving power of the Zeiss fundus camera under the conditions of fluorescein photography has not been measured. Microaneurysms known to be approximately 75 microns in diameter or smaller are visible, however."
in the region of the lamina cribrosa and papilla. The central retinal artery branches spread out on the retina and supply its inner one-half. The intravascular pressure of the retinal arterioles is designated in Figure 6 as Pj. A s the short posterior ciliary arteries penetrate the sclera to supply the choroid and thereby the posterior half of the retina, they give off small branches to the circle of Zinn-Haller. The pressure designated P» in Figure 6 is the pressure in the circle of Zinn-Haller which surrounds the optic nerve. The vasculature of the distal segment of the optic nerve in the monkey is similar to that of man. The central retinal artery does not branch at the level of the lamina cribrosa and the papilla. The branches to the lamina cribrosa and papilla originate from intrascleral ciliary artery anastomoses surrounding the optic nerve. Anatomic studies have demonstrated that the branches from the ciliary arteries supplying the optic nerve must course through the sclera and are of small caliber. This is in contrast to the larger arterioles found in the retina which branch from the central retinal artery. The results of both the latex injections and fluorescein angiography indicated that elevated levels of intraocular pressure decrease the filling of vessels
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of the papilla and lamina cribrosa. The latex injection studies further demonstrated a decrease in optic nerve capillary filling posterior to the lamina cribrosa. Since it has been demonstrated that the intraocular pressure is not transmitted posterior to the lamina cribrosa, another explanation for the poor capillary filling in this area is necessary. It may be that the capillaries of the distal optic nerve segment posterior to the lamina cribrosa receive the majority of their blood from anastomoses with papillary and lamina cribrosa vessels. The latter are directly subjected to elevated intraocular pressure and thus may secondarily decrease the blood flow to the posterior vessels. These studies suggest that the blood pressure in the disc vessels of ciliary artery origin is at a lower level than the blood pressure in the retinal vessels from the central retinal artery. Furthermore, the tissue pressure of the papilla is many times greater than tissue pressures found in other parts of the body, since the papilla is subjected to the intraocular pressure. It might be expected, then, that slight increases in the intraocular pressure found in glaucomatous eyes could compromise the vasculature. The result may be atrophy of tissue with cupping of the disc and visual field loss. SUMMARY This study investigated the effect of elevated intraocular pressure on the vascular conductivity in the distal segment of the optic nerve in the monkey. Both injection, followed by fixation and clearing, and fluorescein angiography in the living animal were used. The anatomic findings agreed with those previously reported, indicating that the papilla and distal optic nerve receive the majority of their blood supply from the
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posterior ciliary circulation and not the central retinal artery. Both latex injections and fluorescein angiography were done with the intraocular pressure elevated. It was demonstrated that elevated intraocular pressure compromises the small vessel structure of the distal optic nerve segment and it is hypothesized that this accounts for the glaucomatous disc changes seen in man. REFERENCES 1. Ernest, J. T. and Potts, A. M . : Pathophysiology of the distal portion of the optic nerve. I. Tissue pressure relationships. Am. J. Ophth. 66:373, 1968. 2. Bárány, E. : Applanation tonometry and ophthalmoscopy of the vervet monkey (Cercopithecus ethiops) in phencyclidine catalepsia. Invest. Ophth. 2:322, 1963. 3. Potts, A. M . : Anatomic methods for study of the bulbus oculi. Am. J. Ophth. 65:155, 1968. 4. Spalteholz, W . : Uber das Durchsichtigmachen von Menschlichen und Tierischen Präparaten. Leipzig, Hirzel, 1914, ed. 2. 5. Novotny, H . R. and Alvis, D. L. : A method of photographing fluorescence in circulating blood in the human retina. Circulation 24:82, 1961. 6. Dollery, C. T., Hodge, J. V . and Engel, M . : Studies of the retinal circulation with fluorescein. Brit. Med. J. 2:1210, 1962. 7. Hayreh, S. S. and Vrabec, F. : The structure of the head of the optic nerve in rhesus monkey. Am. J. Ophth. 61:136, 1966. 8. Hayreh, S. S. : The orbital vessels of rhesus monkeys. Exper. Eye Res. 3:16, 1964. 9. Hamasaki, D. I. and Fujino, T. : Effect of intraocular pressure on ocular vessels: Filling with India ink. Arch. Ophth. 78:369, 1967. 10. Toussaint, D., Kuwabara, T. and Cogan, D. G. : Retinal vascular patterns. Arch. Ophth. 65:575, 1961. 11. David, N. J., Norton, E.W.D., Gass, J. D. and Beauchamp, J. : Fluorescein angiography in central retinal artery occlusion. Arch. Ophth. 77:619, 1967. 12. Hayreh, S. S. and Walker, W . M . : FluoresOphth. 63 :982, 1967. cent fundus photography in glaucoma. Am. J. 13. Scott, D. J., Dollery, C. T., Hill, D. W . , Hodge, J. V . and Fraser, R. : Fluorescein studies of diabetic retinopathy. Brit. Med. J. 1:811, 1964.