- Email: [email protected]
Preretinal Arterial Loops and Retinal Arterial Occlusion
PRERETINAL ARTERIAL LOOPS AND RETINAL ARTERIAL OCCLUSION GARY C. B R O W N , M.D., L A R R Y M A G A R G A L , M.D., J A M E S J. A U G S B U R G E R , M.D., AND J E R R Y A. S H I E L D S , M.D. Philadelphia,
Pennsylvania
The preretinal arterial loop, first de scribed by Liebreich in 1871, 1 is a con genital vascular anomaly that origi nates from a main branch of the central retinal artery on the optic disk. 2 These arterial loops, which are usually unilater al, are more common than preretinal ve nous loops. 3 Although they are usually asymptomatic, three cases of preretinal arterial loops associated with branch reti nal arterial occlusion have previously been described. 4 " 6 We report herein a case of unilateral branch retinal artery occlusion in a patient with bilateral pre retinal arterial loops. CASE REPORT A 19-year-old man came to the emergency room after noting loss of vision in the upper visual field of the right eye upon awakening that morning. There was no history of trauma, earlier visual dysfunction, headache, or any systemic disease. Ocular examination revealed a visual acuity of 6/6 (20/20) in both eyes. The pupils were equal, round, and reactive to light, but there was an afferent pupillary defect in the right eye. Slit-lamp examina tion was normal in each eye and applanation pres sure was 16 mm Hg in both eyes. Ophthalmoscopy of the right eye revealed an arterial trunk which passed from the optic disk, extended for 1 to 2 mm into the vitreous cavity and abruptly returned to the optic disk to form the inferior papillary artery. There was nerve fiber layer edema in the distribution of the inferior papillary artery (Fig. 1), consistent with a branch retinal ar tery occlusion. There were no emboli or plaques From the Vascular and Oncology Units of the Retina Service, Wills Eye Hospital and Research Institute, Philadelphia, Pennsylvania. This study was supported in part by the Retina Research and Development Foundation, Philadelphia, and the Pennsylvania Lions Sight Conservation and Eye Research Foundation, Inc. Reprint requests to Larry Magargal, M.D..Vascular Unit, Retina Service, Wills Eye Hospital, 1601 Spring Garden St., Philadelphia, PA 19130. 646
Fig. 1 (Brown and associates). Preretinal arterial loop with associated nerve fiber layer edema in the distribution of the inferior papillary artery in the right eye. The small dark object adjacent to the inferonasal disk is retinal hemorrhage.
within the vessel. Intravenous fluorescein angiography showed no filling of the preretinal loop throughout the study and segmentation of the blood column within the inferior papillary artery (Fig. 2). Examination of the left eye revealed a pulsating preretinal vascular loop over the disk, extending 3 mm into the vitreous cavity (Fig. 3). Fluorescein angiography of this eye disclosed filling of the superior papillary artery before the appearance of background choroidal fluorescence (Fig. 4, top left). As the angiogram progressed we could see that the superior fundus and the macula were supplied by this large superior cilioretinal artery and a smaller inferotemporal cilioretinal artery (Fig. 4, top right). Fluorescein dye appeared in the loop shortly after filling of the cilioretinal arteries, but approximately 3.5 seconds elapsed before the loop demonstrated fluorescence as intense as within the cilioretinal arteries (Fig. 4, top right, bottom left and right), indicating a relatively slow flow through this arterial system. Inferonasal choroidal background fluores cence and inferior papillary retinal arterial filling were absent until the arterial loop was filled with
AMERICAN JOURNAL O F OPHTHALMOLOGY 87:646-651, 1979
VOL. 87, NO. 5
PRERETINAL ARTERIAL LOOPS
Fig. 2 (Brown and associates). Intravenous fluorescein angiography of Figure L No filling is seen within the arterial loop (black arrow) and segmenta tion of the arterial blood column is present within one of the branches of the inferior papillary system (white arrow). fluorescein dye (Fig. 4, top right, bottom left and right), suggesting that blood supply to both of these areas was derived from the preretinal arterial loop. Results of complete physical examination were normal, as were results of laboratory tests for cardio vascular and collagen disease. An extensive hematologic examination, including hemoglobin electrophoresis, disclosed a nonspecific mild rise in factor VIII procoagulant activity. No therapy was undertaken and the patient was next seen months later. Visual acuity in the right eye was 6/5-2 (20/16)-2 at this time. The nerve fiber layer edema had subsided and the preretinal arterial loop appeared as a subtle white ghost vessel, devoid of blood. Fluorescein angiography (Fig. 5) showed antegrade filling of the inferior papillary artery from a newly demonstrable cilioretinal artery originating from the inferonasal aspect of the optic disk. No fluorescein was seen within the former arterial loop throughout the study. Visual field examination dis closed an absolute superior defect (Fig. 6). On the most recent examination, eight months after the occlusion in the right eye, the cilioretinal artery supplying the inferior retina was more promi nent and the blood column had increased in size (Fig. 7). Fluorescein angiography showed filling of the superior and inferior cilioretinal system before the filling of the small central retinal arterial branch supplying the fovea (Fig. 7, center and bottom). Despite the early filling of the inferior arterial sup ply to the retina, the arteriovenous transit time was delayed in comparison to the superior retina. Visual
647
Fig. 3 (Brown and associates). Spiral preretinal arterial loop extending into the vitreous and then returning to the optic disk in the left eye (double arrows). The superior retina is supplied by a large branching cilioretinal artery with its origin (single arrow) at the superior disk margin.
acuity was 6/5-2 (20/16)-2 in this eye and the abso lute superior visual field defect remained. The left eye was unchanged from the previous visit. DISCUSSION
In the one case of a preretinal arterial loop described histologically, the vessel originated from a main branch of the central retinal artery and continued as a loop within the course of that branch. 2 Embryologically, these loops may be de rived at about the 100 mm stage from within the area of Bergmeister's papilla and have no connection with the hyaloid artery. 7 The loops may be single or take one or more spiral turns, as shown in our pa tient. They may enter and exit at the disk or enter by the disk and continue into the retina as a branch vessel. 8 Preretinal vascular loops had been re ported in only 63 eyes by 1953. 3 Of these, 52 (83%) eyes contained arterial loops and 11 (17%) contained venous loops. Bilater-
648
AMERICAN JOURNAL OF OPHTHALMOLOGY
MAY, 1979
Fig. 4 (Brown and associates). Fluorescein angiogram of the left eye. Top left, Filling of the superior papillary cilioretinal artery before the appearance of choroidal fluorescence. Top right, The superior papillary and inferotemporal (black arrow) cilioretinal arteries are well filled. Dye is faintly appearing within the loop, but the inferior papillary artery and the inferonasal choroid (outlined by white arrows) are relatively hypofluorescent. Bottom left, Hyperfluorescence is more prominent within the prearterial loop and inferotemporal retinal artery. The inferonasal choroid is still relatively hypofluorescent. Bottom right, the preretinal arterial loop is completely filled with fluorescein dye, as now are the inferior retinal arteries and the inferonasal choroid. The progression of blood through the loop and then the inferior retinal arteries and the inferonasal choroid suggests that the loop may supply blood to both the retina and the choroid.
al arterial loops were present in about 8% (4 of 48) of patients with preretinal arteri al loops. Arterial occlusion was associated in 4% (2 of 52) of eyes with arterial loops. Oxilia 9 estimated that 70% of preretinal arterial loops involved the inferior retinal vessels, and 30% involved the superior retinal vessels. In this case, as well as in
the three previously reported cases of ar terial occlusion associated with loops, 4 - 6 only the inferior retinal vessels were in volved. One of the four patients had visual acuity of counting fingers, although the others maintained visual acuity of 6/6 (20/20) or better in the involved eye. If the involved vessel supplies a significant por-
VOL. 87, NO. 5
PRERETINAL ARTERIAL LOOPS
649
RE 120
105
255
Fig. 5 (Brown and associates). Right eye three months after inferior papillary arterial occlusion. Fluorescein angiography discloses a small cilioretinal vessel (arrow) now supplying the inferior retina. This vessel could not be visualized at the initial examination (Figs. 1 and 2).
tion of the fovea, the prognosis for good visual acuity diminishes. The cause of occlusion is thought to be kinking or twisting of the loops, with subsequent obstruction. 6 Although this may have been the situation in our case, the precise mechanism is not clear. Flow through the loop may also be somewhat slow, as evidenced by delayed filling of the inferior papillary artery on fluoresce in angiography of the left eye in our patient. Presently detectable systemic diseases associated with retinal arterial occlusion appear not to have a significant role. Atherosclerotic changes are doubt ful, as three of the four patients with occlusions have ranged in age between 19 to 22 years. Emboli have not been ob served in any of these cases. This case shows that a major portion of the retinal arterial blood supply in each eye is cilioretinal in origin. Although it is impossible to know whether the occluded
90
75
270
285
60
Fig. 6 (Brown and Associates). Visual field of the right eye, showing an absolute superior altitudinal field defect.
arterial loop in the right eye was cilio retinal, the probable collateral vessel sup plying the inferior retina months after the occlusion is characteristically cilioretinal. Most of the retina in each eye is rarely supplied by cilioretinal arteries; Collier 10 found only one such case in 250 eyes with cilioretinal arteries. In the right eye of our patient the foveal vision was spared by a small branch of the central retinal artery, whereas much of the rest of the fundus was supplied by a cilioretinal system. Central retinal or branch retinal artery occlusion with cilioretinal foveal spar ing 11 is more common. The presence of an artery that supplies both the retina and the choroid, as may be the case in the left eye of our patient, is rare except for retinochoroidal anastomo ses with disciform lesions of the macula and after focal argon laser photocoagulation. Anastomoses between retinal and ciliary circulations 1 2 ' 1 3 and between the cilioretinal and choroidal systems 10 have been described. We are currently attempt ing to determine whether preretinal arter ial loops are, other than by chance, associ-
650
AMERICAN JOURNAL OF OPHTHALMOLOGY
MAY, 1979
Fig. 7 (Brown and associates). Right eye eight months after inferior papillary arterial occlusion. Top, The cilioretinal artery (arrows) supplying the inferior retina. Center, Filling of both the superior and inferior retina by cilioretinal arteries (black arrows point to the origins of the arteries on the optic disk). The blood column within the inferonasal cilioretinal artery is larger than five months earlier (Fig. 5). The central retinal derived artery (white arrow) supplying the fovea is not yet filled. Bottom, Filling of the central retinal arterial branch (arrow), which spared foveal vision after the occlusion.
ated with vessels supplying both retina and choroid. SUMMARY
A 19-year-old man had bilateral con genital preretinal arterial loops emerging from the optic disk into the vitreous cavi ty. The loop in the right eye was asso ciated with an inferior papillary arterial occlusion. Extensive examination to de termine a systemic cause of the occlusion
was negative; twisting or kinking of the loop may have had a role. The major arterial blood supply in each eye was cilioretinal in origin and in the left eye, the inferior preretinal arterial loop sup plied blood to both the retina and the choroid. In three previous cases and our case of preretinal arterial loops associated with branch retinal arterial occlusion, none had emboli or associated systemic factors.
VOL. 87, NO. 5
PRERETINAL ARTERIAL LOOPS
All involved solely arterial supply to the inferior retina. The ages of three of the four patients ranged from 19 to 22 years. REFERENCES 1. Liebreich, R.: Demonstration of diseases of the eye. Persistent hyaloid artery and vein. Trans. Pjthpl. Soc. London 22:222, 1871. 2. Goldstein, I., and Wexler, D.: The preretinal artery. An anatomic study. Arch. Ophthalmol. 1: 324, 1929. 3. Bisland, T.: Vascular loops in the vitreous. Arch. Ophthalmol. 49:514, 1953. 4. Bar, C : Ein Fall von in den Glaskorper vordringender Arterienschlinge in einem durch Embolie der Zentralarterie erblindeten. Auge. Klin. Monatsbl. Augenheilkd. 39:307, 1901. 5. Walker, C : Thrombosis of the inferior tempor al branch of the arteria centralis retinae in an eye with a persistent hyaloid artery and vein; caused by exposure to direct sunlight. Trans. Ophthalmol. Soc. U.K. 23:279, 1903. 6. Harcourt, R., and Locket, N.: Occlusion of a
651
preretinal arterial loop. Br. J. Ophthalmol. 51:562, 1967. 7. Mann, I.: Developmental Abnormalities of the Eye. Philadelphia, J. B. Lippincott Co., 1957, pp. 133-136. 8. Duke-Elder, S.: Normal and Abnormal Devel opment. Congenital Deformities. In System of Oph thalmology, vol. 3, pt 2. St. Louis, C. V. Mosby, 1963, pp. 782-787. 9. Oxilia, E.: Anomalie vascolari della retina; ansa arteriosa prepapillare. Ann Ottal. Clin. Ocul. 73:408, 1946. 10. Collier, M.: Frequence des vaisseaux cilioretiniens, leur rapport avecles ametropies, leur association avec d'autres anomalies du fond de l'oeil. Bull. Dox. Ophthalmol. Fr. 9:598, 1957. 11. Brown, G. C , and Shields, J. A.: Cilioretinal arteries and retinal arterial occlusion. Arch. Oph thalmol. 98:84, 1979. 12. Wybar, K. C. L.: Anastomoses between the retinal and ciliary arterial circulations. Br. J. Oph thalmol. 40:65, 1956. 13. Green, W. R., and Gass, J. D. M.: Senile disciform degeneration of the macula. Arch. Ophthal mol. 86:487, 1971.
Pennsylvania
The preretinal arterial loop, first de scribed by Liebreich in 1871, 1 is a con genital vascular anomaly that origi nates from a main branch of the central retinal artery on the optic disk. 2 These arterial loops, which are usually unilater al, are more common than preretinal ve nous loops. 3 Although they are usually asymptomatic, three cases of preretinal arterial loops associated with branch reti nal arterial occlusion have previously been described. 4 " 6 We report herein a case of unilateral branch retinal artery occlusion in a patient with bilateral pre retinal arterial loops. CASE REPORT A 19-year-old man came to the emergency room after noting loss of vision in the upper visual field of the right eye upon awakening that morning. There was no history of trauma, earlier visual dysfunction, headache, or any systemic disease. Ocular examination revealed a visual acuity of 6/6 (20/20) in both eyes. The pupils were equal, round, and reactive to light, but there was an afferent pupillary defect in the right eye. Slit-lamp examina tion was normal in each eye and applanation pres sure was 16 mm Hg in both eyes. Ophthalmoscopy of the right eye revealed an arterial trunk which passed from the optic disk, extended for 1 to 2 mm into the vitreous cavity and abruptly returned to the optic disk to form the inferior papillary artery. There was nerve fiber layer edema in the distribution of the inferior papillary artery (Fig. 1), consistent with a branch retinal ar tery occlusion. There were no emboli or plaques From the Vascular and Oncology Units of the Retina Service, Wills Eye Hospital and Research Institute, Philadelphia, Pennsylvania. This study was supported in part by the Retina Research and Development Foundation, Philadelphia, and the Pennsylvania Lions Sight Conservation and Eye Research Foundation, Inc. Reprint requests to Larry Magargal, M.D..Vascular Unit, Retina Service, Wills Eye Hospital, 1601 Spring Garden St., Philadelphia, PA 19130. 646
Fig. 1 (Brown and associates). Preretinal arterial loop with associated nerve fiber layer edema in the distribution of the inferior papillary artery in the right eye. The small dark object adjacent to the inferonasal disk is retinal hemorrhage.
within the vessel. Intravenous fluorescein angiography showed no filling of the preretinal loop throughout the study and segmentation of the blood column within the inferior papillary artery (Fig. 2). Examination of the left eye revealed a pulsating preretinal vascular loop over the disk, extending 3 mm into the vitreous cavity (Fig. 3). Fluorescein angiography of this eye disclosed filling of the superior papillary artery before the appearance of background choroidal fluorescence (Fig. 4, top left). As the angiogram progressed we could see that the superior fundus and the macula were supplied by this large superior cilioretinal artery and a smaller inferotemporal cilioretinal artery (Fig. 4, top right). Fluorescein dye appeared in the loop shortly after filling of the cilioretinal arteries, but approximately 3.5 seconds elapsed before the loop demonstrated fluorescence as intense as within the cilioretinal arteries (Fig. 4, top right, bottom left and right), indicating a relatively slow flow through this arterial system. Inferonasal choroidal background fluores cence and inferior papillary retinal arterial filling were absent until the arterial loop was filled with
AMERICAN JOURNAL O F OPHTHALMOLOGY 87:646-651, 1979
VOL. 87, NO. 5
PRERETINAL ARTERIAL LOOPS
Fig. 2 (Brown and associates). Intravenous fluorescein angiography of Figure L No filling is seen within the arterial loop (black arrow) and segmenta tion of the arterial blood column is present within one of the branches of the inferior papillary system (white arrow). fluorescein dye (Fig. 4, top right, bottom left and right), suggesting that blood supply to both of these areas was derived from the preretinal arterial loop. Results of complete physical examination were normal, as were results of laboratory tests for cardio vascular and collagen disease. An extensive hematologic examination, including hemoglobin electrophoresis, disclosed a nonspecific mild rise in factor VIII procoagulant activity. No therapy was undertaken and the patient was next seen months later. Visual acuity in the right eye was 6/5-2 (20/16)-2 at this time. The nerve fiber layer edema had subsided and the preretinal arterial loop appeared as a subtle white ghost vessel, devoid of blood. Fluorescein angiography (Fig. 5) showed antegrade filling of the inferior papillary artery from a newly demonstrable cilioretinal artery originating from the inferonasal aspect of the optic disk. No fluorescein was seen within the former arterial loop throughout the study. Visual field examination dis closed an absolute superior defect (Fig. 6). On the most recent examination, eight months after the occlusion in the right eye, the cilioretinal artery supplying the inferior retina was more promi nent and the blood column had increased in size (Fig. 7). Fluorescein angiography showed filling of the superior and inferior cilioretinal system before the filling of the small central retinal arterial branch supplying the fovea (Fig. 7, center and bottom). Despite the early filling of the inferior arterial sup ply to the retina, the arteriovenous transit time was delayed in comparison to the superior retina. Visual
647
Fig. 3 (Brown and associates). Spiral preretinal arterial loop extending into the vitreous and then returning to the optic disk in the left eye (double arrows). The superior retina is supplied by a large branching cilioretinal artery with its origin (single arrow) at the superior disk margin.
acuity was 6/5-2 (20/16)-2 in this eye and the abso lute superior visual field defect remained. The left eye was unchanged from the previous visit. DISCUSSION
In the one case of a preretinal arterial loop described histologically, the vessel originated from a main branch of the central retinal artery and continued as a loop within the course of that branch. 2 Embryologically, these loops may be de rived at about the 100 mm stage from within the area of Bergmeister's papilla and have no connection with the hyaloid artery. 7 The loops may be single or take one or more spiral turns, as shown in our pa tient. They may enter and exit at the disk or enter by the disk and continue into the retina as a branch vessel. 8 Preretinal vascular loops had been re ported in only 63 eyes by 1953. 3 Of these, 52 (83%) eyes contained arterial loops and 11 (17%) contained venous loops. Bilater-
648
AMERICAN JOURNAL OF OPHTHALMOLOGY
MAY, 1979
Fig. 4 (Brown and associates). Fluorescein angiogram of the left eye. Top left, Filling of the superior papillary cilioretinal artery before the appearance of choroidal fluorescence. Top right, The superior papillary and inferotemporal (black arrow) cilioretinal arteries are well filled. Dye is faintly appearing within the loop, but the inferior papillary artery and the inferonasal choroid (outlined by white arrows) are relatively hypofluorescent. Bottom left, Hyperfluorescence is more prominent within the prearterial loop and inferotemporal retinal artery. The inferonasal choroid is still relatively hypofluorescent. Bottom right, the preretinal arterial loop is completely filled with fluorescein dye, as now are the inferior retinal arteries and the inferonasal choroid. The progression of blood through the loop and then the inferior retinal arteries and the inferonasal choroid suggests that the loop may supply blood to both the retina and the choroid.
al arterial loops were present in about 8% (4 of 48) of patients with preretinal arteri al loops. Arterial occlusion was associated in 4% (2 of 52) of eyes with arterial loops. Oxilia 9 estimated that 70% of preretinal arterial loops involved the inferior retinal vessels, and 30% involved the superior retinal vessels. In this case, as well as in
the three previously reported cases of ar terial occlusion associated with loops, 4 - 6 only the inferior retinal vessels were in volved. One of the four patients had visual acuity of counting fingers, although the others maintained visual acuity of 6/6 (20/20) or better in the involved eye. If the involved vessel supplies a significant por-
VOL. 87, NO. 5
PRERETINAL ARTERIAL LOOPS
649
RE 120
105
255
Fig. 5 (Brown and associates). Right eye three months after inferior papillary arterial occlusion. Fluorescein angiography discloses a small cilioretinal vessel (arrow) now supplying the inferior retina. This vessel could not be visualized at the initial examination (Figs. 1 and 2).
tion of the fovea, the prognosis for good visual acuity diminishes. The cause of occlusion is thought to be kinking or twisting of the loops, with subsequent obstruction. 6 Although this may have been the situation in our case, the precise mechanism is not clear. Flow through the loop may also be somewhat slow, as evidenced by delayed filling of the inferior papillary artery on fluoresce in angiography of the left eye in our patient. Presently detectable systemic diseases associated with retinal arterial occlusion appear not to have a significant role. Atherosclerotic changes are doubt ful, as three of the four patients with occlusions have ranged in age between 19 to 22 years. Emboli have not been ob served in any of these cases. This case shows that a major portion of the retinal arterial blood supply in each eye is cilioretinal in origin. Although it is impossible to know whether the occluded
90
75
270
285
60
Fig. 6 (Brown and Associates). Visual field of the right eye, showing an absolute superior altitudinal field defect.
arterial loop in the right eye was cilio retinal, the probable collateral vessel sup plying the inferior retina months after the occlusion is characteristically cilioretinal. Most of the retina in each eye is rarely supplied by cilioretinal arteries; Collier 10 found only one such case in 250 eyes with cilioretinal arteries. In the right eye of our patient the foveal vision was spared by a small branch of the central retinal artery, whereas much of the rest of the fundus was supplied by a cilioretinal system. Central retinal or branch retinal artery occlusion with cilioretinal foveal spar ing 11 is more common. The presence of an artery that supplies both the retina and the choroid, as may be the case in the left eye of our patient, is rare except for retinochoroidal anastomo ses with disciform lesions of the macula and after focal argon laser photocoagulation. Anastomoses between retinal and ciliary circulations 1 2 ' 1 3 and between the cilioretinal and choroidal systems 10 have been described. We are currently attempt ing to determine whether preretinal arter ial loops are, other than by chance, associ-
650
AMERICAN JOURNAL OF OPHTHALMOLOGY
MAY, 1979
Fig. 7 (Brown and associates). Right eye eight months after inferior papillary arterial occlusion. Top, The cilioretinal artery (arrows) supplying the inferior retina. Center, Filling of both the superior and inferior retina by cilioretinal arteries (black arrows point to the origins of the arteries on the optic disk). The blood column within the inferonasal cilioretinal artery is larger than five months earlier (Fig. 5). The central retinal derived artery (white arrow) supplying the fovea is not yet filled. Bottom, Filling of the central retinal arterial branch (arrow), which spared foveal vision after the occlusion.
ated with vessels supplying both retina and choroid. SUMMARY
A 19-year-old man had bilateral con genital preretinal arterial loops emerging from the optic disk into the vitreous cavi ty. The loop in the right eye was asso ciated with an inferior papillary arterial occlusion. Extensive examination to de termine a systemic cause of the occlusion
was negative; twisting or kinking of the loop may have had a role. The major arterial blood supply in each eye was cilioretinal in origin and in the left eye, the inferior preretinal arterial loop sup plied blood to both the retina and the choroid. In three previous cases and our case of preretinal arterial loops associated with branch retinal arterial occlusion, none had emboli or associated systemic factors.
VOL. 87, NO. 5
PRERETINAL ARTERIAL LOOPS
All involved solely arterial supply to the inferior retina. The ages of three of the four patients ranged from 19 to 22 years. REFERENCES 1. Liebreich, R.: Demonstration of diseases of the eye. Persistent hyaloid artery and vein. Trans. Pjthpl. Soc. London 22:222, 1871. 2. Goldstein, I., and Wexler, D.: The preretinal artery. An anatomic study. Arch. Ophthalmol. 1: 324, 1929. 3. Bisland, T.: Vascular loops in the vitreous. Arch. Ophthalmol. 49:514, 1953. 4. Bar, C : Ein Fall von in den Glaskorper vordringender Arterienschlinge in einem durch Embolie der Zentralarterie erblindeten. Auge. Klin. Monatsbl. Augenheilkd. 39:307, 1901. 5. Walker, C : Thrombosis of the inferior tempor al branch of the arteria centralis retinae in an eye with a persistent hyaloid artery and vein; caused by exposure to direct sunlight. Trans. Ophthalmol. Soc. U.K. 23:279, 1903. 6. Harcourt, R., and Locket, N.: Occlusion of a
651
preretinal arterial loop. Br. J. Ophthalmol. 51:562, 1967. 7. Mann, I.: Developmental Abnormalities of the Eye. Philadelphia, J. B. Lippincott Co., 1957, pp. 133-136. 8. Duke-Elder, S.: Normal and Abnormal Devel opment. Congenital Deformities. In System of Oph thalmology, vol. 3, pt 2. St. Louis, C. V. Mosby, 1963, pp. 782-787. 9. Oxilia, E.: Anomalie vascolari della retina; ansa arteriosa prepapillare. Ann Ottal. Clin. Ocul. 73:408, 1946. 10. Collier, M.: Frequence des vaisseaux cilioretiniens, leur rapport avecles ametropies, leur association avec d'autres anomalies du fond de l'oeil. Bull. Dox. Ophthalmol. Fr. 9:598, 1957. 11. Brown, G. C , and Shields, J. A.: Cilioretinal arteries and retinal arterial occlusion. Arch. Oph thalmol. 98:84, 1979. 12. Wybar, K. C. L.: Anastomoses between the retinal and ciliary arterial circulations. Br. J. Oph thalmol. 40:65, 1956. 13. Green, W. R., and Gass, J. D. M.: Senile disciform degeneration of the macula. Arch. Ophthal mol. 86:487, 1971.