Ocular circulation

OCULAR CIRCULATION PPT FOR OPHTHALMOLOGY STUDENTS

1. OCULAR CIRCULATION PRESENTED BY MAJ ANJANI KUMAR RESIDENT (OPHTH) MODERATOR LT COL M JHA CL SPL (OPHTH)

2. References Skalicky , Simon

3. Vascular Anatomy of the Eye Two separate vascular systems supply the eye: The retinal vessels , including the central retinal artery ( CRA ), central retinal vein ( CRV ), and branches (ii) The ciliary ( uveal ) vessels, including the short and long posterior and anterior ciliary arteries • Both systems arise from the ophthalmic artery, a branch of the internal carotid Artery.

4. 65-85% 800-1000ml /sec 5% 80ml/sec

5. OPHTHALMIC ARTERY First intracranial branch of the internal carotid just as the artery exits from the cavernous sinus • optic foramen below and lateral to the optic nerve • Pass over the optic nerve to its medial side • Between the MR and SO • Terminates by dividing into dorsonasal and supratrochlear

6. OPHTHALMIC ARTERY

7. Branches of ophthalmic artery: • Central retinal a. • Supra-orbital artery • Posterior ciliary artery – Long posterior ciliary a. (2 arteries) – Short post. Ciliary a. (10-20 arteries) • Muscular arteries – Anterior ciliary a. (7 arteries) • Lacrimal artery (terminate into zygomatic branches) • Ant. And post. Ethmoidal arteries • Superior and inferior palpebral arteries • Dorsonasal artery • Supratrochlear artery

8. CENTRAL RETINAL ARTERY Four branch retinal arteries, each supplying a quadrant of the retina, are derived from a central retinal artery (CRA), which is the first branch of the ophthalmic artery after it leaves the internal carotid

9. The CRA enters the ventromedial aspect of the retrobulbar optic nerve approximately 1.2 cm posterior to the globe. As the CRA emerges from the optic nerve head (ONH), it divides into superior and inferior branches, and then immediately branches again into superior and inferior nasal and temporal vessels. The temporal branches arch around the macula and create a foveal avascular zone, which is a 0.4 mm diameter, capillary-free zone of pure cone photoreceptors.

10. Arteries and veins remain in the nerve fiber layer, while arterioles and venules extend into the deeper layers of the retina,forming two major microvascular networks; superficial capillaries -in the ganglion cell and nerve fiber layers (2) deep, more dense, capillaries in the inner nuclear layer. In the perifoveal and peripheral regions of the retina, these capillary networks thin to a single layer

11. CENTRAL RETINAL ARTERY

12. Retinal vessels ( i ) Central retinal artery • Retinal arteries have a well-developed smooth muscle layer and lack an internal elastic lamina • The CRA supplies the inner retina ; the outer retina is avascular , nourished from the choroid • 10–20 % of individuals have a cilioretinal artery , arising from the choroidal circulation; this typically enters the inner retina at the temporal optic disc margin and supplies some of the macula (ii) Retinal capillaries and veins • Capillaries are arranged in lamellae within the inner retina • Astrocytes surround retinal vessels and maintain their integrity

13. Location of retinal capillary layers Capillary layer Location Innermost Peripapillary nerve fiber layer Middle Ganglion cell layer Outer Inner nuclear layer

14. Ciliary vessels The ciliary vessels include the vascular beds of the uveal tract . ( i ) The anterior ciliary vessels • Seven anterior ciliary arteries provide the major blood supply to the anterior uvea . • Two travel with each rectus muscle (the lateral rectus has only one) and pierce the sclera anteriorly . • They anastomose with the long posterior ciliary arteries to form the major iridial circle • This forms a ring around the iris peripheral margin supplying the iris and ciliary body .

15. The posterior ciliary vessels • 10–20 short posterior ciliary arteries enter the sclera to form an anastomotic ring ( circle of Zinn - Haller ) around the optic nerve. This supplies the anterior optic nerve and posterior choroid . • Two long posterior ciliary arteries supply the iris , ciliary body , and anterior choroid • Venous blood from the choroid and anterior uvea drains through four vortex vein.

16. The choroid The choroid is a highly vascular uveal layer between the retina and sclera. • It provides oxygen and nutrients to the outer retina and is a heat sink absorbing excessive light energy focused onto the retina . • The anterior surface, the choriocapillaris , is a dense, lobular, single-layered capillary network . • Feeding arteries and draining venules located deep to the choriocapillaris supply the choroid in a segmented fashion .

17. Human choroidal vascular anatomy The choroid is supplied posteriorly by 10–20 short posterior ciliary branches of the ophthalmic artery, that ramify the peripapillary sclera . A nasal and temporal long posterior ciliary artery supplies the anterior choroid and uvea . Portions of the short posterior ciliary arteries contribute to the circle of Haller and Zinn , but the majority give rise to the choriocapillaris , a single layer of choroidal capillaries containing fenestrated endothelium without tight junctions and supplying the outer third of the retina. This capillary network is immediately adjacent to Bruch’s membrane.

18. CHOROIDAL CIRCULATION

19. Choriocapillaris The choriocapillaris and its unique structure are crucial in enabling the choroid to perform its functions. It was first described in 1702 by Hovius and named in 1838 by Eschricht . The choriocapillaris is the capillary layer of the choroid. The capillaries are rather large (40–60 μm in diameter) and have very thin walls. The large diameter of the lumen allows at least two to three red blood cells to pass through at a time, whereas most other capillary systems in the body allow only one RBC.

20. Multiple fenestrations(600–800 Å in diameter) with covering diaphragms are present on the capillary wall, especially on the internal side . Fenestrations are also noted on the other side of the capillary but are much less frequent. The endothelial cell nucleus usually lies on the outer side of the capillary, so less room is available for fenestrations on that side. These fenestrations leak fluorescein molecules during fluorescein angiography. Gap junctions are present. Pericytes occasionally are seen on the outer wall. Connective tissue is present between vessels and provides support for the vascular system. Fibroblasts and nerve fibers can also be observed between capillaries

21. Choroidal circulation: Facts Choroidal circulation constitutes 85% of the blood circulation of the eye. Choroidal blood flow is higher than that in tissues like retina and brain. Choroidal blood-flow ranges from 800 to 2000 mL /min/100 g of tissue. Choroid provides the metabolic requirements of the full retinal thickness only in the macular region. In embryonic life, choroid serves as an additional site for the erythropoiesis .

22. The optic nerve head • Most of the anterior optic nerve is supplied by the circle of Zinn -Haller and pial vessels • There is a small physiological break in the blood - neural barrier at the lateral optic nerve head, adjacent to the choroid (border tissue of Elschnig ). Choroidal extravascular solutes may diffuse into the nerve tissue there . • Branches of the central retinal artery supply the superficial optic nerve head .

23. CIRCLE OF ZINN Lies in sclera Formed by circular anastomosis between the short cilliary arteries Gives branches to choroid, optic nerve & pial network. Also supplies lamina cribrosa , ONH, & surrounding retina. Cilioretinal arteries may arise to supply macula

24. The central retinal vein (CRV) leaves the eye through the optic nerve to drain venous blood into the cavernous sinus. Venous drainage of the choriocapillaris is mainly through the vortex vein system. Minor drainage occurs through the ciliary body through the anterior ciliary veins The vortex veins drain into the superior (SOV) and inferior ophthalmic (orbital) veins (IOV). VENOUS DRAINAGE

25. Control of Circulation With high metabolic requirements and relatively low flow, retinal and optic nerve perfusion must remain constant despite changes in perfusion pressure. Ocular perfusion pressure and intraocular pressure BF =(Pa - IOP) / R • A rise in IOP or reduction in mean arterial pressure reduces the ocular perfusion pressure . • This would cause reduced retinal or optic nerve perfusion if vascular resistance was unchanged; however, autoregulation results in vascular dilation, reduced resistance, and unchanged perfusion. • In contrast the choroid has limited autoregulation , and perfusion reduces when Pa drops or IOP rises. This does not cause significant ischemia except in extreme changes in Pa or IOP.

26. AUTOREGULATION • Retinal and optic nerve head vessels have the ability to autoregulate . • They maintain constant blood flow despite changes in oxygenation or perfusion pressure • The endothelium regulates vascular tone in response to myogenic , metabolic , and light stimuli: Myogenic stimuli (changes in vessel wall pressure) • Decreased perfusion pressure results in vascular dilatation. • Increased perfusion pressure results in reduced vascular dilatation. (ii) Metabolic stimuli (changes in lactic acid, O 2 , and CO 2 levels). • Low O 2 and high CO 2 tensions result in vascular dilatation. • Low CO 2 and high O 2 tensions result in reduced vascular dilatation. (iii) Light stimuli • Flickering light increases retinal metabolism resulting in retinal capillary dilatation.

27. (iv) Mechanisms of autoregulation • The vascular endothelium orchestrates vasodilation by release of prostacyclin and nitric oxide . Both cause endothelial cell relaxation in response to myogenic and metabolic stimuli. • Endothelins released by the endothelium are also involved in control of vascular tone. (v ) Limited choroidal autoregulation • The subfoveal choroid has a limited capacity for autoregulation . • In general autoregulatory mechanisms are not found in the choroidal circulation. • The choroid with high blood flow and O 2 supply can tolerate some perfusion decrease without tissue compromise

28. CHARACTERISTICS OF THE RETINAL AND CHOROIDAL CIRCULATIONS Retinal circulation Choroidal circulation Tissue supplied Inner retina Outer retina Blood flow (% total ocular supply) 4 % 85 % 10× retinal flow (per unit mass) Perfusion speed Slow (3–5 s) Fast (1 s before retinal perfusion) O 2 consumption (% arteriovenous O 2 gradient) 38 % 5 % Retinal O 2 supply (% total) 35 % of total retinal supply 65 % of total retinal supply

29. Capillary bed Retina Choroid Structure Stratified capillary network choriocapillaris : a large endothelial-lined space interrupted by stromal pillars luminal diameter 5 um 10–20 um Passage of red blood cells (7–8 um in diameter) Deform under resistance Move freely in sheet flow Endothelial barrier Continuous, forming blood-retinal barrier Fenestrated allowing free flow of fluid and solutes into extravascular space Intramural pericytes Present Absent

30. Large Vessels retina choroid Anastamoses End-on capillary supply with no physiological anastamoses Blockages not bypassed Lobular segmental supply of choriocapillaris with some arteriovenous anastamoses Watershed areas between lobules exist Change in vessel caliber Progressive reduction from large arteries to capillaries Abrupt change from short, wide arterioles to capillaries Perfusion pressure Moderate High Autoregulation Myogenic and metabolic mechanisms Limited capacity for autoregulation in the subfoveal choroid, otherwise none Neural vasomotor control None Sympathetic and parasympathetic innervation

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