OCULAR BLOOD SUPPLY.pdf

Ocular Blood Supply Karen Gil MD, MHSN Orbit Blood Supply • Circulation to the head and neck is supplied by the common carotid artery • Common carotid artery divides: – Internal carotid artery – External carotid artery Common Carotid Artery • Internal carotid artery supplies: – Structures of the...

Ocular Blood Supply Karen Gil MD, MHSN Orbit Blood Supply • Circulation to the head and neck is supplied by the common carotid artery • Common carotid artery divides: – Internal carotid artery – External carotid artery Common Carotid Artery • Internal carotid artery supplies: – Structures of the cranium – Eye and related structures • External carotid artery supplies: – Superficial areas of the head and neck – Small portion of the circulation to the ocular adnexa Internal Carotid Artery • Enters the skull through the carotid canal (petrous portion of temporal bone) • Then immediately enters the cavernous sinus • The ophthalmic artery branches just as it emerges form the cavernous sinus (usually the first major branch form the internal carotid artery) Ophthalmic Artery • Enters the orbit within the dural sheath of the optic nerve • Passes through the optic canal (below and lateral to the nerve) • In the orbit runs inferolateral to the optic nerve for a short distance • Then crosses either above or below the nerve Ophthalmic Artery Branches 1. 2. 3. 4. 5. 6. 7. Central retinal artery Lacrimal artery Ciliary arteries Ethmoid arteries Supraorbital artery Muscular arteries Medial palpebral arteries (superior and inferior) 8. Supratrochlear artery 9. Dorsonasal artery Long Posterior ciliary arteries Short posterior ciliary arteries Central Retinal Artery • The smallest branch • Leaves the ophthalmic artery as it lies below the optic nerve • Enters the meningeal sheet of the nerve about 10-12mm behind the globe • Supply the inner retina Central Retinal Artery Central Retinal Artery • In the optic nerve CRA provide branches to the nerve and pia mater (collateral branches) • CRA passes through the lamina cribosa and enters the optic disc just nasal to center branching: – Superior – Inferior Col. Br.=collateral branches, ON= optic nerve, D =dura mater, A=aracnoid mater, SAS=subarachnoid space, Pia=Pia mater, CRA= central retinal artery,CRV=central retinal vein, PCA= posterior ciliary artery, R=retina, C=choroid, S=sclera, LC= lamina cribosa, PR=prelaminar region. Central Retinal Artery • Superior and inferior branches divides further into nasal and temporal branches • These vessels continue to bifurcate • Two capillary networks are formed: – Deep – Superficial • No blood vessels are found in the macula Posterior Ciliary Arteries • Posterior ciliary arteries branches: – Short ciliary arteries (10-20) – Long ciliary arteries (2) Short Posterior Ciliary Arteries • Arise as one or two branches and then form 10 to 20 branches • Enter the sclera in a ring around the optic nerve and form the arterial network within the choroidal stroma • They divide to form posterior choriocapillaris, which nourishes retina as far anteriorly as equator Short Posterior Ciliary Arteries • Other branches form the circle of Zinn (ZinnHaller) encircles the optic nerve at the choroid • Supply the optic nerve head Short Posterior Ciliary Arteries IMC, intramuscular ciliary arterial circle; MAC, major arterial circle of the iris; PCA, posterior ciliary artery; ACA, anterior ciliary arteries; lpca, long posterior ciliary artery Long Posterior Ciliary Arteries • Almost the entire blood supply of the eye comes from the uveal vessels (except inner retina) • Two long posterior ciliary arteries – One enters nasally – One enters temporally Near the optic nerve Long posterior ciliary artery Long Posterior Ciliary Arteries • Long posterior ciliary arteries give 3 to 5 branches at the ora serrata and passes directly back to form anterior choriocapillaris • This capillaries nourish retina from equator forward Long Posterior Ciliary Arteries • Run between the sclera and the choroid anterior globe • Enter the ciliary body and branch superiorly and inferiorly • Anastomose with each other and with the anterior ciliary arteries and form the major arterial circle of the iris Major Arterial Circle of the Iris • Located in the ciliary stroma • Is the source of the radial vessels found in the iris Anterior Ciliary Arteries • Branch from the vessels supply the rectus muscles • Exit the muscles near the insertions, run along the tendons • Then loop inward to pierce the sclera (just outer to the limbus) Anterior Ciliary Arteries • Before entering the sclera • Send branches into the conjunctiva • Forming a network of vessels in the limbal conjunctiva Anterior Ciliary Arteries • Other branches enter the episclera to form a network of vessels before entering the uvea • Then enter the ciliary body and anastomose with the branches of the long posterior ciliary arteries (major circle of the iris) Anterior Ciliary Arteries • Generally, 2 anterior ciliary arteries emanate from each of the rectus muscles, with exception of the lateral muscle, which provide only one • Supply blood to the conjunctiva, episclera, ciliary body and iris Vasculature of the uveal tract. The long posterior ciliary arteries, one of which is visible (A), branch at the ora serrata (b) and feed the capillaries of the anterior part of the choroid. Short posterior ciliary arteries (C) divide rapidly to form the posterior part of the choriocapillaris. Anterior ciliary arteries (D) send recurrent branches to the choriocapillaris (e) and anterior rami to the major arterial circle (f). Branches from the circle extend into the iris (g) and to the limbus. Branches of the short posterior ciliary arteries (C) form an anastomotic circle (of Zinn) (h) round the optic disc, and twigs from this (i) join an arterial network on the optic nerve. The vortex veins (J) are formed by the junctions (k) of suprachoroidal tributaries (l). Smaller tributaries are also shown (m, n). The veins draining the scleral venous sinus (o) join anterior ciliary veins and vorticose tributaries. (from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.) LR-lateral rectus, SR-superior rectus, SO superior oblique, MR-medial rectus, IR-inferior rectus, IO- inferior oblique Diagram summarizing the blood supply of the orbit as seen in a superior view: 1, central retinal artery; 2, posterior ciliary arteries (usually emerge as two trunks that divide into the short posterior ciliary arteries (seven or more) and the long posterior ciliary arteries (usually two, medial and lateral)); 3, lacrimal artery; 4, recurrent branches (to meninges); 5, muscular branches (give rise to anterior ciliary arteries); 6, supraorbital artery; 7, posterior ethmoidal artery; 8, anterior ethmoidal artery; 9, superior and inferior medial palpebral arteries; 10, dorsalis nasi; 11, supratrochlear; 12, superior and inferior lateral palpebral arteries; 13, zygomatic branches of the lacrimal artery. Sites of anastomosis between branches of the internal and external carotid arteries External carotid Internal carotid Region of branch branch anastomosis Angular artery (facial) Dorsalis nasi (ophthalmic) Medial palpebral margin Transverse facial artery (superficial temporal) Lacrimal artery (ophthalmic) Lateral palpebral margin Middle Lacrimal artery meningeal (ophthalmic) artery and deep temporal artery Orbit Venous drainage from the eye • The orbit has to ophthalmic veins – Superior ophthalmic vein (larger) – Inferior ophthalmic vein • Drain into the cavernous sinus Venous drainage from the eye Cavernous Sinus Venous drainage from the eye Superior Ophthalmic Vein • Form by the join of – Supraorbital vein • Enter the orbit thru the supraorbital notch – Angular vein • Passes through the septum • Runs with the ophthalmic artery • Receive blood from – Anterior and posterior ethmoid veins – Muscular veins (superior and medial muscles) – Lacrimal vein – Central retinal vein – Superior vortex veins Venous drainage from the eye Inferior Ophthalmic Vein • Drains blood from – – – – Lower and lateral muscles Inferior conjunctiva Lacrimal sac Inferior vortex veins • May form two branches – One drains the superior ophthalmic vein – Other drains the pterygoid venous plexus Venous drainage from the eye Central Retinal Vein • 33% larger than CRA • Venous branches of the retinal tissue join to exit thru a single central retinal vein • Leave the optic nerve approximately 10-12 mm behind the lamina cribosa • Join the superior ophthalmic vein and/ or drains directly into the cavernous sinus Venous drainage from the eye Vortex Veins • Drain the choroid • Posteriorly 4 to 7 vortex (45) veins drain the venous system (the choroid, ciliary body and iris) into the superior and inferior ophthalmic veins • Exit the globe 6 mm posterior to the equator Venous drainage from the eye Anterior Ciliary Veins • Receive blood form the anterior conjunctiva, limbal arcades and anterior episcleral collecting veins • Join the muscular veins Venous drainage from the eye Infraorbital Vein • Form by veins that drain the face • Enters the infraorbital foramen • Drains into the pterygoid venous plexus Venous Drainage form the Eye • Blood from the ophthalmic veins drain into the cavernous sinus that drains into the superior petrosal sinus that drain into the internal jugular vein • Descends alongside the internal carotid artery Lymphatic Drainage of the Eye • No lymphatic vessels on the eye • Lymphatics on – Conjunctiva – Eyelids • Submandibular lymph nodes receive – Medial aspects of the lids – Medial canthal structures (include lacrimal sac) • Parotid and preauricular lymph nodes receive – Lateral eyelids – Lacimal gland Regulation of Ocular Circulation • Blood flow through a blood vessel depends upon the perfusion pressure (PP), the pressure that drives blood through the vessel, and the resistance (R) generated by the vessels • The ocular perfusion pressure can be reduced by either reduction of arterial pressure or increase in intraocular pressure Hemodynamic Patterns F = P arteries (entering a tissue) – P veins (leaving this tissue) R (resistance) F= blood flow P= blood pressure R= resistance Mean arterial pressure of the arteries entering the eye= 65 mmHg Pressure of the episcleral veins leaving the eye is around = 15mm Hg Hemodynamic Patterns • Perfusion pressure – Numeric value that indicates how easily blood can pass through a tissue – Approximately 50 mmHg (is the difference between the arteries and veins pressure) – This value has being used in glaucoma research: • Diastolic blood pressure – Intraocular blood pressure = OPP (ocular perfusion pressure) • Current research is showing that glaucoma patients with low OPP are 1.5 times more likely to develop progressive nerve damage from ischemia Autoregulation of Ocular Circulation • Autoregulation – is that property of a vascular bed that permits constant or nearly constant blood flow throughout a wide range of perfusion pressures • Blood flow in the retina appears to be primarily controlled by metabolic needs, especially the need for oxygen Autoregulation of Ocular Circulation • Autoregulation is a complex physiologic function of the microcirculation • The precise mechanisms behind autoregulation are not known at present • The two concepts that have been introduced to explain this phenomenon are – the myogenic theory – the local metabolite theory Autoregulation of Ocular Circulation • The myogenic theory – Proposes that vasodilatation and vasoconstriction are affected by cell-to-cell communication between adjacent smooth muscle cells to maintain blood flow in the presence of changing intravascular pressures Ocular Circulation • The local metabolite theory – Hypothesizes that metabolites or other substances (currently unknown) are produced by the retina when hypoxemic or otherwise metabolically stressed – These metabolites induce local alterations in blood flow in an attempt to maintain a constant environment of the retina Ocular Circulation • Blood flow is influenced by – Vascular pressure – Tone in the vasoactive nerves – Vasoactive substances – Metabolic activity Regulation of Ocular Circulation • Vasoactive substances– vascular endothelium is involved in the regulation of vascular tone, platelet activity and vascular permeability – This endothelium maintains vascular tone by releasing potent vasoactive agents (endothelins, angiotensin II) Average Pressure in Different Vessels of the Eye Vessel Pressure Central Retinal Artery Systolic 65-70 mmHg Diastolic 35-45 mmHg Retinal Arterioles Systolic 88 mmHg Diastolic 64 mmHg Intraocular veins About 2mmHg higher than IOP Intrascleral veins 7 to 8 mmHg Vortex veins 8.5 mmHg Episcleral veins 11 mmHg Choroid capillaries About 55 mmHg Retinal capillaries Lesser than 55 mmHg Metabolic Activity • Breathing pure oxygen – constricts the retinal vessels and – increases the oxygen tension of choroidal venous blood • When O2 levels are low → more blood production • Less O2 → vessels dilate; too much O2 → vessels constrict Neural Control of Ocular Circulation • Mediated by the autonomic nervous, part of the PNS (involuntary actions) – Parasympathetic – Sympathetic – sensory innervation of ocular and orbital blood vessels • Parasympathetic and sympathetic inputs to ocular vessels are controlled by PNS circuitry that is responsive to interoceptive and exteroceptive stimuli Neuronal Regulation of Ocular Circulation • Parasympathetic input to choroid appear to maintain high ocular blood flow during loss systemic blood pressure and may also mediate increase blood flow in response to increases retinal activity • Sympathetic input to choroid appears to prevent excessive ocular blood flow during systemic hypertension; helps auto regulation system in maintaining the intraocular flow and volumeconstant Neuronal regulation of Ocular Circulation • Impaired autonomic control of choroidal blood flow leads to retinal pathology and dysfunction • Impairments in the neuronal control of choroidal blood flow occur with aging and various ocular or systemic diseases such as glaucoma, age related macular degeneration, hypertension and diabetes Automatic Nervous System Control • Sympathetic innervation – Vasoconstriction of blood vessels – Maintaining reasonable blood flow thru the uvea (choroid, ciliary body and iris) during sudden spikes in blood pressure – Retinal blood flow is not affected (doesn't innervate the CRA past the lamina cribosa) Autonomic Nervous System Control • Parasympathetic innervation – Vasodilatation of blood vessels – Most of is effect are in the anterior uvea (iris and ciliary body) – Minimal contribution to the choroidal and retinal blood flow Autoregulation of blood flow • Blood flow through the retina and optic nerve can be maintained at a constant rate despite moderate variations in mean arterial pressure and intraocular pressure because of autoregulation Autoregulation blood flow • Acute angle closure attack The high intraocular pressure will infringe on the CRA reduction in blood supply to the retinal tissue retinal vessels increase their vessel diameter to allow more blood flow (autoregulation) Autoregulation of blood flow • IOP is high enough for a long period of time CRA will obtains its critical closing pressure and shut down – Critical closing pressure : pressure at which a blood vessel collapses, and blood flow stops • Acute angle closure attack – IOP increases reduction in the arterial pressure entering the retina – If IOP is elevated for a long enough period of time CRAO (central retinal artery occlusion) occur perfusion pressure hypoxia retina – CRAO is the primary threat to vision loss in an acute angle closure attack CRAO • Obstruction of the central retinal artery results in inner layer edema • Ischemic necrosis results in retina opacification and yellow-white in appearance • The foveola assumes a cherry-red spot (foveolar retina is nourished by the choriocapillaris) Ocular Veins • Pressure in the ocular veins (outside the eye) are lower than the IOP – to drain into the trabecular meshwork and into the venous system • IOP is lower than the perfusion pressure of the retinal and uveal arteries – allow nutrients to be delivered form the choriocapillaris to the retina Uveal blood flow • Fenestrated capillaries – are capillaries that contain pores that span the endothelial lining – The pores permit the rapid exchange of water and solutes Uveal blood flow • Choroid – The majority of blood flow in the ocular vessels is always in the choriocapillaris (60%) – Capillaries have huge fenestration • provide the outer retina with oxygen, glucose, vitamin A, etc. • Ciliary Body – Major arterial circle of the Iris = ACA + LPCA – Fenestrated capillaries Uveal blood flow • Iris – Minor arterial circle of the iris – Blood flows form the major circle to the minor circle to the pupillary margin and then back again – Iris and retinal capillaries are non-fenestrated Uveal blood flow • Cilioretinal Artery – Arise either form the vessels entering the choroid or from the circle of Zinn – Arise form the ciliary circulation and not from the retinal supply – Occur 15 – 50% of the population – Supply the macular area – ORA direct blood supply to the macular area will be maintained in those individuals with such a cilioretinal artery Retinal blood flow • Dual blood supply – Central Retinal Artery = inner retina – Choriocapillaris (choroid) = outer retina • Blood retinal barrier – Barrier formed by tight junctions between endothelial cells lining the retinal vessels as well as between RPE cells • keeping blood of the choriocapillaris out of the retina, preventing damage cause blood is toxic to the retina RPE= retinal pigment epithelium Blood Retinal Barrier • Diabetic Retinopathy – Occurs because of damage to the bloodretinal barrier – Pericytes (contractile cells) are lost, and retinal capillary basement membranes become damaged Retinal Blood Supply • Vessel walls are transparent – column of blood within the vessel can be seeing • Lighter-colored blood is the oxygenated blood of the artery • Venous deoxygenated blood is slightly darker • The artery generally lies superficial to the vein Retinal Blood Supply • Aging and diseases like hypertension – the arterial wall may thicken and constrict the vein (arterio-venous nicking) Arterio-venous nicking – the rigid arteriole compresses the vein Central Retinal Vein Occlusion • Central retinal artery and vein share a common adventitial sheath as they exit the optic nerve head and pass through a narrow opening in the lamina cribosa • Because of this narrow entry the vessels are in a tight compartment with limited space for displacement • This anatomical position predisposes to thrombus formation in the central retinal vein by various factors, including slowing of the blood stream, changes in the vessel wall, and changes in the blood Central Retinal Vein Occlusion • CRVO is a common retinal vascular disorder • Clinically – variable visual loss – the fundus may show retinal hemorrhages, dilated tortuous retinal veins, cotton-wool spots, macular edema, and optic disc edema Branch Retinal Vein Occlusion • The branches of the CRA and vein are joined in a common connective tissue sheath at the point where the vessels cross each other • Generally artery crosses the vein • Some diseases as arteriosclerosis may compress the vein causing first a deflection of the vessel and progress later to a venous occlusion • Restriction of flow results in retinal edema and hemorrhage in the are surrounding the occlusion Branch Retinal Arterial Occlusion • One of these branches of the CRA becomes occluded • Most commonly secondary to an embolus • The most common include cholesterol emboli • BRAO is most likely to occur at the bifurcation of an artery because bifurcation sites are associated with a narrowed lumen • In 90% of cases, branch retinal artery occlusion involve the temporal retinal vessels BRAO • Ischemia of the inner layers of the retina leads to intracellular edema as a result of cellular injury and necrosis • This intracellular edema has the ophthalmoscopic appearance of grayish whitening of the superficial retina Anterior Ischemic Neuropathy • AION results from nonperfusion or hypoperfusion of the ciliary blood supply to the optic nerve head by the circle of Zinn Anterior Ciliary Artery • Inflammations generate an increase of the blood supply to the affected areas, causing hyperemia Anterior Ciliary Arteries • Conjunctivitis – the superficial blood vessels are injected, giving the conjunctiva a bright-red color that often increases toward the fornix – The vessels move with conjunctival movement and can be blanched with a topical vasoconstrictor Anterior Ciliary Arteries • Uveitis – The deeper scleral and episcleral vessels are injected, giving the circumlimbal area a purplish or rose-pink color – These vessels do not move with the conjunctiva and are not blanched with a topical vasoconstrictor