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Anatomy of the Ocular Blood Supply
Anatomy
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11. Clinical Analogy & Overview
CLINICAL ANALOGY
- Think of the ocular circulation as a city's water supply system.
- The ophthalmic artery is the main pipeline entering the city from the internal carotid reservoir.
- It gives rise to multiple branches, including the central retinal artery and the ciliary arteries.
- The central retinal artery (CRA) is the inner city grid - an end-artery with no collaterals.
- The ciliary system is the outer suburbs - it supplies the choroid and contributes to the anterior segment circulation.
- The retina is doubly dependent - its inner layers are fed by the central retinal artery (CRA), and its outer layers are fed by the choroid.
- Obstruct one supply and the other cannot fully compensate.
Hierarchy of Ocular Blood Supply
- The eye is supplied by the ophthalmic artery (OA), the first major branch of the internal carotid artery (ICA).
- Occasionally, the ophthalmic artery (OA) arises from the middle meningeal artery (external carotid territory) - an important surgical and embryological variant.
- The ophthalmic artery (OA) enters the orbit through the optic canal, initially inferolateral to the optic nerve, then crosses superiorly (in ~85%) or inferiorly (~15%) to run medially.
| Branch | Territory Supplied | Clinical Significance |
|---|---|---|
| Central Retinal Artery (CRA) | Inner retina (nerve fibre layer to inner nuclear layer) | • End-artery • Central retinal artery occlusion (CRAO) → acute retinal ischaemia |
| Short Posterior Ciliary Arteries (Short PCAs × 6–12) | Choroid, retinal pigment epithelium, outer retina | • Watershed zone • Non-arteritic anterior ischaemic optic neuropathy (AION) with hypotension |
| Long Posterior Ciliary Arteries (LPCAs) × 2 | Anterior uvea, iris, ciliary body | • Anterior segment ischaemia (ASI) if disrupted |
| Anterior Ciliary Arteries (ACAs) × 7 | Anterior uvea via rectus muscles | • Disrupted in strabismus surgery → anterior segment ischaemia (ASI) |
| Lacrimal Artery | Lacrimal gland, lateral eyelids | • Gives recurrent meningeal branch |
| Supraorbital & Supratrochlear | Forehead, upper eyelid | • Potential collateral channels via anastomoses with external carotid branches |
| Medial Palpebral Arteries | Eyelids; contribute to palpebral arcades | • Contribute to the marginal and peripheral arcades with the lateral palpebral arteries |
| Muscular branches | Extraocular muscles | • Source of anterior ciliary arteries (ACAs) |
| Branch | Territory Supplied | Clinical Significance |
|---|---|---|
| Central Retinal Artery (CRA) | Inner retina (nerve fibre layer to inner nuclear layer) | • End-artery • Central retinal artery occlusion (CRAO) → acute retinal ischaemia |
| Short Posterior Ciliary Arteries (Short PCAs × 6–12) | Choroid, retinal pigment epithelium, outer retina | • Watershed zone • Non-arteritic anterior ischaemic optic neuropathy (AION) with hypotension |
| Long Posterior Ciliary Arteries (LPCAs) × 2 | Anterior uvea, iris, ciliary body | • Anterior segment ischaemia (ASI) if disrupted |
| Anterior Ciliary Arteries (ACAs) × 7 | Anterior uvea via rectus muscles | • Disrupted in strabismus surgery → anterior segment ischaemia (ASI) |
| Lacrimal Artery | Lacrimal gland, lateral eyelids | • Gives recurrent meningeal branch |
| Supraorbital & Supratrochlear | Forehead, upper eyelid | • Potential collateral channels via anastomoses with external carotid branches |
| Medial Palpebral Arteries | Eyelids; contribute to palpebral arcades | • Contribute to the marginal and peripheral arcades with the lateral palpebral arteries |
| Muscular branches | Extraocular muscles | • Source of anterior ciliary arteries (ACAs) |
2. Central Retinal Artery (Cra)
- The central retinal artery branches from the ophthalmic artery in the orbit, ~10 mm behind the globe.
- It pierces the optic nerve sheath to travel within the nerve.
- It enters the eye at the optic disc.
- It divides into superior and inferior trunks, each subdividing into nasal and temporal branches - giving 4 main branches.
- Each is a functional end-artery with no overlap or anastomoses between them.
HIGH-YIELD PEARL: CILIORETINAL ARTERY
- Cilioretinal artery: Present in 15–30% of individuals.
- Arises from the short posterior ciliary circulation (not central retinal artery).
- It supplies the papillomacular bundle and a portion of the macula.
- Critical pearl: In central retinal artery occlusion, presence of a cilioretinal artery can preserve central vision.
- Conversely, in central retinal vein occlusion, isolated cilioretinal artery occlusion can occur (paradoxically impaired by raised venous pressure).
- In central retinal vein occlusion, cilioretinal artery occlusion can cause marked visual impairment despite a patent central retinal artery, particularly when the entire parafoveal capillary network is affected.
Retinal Microvasculature
The retinal vasculature has unique structural features that are important for examination:
| Feature | Retinal Vessels | Choroidal Vessels |
|---|---|---|
| Wall structure | Inner blood-retinal barrier • Tight junctions • No fenestrations | Fenestrated choriocapillaris • Retinal pigment epithelium forms the outer blood-retinal barrier |
| Autoregulation | Present • Myogenic • Metabolic | Limited/absent |
| Pericyte ratio | 1:1 pericyte:endothelial cell • Highest in body | Lower pericyte density |
| Oxygen extraction | ~35–40% • High oxygen extraction | ~3–4% • Very high flow • Low extraction |
| Avascular zones | Foveal avascular zone (~500 µm) • Fovea • Retinal nerve fibre bundles supplied by radial peripapillary capillaries | Dense, continuous choriocapillaris network throughout the choroid • Intercapillary spaces remain |
| Autoregulatory range | Wide autoregulatory range of ocular perfusion pressures | Limited autoregulation • Largely pressure-passive |
| Zones fed | Inner retina • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer • Deep retinal plexus at the inner nuclear layer–outer plexiform layer interface | Outer retina • Outer nuclear layer • Photoreceptor inner segment/outer segment region • Retinal pigment epithelium • All choroid |
| Feature | Retinal Vessels | Choroidal Vessels |
|---|---|---|
| Wall structure | Inner blood-retinal barrier • Tight junctions • No fenestrations | Fenestrated choriocapillaris • Retinal pigment epithelium forms the outer blood-retinal barrier |
| Autoregulation | Present • Myogenic • Metabolic | Limited/absent |
| Pericyte ratio | 1:1 pericyte:endothelial cell • Highest in body | Lower pericyte density |
| Oxygen extraction | ~35–40% • High oxygen extraction | ~3–4% • Very high flow • Low extraction |
| Avascular zones | Foveal avascular zone (~500 µm) • Fovea • Retinal nerve fibre bundles supplied by radial peripapillary capillaries | Dense, continuous choriocapillaris network throughout the choroid • Intercapillary spaces remain |
| Autoregulatory range | Wide autoregulatory range of ocular perfusion pressures | Limited autoregulation • Largely pressure-passive |
| Zones fed | Inner retina • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer • Deep retinal plexus at the inner nuclear layer–outer plexiform layer interface | Outer retina • Outer nuclear layer • Photoreceptor inner segment/outer segment region • Retinal pigment epithelium • All choroid |
Blood-Retinal Barrier (BRB)
- • Inner blood-retinal barrier (BRB): Tight junctions (zonulae occludentes) between retinal capillary endothelial cells equivalent to central nervous system (CNS) blood-brain barrier.
- Disruption → diabetic macular oedema, branch retinal vein occlusion, cystoid macular oedema.
- Outer blood-retinal barrier (BRB): Tight junctions of retinal pigment epithelium (RPE) cells; the retinal pigment epithelium (RPE) rests on Bruch's membrane.
- Disruption → choroidal neovascularisation, retinal pigment epithelium (RPE) detachment, central serous retinopathy (CSR).
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Keep reading Anatomy of the Ocular Blood Supply
10 more sections are included with the Clinical plan.
- Choroidal Vasculature
- Anterior Segment Blood Supply
- Venous Drainage of the Eye
- Optic Nerve Head Blood Supply
- Ocular Autoregulation
- Clinical Correlations & Vascular Syndromes