The ocular surface is supplied by two vascular plexuses: the conjunctival vessels (arising from the anterior ciliary arteries and the palpebral arcades) supplying the conjunctival epithelium and stroma, and the deeper episcleral vessels (from the anterior ciliary arteries) supplying the superficial sclera. The limbal vascular arcade — the terminal anastomosis of the conjunctival and episcleral plexuses at the corneoscleral junction — is the source of corneal neovascularisation and the nutritional supply to the peripheral cornea.
The pattern of vascular injection distinguishes episcleritis from scleritis: episcleritis produces superficial, movable, salmon-pink conjunctival vessel injection that blanches with topical phenylephrine; scleritis produces deep, immovable, violaceous episcleral injection that does not blanch with phenylephrine. The episcleral vessels are the anastomotic connection used in aqueous outflow in non-penetrating glaucoma surgery. Conjunctival vessels are the target of anti-VEGF injections for conjunctival neovascularisation and pterygium vessels.
The phenylephrine blanching test distinguishes scleritis from episcleritis: topical 2.5% phenylephrine blanches the superficial conjunctival vessels in episcleritis but does not affect the deep episcleral vessels of true scleritis, allowing bedside differentiation of these conditions with markedly different systemic association profiles and treatments.
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