Choroid

Choroidea

Also known as: choriocapillaris, Bruch membrane

The thin, highly vascular and pigmented posterior part of the uvea, between sclera and retina, whose capillary layer (choriocapillaris) nourishes the photoreceptors and retinal pigment epithelium.

Overview

Thickness≈0.2–0.3 mm (subfoveal)

Blood flowHighest per gram of any tissue

Vortex veinsUsually 4 (4–6)

The choroid extends from the optic disc to the ora serrata and is ≈0.2–0.3 mm thick (thickest beneath the macula). From outside inwards it comprises: the suprachoroid (lamina fusca) — pigmented connective-tissue lamellae bridging to the sclera; the vascular stroma, with an outer layer of large vessels (Haller layer) and an inner layer of medium vessels (Sattler layer); the choriocapillaris — a single layer of wide (≈20 µm), fenestrated capillaries arranged in lobules; and Bruch membrane, a five-layered basal lamina complex separating the choriocapillaris from the retinal pigment epithelium.

Choroidal blood flow is enormous (the highest per gram of any tissue) and its venous blood remains highly oxygenated, so it can supply the very high metabolic demand of the photoreceptors — the outer one-third of the retina, including the avascular fovea, depends entirely on it. Flow is regulated mainly by sympathetic vasoconstriction and parasympathetic (pterygopalatine) vasodilation rather than by local autoregulation; it also cools the retina.

The choroid's melanocytes absorb light that has passed through the retina. The orange-red fundus reflex comes from choroidal blood seen through the RPE; in light-pigmented fundi the choroidal vessels are visible ("tigroid" fundus).

Location

Between the sclera (outside) and the retinal pigment epithelium (inside), from the optic disc to the ora serrata.

Parts & subdivisions

Suprachoroid (lamina fusca)

Loose pigmented layer; potential suprachoroidal space through which uveoscleral outflow passes.

Haller and Sattler layers

Outer large-vessel and inner medium-vessel layers of the choroidal stroma.

Choriocapillaris

Fenestrated capillary bed supplying the RPE and photoreceptors.

Bruch membrane

Elastic and collagenous basement-membrane complex; drusen accumulate beneath the RPE on it in AMD; breaks allow choroidal neovascularisation.

Anatomy

Innervation

Autonomic: sympathetic (vasoconstriction) and parasympathetic from the pterygopalatine ganglion (vasodilation); sensory from short ciliary nerves.

Arterial supply

Short posterior ciliary arteries (≈15–20 branches piercing the sclera around the optic nerve) and recurrent branches of the long posterior and anterior ciliary arteries.

Venous drainage

Vortex veins (4–6) → superior and inferior ophthalmic veins.

Histology

Loose connective tissue rich in melanocytes, fibroblasts, mast cells and vessels; fenestrated choriocapillaris; Bruch membrane.

Clinical significance

Choroidal neovascularisation

New vessels from the choriocapillaris grow through breaks in Bruch membrane under the RPE/retina — the hallmark of wet AMD and also seen in high myopia and ocular histoplasmosis; treated with anti-VEGF injections.

Choroidal melanoma

The most common primary intraocular tumour in adults; may present with visual field loss or be found on routine fundoscopy as a pigmented, dome- or mushroom-shaped mass. Metastasises to the liver.

Cherry-red spot

In central retinal artery occlusion the ischaemic inner retina becomes opaque white, but the thin foveola lets the intact choroidal circulation show through as a cherry-red spot (also seen in Tay–Sachs disease).

Test yourself

Which feature best distinguishes wet from dry age-related macular degeneration?

Presence of drusen
Loss of peripheral vision
Raised intraocular pressure
Choroidal neovascularisation through Bruch membrane

Answer: Choroidal neovascularisation through Bruch membrane

Wet (neovascular) AMD features choroidal new vessels breaching Bruch membrane, leaking fluid and blood and causing rapid distortion and central loss; it responds to anti-VEGF injections. Drusen and geographic atrophy characterise dry AMD; peripheral vision is preserved in both.