The Choroid - Summary Notes

The Choroid: General Information

• Posterior portion of the uveal tract (iris, ciliary body, choroid).

• Pigmented vascular layer lining the posterior globe.

• Extends from the optic nerve to the ora serrata.

• Composed of blood vessels in loose connective tissue (collagen, elastic, reticular fibres, non-vascular smooth muscle, melanocytes, macrophages, mast cells, lymphocytes & plasma cells).

• Extensive nerve supply via short posterior ciliary nerves (sympathetic, parasympathetic, and sensory trigeminal fibres).

• Thickness varies:

  • 0.30 \pm 0.1 mm posteriorly to

  • 0.1 mm anteriorly.

• Becomes thinner with age and increasing myopia.

• Easily detached from sensory retina & sclera, but firmly bound at:

  • Optic nerve head

  • Entry points of ciliary arteries and nerves & vortex veins

Functions of the Choroid

Blood Supply

• Main arterial supply from the ophthalmic artery via:

  • Long posterior ciliary arteries

  • Short posterior ciliary arteries

  • Anterior ciliary arteries

• Choroidal arteries divide to form the inner choriocapillaris.

  • Fenestrated endothelium allows plasma leakage.

  • Lobular supply basis.

• Blood flow regulated by perfusion pressure and arterial vessel calibre (neural control).

  • Sympathetic stimulation: vasoconstriction.

  • Parasympathetic stimulation: vasodilation.

• High blood flow (677 mg/min), 10X cerebral blood flow.

Nerve Supply

• Innervated by long and short ciliary nerves piercing the sclera.

  • Parasympathetic innervation: pterygopalatine ganglion.

  • Sympathetic innervation: superior cervical ganglion.

• Long ciliary nerves (nasociliary branch of trigeminal nerve):

  • Sensory fibres to cornea, iris, & ciliary body.

• Short ciliary nerves (from ciliary ganglion):

  • Sensory (nasociliary), sympathetic, and parasympathetic fibers (III and VII).

Microscopic Detail - 5 Main Layers

• Suprachoroid (Epichoroid or Lamina Fusca)

• Stroma

  • Haller's layer (large vessels)

  • Sattler's layer (medium vessels)

• Choriocapillaris (layer of smaller blood vessels)

• Bruch's membrane

Suprachoroid

• 10-35 microns thick.

• Loose connective tissue (fibroblasts, collagen, elastic fibres, melanocytes, nerve fibres).

Stroma

• Open collagenous fibre structure.

• Pigmented cells, elastic fibre network & non-vascular muscle.

• Supports blood vessels of Haller & Sattler layers:

  • Haller's Layer:

  • Outer layer - larger blood vessels (100-300 µm diameter).

  • Sattler's Layer:

  • Smaller blood veins, venules, smaller arteries & arterioles.

Choriocapillaris

• Closely packed network of capillaries separated from RPE by Bruch's membrane.

• Wide bore capillaries (18-50 microns diameter), numerous at fovea and macula.

• Thicker at fovea, thinner in periphery.

• Segmental construction.

• Highly fenestrated endothelia (pores of 55-60 nm).

• Venules arise from choriocapillaris & unite eventually - vortex vein.

Bruch's Membrane

• 2-4 microns thick posteriorly, thinning to 1-2 microns anteriorly.

• No cells or blood vessels, contains collagen & elastin.

• Layers by electron microscopy:

  • Basement membrane of the RPE (0.3µ)

  • Inner collagenous zone (1.5µ)

  • Elastic layer (0.8µ)

  • Outer collagenous zone (0.7µ)

  • Basement membrane of the capillary endothelium (0.14µ)

• Anterior to ora serrata only one layer continues as the basement membrane of the ciliary epithelium

• Small age-related increase in thickness.

• Drusen: larger local increases in inner collagenous layer separating photoreceptors from choriocapillaris.

In-Vivo Choroidal Imaging

• OCT imaging allows full choroidal thickness imaging in-vivo.

• Applications in clinical studies provide insights into changes with:

  • Increasing age

  • Refractive error

  • Eye disease

Choroidal Thickness and Refractive Error

• Choroidal thickness: Hyperopia > Emmetropia > Myopia.

• Thickness decreases with increased axial length/myopia.

Choroidal Thickness and Eye Disease