Choroid

The choroid is the vascular layer of the eye that lies between the sclera and the retina. It is part of the uveal tract, along with the iris and ciliary body, and provides most of the blood supply to the outer retina.

Anatomy: The choroid consists of three vascular layers: the choriocapillaris, a dense network of tiny capillaries adjacent to the retinal pigment epithelium; Sattler's layer of medium-sized vessels; and Haller's layer of larger vessels. The stroma contains melanocytes and connective tissue. The inner surface is in contact with Bruch's membrane, and the outer border abuts the sclera.

Blood supply and drainage: The choroid receives arterial blood from the posterior ciliary arteries, branches of the ophthalmic artery, and drains via vortex veins that exit the eye through the sclera. It is characterized by a high blood flow compared with other ocular tissues.

Function: The choroid provides oxygen and nutrients to the outer retina and the retinal pigment epithelium, helps remove metabolic waste, absorbs excess light to reduce scattering, and participates in ocular heat regulation.

Clinical relevance: Disorders include choroiditis (inflammation), choroidal neovascularization associated with age-related macular degeneration, central serous chorioretinopathy, and neoplasms such as choroidal melanoma. Imaging and examination may assess choroidal thickness and vascular flow.

Imaging and assessment: Ophthalmoscopy, fluorescein and indocyanine green angiography, and optical coherence tomography (including enhanced depth imaging) are used to visualize the choroid.