Choroidea

Choroidea, commonly referred to as the choroid, is the vascular middle layer of the eye and a major component of the uveal tract, situated between the sclera and the retina. It is a dark, pigmented membrane that provides most of the blood supply to the outer retina and helps absorb stray light to reduce reflection.

Anatomy and structure: The choroidea comprises a vascular stroma and a dense choriocapillaris network, with Bruch's membrane forming an innermost boundary adjacent to the retinal pigment epithelium. It lies external to Bruch's membrane and internal to the sclera, with the suprachoroidal space lying between it and the sclera. Melanocytes within the choroid contribute to its color in many species. The arterial supply comes primarily from the short and long posterior ciliary arteries, and venous drainage is via vortex veins.

Function: The choroidea primarily nourishes the outer retinal layers, including the photoreceptors, through the choriocapillaris. It also contributes to thermal regulation of ocular tissues, light absorption to minimize glare, and immune surveillance within the eye.

Clinical relevance: Diseases of the choroidea include choroiditis (inflammation of the choroid), choroidal neovascularization (often associated with age-related macular degeneration or lossy myopia), and choroidal tumors such as choroidal melanoma or nevus. Conditions like serous choroidal detachments and various inflammatory or neoplastic processes can affect choroidal thickness and perfusion. Diagnosis and assessment employ imaging techniques such as fluorescein and indocyanine green angiography, optical coherence tomography, and ultrasound.