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Choroid of the eye: Anatomy and function

eye anatomy illustration of the pupil, iris, ciliary body and choroid

The choroid is the middle layer of tissue in the wall of the eye. It’s found between the sclera (the whites of the eyes) and the retina (the light-sensitive tissue in the back of the eye).

This thin layer of tissue is made up almost entirely of blood vessels. These blood vessels supply oxygen and nutrients to the outer part of the retina. In short, the choroid is the life source that keeps the retina healthy and functioning.

Choroid anatomy

The choroid is part of the uvea, which also consists of the iris and the ciliary body. The iris and ciliary body are located in the front of the eye and work together to dilate and constrict the pupils.

The choroid layer begins in the peripheral edges of the eyeball and lines the entire back of it, sandwiched between the sclera and the retina. The thickness of the choroid varies depending on what part of the eye it’s lining. For example, it’s the thickest in the back of the eye (approximately 0.2 mm) and narrows to approximately 0.1mm as it gets to the peripheral part of the eyeball. 

There are four different layers of the choroid:

  • Bruch’s membrane – Thin layer of tissue located on the innermost part of the choroid.

  • Choriocapillaris – Layer made up of capillaries (tiny blood vessels that connect arteries to veins).

  • Sattler’s layer – Layer of medium blood vessels.

  • Haller’s layer – Outermost layer of the choroid that contains large blood vessels.

Choroid function

The eyes and clear vision depend on sufficient blood supply in order to operate. Choroidal circulation accounts for 85% of blood flow within the eye, making it a vital structure to the function of your eyes.

Other important functions of the choroid include:

  • Providing nutrients for the retina, macula and optic nerve.

  • Regulating the temperature of the retina.

  • Helping control pressure within the eye.

  • Absorbing light and limiting reflections within the eye that could harm vision. This part of the choroid is what causes “red eyes” when a photograph is taken using the flash unit of a camera.

Conditions that may affect the choroid

Choroid eye conditions can develop sporadically, genetically or as a result of eye trauma. Each of the following conditions can be evaluated by an eye doctor, who will then determine and prescribe proper treatment:

Choroidal detachment and hemorrhage – Separation of the choroid from the sclera; this may happen as a result of low eye pressure (serous choroidopathy, which is fluid-filled) or high eye pressure (hemorrhagic choroidopathy, which is blood-filled).

Choroidal rupture – A tear in the choroid, Bruch’s membrane and the retinal pigment epithelium (RPE) that result from an eye injury.

Choroid plexus papilloma – Rare, benign brain tumor that develops in the choroid plexus (tissue that makes cerebrospinal fluid).

Chorioretinitis – Inflammation of the choroid caused by infection or an autoimmune disease.

Choroideremia – A hereditary, progressive deterioration of the choroid; this condition primarily affects men. 

Importance of routine eye exams

While the choroid is an important part of the eye, it’s difficult to identify any problems with it unless you have an eye exam. During a fundoscopic eye exam, your eye doctor will use a tool that allows them to view the back of the eye, where the choroid is located.

Using this method, they can determine the health of the choroid and prescribe treatment if they detect a problem.

READ MORE: Retinopathy

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