Choroid Overview and Disorders

The choroid is the vascular layer of the eye that lies between the retina and the sclera. The choroid is thickest in the back of the eye, where it is about 0.2 mm, and narrows to 0.1 mm in the peripheral part of the eye. It contains the retinal pigmented epithelial cells and provides oxygen and nourishment to the outer retina. The choroid forms the uveal tract, which includes the iris and the ciliary body.

The choroid is made of four different layers:

• Haller's layer (large blood vessel layer)
• Sattler's layer (medium-size blood vessels)
• Choriocapillaris (capillaries)
• Bruch's membrane (membrane on the innermost part of the choroid)

The dark-colored melanin pigment in the choroid absorbs light and limits reflections within the eye that could degrade vision. The melanin is also thought to protect the choroidal blood vessels against light toxicity. The choroidal pigment is what causes "red eyes" when flash photographs are taken.

Besides providing a major supply of oxygen and blood supply to the retina, the choroidal blood flow may cool and warm the retina. The choroid also has cells that secrete substances that are thought to be involved in the growth of the sclera. The choroid can also change thickness, and these changes can move the retina forward and back, bringing the photoreceptors into the plane of focus. This does not produce much of a focusing effect but is more important in eye growth in children.

Your eyes and good vision rely upon a sufficient blood supply to function. Therefore, the choroid must remain healthy. When the area of the eye involving the choroid is infected in any way, the macula and optic nerve may suffer. When the macula and optic nerve are compromised or negatively impacted, the result is often a severe decrease in vision and sometimes even total blindness. In addition, many other serious illnesses can occur as a result of an infection.

Diseases and Disorders of the Choroid

• Hemorrhagic choroidal detachment is a hemorrhage in the choroidal space caused by the rupture of choroidal vessels. Although it can occur spontaneously, it is extremely rare. It usually occurs as a consequence of eye trauma. It can also occur rarely during eye surgery. A hemorrhagic choroidal detachment can produce profound symptoms. Treatment consists of topical steroid eye drops, cycloplegic eye drops, and eye-pressure-lowering eye drops. Ultimately, depending on the severity of the detachment, surgery may be recommended. 
• Choroidal rupture is a complete break in the choroid, Bruch's membrane, and the retinal pigment epithelium that occurs as a result of blunt eye trauma, such as getting hit with a fist. Unfortunately, many choroidal ruptures involve the center of the retina, called the macula. The macula allows us to have high quality, central vision. The injury leads to a loss of the photoreceptors in the macula and loss of central vision. If the rupture is not in the macula, central vision is retained.
• Choroidal nevi are a collection of pigmented or non-pigmented cells in the choroid, the vascular layer under the retina. Most choroidal nevi only need to be monitored. Your eye doctor will photograph the area of concern and check it frequently. Most do not need any treatment. If the choroidal nevus has orange pigmentation, appears elevated, or has an unusual shape, it is possible that it could become a malignant choroidal melanoma. In this case, aggressive treatment is needed.
• Choroidal dystrophies are a group of inherited diseases that affect the choroid. Choroideremia, gyrate atrophy, central areolar choroidal dystrophy, diffuse choroidal atrophy, and pigmented paravenous retinochoroidal atrophy are examples of choroidal dystrophies. Severe vision loss can occur in some of these dystrophies.
• Chorioretinitis is the most common disease that attacks the choroid. This type of inflammation often produces floating dark spots and blurry vision. Chorioretinitis can be associated with a condition called uveitis. Inflammation to the choroid can be due to an infectious cause or an autoimmune-related inflammatory condition. Different types of treatments are recommended depending on the cause.