The choroid comprises the largest part of the middle eye skin and is located between the retina and the sclera. The main function of the skin, which is rich in small and large blood vessels, is to supply the eye, especially the retina, with blood and oxygen. Typical diseases of the choroid include inflammation of various kinds, mechanical injury from accidents, or choroidal melanoma.
What is the choroid?
The choroid has the medical term choroid or chorioidea. Together with the iris and the ciliary or ray body (corpus ciliare), it forms the middle skin of the eye (tunica media bulbi or uvea), making up the largest portion of this structure. The choroid lies directly adjacent to the sclera inside the eye and has a dark, brown-black pigmentation. It is the middle layer between the sclera and the retina and encloses almost the entire vitreous body except for a small portion in the anterior part of the eye. The choroid gets its name because it is crisscrossed by numerous small vessels that make it the best perfused structure in the entire body.
Anatomy and structure
In humans, the choroid of the eye is composed of four different layers: On the very outside is the lamina suprachoroidea, which is composed of pigmented connective tissue. The lamina vasculosa has a similar structure, in which the connective tissue is traversed by the larger arterial and venous blood vessels of the choroid. In contrast, an extensive network of very fine capillary vessels traverses the deeper layer of the choroid, the lamina choroidocapillaris, which lies toward the retina. Immediately adjacent to the pigmented layer of the retina is the lamina basalis, also known as the lamina vitra or complexus basalis. It connects directly to the retina via Bruch’s membrane, which provides nutrition to the retina. This is crucial for the nourishment of this important structure of the eye. In addition to its extensive array of blood vessels of various sizes, the choroid is composed of fibrocytes and collagen, which form the connective tissue of the skin, and melanocytes, which are the basis for pigmentation.
Function and tasks
The main function of the choroid is to supply the eye, focusing on the retina. Due to its location inside the eye, this is namely dependent on a continuous and sufficient supply of blood and also oxygen. The choroid can do this optimally because it has a dense network of larger and smaller blood vessels. These lead the arterial, oxygen-enriched blood to the retina and transport the venous blood back. Through the blood, the retina receives all the nutrients it needs to function optimally. Since the retina is in constant use every day, it needs a consistent supply of nutrients, which is why the supplying choroid is the area in the body with the most blood supply for good reason. The second important function of the choroid is derived from its many melanocytes and the resulting strong pigmentation: The black-brown protection is effective in preventing stray light from entering the interior of the eye. Among other things, stray light has the unpleasant effect of making it difficult to recognize objects that are in poor contrast to one another. This impairs vision especially in twilight and at night, especially when oncoming traffic adds an additional glare effect when driving a car. Thus, the pigmented choroid has a protective function with far-reaching effects.
Diseases
A typical disease of the posterior region of the eye, where much of the choroid is located, is inflammation. If the inflammatory reaction affects only the choroid, it is called choroiditis; if the choroid and retina are affected at the same time, it is chorioretinitis. Often these inflammations develop on the basis of another disease, such as toxoplasmosis, but a bacterial cause must always be considered. Inflammation of the choroid and retina may or may not cause symptoms. Characteristic are visual disturbances that can lead to complete blindness – namely when the retina can no longer fulfill its function due to degenerative changes.Uveitis is the inflammation of the complete middle eye skin, which can also spread to the retina as well as to the vitreous body. Possible signs, which also depend on the respective localization of the inflammation in the anterior or posterior eye area, include blurred vision, watery eye, foreign body sensation and sensitivity to light. Again, the causes may be systemic disease or bacterial invasion. In children, the condition is not uncommonly associated with rheumatism. In addition to inflammation, the choroid may also be affected by trauma and may have bruising, for example. A malignant disease with a probability of one affected person per 100,000 people per year is choroidal melanoma, which occurs due to a degeneration of the melanocytes of the skin. The disease can be detected with ultrasound examination or fluorescence angiography. It is a serious disease as it tends to form metastases and is often fatal in these cases. Treatment options include surgery as well as laser therapy and, in combination, radiation therapy.
