The choroid is a layer of tissue situated in the back of the eye, forming a part of the uvea, which is the eye’s middle layer. It is a complex and highly vascular structure that performs several functions related to vision and eye health. Although not as commonly known as the retina or cornea, the choroid’s role is necessary for maintaining the eye’s normal operation.
Anatomy and Function of the Choroid
The choroid is a thin layer of tissue located between the retina, the light-sensitive tissue at the back of the eye, and the sclera, the white outer layer of the eyeball. It extends from the optic nerve to the edges of the eyeball. The thickness of the choroid varies, measuring approximately 0.2 mm in the back of the eye and narrowing to about 0.1 mm toward the periphery. Structurally, it is composed of four distinct layers: Bruch’s membrane, the choriocapillaris, Sattler’s layer, and Haller’s layer.
The primary function of the choroid is to supply blood, oxygen, and nutrients to the outer layers of the retina, which includes the photoreceptor cells responsible for detecting light. Choroidal circulation accounts for about 85% of the blood flow within the eye, ensuring these cells receive continuous nourishment.
Another function of the choroid is to absorb stray light that enters the eye, which prevents visual interference and blurring. This is accomplished by the pigment melanin, found in high concentrations within the choroid. The choroid also contributes to regulating the retina’s temperature, acting as a cooling system for this metabolically active tissue.
Common Conditions Affecting the Choroid
One condition affecting the choroid is choroidal neovascularization (CNV), characterized by the growth of abnormal blood vessels from the choroid into the retina. This is a hallmark of the “wet” form of age-related macular degeneration (AMD), a leading cause of vision loss in older adults. These new vessels are fragile and can leak fluid and blood, causing damage to the macula, the part of the retina responsible for sharp, central vision.
Choroiditis is another condition that involves inflammation of the choroid. This inflammation can be caused by infections from bacteria, viruses, or fungi, or it can be associated with systemic autoimmune diseases like rheumatoid arthritis or lupus. In some instances, the specific cause of choroiditis cannot be identified, in which case it is termed idiopathic.
A more serious, though rare, condition is choroidal melanoma, a type of cancer that develops from the pigment-producing cells (melanocytes) in the choroid. It is the most common primary cancer within the eye in adults. These tumors can grow and interfere with the function of the retina, and they have the potential to spread to other parts of the body, particularly the liver. The tumor can appear as a pigmented, dome-shaped lesion during an eye examination.
Symptoms and Diagnosis of Choroidal Issues
Issues with the choroid can manifest through several visual symptoms. One is distorted vision, known as metamorphopsia, where straight lines appear wavy or bent. Patients might also experience blind spots, or scotomas, in their field of vision, or notice an increase in floaters, which are dark spots that seem to drift across the line of sight. Blurred vision and sensitivity to light (photophobia) are also frequent complaints.
To diagnose problems with the choroid, an ophthalmologist will conduct a comprehensive eye examination, which includes dilating the pupils to get a clear view of the back of the eye. Two diagnostic tools are often used for a more detailed assessment. Optical Coherence Tomography (OCT) is a non-invasive imaging test that uses light waves to take cross-section pictures of the retina. This allows the doctor to see the different layers of the retina and choroid, identifying any fluid buildup or structural changes.
Another diagnostic test is fluorescein angiography. This procedure involves injecting a fluorescent dye into a vein in the arm. As the dye circulates through the blood vessels in the eye, a special camera takes a series of photographs. This technique helps to visualize blood flow and can reveal abnormal, leaking blood vessels characteristic of conditions like choroidal neovascularization.
Treatment and Management Strategies
The treatment for choroidal conditions is tailored to the specific diagnosis. For choroidal neovascularization associated with wet AMD, the standard treatment involves injections of anti-VEGF (vascular endothelial growth factor) medications into the eye. These drugs work by blocking a protein that promotes the growth of abnormal blood vessels, reducing leakage and slowing vision loss.
In cases of choroiditis, the primary goal is to reduce inflammation. This is often achieved with corticosteroid medications, which can be administered as eye drops, oral pills, or injections depending on the severity of the condition. If an infection is the cause, antibiotics, antiviral, or antifungal medications will be prescribed.
The management of choroidal melanoma depends on the size and location of the tumor. For smaller melanomas, options may include careful observation or laser therapy. Larger tumors are often treated with radiation therapy, such as plaque brachytherapy, where a small, radioactive disc is temporarily stitched to the outside of the eye over the tumor. In advanced cases, surgical removal of the eye (enucleation) may be necessary.