The uvea, also known as the uveal tract, represents the middle, pigmented layer of the eyeball. It is positioned between the outer protective layer, composed of the sclera (the white of the eye) and cornea, and the inner light-sensitive retina. This layer is responsible for controlling the amount of light entering the eye and supplying blood flow to other ocular structures.
Anatomy and Function of the Uvea
The Iris
The iris is the colored part of the eye, which is a thin, circular structure composed of connective tissue and muscles. It is the most visible component of the uvea, and its color is determined by the amount of melanin pigment present. The iris surrounds the pupil, which is the small, black opening in the center of the eye.
The primary function of the iris is to control the size of the pupil, thereby regulating the amount of light that enters the eye and reaches the retina. It acts like the aperture of a camera, automatically adjusting to changes in light conditions. In bright light, muscles within the iris constrict the pupil to limit light entry, while in dim light, other muscles dilate the pupil to allow more light in.
The Ciliary Body
Located behind the iris and connecting it to the choroid, the ciliary body is a ring-shaped structure that cannot be seen externally. It contains the ciliary muscle, which plays a role in focusing the eye. This muscle is connected to the eye’s lens by tiny fibers called zonular fibers.
The ciliary body performs two main functions. It produces aqueous humor, a clear fluid that fills the front part of the eye, providing nutrients to the lens and cornea and helping maintain intraocular pressure. Additionally, the ciliary muscle controls the shape of the lens, a process known as accommodation, which allows the eye to focus on objects at different distances.
The Choroid
The choroid is a thin, pigmented layer of vascular connective tissue that lines the back of the eye, situated between the sclera and the retina. This layer is exceptionally rich in blood vessels, making it one of the most highly vascularized tissues in the body.
The main function of the choroid is to supply oxygen and nutrients to the outer layers of the retina, which have a high metabolic demand. It also plays a role in light absorption, with its high melanin content helping to reduce reflections within the eye and improve the clarity of the retinal image. Furthermore, the choroid helps regulate the temperature of the retina and modulates intraocular pressure.
Inflammatory Conditions of the Uvea
Uveitis refers to inflammation of the uvea, the middle layer of the eye. It is not a singular disease but rather a group of inflammatory conditions that can affect one or both eyes. When left untreated, uveitis can lead to complications such as cataracts, glaucoma, and potentially permanent vision loss.
Uveitis is categorized based on the specific part of the uvea affected. Anterior uveitis, also known as iritis, is the most common type and involves inflammation of the iris and the front part of the ciliary body. Symptoms often appear suddenly and can include eye pain, redness, blurred vision, and sensitivity to light. The pupil may also appear constricted or irregularly shaped.
Intermediate uveitis primarily impacts the vitreous humor and the peripheral retina, and it may also affect the blood vessels in the retina. Individuals with this type often experience blurred vision and the presence of floaters, which are small dark spots or lines that drift across the field of vision.
Posterior uveitis involves inflammation of the choroid and/or the retina, located at the back of the eye. This type can significantly affect vision and often presents with symptoms like floaters, reduced visual sharpness, and sometimes flashing lights.
When inflammation extends throughout the entire uveal tract, affecting the iris, ciliary body, and choroid, it is classified as panuveitis. This widespread inflammation can cause a combination of symptoms seen in other types, including eye irritation, pain, redness, and significant vision changes. Panuveitis can lead to long-term damage and vision impairment.
The causes of uveitis are diverse and can include autoimmune disorders such as rheumatoid arthritis, lupus, sarcoidosis, and Behcet’s disease. Infections from viruses like herpes simplex and varicella-zoster, bacteria such as those causing syphilis and tuberculosis, or fungi can also trigger uveitis. In many instances, however, a specific cause cannot be identified.
Treatment for uveitis typically focuses on reducing inflammation and preventing complications. Corticosteroid eye drops are a common initial treatment for anterior uveitis to help control redness, pain, and swelling. For more severe or posterior forms, oral steroids, injections into or around the eye, or immunosuppressive medications may be prescribed. Dilating eye drops can also be used to relieve pain and prevent iris adhesions.
Uveal Growths and Tumors
The uveal tract can develop various growths, ranging from benign to malignant. A common benign growth is a uveal nevus, often referred to as an eye freckle or mole. These pigmented lesions are typically harmless and found during routine eye examinations. While most nevi remain stable, some may have features that suggest a higher risk of transforming into melanoma, such as larger size or elevation.
Uveal melanoma is the most common primary eye cancer in adults. This malignancy originates in the melanin-producing cells within the uvea, specifically in the choroid, ciliary body, or iris. Choroidal melanomas are the most frequent, while ciliary body and iris melanomas are less common.
The exact cause of uveal melanoma is not fully understood, but several risk factors have been identified. Individuals with light skin and eye color (blue or green eyes) have a greater risk. Certain genetic predispositions can also increase susceptibility. While the link to UV light exposure is less clear than for skin melanoma, it may play a role. Age is another factor, with most cases diagnosed in individuals aged 60 and older.
Uveal melanoma often presents without noticeable symptoms in its early stages, making routine dilated eye exams important for detection. As the tumor grows, symptoms may emerge, including blurred or distorted vision, the appearance of new dark spots on the iris, or changes in pupil shape. Patients might also experience floaters or flashes of light in their vision.
Diagnosis of uveal melanoma typically involves a comprehensive eye examination, including a dilated pupil exam and specialized imaging techniques. An ophthalmologist looks for characteristics like tumor size, elevation, the presence of subretinal fluid, or orange pigment on the lesion’s surface, which can indicate malignancy. In rare cases, a biopsy may be performed for confirmation.
Treatment for uveal melanoma depends on factors such as the tumor’s size, location, and whether it has spread. Common approaches include radiation therapy, which involves placing a radioactive disc on the eye to deliver targeted radiation. Laser therapies may also be used. In some instances, surgical removal of the tumor (resection) or, for larger tumors or those causing significant vision loss, removal of the entire eye (enucleation) may be necessary. Immunotherapy drugs are considered if the melanoma has spread to other parts of the body.
Maintaining Uveal Health
Proactive measures are important for preserving the health of the uveal tract and overall vision. Regular comprehensive, dilated eye exams are important. These exams allow eye care professionals to thoroughly inspect the retina, optic nerve, and other structures at the back of the eye, which can reveal subtle signs of conditions like uveal melanoma or inflammation before symptoms become apparent. Early detection makes many eye conditions more manageable and treatable, helping to preserve vision.
Protecting your eyes from ultraviolet (UV) radiation is another important step. Wearing sunglasses that block 100% of both UV-A and UV-B rays is advised whenever outdoors, even on cloudy days. A wide-brimmed hat can provide additional protection by blocking about half of UV rays from above and around the eyes. This practice helps reduce the long-term risk of various eye diseases.
Overall systemic health significantly impacts eye health. Conditions like diabetes and high blood pressure can affect the blood vessels in the retina, potentially leading to vision problems. Autoimmune disorders can also cause inflammation in the eyes, including uveitis. Managing these systemic conditions through proper medical care can contribute to maintaining healthy eyes and preventing complications.