Cornea

The cornea is a clear layer on the outside of the eye. The average human cornea measures roughly eleven or twelve millimeters in diameter. The edges of the cornea meet with the sclera, which is the white part of the eye. The cornea plays an important role in protecting the eye and focusing light to aid in vision. Disease and injury can damage the cornea, which can impair vision. If a person's cornea becomes damaged from an injury or an illness, he or she may benefit from a corneal transplant, during which a cornea from a donor replaces the person's original cornea.

Background

The cornea is an important part of the eye. It protects the eye and aids in vision. The cornea is the outmost layer of the eye and covers the iris, the round colored part of the eye. Light reflects off surfaces and into the eye through the cornea. The curved cornea then bends the light. The cornea's bending of light helps the light correctly enter the other parts of the eye. If the cornea does not bend the light in the appropriate way, vision can be impaired.

The cornea is clear so light can enter the eye. Since it is made up of clear tissue, the cornea does not have blood vessels feeding it nutrients like other tissue in the body does. Instead, the cornea receives its nutrients from tears and the aqueous humor, which lies behind the cornea and is the liquid in the eye. Tears are very important to the health of the cornea. Each time a person blinks, tears distribute across the cornea. So it is important for people to maintain tear production to keep the cornea healthy and functioning properly.

Overview

The cornea has five layers. The first is the epithelium. This outmost layer is important because it helps prevent foreign materials such as dust and bacteria from entering the eye. It also takes in nutrients and oxygen from tears. The epithelium is packed with nerve endings. The layer below the epithelium is called Bowman's layer. It is a layer of transparent tissue that is mostly made up of collagen, a type of protein. If this layer is damaged, it can cause scars, which could impede vision, depending on their severity and location. The stroma is the next layer of the cornea. It is the thickest layer and made mostly of water and collagen. The stroma helps gives the cornea its shape and form. The next layer is the Descemet's membrane. This is a strong, thin layer of tissue that helps protect the rest of the eye. This layer of the eye heals quickly. The final layer of the cornea is the endothelium. This innermost layer of the cornea helps regulate the amount of liquid in the stroma. Cells in the endothelium that are damaged or killed do not heal.

Because the cornea is the outer layer of the eye along with the sclera, which is the white part of the eye, it can easily be injured or damaged. Different illnesses and diseases can also affect the cornea. A person with a cornea injury or illness may experience redness of the eye, tearing, sensitivity to light, pain, or blurred vision. A number of different illnesses commonly affect the cornea:

• Ocular herpes, or herpes of the eye, is an infection caused by herpes simplex virus I (HSV I) or herpes simplex virus II (HSV II). This illness causes sores on the cornea and can reoccur. This disease has no cure, but antiviral medication and steroids can help control it.
• Herpes zoster, or shingles, is another illness that can affect the eye. People who develop shingles had chicken pox at some point in their lives. The virus that causes chicken pox stays dormant inside the body. Then, it travels to the body's nerves in specific parts of the body. It can sometimes affect the eye and cause sores on the cornea. This condition will generally go away on its own, but antiviral medication and steroids can help people recover more quickly.
• Keratoconus is a disease that affects the cornea's structure. The disease thins the cornea, causing its shape to change. Since the cornea helps focus light coming into the eye, changes in its shape will cause changes in vision. This disease can happen at any age and progresses over time. Often the disease can be treated with glasses or special contact lenses. Sometimes, however, the disease progresses so far that a person requires a corneal transplant.
• Fuch's dystrophy is another disease that affects the cornea's structure. It causes endothelial cells to die in the cornea. The disease progresses over many years, but generally does not affect people's vision until later in life. As the endothelial cells die, fluid builds and causes the cornea to swell, which can impair vision. People with this disease may require a corneal transplant.
• Lattice dystrophy is a disorder that causes the buildup of a protein called amyloid in the stroma. The deposits create a pattern similar to lattice. The buildup can also erode the cornea's epithelial layer. Lattice dystrophy can be painful and cause vision problems. The erosion and pain can be treated with drops and ointment. Sometimes people with lattice dystrophy are treated with a corneal transplant.

The cornea can often heal itself after injuries or diseases, sometimes it cannot and the damage is permanent. Sometimes a corneal transplant is necessary. During this procedure, a patient's diseased or damaged cornea is replaced with a healthy cornea from a donor. Although the demand for corneal transplant remains very high, fewer people are able to donate corneas today because of the popularity of LASIK surgery, which is a surgery that improves vision but can weaken the cornea. People who have had LASIK often cannot donate their corneas. However, scientists are working to develop artificial cornea that can be used to replace those that are damaged or diseased.

Bibliography

Asbell, P. and D. Brocks. "Cornea Overview." Encyclopedia of the Eye, edited by Joseph Besharse and Reza Dana. Academic Press, 2010.

Boyd, Kierstan. "About Corneal Transplantation." American Academy of Ophthalmology, 1 Sept. 2016. https://www.aao.org/eye-health/treatments/about-corneal-transplantation. Accessed 31 May 2017.

Boyd, Kierstan. "Parts of the Eye." American Academy of Ophthalmology, 20 Dec. 2016. org/eye-health/anatomy/parts-of-eye" https://www.aao.org/eye-health/anatomy/parts-of-eye. Accessed 31 May 2017.

Boyd, Kierstan. "What Causes Cornea Problems?" American Academy of Ophthalmology, 1 Sept. 2016. https://www.aao.org/eye-health/treatments/what-causes-cornea-problems. Accessed 31 May 2017.

"Common Cornea Problems." WebMD, http://www.webmd.com/eye-health/cornea-conditions-symptoms-treatments#1. Accessed 31 May 2017.

"Corneal Disease." The Cleveland Clinic Foundation, https://my.clevelandclinic.org/health/articles/corneal-conditions. Accessed 31 May 2017.

"Facts about the Cornea and Corneal Disease." National Eye Institute, May 2016. https://nei.nih.gov/health/cornealdisease. Accessed 31 May 2017.

Sardegna, Jill and Susan Shelly. The Encyclopedia of Blindness and Vision Impairment. Facts On File, 2002.