Cornea and Corneal Diseases

What is the cornea?

The outermost layer of your eye is called the cornea. It is clear and shaped like a dome. The cornea receives nutrients and lubrication from tears and the aqueous humor (fluid in the part of the eye behind the cornea). It is composed of three basic layers with two membranes between them.

What is the function of the cornea?

The cornea plays multiple important roles related to protection of the eye as well as vision.

• It helps focus light that comes into the eye and is responsible for up to 75 percent of the eye’s focusing power. Light passes through the cornea and the lens and then focuses on the retina—the part of the back of the eye which receives light and processes it, sending messages to the optic nerve in the brain. Your brain translates these messages into the images you see with your eyes.
• It acts as a barrier against germs, dirt, and foreign particles that may harm the eye.
• It serves as a filter to screen out damaging ultraviolet rays from the sun.

Minor injuries or scratches can affect the cornea, and they usually heal on their own. Deeper injuries can cause scarring and a haze on the cornea that impairs vision. Corneal injuries can cause symptoms such as:

• Eye pain
• Light sensitivity
• Reduced or blurred vision
• Eye redness or inflammation
• Headache, nausea, and fatigue

See an eye care professional if you are having any of these symptoms and they don’t seem to get getting better.

What types of diseases may affect the cornea?

Common corneal diseases include:

• Keratitis, an inflammation of the cornea that may be caused by minor injury or bacteria, viruses, fungi, or parasites. Neurotrophic keratitis is a rare degenerative disease resulting from a loss of sensation in the cornea. It causes progressive damage to the top layer of the cornea, including corneal thinning, ulceration, and perforation in severe cases.
• Dry eye, which results when the eyes don’t produce enough tears or the tears are not of sufficient quality.
• Herpes zoster (shingles). The virus that causes shingles spreads through nerve fibers and can emerge in the cornea.
• Ocular herpes. People infected with the herpes virus may develop sores on the eyelids or the surface of the cornea.
• Iridocorneal endothelial syndrome (ICE). ICE typically develops between the ages of 30 and 50 and causes visible changes in the iris (colored part of the eye), corneal swelling, and glaucoma.
• Pterygium. This pinkish, triangular growth of tissue on the cornea is most common in sunny climates and in adults age 20-40.
• Stevens-Johnson syndrome. Also called erythema multiforme major, this disorder of the skin also affects the eyes, causing painful blisters on the eyelids, conjunctivitis, corneal blisters and erosions, and holes in the cornea. It is often an allergic reaction to medication and may also be related to viral infections.

What are corneal dystrophies?

When one or more parts of the cornea lose their normal clarity due to a buildup of material that causes clouding, it is called a corneal dystrophy. These diseases are typically inherited, affect both eyes, and gradually progress. They may cause no vision problems at all, distorted or blurred vision, or permanent vision loss. Examples of corneal dystrophies include:

• Keratoconus. The most common corneal dystrophy, keratoconus is a progressive thinning of the cornea and is most prevalent in teenagers and young adults. It usually affects both eyes.
• Fuchs’ dystrophy. Gradual deterioration of the cells in the innermost layer of the cornea is the primary feature of Fuchs’ dystrophy. The cornea thickens and vision gets blurry. Other symptoms include glare, vision distortion, tiny painful corneal blisters, and a hazy-looking cornea. 
• Lattice dystrophy. This disorder causes a lattice-like pattern of deposits made of a protein called amyloid which collect on the thickest layer of the cornea (stroma). It usually begins in early childhood and can cause sometimes painful corneal erosion, which can be relieved with eye drops and ointments. By age 40, some people have such visual impairment from scarring that they need a corneal transplant.
• Map-dot-fingerprint dystrophy. With this dystrophy, the basement membrane of the cornea, one of the outermost layers, develops abnormally and forms folds in the tissue that may look like continents on a map, clusters of dots, or concentric lines that look like small fingerprints. People with this disorder may have blurry vision, eye pain in the morning that subsides, light sensitivity, excessive tearing, and a feeling like something is in the eye. It typically affects adults ages 40-70 and can flare up every now and then, or not cause any symptoms at all. When eye drops and ointments are not enough to control eye discomfort, surgery may be needed to treat the affected areas.

How are corneal disorders diagnosed?

The best way to determine if your eye symptoms are caused by a corneal disorder is to see an eye care professional for a thorough eye exam.

The choice of treatment depends on the corneal disease and may include:

• Eye drops and ointments to relieve discomfort and treat infection.
• Cenegermin was approved in 2018 as the first drug to treat neurotrophic keratitis. It is a nerve growth factor administered in eye drops and has been shown to promote healing of the cornea.
• Oral antiviral medications to treat viral infections affecting the cornea (such as shingles and ocular herpes).
• Laser surgery, such as phototherapeutic keratectomy (PTK) to reshape and restore the cornea. PTK is used to treat corneal erosions and dystrophies.
• Corneal transplantation. During a corneal transplant, the damaged portion of the cornea is replaced with healthy human tissue from a donor. Today surgeons perform lamella keratoplasty (“partial thickness transplant”) whenever possible, removing only diseased tissue and leaving healthy corneal tissue in place. This approach leaves the cornea more intact structurally and is associated with fewer complications and better improvement of vision. Corneal transplantation is an option for some patients with keratoconus, severe corneal scarring, certain corneal dystrophies, edema after cataract surgery, and any failure of the cornea after eye surgery.
• Artificial cornea. People who cannot have a cornea transplant from a donor or who did not do well with a prior corneal transplant may be candidates for an artificial cornea (keratoprosthesis or KPro).

Source: The National Eye Institute (NEI)