The cornea is the outermost layer of the eye. It's the clear, dome-shaped surface that covers the front of the eye. The cornea helps shield the rest of the eye from germs, dust and other harmful matter. The cornea functions like a window that controls and focuses the entry of light into the eye. It contributes between 65-75% of the eye's total focusing power.
The cornea is made up of five layers
- Epithelium- the outermost, protective layer
- Bowman membrane- the strong second protective layer
- Stroma- the thickest layer of the cornea. Made up of water, collagen fibers and other connective tissue
- Decemet membrane- a thin, strong inner layer that is also protective
- Endothelium- the innermost layer made up of cells that pump excess water out of the cornea
Corneal Diseases and disorders
One of the most common eye injuries, a corneal abrasion is when the cornea is scratched. This causes pain, the sensation of an object in the eye, bloodshot eyes, and extreme sensitivity to light. They can be caused by large objects as well as objects as small as a mote of dust.
Minor abrasions can be treated with lubricating drops, although they are also sometimes treated with antibiotic drops to prevent infection. More serious abrasions may need antibiotic ointment and steroids to decrease inflammation and scarring. In some cases, a special bandage contact lens must be worn. Most corneal abrasions heal completely, with no permanent vision loss, although some deeper abrasions can leave scars.
Corneal dystrophies are genetic conditions that affects the cornea’s clarity. They can affect any layer of the cornea. Onset age and symptoms vary among the different forms. They have some similar characteristics, however. For example, they affect both eyes, progress slowly, are limited to the eyes, and are genetic.
Corneal erosion occurs when the outermost layer of the cornea, known as the epithelium, detaches from the tissue below. The most common symptom is pain, particularly in the morning. Because the eye dries, the lid can stick to the epithelium and tear it off when the lid opens. Other symptoms include light sensitivity, blurred vision, watery eyes, and a feeling of something in the eye. Erosion can happen to anyone, but is likelier to happen in people with a history of eye injury or corneal disease, have had an eye ulcer, or where improperly-fitted contact lenses.
Corneal erosion can be treated with lubricating ointments to prevent the eyelid from sticking to the epithelium while the eye heals. Your eye may be patched and you may also be given antibiotics while the eye heals. For recurrent erosion, your doctor may recommend surgery.
A corneal laceration is a cut on the cornea, usually caused by a sharp or heavy object striking the eye. Lacerations are deeper than abrasions. If deep enough it can cut completely through the cornea and tear the eyeball itself. Corneal lacerations are very serious. If you have a corneal laceration, seek medical attention immediately. Do not remove the object, rinse the eye with water, or apply pressure to the eye. Do not take aspirin, ibuprofen, or other anti-inflammatory drugs, as they can thin the blood, which can increase bleeding.
Corneal ulcers are caused when the cornea is infected. This can happen from wearing contact lenses, eye trauma, and other corneal conditions. Symptoms include redness, pain, light sensitivity, the feeling of a foreign object in the eye, a discharge, or mild to severe decreased vision.
Mild infections are treated with regular antibiotic drops. If the ulcer is more severe, it may be treated with specially-made antibiotics. Vision may or may not return to normal, depending on how much scarring the patient has. In rare cases, a corneal transplant may be needed.
Fuchs’ dystrophy is a condition in which the endothelial cells on the back layer of the cornea are abnormal or undergo degenerative changes. These cells maintain proper fluid levels in the cornea and keep vision clear by pumping out excess fluid. Without this, the cornea could swell. While most patients have a mild form that never affects vision, it can when the patient reaches middle age. It can cause pain in later stages.
While it can’t be prevented, Fuchs’ dystrophy can be treated with saline drops. Corneal transplants may be necessary at later stages.
Keratoconus is a progressive thinning and distortion of the cornea. Doctors do not yet know what causes keratoconus, although in some cases it appears to be genetic. About 1 out of 10 people with keratoconus have a parent who has it, too.
In addition to blurry or distorted vision, keratoconus symptoms can include an increased sensitivity to light and glare, and eye redness or swelling. In later stages, keratoconus symptoms can include nearsightedness orastigmatism, and an inability to wear contact lenses.
Keratoconus can be diagnosed through a routine eye exam. It is treated with corneal transplants, special corrective contact lenses, or corneal cross-linking.
A corneal transplant is needed if vision cannot be corrected satisfactorily with eyeglasses or contact lenses, or if pain cannot be relieved by medications or special contact lenses. There are many conditions that can affect the clarity of the entire cornea. For instance, trauma or injury to the cornea (causing scarring), Fuchs' dystrophy (causes corneal failure), and Keratoconus cause a steep curving of the cornea. Sometimes cataract surgery can cause corneal failure. A corneal transplant is done using a human donor cornea. Before a cornea is released for transplant, tests are done for viruses and other potentially infectious diseases. The cornea is also checked for clarity.
During a traditional full corneal transplant surgery (penetrating keratoplasty), a circular portion is removed from the center of the diseased cornea. A matching circular area is removed from the center of a healthy, clear donor cornea. It is then placed into position and sutured into place.
An endothelial keratoplasty (EK) involves just the abnormal inner lining of the cornea being removed. A thin disc of donor tissue containing the healthy endothelial cell layer is placed on the back surface of the cornea. An air bubble pushes the endothelial cell layer into place until it heals in an appropriate position.
With a lamellar corneal transplant procedure, the superficial layers of the cornea are removed and replaced with donor tissue. Sutures are used to secure the new tissue into place. Corneal transplants have proven to be very successful in patients with poor vision, or whose corneas have been significantly damaged because of corneal dystrophies.
Descemet’s Membrane Endothelial Keratoplasty
Descemet’s Membrane Endothelial Keratoplasty, or DMEK, is a partial-thickness cornea transplant procedure that involves removal of the patient's Descemet membrane and endothelium, which is replaced by a donor corneal endothelium and Descemet membrane. Unlike other, similar procedures, the procedure produces a result that is more similar to the original, naturally-occurring cornea, which often allows for a better visual outcome.
Not all patients are suitable for DMEK. Patients who are good candidates for this surgery are those with Fuchs’ endothelial dystrophy, Bullous keratopathy, a failed prior corneal transplant, and corneal swelling. Patients who are not candidates for DMEK surgery could still be treated with other corneal transplantation techniques such as DSEK or full thickness penetrating keratoplasty.
© eyeSmart® Eye health information from the American Academy of Ophthalmology. The Eye M.D. Association.