The cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye.
Cross Section of Cornea
Viewed from the front of the eye, the cornea appears slightly wider than it is tall. This is because the sclera (the “white” of the eye) slightly overlaps the top and bottom of the anterior cornea. The center thickness of the average cornea is about 550 microns, or slightly more than half a millimeter. The cornea has five layers. From front to back, these layers are:
- The corneal epithelium
- Bowman’s layer
- The corneal stroma
- Descemet’s membrane
- The corneal endothelium
As already mentioned, the clear cornea allows light to enter the eye for vision. But it has another very important function as well — the cornea provides approximately 65 to 75 percent of the focusing power of the eye.
Corneal Epithelium. The corneal epithelium provides an optimal surface for the tear film to spread across the surface of the eye to keep it moist and healthy and to maintain clear, stable vision.
Bowman’s Layer. The dense nature of Bowman’s layer helps prevent corneal scratches from penetrating into the corneal stroma. Corneal abrasions that are limited to the outer epithelial layer generally heal without scarring, but scratches that penetrate Bowman’s layer and the corneal stroma typically leave permanent scars that can affect vision.
Corneal Endothelium. The single layer of cells that forms the endothelium maintains the fluid content within the cornea. Damage to the corneal endothelium can cause swelling (edema) that can affect vision and corneal health
A number of conditions can affect the cornea. Among the more common corneal problems are:
Appearance of Arcus Senilis (Corneal Arcus) Arcus Senilis: As people get older, a white ring often develops in the periphery of the cornea. This is called arcus senilis (also called corneal arcus), and it’s the most common aging change in the cornea. Arcus senilis typically is separated from the limbus by an area of clear cornea.
Corneal Abrasion. A scratched cornea can be very painful and can lead to an eye infection.
Dry Eyes: Though the cause of dry eyes typically begins in the tears gland and eyelids, it can lead to damage of the corneal epithelium, which causes eye discomfort and vision disturbances.
Corneal Ulcer: A corneal ulcer is a serious abscess-like infection of the cornea that can lead to significant pain, scarring and vision loss.
Corneal Dystrophy: A dystrophy is a weakening or degeneration of a tissue. The most common corneal dystrophy — called Fuch’s dystrophy — affects the corneal endothelium, causing corneal swelling, foggy vision, light sensitivity and other problems.
Acanthamoeba Keratitis. This is a very serious and painful corneal infection that can cause significant pain and vision loss.
Fungal Keratitis. This is another dangerous corneal infection that (like Acanthamoeba keratitis) tends to affect contact lens wearers more often than people who wear glasses.
Keratoconus. This is a thinning and deformation of the cornea that causes vision problems that can’t be corrected with regular eyeglasses or contact lenses. In some cases, vision problems from keratoconus can be corrected with scleral contact lenses or hybrid contacts. But in severe cases, a cornea transplant may be required.
What other diseases can affect the cornea?
Herpes Zoster (Shingles): Shingles is a reactivation of the varicella-zoster virus, the same virus that causes chickenpox.
Pterygium: A pterygium is a pinkish, triangular tissue growth on the cornea. Some pyterygium grow slowly throughout a lifetime. A pterygium rarely grows so large that it covers the pupil of the eye.
What treatments are there for advanced corneal disease?
Laser Surgery: Phototherapeutic keratectomy (PTK) is a surgical technique that uses UV light and laser technology to reshape and restore the cornea.
Corneal Transplant Surgery: A full thickness corneal transplant, with the sutures still visible. Corneal transplant surgery removes the damaged portion of the cornea and replaces it with healthy donor tissue. Corneas are the most commonly transplanted tissue worldwide. More than 47,000 corneal transplants were performed in the U.S. in 2014.
In the past, the standard approach to corneal transplants was to surgically replace the entire cornea with donor tissue, a technique known as penetrating keratoplasty. This is called a full thickness transplant, and may still be the only option for people with advanced keratoconus and scarring, severe herpetic scarring, or traumatic injury that affects the whole cornea.
However, most people who need a cornea transplant undergo a newer procedure called lamellar keratoplasty. This is called a partial thickness transplant. In this procedure, the surgeon selectively removes and replaces the diseased layer(s) of the cornea and leaves the healthy tissue in place. Replacing only diseased layers with a donor graft leaves the cornea more structurally intact and leads to a lower rate of complications and better visual improvement.
Anterior Lamellar Keratoplasty: Removes damaged stromal tissue and replaces it with healthy stroma from a donor. This procedure is used for:
- Severe corneal scarring
- Corneal dystrophies that affect the stroma
Endothelial Lamellar Keratoplasty: Rremoves diseased endothelial tissue and replaces it with healthy endothelium from a donor. This procedure is used for:
- Post-cataract edema
- Corneal failure after surgery for cataract, glaucoma or retinal detachment
Corneal transplants are generally done under local anesthetic as an outpatient procedure. With full thickness transplants, the damaged cornea is removed and replaced with a donor cornea. Tiny stitches secure the transplant. Partial thickness transplants use fewer stitches. Either type of surgery usually takes 30 minutes.
Artificial Cornea: A keratoprosthesis (KPro) is an artificial cornea. A KPro may be the only option available for people who have not had success with corneal tissue implants or who have a high risk of tissue rejection (such as those with Stevens-Johnson syndrome or severe chemical burns).