What is the function of the cornea?
What problems may affect the cornea?
Structure of the Cornea
Corneal Dystrophies  
Corneal Transplantation

What is the function of the cornea?

Because the cornea is as smooth and clear as glass but is strong and durable, it helps the eye in two ways:

1. It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the sclera, or white part of the eye.

2. The cornea acts as the eye's outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye's total focusing power.

When light strikes the cornea, it bends--or refracts--the incoming light onto the lens. The lens further refocuses that light onto the retina, a layer of light sensing cells lining the back of the eye that starts the translation of light into vision. For you to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images.

The refractive process is similar to the way a camera takes a picture. The cornea and lens in the eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image.

The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lens and the retina would be highly susceptible to injury from UV radiation.

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Structure of the Cornea

Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and protein. The cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. It must remain transparent to refract light properly, and the presence of even the tiniest capillaries would interfere with this process.

            Corneal Thickness                            Corneal Layers

EPITHELIUM As the cornea's outermost region--comprising about 10 percent of the tissue's thickness--the epithelium functions primarily to: (1) block the passage of foreign material--such as dust or water--into the eye and other layers of the cornea, and (2) provide a smooth surface that absorbs oxygen and other needed cell nutrients that are contained in tears. This layer, which is about five cells deep, is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched.  

BOWMAN'S LAYER Lying directly below the basement membrane of the epithelium is a transparent sheet of tissue known as Bowman's layer. It is composed of strong layered protein fibers called collagen. Once injured, Bowman's layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur.

STROMA Located behind the epithelium, the stroma comprises about 90 percent of the cornea. It consists primarily of water (78 percent); layered protein fibers (16 percent) that give the cornea its strength, elasticity, and form; and cells that nourish it. The unique shape, arrangement, and spacing of the protein fibers are essential in producing the cornea's light-conducting transparency.  

DESCEMET'S MEMBRANE Under the stroma is Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries. Descemet's membrane is composed of collagen fibers (different from those of the stroma) and is made by the endothelial cells that lie below it. Descemet's membrane is regenerated readily after injury.

ENDOTHELIUM This single layer of cells is located between the stroma and the aqueous humor (see diagram). Because the stroma tends to absorb water, the endothelium's primary task is to pump excess water out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque.  

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What problems may affect the cornea?

The cornea copes very well with minor injuries or abrasions. If dirt scratches the highly sensitive cornea, epithelial cells slide over quickly and patch the injury before infection occurs and vision is affected.

But if the scratch penetrates the cornea more deeply, the healing process will take longer, resulting in greater pain, blurred vision, tearing, redness, and extreme sensitivity to light.

These symptoms require professional treatment. Some of the more serious problems that affect the cornea are:

Microbial Infections (keratitis)

When the cornea is damaged, such as after a foreign object has penetrated the tissue, bacteria or fungi can pass into the cornea, causing a deep infection and inflammation. This condition may cause severe pain, reduce visual clarity, produce a corneal discharge, and perhaps erode the cornea.

As a general rule, the deeper the corneal infection, the more severe the symptoms and complications. It should be noted that microbial infections, although relatively infrequent, are the most serious complication of contact lens wear.

Minor corneal infections are commonly treated with anti-bacterial or anti-fungal eye drops. If the problem is more severe, a person may receive more intensive antibiotic treatment to eliminate the infection and may need to take steroid eye drops to reduce inflammation. Frequent visits to an eye care professional may be necessary for several months to eliminate the problem.  

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Conjunctivitis ("pink eye")

This term describes a group of inflammatory and often contagious diseases of the conjunctiva (the protective membrane that lines the eyelids and covers exposed areas of the sclera, or white of the eye). These diseases can be caused by a bacterial or viral infection, drug allergy, environmental irritants, or a contact lens product.

At its onset, pink eye is usually painless and does not adversely affect vision. The infection will come and go in most cases without requiring medical care. But for some forms of pink eye, such as epidemic keratoconjunctivitis, treatment will be needed. If treatment is delayed, the infection may worsen and cause corneal inflammation and a loss of vision. Depending on the type of pink eye that a person develops, treatment often consists of antibiotics and steroids.

Herpes Zoster (shingles)

This infection is produced by the varicella-zoster virus, the same virus that causes chicken pox. After an initial outbreak of chicken pox (often during childhood), the virus remains dormant within the nerve cells of the central nervous system. But in some people, the varicella-zoster virus will reactivate at some time during their lives. When this occurs, the virus travels down long nerve fibers and infects some part of the body, producing a blistering rash (shingles), fever, painful inflammations of the affected nerve fibers, and a general feeling of malaise.

Varicella-zoster virus may travel to the head and neck, perhaps involving an eye, part of the nose, mouth, cheek, and forehead. In about 40 percent of those with shingles in this area, the virus infects the cornea. These zoster-related corneal lesions will usually clear up on their own. But without early anti-viral treatment, a person runs the risk of the virus infecting cells deep within the tissue, causing inflammation and scarring of the cornea. The disease may also cause decreased corneal sensitivity. For many, this decreased sensitivity will be permanent.

Although shingles can occur in anyone exposed to the varicella-zoster virus, several studies have established two general risk factors for the disease: (1) advanced age and (2) a weakened immune system. Studies show that people over age 80 have a five times greater chance of having shingles than adults between the ages of 20 and 40. Unlike herpes simplex I, the varicella-zoster virus does not usually flare up more than once in adults with normally functioning immune systems.

Be aware that corneal complications may arise months after the shingles are gone. For this reason, it is important that patients schedule followup eye examinations.  

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Corneal Dystrophies

There are over 20 corneal dystrophies that affect all parts of the cornea. Some of the most common are:

Keratoconus
The disorder arises when the middle of the cornea thins and gradually bulges outward, forming a rounded cone shape. This abnormal curvature changes the cornea's refractive power, producing moderate to severe distortion (astigmatism) and blurriness (near- and farsightedness) of vision.
These changes may also disrupt the normal, light-conducting arrangement of corneal protein, causing swelling and a sight-impairing scarring of the tissue.

Studies indicate that keratoconus stems from one of several causes: (1) an inherited corneal abnormality. About 7 percent of those with the condition have a family history of keratoconus; (2) an eye injury, i.e., excessive eye rubbing or wearing hard contact lenses for many years; (3) certain eye diseases, such as retinitis pigmentosa, retinopathy of prematurity, vernal keratoconjunctivitis; or (4) systemic diseases, such as Leber's congenital amaurosis, Ehlers-Danlos Syndrome, Down's syndrome, osteogenesis imperfecta, and Addison's disease.

In most cases, the cornea will stabilize after a few years without ever causing severe vision problems. But in about 10 to 20 percent of people with keratoconus, the cornea will eventually become too scarred or will not tolerate a contact lens. If either of these problems occur, a person will probably need to replace the diseased tissue with a donor cornea, called a corneal transplant.

This operation is successful in about 9 out of 10 people with advanced keratoconus. Several studies have also reported that about 80 percent of these patients have 20/40 vision or better with contact lenses after the operation. In fact, about 60 percent of transplant recipients will need to wear contact lenses after the surgery to correct astigmatism and nearsightedness.

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Map-Dot-Fingerprint Dystrophy

A membrane separates the epithelium from the stroma, in part. It serves as the foundation on which the epithelial cells anchor and organize themselves. In map-dot-fingerprint dystrophy, the membrane develops abnormally. Like building a house on a damaged foundation, the epithelial cells anchor to an irregular membrane that, in turn, may make the epithelium slightly irregular.

Map-dot-fingerprint dystrophy gets its name from the unusual appearance of the cornea during an eye examination. Most often, the epithelium will have a map-like appearance, i.e., large, slightly cloudy bodies that look like a continent on a map. This configuration is actually the irregular pattern of the membrane extending into the epithelium. There may also be a sequence of opaque dots--formed from cellular debris--underneath or close to the map-like patches. Less frequently, the irregular membrane will form concentric lines in the central cornea that resemble small fingerprints.

Most people will never know that they have this corneal dystrophy, since they will never have any pain and vision loss.

But for others, the disease will cause recurrent epithelial erosions--possibly from poor epithelial adhesion to the membrane.

These erosions can be a chronic problem. They alter the cornea's normal curvature, causing periodic blurred vision. They may also expose the nerve endings that line the tissue, causing moderate to severe pain for several days. Generally, the pain will be worse in the morning. Other symptoms include: sensitivity to light, excessive tearing, and foreign body sensation in the eye.

Typically, the problem will flair up occasionally for a few years in adults and then go away on its own, with no lasting loss of vision.

However, if treatment is needed, the doctor will try to control the pain associated with the corneal erosion. He or she may do this by patching the eye to immobilize it or by prescribing lubricating eye drops and ointments. With effective care, the pain will subside in about 10 days, although periodic flashes of pain may occur for several weeks thereafter.

Fuch's Dystrophy
Fuch's Dystrophy is a slowly progressing disease that usually affects both eyes and is slightly more common in women than in men. Although doctors can often see early signs of Fuch's dystrophy in people in their 30s and 40s, the disease rarely affects vision until a person reaches their 50s and 60s.

Fuch's dystrophy occurs when endothelial cells gradually deteriorate without any apparent reason, such as trauma or inflammation. As more endothelial cells are lost over the years, the cornea becomes less efficient at pumping water out of the stroma. This causes the cornea to swell and to distort vision. Eventually, the epithelium also takes on water, resulting in great pain and severe visual impairment.

Epithelial swelling damages vision in two ways: (1) changing the cornea's normal curvature, and (2) causing a sight-impairing haze to appear in the tissue. Epithelial swelling will also produce tiny blisters on the corneal surface. When the blisters burst, they are extremely painful.

At first, a person with Fuch's dystrophy will awaken with blurred vision that will gradually clear during the day. This occurs because the cornea is normally thicker in the morning, and it retains fluids during sleep that evaporate in the tear film while we are awake. But as the disease worsens, this swelling will remain constant and reduce vision throughout the day.

When treating the disease, doctors will try first to reduce the swelling with ointments or soft contact lenses. They may also instruct a person to use a hair dryer, held at arm's length or directed across the face, to dry out the epithelial blisters. This can be done two or three times per day.

But when the disease makes even the most simple tasks hard to complete, a person may need to consider having a corneal transplant to restore sight. The short-term success rate of corneal transplantation is quite good for people with Fuch's dystrophy. But, some studies do suggest that the long-term survival of the donor cornea can be a problem.  

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Lattice Dystrophy

Although lattice dystrophy can occur at any time in life, the condition usually arises in children between the ages of 2 and 7. It is characterized by an accumulation of abnormal protein fibers (amyloid) throughout the middle and anterior stroma. However, the disease is NOT related to amyloidosis, a serious systemic disease.

Lattice dystrophy gets its name from the amyloid deposits, which during an eye examination can appear as clear, comma-shaped dots and branching filaments that overlap each other in the stroma, creating a lattice effect. Over time, the lattice lines will grow opaque and involve more of the stroma. They will also gradually coalesce, giving the cornea a slight cloudiness that may also reduce vision somewhat.

In some people, abnormal protein also accumulates under the epithelium. This may result in poor adhesion between the stroma and epithelium, causing periodic epithelial erosions. The erosion will: (1) alter the cornea's normal curvature, resulting in temporary vision problems such as astigmatism and nearsightedness, and (2) expose the nerves that line the cornea, causing severe pain. In fact, even the involuntary act of blinking can be painful.

To ease this pain, a doctor may prescribe eye drops and ointments to reduce the friction on the eroded cornea. In some cases, an eye patch may be used to immobilize the eye. With effective care, the pain will subside in about 10 days, although occasional sensations of pain may occur for about the next 6 to 8 weeks.

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Corneal Transplantation

The cornea is normally a clear layer of tissue covering the front of the eye, similar to a watch crystal.  Its purpose is to refract or bend light rays as they enter the eye, allowing them to focus on the retina.  In cases where the cornea has become clouded as a result of disease, swelling, scarring, infection, or chemical burns, a corneal transplant (also called keratoplasty) is sometimes necessary to restore functional vision.  For this procedure, the surgeon carefully removes the central corneal tissue and replaces it with a precisely shaped replica of donor tissue. 

The procedure usually takes approximately one hour and is performed with local anesthesia on an outpatient basis.  After the surgery, a plastic shield or glasses should be worn at all times to avoid accidentally rubbing, bumping or hitting the eye.  Drops are prescribed to prevent rejection of the donor cornea.

During the postoperative period, the surgeon monitors the cornea’s healing with special computer mapping called corneal topography.  This allows the doctor to evaluate the shape of the new cornea and remove sutures as needed to control astigmatism. 

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