Cornea

Shaped like a dome, the cornea is the transparent layer that covers the front of the eye. It reflects light once it enters the eye. The cornea needs tears to help nourish and clean it from undesirable elements. It also receives nourishment from the aqueous humor, which fills the chamber behind it. Because of the cornea and the lens, we can see objects that are near or far.

The cornea works in three ways:

It shields the eyes from undesirable elements like germs and dirt with cooperation from the other parts of the eye.

It focuses the entry of light in the eye.

It filters out the UV rays from the sun.

Decoding the Layers of the Cornea

Code One (The epithelium):

The epithelium, the layer that covers the surface of the cornea, is about 5-6 cell layers thick and filled with tiny nerve endings, making the cornea very sensitive to pain.

The epithelium blocks the passage of dust and germs and provides a smooth surface that absorbs oxygen and cell nutrients from tears, and then distributes these nutrients to the rest of the cornea.

The basement membrane is the part where the epithelial cells anchor and organize.

Code Two (Bowman´s Membrane):

This layer lies beneath the epithelium and is very difficult to penetrate. The difficult access to the Bowman´s membrane protects the cornea from injury.

But once injured, it resiliently regenerates. It leaves a scar when the injury is deeper. The scar becomes opaque areas, causing the cornea to lose its clarity and luster.

Code Three (The stroma):

Lying beneath the Bowman, the stroma is the thickest layer. Composed of tiny collagen fibrils that run parallel to each other, this precision formation gives the cornea its clarity, strength, elasticity, and form.

Code Four(Descemet’s membrane):

Lying beneath the stroma, Descemet's membrane is a thin but strong sheet of tissue that acts as protection against infection and injuries.

It 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.

Code Five (The endothelium)

This is the extremely thin, innermost layer of the cornea. Endothelial cells are essential in keeping the cornea clear. It pumps this excess fluid out of the stroma, which has the danger of swelling with water.

Once endothelium cells are destroyed by disease or trauma, they cannot be recovered. Too much damage to endothelial cells can lead to corneal edema (swelling caused by excess fluid) and blindness ensues, with corneal transplantation the only available therapy.

Most Common Threats to the Cornea

Nearsightedness- far away objects appear blurred because the cornea is too curved or eyes are too long

Farsightedness- near objects appear blurred because the cornea is not curved enough or the eye is too short.

Astigmatism- objects appear blurred and distorted because the cornea is uneven and curved.

Sneeze/Allergy/Pollen- causes redness, itching, and watery discharge. Other causes may come from cosmetic products such as mascara, nail polish, or contact lenses.

Pink Eye/Conjunctivitis- an infection of the conjunctiva, which is the protective membrane that lines the eyelids and covers exposed areas of the sclera (also called the white of the eye). Redness, swelling, and itching of the conjunctiva may occur. Exposure to viral infections, bacteria, allergies, irritants, contact lens products, eye drops, and eye ointments may all cause Pink Eye, and can cause severe damage and visual loss if not treated.

Keratitis/Corneal Infection- targets the cornea and causes painful inflammation, corneal discharges, and visual loss. Keratitis may be caused by a simple poke in the eye or from fungi from contact lens wear, among others. Once found, anti-bacterial eye drops may be used for a remedy.

“Dry Eye”-the eye stops producing tears and becomes un-lubricated, dry, and prone to infections. “Dry Eye” usually begins with a simple irritating, scratchy feeling in the eye. Usually after a long, dry spell, tears will heat up the eye with a burning sensation, or excessive tearing will occur. “Dry Eye” is most common during the dry season and often occurs among women with menopause and people taking medication. Artificial Tears or other over-the-counter eye drops and sterile ointments can usually remedy “Dry eye.”

Corneal Dystrophy-causes cloudy build up in the cornea and progresses gradually, affecting the first layer of the cornea and then spreading to the other layers.

When a cornea is damaged to the point that it can no longer function, a corneal transplant can help to restore sight and ocular function.

Corneal Transplantation

Conditions that Require a Transplant

An individual whose vision has deteriorated to the point that they can no longer see properly may require a corneal transplantation.

Some conditions that may cause the cornea to deteriorate include:

Keratoconus-the central cornea thins and bulges forward, preventing light from being focused correctly into the eye. Usually both eyes are affected.

Pseudophakic Bullous Keratopathy-there is damage to the cells responsible for keeping the cornea clear and healthy, causing the cornea to become cloudy and losing clarity

Fuch’s Dystrophy-a hereditary disease of the inner layer of the cornea. Treatment requires penetrating keratoplasty.

Herpetic Eye Disease-the herpes virus causes scarring of the cornea

Corneal injury may also occur from chemical burns, mechanical trauma, or infection by viruses, bacteria, fungi, or protozoa.

Surgery may be warranted when damage to the cornea is too severe to be treated with corrective lenses.

Description

Corneal transplant is one of the most frequently performed human transplant procedures. Since 1961, more than 549, 889 corneal transplants have been performed, restoring sight to men, women, and children.

In a corneal transplant, a circular disc of tissue is removed from the center of the eye and then replaced by a disc from the donor eye using an instrument called a trephine. In one particular form of corneal transplantation, called a penetrating keratoplasty, the disc removed as well as the replacement disc comprise the entire thickness of the cornea. More than 90% of all corneal transplants in the United States are of this type. In lamellar keratoplasty, however, only the outer layer of the cornea is removed and replaced.

The donor cornea is then attached with fine sutures. Surgery can be performed under local anesthesia with the patient awake, or under general anesthesia with the patient in a state of unconsciousness. Surgery typically requires 30-90 minutes.

A related but less common procedure called epikeratophakia, which is usually performed on children, entails suturing the donor cornea directly onto the surface of the existing cornea. The only tissue that is removed from the host is an extremely thin epithelial cell layer on the outside of the cornea. There is no damage to the cornea, and this procedure can be reversed. In adults, the use of a contact lens usually achieves the same result.

Aftercare

Following surgery, the patient will wear an eye patch overnight. An eye shield or glasses must be worn until the wound has healed. Antibiotic eye drops (to prevent infection) and corticosteroids (to reduce inflammation and prevent graft rejection) are prescribed for patient use for several weeks following surgery.

The eye will feel scratchy and irritated for the first few days following surgery. Vision will remain blurry for several months. Sutures are left in place for 6 months to 2 years.

Success of Corneal Transplantations

Corneal transplants are highly successful. The survival rate after corneal transplantation is 91.2% after one year and 80% after four years. Some conditions such as keratoconus have a higher success rate of 98.1%. However, there are always risks associated with surgery. Complications that can occur include infection, glaucoma, retinal detachment, cataract formation, and rejection of the donor cornea (5-30% of patients).

If a rejection does occur, it can be blocked by steroid treatment. Rejection reactions may occur shortly after surgery, or it may not occur until 10 to 20 years after the transplant. If complete rejection does occur, the surgery may be repeated.

Corneal Donation

Corneal transplantation surgery would not be possible without donors who have donated corneal tissue. The transplantation process depends upon the priceless gift of corneal donation from one human to the next. Donated human eyes and corneal tissue are used for research, education, and transplantation.

When consent for donation is given, corneas must be surgically removed within twelve hours of the donor’s death. Very few conditions, such as infectious disease, exclude individuals from donation. Cataracts, poor eyesight, or age do not prevent an individual from being a donor.

Although more than 46,000 transplants were performed just last year, the need for corneal tissue is never satisfied. With advances being made in artificial corneas, donations tend to be reserved for patients who require donor cornea transplantation. Success rates are higher with donor corneas.

How to Become a Donor

If you wish to become a donor, the most important thing to do is to tell your family of your wishes since it is not always routine for the hospital to approach the family about eye donation.

References

An Eye for an Eye- http://www.nepalitimes.com/issue188/health.htm

Eye Bank Association of America- http://www.restoresight.org/general/faqs.htm

Corneal transplantation- http://www.healthAtoz.com

Corneal transplantation and Donation- http://www.betterhealth.vic.gov.au

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