The cornea is a domed layer of clear tissue that covers the sclera like a protective window. It juts out from the eyeball as a tiny bulge and is made up of several layers of tissue packed with sensitive nerve endings.
Besides being transparent, the cornea is devoid of blood vessels and is highly selective in its sensitivity, which is good news for the contact wearer.
Corneal Degeneration is the term used for the breakdown of the cornea. Corneal degenerations are fairly rare disorders that may be caused by genetic or environmental conditions. Such disorders are mainly inherited.
Among these hereditary diseases are the two most common, keratoconus and Fuch’s endothelial dystrophy.
Little is known about the causes and development of corneal degenerations. It is thought that some degenerations may result from irregularities within corneal fibroblasts, the cells that develop into fibrous tissue. The fibroblasts are central to the processes of healing a corneal injury.
In addition, some corneal degenerations may be caused by environmental influences or can result from corneal injuries or inflammations (keratitis) or from such accidental injuries as alkali burns.
- Corneal edema is a condition in which the cornea becomes overly hydrated. In order to remain transparent, the cornea is normally kept relatively dry through oxygen supplied in tears and the draining of water by the corneal endothelium. Any changes to the dehydration process or an influx of fluid can impede the process and allow fluid to accumulate.
Symptoms of corneal edema include blurred vision or the appearance of halos around lights. Later symptoms include severe pain as corneal nerves are damaged. Since the early symptoms are similar to those of a cataract, an ocular examination is necessary for proper diagnosis.
Corneal edema can be caused by infection, corneal endothelial dysfunction (Fuch’s endothelial dystrophy), a viral corneal inflammation (keratitis), a rise in intraocular pressure (as in glaucoma), postoperative changes, trauma, overwear of contact lenses, and ill-fitting contact lenses. The condition is treated according to the cause. Most are treated with various drugs or surgery. Re-educating lens wearers and proper adjustment to the fit will cure the condition caused by improper contact lenses.
Corneal edema may become a chronic condition. As the condition progresses and worsens, the corneal nerves become ruptured and exposed, causing extreme pain. Treatment of acute edema may include a tarsorrhaphy, in which a portion of the lids are stitched together, or a corneal graft.
Corneal transplant, or keratoplasty is the most common form of human transplant surgery performed today, using a donor cornea to replace a diseased one. Although corneal transplants are generally successful in restoring vision, about ten percent are rejected. However, most rejections can be prevented if treated early. Signs of rejection include sensitivity to light, redness, persistent discomfort, and changes in vision.
Immunosuppressive drugs are available to reduce the possibility of rejection, including an oral vaccine developed especially for that purpose. Donor transplants come from people who have decided that when they die, they want their corneas donated to others. The donor transplants are often kept in eye banks (see below) that help match appropriate corneas to needed hosts. They are selected according to age, cause of death, condition of the eye, and time between death and transplant surgery.
Blood and tissue typing are not required as with other transplanted organs, but time is a critical factor since the cornea should be transplanted within four days of donation. Donor corneas are most desirable from individuals between the ages of 25 to 35 and who died from injury or disease.
The donor corneas are screened for eye disease and the presence of venereal disease.An individual becomes a candidate for a corneal transplant when all vision in that eye is lost due to injury, corneal dystrophy diseases such as keratoconus, chemical burns, or infection. A transplant may also be performed to repair injuries or tears in the cornea, to relieve chronic pain, or to correct a cosmetically unattractive eye.
A normal cornea is avascular, that is, it lacks blood vessels. In order to perform a successful keratoplasty, the transplant must be placed into the eye in a manner that discourages vascularization, or the growth of blood vessels into the cornea. Vascularization of the transplant may destroy it.
Certain disorders or diseases as keratoconus, or a bulging cornea, lend themselves best to corneal transplants because of the lack of corneal blood vessels involved with this disorder. Other conditions, as corneal scars, are less effectively corrected with transplants because of heavy vascularization present in the cornea.
Surgery may replace part or all of the host cornea. A section of the donor cornea is measured to the precise needs of the host with an instrument called a trephine. This section is removed from the donor cornea with scissors. A corresponding section of host cornea is removed in the same way. The donor cornea is placed into the remaining cavity and sewn into position with tiny sutures. The procedure is performed with either local or general anesthesia and may take an hour to an hour and a half to complete. Although it is usually done on an outpatient basis, many patients require a day or two of hospitalization.
For several weeks after the surgery, the patient is discouraged from bending, lifting, or straining. The patient wears a protective eye patch or shield and is treated with eye drops for several months. The stitches are removed after a period of six months to two years. Recently, ophthalmologists have started using artificial corneas in certain transplant cases.
Corneal ulcers occur when the cornea is worn down; damaged by injury or exposure; or from bacterial, viral, or fungal infections. Ulceration may lead to scarring and loss of sight and is a common cause of blindness around the world. Injuries or overuse of contact lenses usually result in minor corneal abrasions or scratches. The symptoms are extreme pain with a feeling of a foreign body within the eye. Anesthetic drops may be administered to examine the eye that is then generally bandaged shut. The eye may be allowed to heal itself or an antibiotic ointment prescribed to avoid infection.
Ulcers due to exposure occur in cases of injury, facial palsy (Bell’s palsy), or unconsciousness in which the lid does not adequately protect the cornea, allowing bacteria to enter and infection to begin. Exposure-related ulcers may be treated with eyepads and/or anesthetic ointments. Ulcers caused by bacteria are often associated with staphylococcus or streptococcus bacteria. Others may infect the eye when the cornea is weakened by disease or additional infections. Bacterial ulcers are usually treated with antibiotics or antibiotic-steroids.
Ulcers caused by viruses are most often caused the the herpes simplex virus. In infants, the virus is passed from the mother to the child as it travels through the birth canal. A rash may appear around the eyelids of the newborn, and the child may develop a fever. The condition is serious and may result in death, blindness, or damage to other organs of the body. The virus is treated with antiviral drugs and antibiotics.
The herpes simplex virus may appear in adults after development of another illness. The virus, which had remained latent in the body, is reactivated and settles in the eye. This gives rise to keratitis and the formation of star-shaped, or dendritic, ulcers. As the ulcers heal, they may leave scars that could impair vision.
An antiviral drug along with an antibiotic may be prescribed as treatment. Although they cannot cure the virus, they can effectively stop the reproduction of viral cells and prevent infection. Steroids are not recommended since they impair the body’s rejection of the herpes simplex virus.
Fungal-related ulcers are most often caused by yeasts. The condition may require hospitalization and treatment with antibiotics. Healing of these ulcers may result in scarring of the cornea. Damage to the corneal nerve, a vitamin A deficiency, or the onset of other diseases, or disorders may also trigger corneal ulcers. Any situation in which the cornea is inflamed should be monitored for ulcers.