The cornea is the clear, dome-shaped surface that covers the front of the eye. It is the outermost layer and protects the iris. It shields the eye from dust and germs, and it controls and focuses the entry of light through the lens to the retina. There are numerous disorders which can affect the cornea.
The cornea can become infected if it is damaged by a foreign body, or if bacteria or fungus is introduced by a contaminated contact lens. The resulting inflammation and infection is called keratitis.
When keratitis is present, visual acuity can be reduced, discharge from the cornea can occur, and erosion of the cornea can happen. If the infection causes corneal scarring, a corneal transplant may be necessary.
Treatment of minor infections is accomplished with antibacterial or antifungal eye drops. Severe infections may be treated with stronger antibiotics, as well as steroid eye drops to reduce inflammation.
The conjunctiva covers the outer surface of the eye. When it becomes inflamed and irritated, conjunctivitis occurs.
Conjunctivitis can be caused by many things: viruses, bacteria, allergens, and chemical irritants or foreign bodies in the eye.
The cornea becomes red, itchy, or burning, possibly with a discharge and swelling. Treatment consists of the application of eye drops, use of antibiotics, and comfort measures, depending upon the cause of the irritation.
The eye is continuously bathed in tears. The tears work to maintain eye health, protecting against infection and helping the eye to heal.
Tears are three-layered: the outer layer is oily (lipid) and slows down evaporation; the middle layer is watery and nourishes the cornea and conjunctiva; the bottom layer is mucin (secreted by mucous membranes) and helps maintain moisture in the eye.
People with dry eye produce fewer tears, less lipid, or poor quality mucin, which prevents the tears from functioning properly. Dry eye can also occur in dry climates, and with the use of certain medications, such as antihistamines, antidepressants, and decongestants, as well as in people with autoimmune disorders, such as rheumatoid arthritis.
With dry eye, the eye will feel gritty or sandy, and may sting or burn. At times, there may be excessive tearing, because the tears no longer properly adhere to the surface of the eye.
The main treatment for dry eye is the use of artificial tears. Using a humidifier, avoiding wind, and wearing wraparound eyeglasses can also help.
There are more than twenty corneal dystrophies. They are relatively rare and account for only .5% of people who are blind or partially-blind. They tend to run in families and progress gradually. Corneal dystrophies usually affect both eyes equally.
The most common are:
- Map-Dot-Fingerprint Dystrophy: The epithelium is the outermost layer of the cornea. Its function is to block foreign bodies from entering the eye and to absorb oxygen and nutrients from tears. When the foundational layer of the epithelium does not develop properly, epithelial cells cannot adhere to it and epithelial erosions occur, causing small openings between the outermost layer and the rest of the cornea. This causes change in the curvature of the cornea, photophobia (sensitivity to light), excessive tearing, and exposed nerve endings (which can cause pain).
- Keratoconus: When the cornea thins excessively, the middle can bulge outward, forming a cone shaped area that no longer refracts light correctly. The distortion of images is known as astigmatism, which typically accompanies nearsightedness. Keratoconus is the most common corneal dystrophy, and is most prevalent in teens and young adults. It can be corrected with eyeglasses as long as the astigmatism remains mild.
- Lattice Dystrophy: The stroma is one of the inner layers of the cornea. It is the thickest layer and is comprised of tiny collagen fibers that run parallel to each other, giving the cornea clarity. When abnormal protein fibers accumulate in the stroma, overlapping deposits create a lattice effect. Eventually, this leads to cloudiness of the cornea and begins to decrease vision. The deposits can accumulate under the epithelium, as well, causing erosion of the epithelium. This causes symptoms similar to those found in Map-Dot-Fingerprint Dystrophy.
- Fuchs’ Dystrophy: The endothelium is the thin, innermost layer of the cornea. It pumps excess fluid from the stroma to keep the cornea clear. Once endothelial cells are damaged or destroyed they cannot be replaced. In Fuchs’ Dystrophy, the endothelial cells deteriorate for no obvious reason. As they deteriorate, more and more fluid builds up in the stroma and causes corneal swelling; the corneal swelling cause distorted vision. Eventually, the epithelium swells from excess fluid, changing the curvature of the cornea, and epithelial blisters develop on the surface of the cornea, causing pain.
Ocular herpes is the most common infectious cause of corneal blindness. It is caused by the herpes simplex virus, the same virus that causes cold sores.
Painful lesions develop on the eyelid or the surface of the eye. The cornea becomes inflamed and prompt treatment with anti-viral medication is necessary to prevent destruction of epithelial cells.
Like other herpes infections, once a lesion has developed, future outbreaks can occur, sometimes years later. A reoccurrence can follow a period of stress, trauma to the eye, excessive exposure to sunlight, and fever.
Children (and adults) who have had chickenpox carry the inactive varicella-zoster virus in their nerve cells. Sometime later in life the virus can become active again, travelling along nerve fibers and infecting some part of the body, producing a rash (shingles).
When the virus travels to the head, about 40 percent of people will develop an infection in the cornea. It is treated with anti-viral medication to prevent scarring of the cornea. Desensitization of the cornea often occurs, and may be permanent.
Although anyone who has been exposed to the varicella-zoster virus can suffer from shingles, it is most common in the elderly and in those with weakened immune systems.
Pterygium grows on the cornea. It is an overgrowth of tissue, generally triangular in shape. They develop slowly throughout a person’s life, and some stop growing entirely after a certain point. They don’t often grow large enough to affect vision.
There is no identifiable cause for pterygium, but they are far more common in sunny climates. Because they are seen most often in people who spend a significant amount of time outdoors, scientists believe they are caused by excessive exposure to ultra-violet light. Wearing protective eye glasses, sunglasses, and hats with a brim may prevent pterygium from developing.