Corneal diseases are vision-impairing diseases that affect the cornea, the transparent front part of the eye that covers the iris and pupil of the eye.
The human eye is approximately 1 inch (2.54 cm) wide, 1 inch deep, and 0.9 inches (2.3 cm) tall. The cornea is the transparent, dome-shaped tissue at the front of the eye that provides refracting power, and 65–75% of the eye's focusing power. From the cornea, light refracted by an object passes through the pupil, the circular opening in the center of the colored iris, and is sharply focused by the crystalline lens to produce images on the retina, the multi-layered sensory tissue that lines the back of the eye.
The cornea consists of five layers of cells and tissue that contain no blood vessels. It receives its nourishment from tears and aqueous humor, the clear, watery fluid in the front of the eyeball. The outermost layer of the cornea is the epithelium. It protects the eye against foreign materials and absorbs oxygen and cell nutrients for the cornea. Bowman's layer is a transparent layer located directly below the epithelium. It is made of strong layers of protein fibers called collagen. Directly beneath Bowman's layer is the stroma layer that represents 90% of the corneal tissue. It consists mostly of water (78%) and collagen (16%). Under the stroma lies Descemet's membrane, consisting of thin but strong collagen fibers that form a protective barrier against infection and injuries. The innermost layer of the cornea is the endothelium, an extremely thin layer of cells that pump excess fluid out of the stroma.
Good vision depends on the cornea acquiring transparency during eye development and maintaining its clarity throughout adult life. The main function of the cornea is to protect the eye from germs, dust, and other foreign material. It also acts as an outer lens, focusing the entry of light into the eye. The cornea is tough and can recover very well from minor injuries or abrasions. For instance, if dirt scratches it, epithelial cells quickly slide over to repair the injury before infection can set in and affect vision. However, the cornea can be affected by more serious disorders that require medical treatment. Corneal diseases include:
- Allergies. The most common allergies that can affect the cornea include allergies to pollen, soap, medications, contact lenses, animal hair, cosmetics, such as mascara, face creams, and eyebrow pencil, or chemicals and substances that are accidentally introduced into the eye.
- Corneal dystrophies. Corneal dystrophies are slowly progressing degenerative disorders in which one or more parts of the cornea lose their normal clarity. Some 20 corneal dystrophies are known that can affect all parts of the cornea. Some of the most common include keratoconus and Fuchs' dystrophy. Keratoconus causes a thinning of the middle of the cornea which then gradually bulges outward, forming a rounded cone. The condition modifies the refractive power of the cornea, leading to distorted (astigmatism) and blurred (nearsightedness) vision. Fuchs' dystrophy affects the endothelium layer which causes the cornea to swell and distort vision. Dystrophies usually affect both eyes.
- Corneal infections. The cornea can be infected by a variety of agents, such as bacteria or fungi that can pass into the cornea from the hands or a contaminated contact lens. A common viral corneal infection is ocular herpes. When the virus spreads to deeper corneal layers, the infection is referred to as stromal keratitis. Another infection is shingles that results in a rash that follows the path of certain nerves on one side of the body. If the infection spreads to the cornea, it becomes inflamed and scarred.
- Corneal ulcer. These ulcers form when the surface of the cornea has been damaged. There are two types of ulcers: without infection (sterile) or with infection (infectious). Bacterial ulcers are very painful and usually involve a break in the epithelium layer. Sterile ulcers cause little pain and tend to occur near the peripheral edge of the cornea.
- Dry eye . Tears are important to keep the eye moist, help wounds heal, and protect against eye infections. They also nourish the cornea. Dry eye occurs when eyes produce less tears, or tears that are unable to keep the eye sufficiently lubricated.
Corneal disease is common in tropica1 and developing countries. Diseases affecting the cornea are a major cause of blindness worldwide, second only to cataract in overall importance. The prevalence of corneal disease varies from country to country and also from one population to another. While cataract is responsible for nearly 20 million of the 45 million blind people in the world, eye trauma and corneal ulceration are significant causes of corneal blindness. They are often underreported but may account for 1.5–2.0 million new cases of blindness every year.
In the United States, corneal diseases and injuries represent some of the most painful eye disorders and are the leading cause of visits to eye care practitioners. Over 10 million people visit health care centers for corneal problems each year and over 44,000 sight-restoring corneal transplants are performed each year. In 2004, 4,219,000 Americans had sight impaired by corneal dystrophies. Fuchs' dystrophy is slightly more common in women than in men. Although early signs of Fuchs' dystrophy are observed in the 30–40 age group, the disease rarely affects vision until people reach their 50s and 60s. Keratoconus is the most common corneal dystrophy, affecting one in every 2000 Americans. It is more prevalent in teenagers and adults in their 20s. Dry eye is more common in women, especially after menopause . An estimated 400,000 Americans have experienced some form of ocular herpes. Each year, nearly 50,000 new and recurring cases are diagnosed, with the more serious stromal keratitis accounting for approximately 25% of cases. Shingles is common in the elderly and rarely seen in younger adults, with approximately two-thirds of all cases occurring in persons over the age of 50.
Causes and symptoms
Corneal infections are caused by infectious agents. For instance, ocular herpes is caused by the herpes simplex virus, and shingles by the varicella—zoster virus, the same virus that causes chickenpox during childhood. The cause of dry eye has not been identified, but recent studies sponsored by the National Eye Institute (NEI) on the causes and mechanisms of tear deficiency have led to the suggestion that dry eye may involve inflammatory processes. Keratoconus studies show that the disease can stem from one of several possible causes, including hereditary factors, corneal injury, wearing hard contact lenses for many years, and eye diseases such as retinal disease (retinitis pigmentosa), retinal disease in premature infants (retinopathy of prematurity), and inflammation of the cornea and conjunctiva (vernal keratoconjunctivitis). Other diseases that affect the entire body (systemic diseases) have also been associated with keratoconus. Many causes of corneal ulcers have been identified. Wearers of soft contact lenses have an increased risk of ulcers if they neglect the cleaning, handling, and disinfection of their lenses and cases. Soft contact lenses are designed to have very high water content and can therefore easily absorb bacteria, such as Pseudomonas and other infectious agents if not cared for properly.
With its ability for quick repair, the cornea usually heals after most injuries or disease. However, the healing process may be prolonged, resulting in a variety of symptoms that may include:
- blurred vision
- corneal scarring
- decreased vision
- discharge from the eye
- dryness of the eye
- extreme sensitivity to light
- stinging or burning sensations in the eye
Corneal disease is diagnosed by performing a complete physical and a standard eye exam, often including the following tests:
- Ophthalmoscopy: Examination of the inside of the eye, especially the optic nerve, using a lighted instrument called an ophthalmoscope.
- Corneal topography: A technique that projects a series of illuminated rings onto the corneal surface, which are reflected back into the instrument. It can produce color maps showing the relative shape, power and elevation of the surface of the cornea.
- Ultrasound: An imaging technique that can measure corneal thickness.
Treatment of corneal disease depends on the underlying disease or condition. Various types of corneal disease can sometimes be treated with eye drops or tablets, and specialized contact lenses can be used as an alternative if scarring is minimal. Antibiotics and antiviral drugs can treat some kinds of corneal infection. In more severe cases of corneal disease, in which the cornea becomes permanently scarred or cloudy, a corneal graft or transplant may be performed (keratoplasty). In this procedure, a part of the cornea is removed and replaced with a similar piece from a donor eye, obtained from an eye bank. Graft surgery is usually done to improve the sight of the eye, but may also be required for pain or to repair a weak area. Following surgery, eye drops to help promote healing are usually prescribed for several months.
QUESTIONS TO ASK YOUR DOCTOR
- How do you decide whether keratoplasty is advisable?
- How long will I have to wait for treatment?
- How many operations have you performed?
- What are the risks and what is your complication rate?
- When can I resume normal activities and what further treatment is necessary?
Vitamin A and carotenoids are known to promote good vision. Research has also shown that vitamin A deficiency makes people more likely to develop infectious diseases and vision problems. Vitamin A is found in foods such as eggs, meat, milk, cheese, cream, liver, kidney, cod, and halibut fish oil. Retinol, an active form of vitamin A, is found in animal liver, whole milk, and some fortified foods. Carotenoids are color pigments found in plant foods such as carrots and tomatoes. They have been shown to have antioxidant properties that protect cells from damage caused by reactive substances called free radicals.
Phototherapeutic keratectomy (PTK) is one of the latest advances in eye care for the treatment of corneal dystrophies, corneal scars, and certain corneal infections. It involves the use of a laser to vaporize microscopically thin layers of diseased corneal tissue and etch away the surface irregularities associated with many corneal dystrophies and scars. New tissue can then grow over the repaired surface. Recovery from PTK only takes a few days, and not months as is the case with keratoplasty.
According to the NEI, corneal transplants are very common in the United States, with some 40,000 procedures performed each year. Successful outcomes for this operation have increased significantly in recent years. A NEI study suggests that matching the blood type, but not tissue type, of the recipient with that of the cornea donor may improve the success rate of corneal transplants in people at high risk for graft failure. Approximately 20% of corneal transplant patients reject their donor corneas.
Antioxidant —Any substance that reduces damage due to reactive oxygen such as that caused by free radicals.
Aqueous humor —The clear, watery fluid in the front of the eyeball.
Astigmatism —A refractive error caused by an irregular shape of the cornea.
Bowman's layer —Transparent cornea layer directly below the epithelium consisting of strong layered protein fibers called collagen.
Carotenoids —Red to yellow pigments responsible for the characteristic color of many plant organs or fruits, such as tomatoes, carrots, etc.
Cataract —Opacity or cloudiness of the eye lens, which may prevent a clear image from forming on the retina.
Conjunctiva —The thin, transparent tissue that covers the outer surface of the eye.
Cornea —Transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.
Descemet's membrane —Cornea layer beneath the stroma. It consists of thin but strong collagen fibers that serve as a protective barrier against infection and injuries.
Endothelium —Extremely thin, innermost layer of the cornea that pumps excess fluid out of the stroma.
Epithelium —Outermost layer of the cornea that protects the eye against foreign materials and absorbs oxygen and cell nutrients for the cornea.
Infectious —Caused by an infectious agent, such as a bacteria, or capable of being transmitted by infection, such as an infectious disease.
Iris —Pigmented tissue lying behind the cornea that gives color to the eye and controls amount of light entering the eye by varying the size of the pupil.
Keratoplasty —Surgical replacement (transplantation) of the cornea.
Lens —Transparent, biconvex crystalline tissue that helps bring rays of light to a focus on the retina.
Pupil —Variable—sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.
Refracted —Bent back as if broken.
Retina —Light—sensitive tissue at the back of the eye.
Sclera —Tough, opaque tissue that serves as the eye's protective outer coat. Also called “the white of the eye.”
Stroma —Cornea layer beneath Bowman's layer that represents 90% of the cornea. Consists mostly of water (78%) and collagen (16%).
Ultrasound —An imaging method in which high—frequency sound waves are used to outline a part of the body.
Varicella—zoster virus —A virus in the herpes family that causes chicken pox during childhood and may reactivate later in life to cause herpes zoster (shingles).
PTK reports very good outcomes. Vision can return rapidly, especially if the condition is confined to the top layer of the cornea. Studies have shown close to an 85% success rate in corneal repair using PTK for well-selected patients.
Injuries to the cornea can be prevented by wearing protective eye gear when necessary. Corneal infections can be prevented by obtaining rapid treatment for any disorders such as shingles, or herpes virus infection. Regular eye examinations are also essential to diagnose corneal disorders as soon as possible and prevent them from worsening.
Older individuals who suffer from corneal disease may associate symptoms with aging. Thus, care giver skills in identifying and treating corneal conditions in this patient group become very important. A general eye examination should always be included as part of the regular physical check-up.
Bohigian, George M. Handbook of Ocular Infections, Inflammation, and External Diseases. 4th ed., Thorofare, NJ: Slack, 2000.
Galloway, Nicholas R. Common Eye Diseases and their Management. 3rd ed., New York, NY: Springer, 2005.
Parker, James N., and Philip M. Parker, editors. The Official Patient's Sourcebook on Fuchs' Dystrophy. San Diego, CA: Icon Health Publications, 2006.
Parker, James N., and Philip M. Parker, editors. The Official Patient's Sourcebook on Keratoconus. San Diego, CA: Icon Health Publications, 2006.
Rapuano, Christopher J., and Wee-Jin Heng. Cornea: Color Atlas and Synopsis of Clinical Ophthalmology. New York, NY: McGraw-Hill, 2002.
Siegel, Mary-Ellen. Living with Shingles: New Hope for an Old Disease. New York, NY: M. Evans & Co., 2002.
Müller, A., et al. “The effects of corneal parameters on the assessment of endothelial cell density in the elderly eye.” British Journal of Ophthalmology 88, no. 3 (March 2004): 325–330.
Niederer, R. L., et al. “Age-related differences in the normal human cornea: A laser scanning in vivo confocal microscopy study.” British Journal of Ophthalmology 91, no. 9 (September 2007): 1165–1169.
Rüfer, F., et al. “Age-related changes in central and peripheral corneal thickness.” Cornea 26, no. 1 (January 2007): 1–5.
Corneal Disease. University of Connecticut Health Center, Patient Information Page. (Cited February 20, 2008). http://health.uchc.edu/clinicalservices/eyecare/cornealdisease.htm.
The Cornea and Corneal Disease. Cleveland Clinic, Health Extra. January 31, 2007 (Cited February 20, 2008), http://www.clevelandclinic.org/health/health-info/docs/1000/1000.asp?index=5638&src=newsp.
The Cornea and Corneal Disease. National Eye Institute, Fact Sheet. (Cited February 20, 2008). http://www.medhelp.org/gov/cornea.htm.
What is the cornea? National Eye Institute, Information Page. (Cited February 20, 2008). http://www.nei.nih.gov/health/cornealdisease/#0.
Cornea Research Foundation of America, 9002 N. Meridian St., Suite 212, Indianapolis, IN, 46260, (317) 844-5610, (317)814-2806, http://www.cornea.org.
Monique Laberge Ph.D.