What is the cornea?
The cornea is the eye’s outermost layer. It is the clear, dome-shaped surface that covers the front of the eye.
Structure of the Cornea
Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and proteins. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas.
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:
- 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.
- 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.
What are some diseases and disorders affecting the cornea?
Some diseases and disorders of the cornea are:
- Allergies affecting the eye are fairly common. The most common allergies are those related to pollen, particularly when the weather is warm and dry. Symptoms can include redness, itching, tearing, burning, stinging, and watery discharge, although they are not usually severe enough to require medical attention. Antihistamine decongestant eyedrops can effectively reduce these symptoms, as does rain and cooler weather, which decreases the amount of pollen in the air.
- An increasing number of eye allergy cases are related to medications and contact lens wear. Also, animal hair and certain cosmetics, such as mascara, face creams, and eyebrow pencil, can cause allergies that affect the eye. Touching or rubbing eyes after handling nail polish, soaps, or chemicals may cause an allergic reaction. Some people have sensitivity to lip gloss and eye makeup. Allergy symptoms are temporary and can eliminated by not having contact with the offending cosmetic or detergent.
Conjunctivitis (Pink Eye)
- This term describes a group of diseases that cause swelling, itching, burning, and redness of the conjunctiva, the protective membrane that lines the eyelids and covers exposed areas of the sclera, or white of the eye. Conjunctivitis can spread from one person to another and affects millions of Americans at any given time. Conjunctivitis can be caused by a bacterial or viral infection, allergy, environmental irritants, a contact lens product, eyedrops, or eye ointments.
- At its onset, conjunctivitis is usually painless and does not adversely affect vision. The infection will clear in most cases without requiring medical care. But for some forms of conjunctivitis, treatment will be needed. If treatment is delayed, the infection may worsen and cause corneal inflammation and a loss of vision.
- Sometimes the cornea is damaged after a foreign object has penetrated the tissue, such as from a poke in the eye. At other times, bacteria or fungi from a contaminated contact lens can pass into the cornea. Situations like these can cause painful inflammation and corneal infections called keratitis. These infections can reduce visual clarity, produce corneal discharges, and perhaps erode the cornea. Corneal infections can also lead to corneal scarring, which can impair vision and may require a corneal transplant.
- As a general rule, the deeper the corneal infection, the more severe the symptoms and complications. It should be noted that corneal infections, although relatively infrequent, are the most serious complication of contact lens wear.
- Minor corneal infections are commonly treated with anti-bacterial eye drops. If the problem is severe, it may require more intensive antibiotic or anti-fungal treatment to eliminate the infection, as well as steroid eye drops to reduce inflammation. Frequent visits to an eye care professional may be necessary for several months to eliminate the problem.
- The continuous production and drainage of tears is important to the eye’s health. Tears keep the eye moist, help wounds heal, and protect against eye infection. In people with dry eye, the eye produces fewer or less quality tears and is unable to keep its surface lubricated and comfortable.
- The tear film consists of three layers–an outer, oily (lipid) layer that keeps tears from evaporating too quickly and helps tears remain on the eye; a middle (aqueous) layer that nourishes the cornea and conjunctiva; and a bottom (mucin) layer that helps to spread the aqueous layer across the eye to ensure that the eye remains wet. As we age, the eyes usually produce fewer tears. Also, in some cases, the lipid and mucin layers produced by the eye are of such poor quality that tears cannot remain in the eye long enough to keep the eye sufficiently lubricated.
- The main symptom of dry eye is usually a scratchy or sandy feeling as if something is in the eye. Other symptoms may include stinging or burning of the eye; episodes of excess tearing that follow periods of very dry sensation; a stringy discharge from the eye; and pain and redness of the eye. Sometimes people with dry eye experience heaviness of the eyelids or blurred, changing, or decreased vision, although loss of vision is uncommon.
- Dry eye is more common in women, especially after menopause. Surprisingly, some people with dry eye may have tears that run down their cheeks. This is because the eye may be producing less of the lipid and mucin layers of the tear film, which help keep tears in the eye. When this happens, tears do not stay in the eye long enough to thoroughly moisten it.
- Dry eye can occur in climates with dry air, as well as with the use of some drugs, including antihistamines, nasal decongestants, tranquilizers, and anti-depressant drugs. People with dry eye should let their health care providers know all the medications they are taking, since some of them may intensify dry eye symptoms.
- People with connective tissue diseases, such as rheumatoid arthritis, can also develop dry eye. It is important to note that dry eye is sometimes a symptom of Sjögren’s syndrome, a disease that attacks the body’s lubricating glands, such as the tear and salivary glands. A complete physical examination may diagnose any underlying diseases.
- Artificial tears, which lubricate the eye, are the principal treatment for dry eye. They are available over-the-counter as eye drops. Sterile ointments are sometimes used at night to help prevent the eye from drying. Using humidifiers, wearing wrap-around glasses when outside, and avoiding outside windy and dry conditions may bring relief. For people with severe cases of dry eye, temporary or permanent closure of the tear drain (small openings at the inner corner of the eyelids where tears drain from the eye) may be helpful.
- This disorder–a progressive thinning of the cornea–is the most common corneal dystrophy in the U.S., affecting one in every 2000 Americans. It is more prevalent in teenagers and adults in their 20s. Keratoconus 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 (nearsightedness) of vision. Keratoconus may also cause swelling and a sight-impairing scarring of the tissue.
- Studies indicate that keratoconus stems from one of several possible causes:
- An inherited corneal abnormality. About seven percent of those with the condition have a family history of keratoconus.
- An eye injury, i.e., excessive eye rubbing or wearing hard contact lenses for many years.
- Certain eye diseases, such as retinitis pigmentosa, retinopathy of prematurity, and vernal keratoconjunctivitis.
- Systemic diseases, such as Leber’s congenital amaurosis, Ehlers-Danlos syndrome, Down syndrome, and osteogenesis imperfecta.
- Keratoconus usually affects both eyes. At first, people can correct their vision with eyeglasses. But as the astigmatism worsens, they must rely on specially fitted contact lenses to reduce the distortion and provide better vision. Although finding a comfortable contact lens can be an extremely frustrating and difficult process, it is crucial because a poorly fitting lens could further damage the cornea and make wearing a contact lens intolerable.
- 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 corneal transplant may be needed. This operation is successful in more than 90 percent of those with advanced keratoconus. Several studies have also reported that 80 percent or more of these patients have 20/40 vision or better after the operation.
- A pterygium is a pinkish, triangular-shaped tissue growth on the cornea. Some pterygia grow slowly throughout a person’s life, while others stop growing after a certain point. A pterygium rarely grows so large that it begins to cover the pupil of the eye.
- Pterygia are more common in sunny climates and in the 20-40 age group. Scientists do not know what causes pterygia to develop. However, since people who have pterygia usually have spent a significant time outdoors, many doctors believe ultraviolet (UV) light from the sun may be a factor. In areas where sunlight is strong, wearing protective eyeglasses, sunglasses, and/or hats with brims are suggested. While some studies report a higher prevalence of pterygia in men than in women, this may reflect different rates of exposure to UV light.
- Because a pterygium is visible, many people want to have it removed for cosmetic reasons. It is usually not too noticeable unless it becomes red and swollen from dust or air pollutants. Surgery to remove a pterygium is not recommended unless it affects vision. If a pterygium is surgically removed, it may grow back, particularly if the patient is less than 40 years of age. Lubricants can reduce the redness and provide relief from the chronic irritation.