The cornea is the clear, protective outer layer of the eye. Along with the sclera (white of the eye), the cornea serves as a barrier against dirt, germs, and other particles that can harm the eye’s delicate components. The cornea is also capable of filtering out some amounts of the sun's ultraviolet light.
The cornea also plays a key role in vision. As light enters the eye, it is refracted, or bent, by the outside shape of the cornea. The curvature of this outer layer helps determine how well your eye can focus on objects close up and far away. If the cornea becomes damaged through disease, infection, or injury, the resulting scars or discoloration can interfere with vision by blocking or distorting light as it enters the eye.
There are three main layers of the cornea:
- Epithelium: The most superficial layer of the cornea, the epithelium stops outside matter from entering the eye. This layer also absorbs oxygen and nutrients from tears.
- Stroma: The stroma is the largest layer of the cornea and is found behind the epithelium. It is made up mostly of water and proteins that give it an elastic but solid form.
- Endothelium: The endothelium is a single layer of cells located between the stroma and the aqueous humor, the clear fluid found in the front and rear chambers of the eye. The endothelium works as a pump, expelling excess water as it is absorbed into the stroma. Without this specialized function, the stroma could become waterlogged and hazy and opaque in appearance, also reducing vision.
The term "corneal disease" refers to a variety of conditions that affect mainly the cornea. These include infections, degenerations, and many other disorders that may arise mostly as a result of heredity.
What are the symptoms of corneal conditions?
With its ability for quick repair, the cornea usually heals after most injury or disease. However, when there is deep injury to the cornea, the healing process may be prolonged, possibly resulting in a variety of symptoms, including:
- Blurred vision
- Extreme sensitivity to light
- Corneal scarring
If you experience any of these symptoms, seek care from an eye doctor. They may indicate a more serious problem or require special treatment.
What conditions can damage the cornea?
This is an inflammation of the cornea that sometimes occurs with infection after bacteria or fungi enter the cornea. These microorganisms can enter the eye after deep injury, causing infection, inflammation, and ulceration of the cornea. Though uncommon, this type of infection can also arise after injury from wearing contact lenses.
Symptoms of keratitis include:
- Severe pain
- Reduced visual clarity
- Corneal discharge
Treatment usually includes antibiotic or antifungal eye drops.
Ocular herpes (herpes of the eye)
This is a viral infection of the eye that may reoccur. The main cause of ocular herpes is the herpes simplex virus I (HSV I). This is the same virus that causes cold sores, but ocular herpes can also result from the sexually transmitted herpes simplex II virus (HSV II) that causes genital herpes.
Ocular herpes produces sores on the surface of the cornea; over time, the inflammation can spread deeper into the cornea and eye.
There is no cure for ocular herpes, but it can often be controlled with the use of antiviral drugs.
Herpes zoster (shingles)
Shingles is a recurrence of the chicken pox virus in people who have already had the disease. After the bout of chicken pox, this virus usually remains inactive within the nerves of the body. It can later travel down these nerves, infecting specific parts of the body, like the eye. Herpes zoster can cause blisters or lesions on the cornea, fever, and pain from nerve fibers. Corneal lesions usually heal by themselves, but antiviral treatment may reduce the inflammation.
Shingles can occur in anyone exposed to the chicken pox virus, but there is an increased risk in older adults (especially those over age 80) and people with a weakened immune system.
See your doctor if you have any symptoms of shingles.
There are more than 20 corneal dystrophies, diseases that cause structural problems with the cornea. Some of the most common are:
Keratoconus is a progressive disease in which the cornea thins and changes shape. Keratoconus changes the curvature of the cornea, creating either mild or severe distortion, called astigmatism, and usually nearsightedness. Keratoconus may also cause swelling and scarring of the cornea and vision loss.
Causes of keratoconus include:
- Genetics (a person can inherit a tendency to get the condition from a parent.)
- Eye trauma (for example, from rubbing your eyes or long-term use of hard contacts)
- Eye diseases, such as retinitis pigmentosa, retinopathy of prematurity, and vernal keratoconjunctivitis
- Other diseases, like Down syndrome, osteogenesis imperfecta, Addison’s disease, Leber’s congenital amaurosis, and Ehlers-Danlos Syndrome
At first, the condition is correctible with glasses or soft contact lenses. But, as the disease progresses, you may need to wear rigid gas permeable contact lenses.
A corneal transplantation may also be necessary. During this procedure, the damaged cornea is replaced with a donated cornea. This operation is successful in about 9 out of 10 people with advanced keratoconus. Even after a transplant, you most likely will need glasses or contact lenses to see clearly.
Map-dot-fingerprint dystrophy is the abnormal appearance of the basement membrane of the epithelium of the cornea. As this membrane that separates the epithelium and stroma grows irregularly (thicker in some places, thinner in others), findings that resemble maps, dots, and small fingerprints appear in the cornea.
Map-dot-fingerprint dystrophy usually affects adults between the ages of 40 and 70, or children as a result of heredity. It is usually painless and causes no vision loss, and sometimes clears up without treatment. In some cases, however, epithelial erosion may occur. Epithelial erosion can expose the nerves lining the cornea, causing severe pain. The cornea’s normal curvature may be altered, causing astigmatism and nearsightedness.
As the cornea is altered, vision may be blurry and accompanied by:
- Moderate to severe pain
- Increased sensitivity to light
- Excessive tearing
- A feeling that something is in your eye
Treatment may include an eye patch, eye drops, and ointments.
Fuch’s dystrophy is the gradual deterioration of endothelial cells for no apparent reason. As these cells thin over time, the cornea is less capable of removing water form the stroma, causing it to swell and distort vision. Haze and small blisters on the corneal surface may also appear.
As a slowly progressing disease, signs of Fuch’s dystrophy begin to appear in people 30 to 40 years old; however, the disease doesn’t normally affect vision until about 20 years later. It is more common in women than men.
An early sign of Fuch’s dystrophy includes awakening with blurred vision that gradually clears up during the day. As the disease worsens, swelling becomes more consistent and vision blurs.
- Soft contact lenses
- Drying of blisters with a hair dryer two or three times per day
- Corneal transplant
Lattice dystrophy is characterized by the presence of abnormal protein fibers throughout the stroma. Although this condition can occur at any age, early changes can be seen in childhood.
Lattice dystrophy gets its name from the clear overlapping lines of proteins in the stroma. This can make the cornea cloudy and reduce vision. In some people, these proteins may cause epithelial erosion.
Treatment of lattice dystrophy includes:
- Prescription eye drops
- An eye patch
- Corneal transplant
Symptoms may disappear with treatment, but a corneal transplant may be required. Although results of this surgery are very good, lattice dystrophy may reappear in transplant patients.
How is corneal disease diagnosed?
Corneal disease can only be diagnosed after a thorough examination by an ophthalmologist.
Can corneal disease be prevented?
The risk of infectious corneal disease caused by bacteria and viruses can be reduced by protecting the eye from injury and limiting physical contact with people who have contagious forms of conjunctivitis. Avoid sharing eye makeup, contact solution, lens cases, and eye drops with people who are infected. Wash your hands thoroughly with soap and warm water for at least 15 seconds after contact with an infected person.
Although corneal disease resulting from hereditary factors, like dystrophies, cannot be prevented, vision can often be preserved through early detection and treatment.