The cornea is the transparent, protective outer layer of the eye. Alongside sclera (white of the eye), the cornea protects 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. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, it receives its nourishment from the tears and aqueous humor that fill the chamber behind it.
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. 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.
- Stroma: The stroma is the thickest 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.
A corneal disease refers to a number of conditions that affect the cornea. These include infections, degenerations, and many other disorders that may arise mostly as a result of heredity. Some of the common corneal diseases are:
- Keratitis: It 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. Symptoms of keratitis include severe pain, minimized visual clarity and 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 occur twice. Ocular herpes occur because of the herpes simplex virus I (HSV I) which is also responsible for cold sores. Ocular herpes produces sores on the surface of the cornea. There is no cure for ocular herpes, but it can often be managed 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. Post 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 on the cornea, fever, and pain from nerve fibers. Shingles can occur in anyone exposed to the chicken pox virus, but there is an increased risk in older adults and people with a weakened immune system.
- Keratoconus: 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. Keratoconus may also cause swelling and scarring of the cornea and vision loss.
- Initially, the condition is correctable 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.
- Fuch’s dystrophy: Fuch's dystrophy is a gradual deterioration of the innermost layer of the cornea, the corneal endothelium. As these cells weaken over time, the cornea may swell causing blurred vision. In the advanced stages of this condition, haze and small blisters on the corneal surface may appear, causing pain and irritation. Treatment for Fuch’s dystrophy includes constant observation, ointments and eye drops and corneal transplant surgery when the vision is significantly impaired or chronic pain is present.
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 infected people.