Your cornea is the outermost layer of your eye. It is clear and shaped like a dome. The cornea helps to shield the rest of the eye from germs, dust, and other harmful matter. It also helps your eye to focus. If you wear contact lenses, they float on top of your corneas.

Problems with the cornea include

• Refractive errors
• Allergies
• Infections
• Injuries
• Dystrophies - conditions in which parts of the cornea lose clarity due to a buildup of cloudy material

Treatments of corneal disorders include medicines, corneal transplantation, and corneal laser surgery.

The cornea is the clear outer layer at the front of the eye. The cornea helps your eye to focus light so you can see clearly.

The cornea and lens of your eye helps you focus. Refractive errors are vision problems that happen when the shape of the eye keeps you from focusing well. The cause could be the length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens.

Four common refractive errors are

• Myopia, or nearsightedness - clear vision close up but blurry in the distance
• Hyperopia, or farsightedness - clear vision in the distance but blurry close up
• Presbyopia - inability to focus close up as a result of aging
• Astigmatism - focus problems caused by the cornea

The most common symptom is blurred vision. Other symptoms may include double vision, haziness, glare or halos around bright lights, squinting, headaches, or eye strain.

Glasses or contact lenses can usually correct refractive errors. Laser eye surgery may also be a possibility.

The structure of your face helps protect your eyes from injury. Still, injuries can damage your eye, sometimes severely enough that you could lose your vision. Most eye injuries are preventable. If you play sports or work in certain jobs, you may need protection.

The most common type of injury happens when something irritates the outer surface of your eye. Certain jobs such as industrial jobs or hobbies such as carpentry make this type of injury more likely. It's also more likely if you wear contact lenses.

Chemicals or heat can burn your eyes. With chemicals, the pain may cause you to close your eyes. This traps the irritant next to the eye and may cause more damage. You should wash out your eye right away while you wait for medical help.

Your eyes can get infections from bacteria, fungi, or viruses. Eye infections can occur in different parts of the eye and can affect just one eye or both. Two common eye infections are

• Conjunctivitis - also known as pinkeye. Conjunctivitis is often due to an infection. Children frequently get it, and it is very contagious.
• Stye - a bump on the eyelid that happens when bacteria from your skin get into the hair follicle of an eyelash.

Symptoms of eye infections may include redness, itching, swelling, discharge, pain, or problems with vision. Treatment depends on the cause of the infection and may include compresses, eye drops, creams, or antibiotics.

Some eye problems are minor and don't last long. But some can lead to a permanent loss of vision.

Common eye problems include

• Refractive errors
• Cataracts - clouded lenses
• Optic nerve disorders, including glaucoma
• Retinal disorders - problems with the nerve layer at the back of the eye
• Macular degeneration - a disease that destroys sharp, central vision
• Diabetic eye problems
• Conjunctivitis - an infection also known as pinkeye

Your best defense is to have regular checkups, because eye diseases do not always have symptoms. Early detection and treatment could prevent vision loss. See an eye care professional right away if you have a sudden change in vision, if everything looks dim, or if you see flashes of light. Other symptoms that need quick attention are pain, double vision, fluid coming from the eye, and inflammation.

Keratoconus is usually diagnosed in teens and young adults. It causes the middle and lower parts of the cornea to get thinner over time. While a normal cornea has a rounded shape, a cornea with keratoconus can bulge outward and become a cone shape. This different cornea shape can cause vision problems.

Symptoms of keratoconus include:

• Itchy eyes
• Double vision
• Blurry vision
• Nearsightedness (when far-away objects look blurry)
• Astigmatism (when things look blurry or distorted)
• Sensitivity to light

As keratoconus gets worse, it may cause eye pain and more serious vision problems.

Most people with keratoconus can correct their vision problems by wearing glasses, soft contact lenses, or special hard contact lenses that change the shape of the cornea. Your doctor may also recommend a procedure called corneal cross-linking to strengthen your cornea. If your keratoconus causes severe corneal scarring or you have trouble wearing contact lenses, you may need a corneal transplant.

If you have severe damage to your cornea (the clear front layer of your eye), doctors can replace the damaged part with healthy corneal tissue from a donor.

If you have scarring or other damage that affects the whole cornea, doctors can do a full thickness corneal transplant (called a penetrating keratoplasty). If only part of your cornea is damaged, doctors can do a partial thickness transplant (called a lamellar keratoplasty).

What happens during a corneal transplant?

You may get general anesthesia to put you to sleep during the transplant surgery, or you may be awake. If you’re awake, your doctor will put medicine in your eye to make it numb and give you another medicine to help you relax.

Your doctor will use a special tool to keep your eye open during surgery. They will remove the damaged part of your cornea and replace it with healthy donor tissue.

How long does it take to recover?

Corneal transplant is an outpatient surgery, so you can go home the same day. You won’t be able to drive, so you’ll need someone to give you a ride home after surgery.

You’ll need a follow-up appointment the day after surgery to check how your eye is healing.

After surgery, you’ll need to take steps to help your eye recover:

• Use special eye drops prescribed by your doctor
• Avoid rubbing or pressing on your eye
• Wear eyeglasses or a special shield to protect your eye

Depending on the type of transplant, it can take up to a year to fully recover. Talk with your doctor about when you can get back to your normal activities.

When to get help right away

Cornea rejection can cause:

• Eye pain
• Sensitivity to light
• Red eyes
• Cloudy or hazy vision

If you have these symptoms after a corneal transplant, tell your eye doctor right away.

Are there any side effects?

Like any surgery, corneal transplant surgery has risks. One major risk is tissue rejection, when your body sees the new cornea as a foreign object and tries to get rid of it. Your doctor can give you medicine to help stop the rejection and save your cornea.

Corneal transplant can also cause other eye problems, including:

• Infection
• Bleeding from the eye
• Retinal detachment
• Glaucoma

If you have tissue rejection or other severe problems with your new cornea, you may need another transplant. Talk with your doctor about the risks of corneal transplant and whether this treatment is right for you.

There are several common conditions that affect the cornea.

Injuries. Small abrasions (scratches) on the cornea usually heal on their own. Deeper scratches or other injuries can cause corneal scarring and vision problems.

Allergies. Allergies to pollen can irritate the eyes and cause allergic conjunctivitis (pink eye). This can make your eyes red, itchy, and watery.

Keratitis. Keratitis is inflammation (redness and swelling) of the cornea. Infections related to contact lenses are the most common cause of keratitis.

Dry eye. Dry eye happens when your eyes don’t make enough tears to stay wet. This can be uncomfortable and may cause vision problems.

Corneal dystrophies. Corneal dystrophies cause cloudy vision when material builds up on the cornea. These diseases usually run in families.

There are also a number of less common diseases that can affect the cornea — including ocular herpes, Stevens-Johnson Syndrome, iridocorneal endothelial syndrome, and pterygium.

If you’ve had chickenpox, you’re at risk for shingles (also called herpes zoster). Shingles happens when the chickenpox virus gets reactivated in your nerve cells, usually many years after you had the chickenpox. When shingles affects the cornea, it can cause inflammation (swelling) and scarring.

Your eye doctor can prescribe antiviral medicine to help shingles go away faster and prevent damage to your cornea. If you get shingles on your face or in your nose or eyes, it’s important to get a comprehensive eye exam to check for cornea problems.

When a herpes virus (the type of virus that causes cold sores and genital herpes) infects the eye, it can cause sores on the eyelid or the outer layer of the cornea. This is called ocular (eye) herpes.

If an ocular herpes infection spreads deeper into the cornea or the other layers of the eye, it can become a serious eye infection called keratitis. Keratitis can cause corneal scarring and vision loss.

If you have ocular herpes, your eye doctor can prescribe antiviral medicine to help control the virus and prevent damage to your cornea.

ICE is most common in women ages 30 to 50. It happens when a type of corneal cells called endothelial cells move from the cornea into the iris (the colored part of the eye). When these cells move, they can block eye fluid from draining and raise eye pressure.

ICE has 3 main symptoms:

• Changes in the shape of the iris or pupil
• Swelling in the cornea
• Glaucoma

Your eye doctor can prescribe medicine to treat the corneal swelling and glaucoma. If the damage to the cornea is severe, you may need a corneal transplant.

A pterygium is pink-colored growth on the cornea shaped like a wing or triangle. It’s most common in adults ages 20 to 40 who spend a lot of time outdoors in the sun.

A pterygium may cause redness or irritate your eye. Eye drops can help with these symptoms. If a pterygium is large enough to cause vision problems, you may need surgery to remove it.

To protect your eyes and lower your risk of a pterygium, wear sunglasses or a wide-brimmed hat when you’re in the sun.

Stevens-­Johnson syndrome (SJS), also called erythema multiforme major, is a rare skin disorder that also affects the eyes. SJS can cause serious eye problems that may lead to vision loss, including:

• Severe pink eye (conjunctivitis)

• Iritis (inflammation inside the eye)

• Serious corneal damage

SJS can happen as an allergic reaction to a drug or medication, or as part of a viral infection. Anyone can get it, but it’s more common in men and in children and young adults under age 30.

If you have SJS, your eye doctor may prescribe medicines like antibiotics or a corticosteroid, as well as a type of eye drops called artificial tears. With treatment, SJS usually goes away over time, but people who have had SJS before are more likely to get it again.

Some corneal conditions, like corneal dystrophies, run in families. But there are steps you can take to lower your risk of corneal injuries and infections.

To prevent corneal injuries, wear protective eyewear when you:

• Play sports that use a ball or puck, like baseball or hockey
• Do yardwork, like mowing the lawn or using a weedwhacker
• Make repairs, like painting or hammering
• Use machines, like sanders or drills
• Use chemicals, like bleach or pesticides

If you wear contact lenses, always follow the instructions to clean, disinfect, and store your lenses. This can help prevent corneal infections, like keratitis.

Eye doctors can check for corneal conditions as part of a comprehensive eye exam. The exam is simple and painless.

To check for corneal abrasions (scratches), your eye doctor may use a special type of eye drops called fluorescein dye. The dye makes corneal abrasions easier to see.

Many corneal conditions can be treated with prescription eye drops or pills. If you have advanced corneal disease, you may need a different treatment.

Laser treatment. To treat some corneal dystrophies and other conditions, doctors can use a type of laser treatment called phototherapeutic keratectomy (PTK) to reshape the cornea, remove scar tissue, and make vision clearer.

Corneal transplant surgery. If the damage to your cornea can’t be repaired, doctors can remove the damaged part and replace it with healthy corneal tissue from a donor.

Artificial cornea. As an alternative to corneal transplant, doctors can replace a damaged cornea with an artificial cornea, called a keratoprosthesis (KPro).

Did you know?

• Corneal transplants are the most common type of transplant surgery
• In 2014, more than 47,000 people in the United States got a corneal transplant

• Try blinking several times
• Try rinsing your eye with clean water or saline (salt) solution
• Try pulling your upper eyelid down over your lower eyelid
• Don’t rub your eye — you could scratch your cornea
• If an object is stuck in your eye, don’t try to remove it yourself — go to your eye doctor or the emergency room

Go to the eye doctor or the emergency room if you have:

• Intense eye pain
• Change in vision
• Blurry vision
• Very red, watery eyes
• An object stuck in your eye
• A serious eye injury or trauma — like getting hit hard in the eye

NEI-supported researchers are always working to better understand, diagnose, and treat conditions that affect the cornea (the clear front layer of the eye). Learn about some recent developments in corneal research.

Corneal wound healing

NEI supports research to understand how the cornea naturally heals after injury, including how certain proteins help to close a layer of the cornea (called the epithelial layer) over the wound. This research could help find new treatments to fix damaged corneas.

Corneal transplants

While most corneal transplants are successful, organ rejection can cause some corneal transplants to fail. NEI supports research into the causes of rejection and new ways to prevent it. One goal is to find alternatives to the immunosuppressive medicines people have to take after transplant surgery, because these medicines can increase the risk of infection.

Artificial corneas

The need for corneal transplants is greater than the supply of donor corneas. NEI supports research to develop new types of artificial corneas to fill this need, including:

• An artificial cornea that combines live corneal cells with other materials
• A folding artificial cornea that doctors can inject into the eye through a very small opening

Corneal dystrophies

NEI supports research to improve early diagnosis and treatment of corneal dystrophies, like keratoconus and Fuchs’ dystrophy. Some recent research aims to:

• Diagnose keratoconus using a high-resolution ultrasound to measure the thickness of corneal layers
• Understand how genetic risk factors for keratoconus affect corneal cells, which could help find new treatments
• Treat keratoconus with corneal collagen cross-linking, a minimally invasive surgery that strengthens the cornea using UV light
• Find the genetic mutations associated with Fuchs’ dystrophy, which could help find new gene therapy treatments

Ocular herpes

Herpetic stromal keratitis caused by ocular herpes is the most common cause of corneal blindness in the developed world. One group of researchers is developing a vaccine to prevent herpes from infecting the cornea.

Stem cell research

NEI supports research to develop endothelial stem cell lines. This research aims to:

• Create stem cell lines that researchers can use to test new treatments in the lab
• Implant stem cells in the eye to grow new, healthy corneal tissue after the cornea is damaged