What Is a Cornea?

Medically Reviewed by Zilpah Sheikh, MD on November 16, 2023
7 min read

Your cornea is your eye’s clear, protective outer layer. It acts like a barrier against dirt and germs, and it helps filter out some of the sun's damaging ultraviolet light. 

It also plays a key role in vision. As light enters your eye, it gets refracted, or bent, by the cornea’s curved edge. This helps determine how well your eye can focus on objects close up and far away. 

If your cornea is damaged by disease, infection, or an injury, the scars can affect your vision. They might block or distort light as it enters your eye.

Your cornea has six main layers:


The outermost layer protects your eyes from chemicals, water, and microbes and absorbs nutrients from tears and oxygen. It's the most sensitive part of the body.

Bowman's layer

The second layer is made up of a strong protein called collagen. It helps form the shape of your cornea.


The third layer is the thickest layer of your cornea. It's made up of water and proteins that strengthen and support your cornea. It's the most important layer for helping your eyes to focus. 

Dua's layer

This is the thinnest layer of your cornea. It was only recently discovered, and scientists aren't sure what its function is yet. 

Descemet’s membrane

This is a strong layer of tissue that protects your eye against infection and injury. 


The innermost layer is a very thin layer of cells on the back of the stroma. It works like a pump to drain excess fluid from the stroma. Without it, fluid would build up in the stroma and your cornea. Your cornea would get opaque and hazy, and so would your vision.

The term corneal disease refers to many conditions that affect this part of your eye. These include infections, tissue breakdown, and other disorders you get from your parents.

Symptoms depend on the type of cornea problem you have, but you might notice:

  • Pain
  • Blurred vision
  • Tearing 
  • Redness
  • Extreme sensitivity to light
  • Vision that gets worse over time
  • Pus or discharge from your eyes

Some cornea diseases don't have symptoms, especially in the beginning. 

There are several types of corneal diseases.


Sometimes called a corneal ulcer, keratitis causes inflammation and irritation. There are two types of keratitis:

Infectious keratitis: This is an infection that can be caused by bacteria, fungi, and viruses. It can also be caused by not having enough vitamin A. Not taking care of your contact lenses, either by not cleaning them well or wearing them for too long, can also cause infectious keratitis. 

Noninfectious keratitis: An injury to your cornea, something getting into your eye, dryness or irritation of the eye, or too much ultraviolet light getting into your eye causes this. It can also be caused by not taking care of your contact lenses or by allergies. 

Symptoms to look out for:

  • Eye pain
  • Blurred vision
  • Tearing
  • Redness
  • Sensitivity to light
  • Feeling like there's something in your eye

Treatment depends on the type of keratitis you have. Noninfectious keratitis can usually be treated with artificial tears. If you have the infectious type, you might need to use antibiotic or antifungal eyedrops. 

Herpetic eye disease

Ocular herpes (Herpes of the eye): Ocular herpes is caused by the herpes simplex virus (HSV). With this condition, your cornea might be inflamed, and you might have sores on your eyelids. It's important to see your doctor quickly because if it's not treated, it can cause permanent damage. Usually, it is treated with antiviral medication. The condition often is recurring, which means it comes back again and again. 

Herpes zoster (shingles): You can only get this eye disease if you had chickenpox. The itchy illness goes away, but the virus that causes it doesn’t leave your body. It stays in your nerves, but it isn’t active. Later in life, it can travel down those nerves and infect specific body parts like your eye. A shingles rash on the face can cause sores on your cornea. You should see your doctor for treatment because anything that hurts the eye can cause vision problems in the future. Your doctor can prescribe corticosteroid eyedrops and antiviral drugs.

There are several types. These diseases can cause problems with the cornea’s structure:

Corneal ectasia

Corneal ectasia is a group of conditions that affect your cornea. These disorders cause your cornea to thin and bulge outward.

Keratoconus: This type of corneal ectasia disease thins your cornea and changes its shape. It usually starts in your 20s and 30s, but it can also happen when you're a child. Your cornea becomes cone-shaped, and this change in shape distorts your vision. As the disease gets worse, you may have trouble seeing things close to you and far away. The disease can also cause swelling and scars on your cornea.

Causes include:

  • Genetics (it runs in families)
  • Eye trauma (from rubbing your eyes a lot)
  • Certain diseases, like atopic dermatitis, allergic dermatitis, allergic rhinitis, asthma, and Down syndrome

At first, glasses or soft contacts can solve the problem. As the disease goes on, you may need contact lenses specially made for the condition. For people with early keratoconus, a procedure called corneal crosslinking can be done. During the procedure, the doctor puts drops of vitamin B2 (riboflavin) into your eyes and exposes them to small amounts of ultraviolet (UV) light. 

A small number of people with keratoconus might need a cornea transplant. During this procedure, the doctor will replace your damaged cornea with a donated one. This operation is usually successful. But you probably will still need glasses or contacts to see clearly.

Corneal dystrophies 

Corneal dystrophy is a group of diseases caused by proteins building up on the cornea. Some of the most common are:

Map-dot-fingerprint dystrophy: This causes your cornea to develop folds that look like a map's continents, small dots, or tiny fingerprints.

It's most common in people who are over 40. The folds can cause vision problems and lead to the cornea wearing away. This can cause symptoms like:

  • Eye pain the morning that feels better later in the day
  • Blurry vision
  • A feeling of something in your eye
  • Teary eyes
  • Light sensitivity

This condition is usually treated with eyedrops, ointment, a special eye patch, or special contact lenses to protect the eye. In some cases, you might need surgery to correct the problem. 

Fuchs' dystrophy: This condition causes cells in the eye to stop working, which leads to your cornea swelling. It can cause serious vision problems, pain, and swelling of the cornea. You also might:

  • Have blurred vision in the morning that improves later in the day
  • See a glare or a halo around objects in low light
  • Have clouded corneas
  • Be sensitive to light

You're more likely to have this condition if you're over 50. Treatment includes eyedrops, ointment, and special contact lenses you get from your doctor. If your case is severe, your doctor might suggest a cornea transplant. 

Lattice dystrophy: This causes growths on the cornea that look like a grid. They can make your cornea cloudy and reduce your vision, and they could wear down your corneal layers.

Treatments include prescription eyedrops, ointments, eye patches, or a corneal transplant if your condition is more severe.

Bullous keratopathy: Bullous keratopathy causes your cornea to become permanently swollen. It happens because the inner layer of your cornea is damaged. Symptoms can include blurred vision, sensitivity to light, and feeling that you have a foreign object in your eye.

You may get bullous keratopathy after eye surgery, a trauma, or a diagnosis of glaucoma.

Corneal abrasion: A corneal abrasion is a scratch or scrape on the surface of your cornea. This injury can happen due to many causes, such as contact with fingernails, dirt, dust, wood shavings, sand, or metal particles.

Some symptoms of a corneal abrasion are:

  • Pain
  • Watery eyes
  • Red eyes
  • Blurry vision
  • Feeling like something is stuck in your eye
  • Sensitivity to light
  • Swollen eyes

Iridocorneal endothelial syndrome (ICE): ICE is a rare form of glaucoma that usually affects just one eye. It happens when corneal cells move to the surface of the iris. If you have ICE, you may notice blurry vision, pain, or changes in your iris or pupil.

The main symptoms of ICE are:

  • Swelling of the cornea
  • Changes in the iris, the colored part of your eye
  • A form of glaucoma, which is an eye disease that causes slow vision loss

Pterygium: Pterygium, which is also called “surfer’s eye,” is a raised growth on your cornea. Your eyes may be irritated, red, or swollen. If it grows large enough, pterygium can cover part of your cornea and affect your vision.

The main cause of pterygium is a lot of exposure to UV light, wind, and dust.

Your doctor will examine your eye and do some tests to diagnose the problem. Tests for cornea problems include:

Slit lamp exam: Your doctor uses a special microscope with a bright light to see the different parts of your eyes. They will customize the brightness and thickness of the light’s beam. Before this test, you may need to have your pupils dilated, or made more open, with eyedrops. This lets more light into your eye, so your doctor can get a better view.

Fluorescein eye stain test: With this test, your doctor puts a special type of dye, called fluorescein dye, into your eye. Then, they shine a blue light at your eye. The dye makes scratches or other damage easier to see. 

You can't always prevent cornea problems, but you can take steps to reduce your risk:

  • Keep your contact lenses clean.
  • Don’t sleep with contact lenses in, even if the package says you can.
  • Wear safety goggles when using machinery or working with chemicals. This includes doing yard work and using tools like hammers and saws.
  • Wear protective eyewear when playing sports where something might get in your eye.
  • Visit your doctor if you get something stuck in your eye. Don't try to remove it yourself.
  • Get regular eye exams to catch eye problems early.