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What is the cornea?

The cornea is often described as the “window of the eye”, the cornea is a clear, round surface at the front of the eye. The cornea looks simple, but is actually a delicate piece of tissue made of complex layers and systems. It is also extremely important to healthy vision. The cornea, while resilient, is susceptible to many different diseases and disorders.

The cornea is made up of five separate layers, each with it’s own important function:

  • Epithelium. This is the outermost layer of the cornea. Its primary function is to protect the inner layers from bacteria, dust and water. It also provides a smooth surface to absorb oxygen and nutrients from tears, which are then distributed to the inner layers of the cornea.
  • Bowman’s membrane. The Bowman’s membrane is an extremely thin, clear film of tissue made of strong protein fibers called collagen. When injured, the Bowman’s layer has a high risk of scarring, which can lead to vision loss.
  • Stroma. Behind the Bowman’s layer lies the stroma. The stroma is the thickest layer of the cornea, making up for about 90% of the corneal thickness. The stroma is composed of water and collagen, which is important in maintaining its transparency and shape.
  • Descemet’s membrane. Behind the stroma is Descemet’s membrane. This thin, strong membrane protects against infection and injury, and repairs itself easily after injury.
  • Endothelium. The endothelium is the innermost layer. This thin layer contains endothelial cells, which are important in keeping the cornea clear. Fluid from inside the eye leaks slowly into the stroma. The endothelium is responsible for pumping excess fluid out of the stroma. Without a healthy endothelium, fluid would build up, leading to swelling of the cornea and causing vision loss (a condition called Fuch’s dystrophy).

Why are tears so important to the cornea?

Each time we blink, a thin layer of tears spreads over the eye. This tear film allows our eyelids to move smoothly over the corneal surface, prevents infection, and promotes healing. The tear film consists of three components: an outer oily (lipid) layer, a middle water (aqueous) layer and a bottom mucin layer.

Each component of the tear film is important in overall eye health:

  • Lipid layer. The outermost oil, or lipid, layer keeps tears from evaporating too quickly from the eye. When the oil layer is lacking, it leads to a condition called evaporative dry eye syndrome.
  • Aqueous layer. The middle aqueous layer nourishes the cornea and conjunctiva.
  • Mucin layer. The bottom mucin layer helps spread the aqueous layer over the eye, ensuring that the eye remains wet.

When a component of the tear film is not adequate, it can result in irritation and redness. Dry eye syndrome, a chronic condition in which the eye is unable to produce enough tears, may also develop. The tear film is extremely important to eye health, so it is important that any prolonged dryness or redness of the eyes is reported to your eye doctor.

Common conditions of the cornea


Allergies, most commonly brought on by pollen, are a very common culprit for itchy red eyes. Other symptoms include excessive watering, dry eyes, stinging and burning. Allergies do not generally require medical attention, and are most commonly treated with antihistamines (a medication that can be purchased over-the-counter in oral or eyedrop form).


The cornea is fairly resilient, and heals quickly on its own after minor scratches and injuries. However, deeper injuries can lead to corneal scarring. Corneal scarring can lead to further problems, and possible vision loss.


Keratitis is an inflammation of the cornea. The noninfectious type of keratitis can be caused by an injury, or by wearing contact lenses for too long. Infectious keratitis is the result of bacteria, viruses, fungi or parasites entering the eye. This can also be attributed to contact lens wearing, especially if poor hygiene is a factor. This infection is generally treated with antibacterial eye drops.

Dry eye syndrome

Dry eye syndrome is extremely common. It occurs when the eye either cannot produce enough tears, or produces poor quality tears. Symptoms of dry eye syndrome generally include a feeling of dryness, redness, the feeling that something is in your eye, and pain in the eye. Mild to moderate dry eye symptoms can usually be managed with over-the-counter lubricating eye drops. Severe cases of dry eye syndrome should be treated by a doctor, as prolonged dryness can cause more serious problems over a long period of time.


A corneal dystrophy is a condition in which the cornea loses clarity due to a buildup of materials which cloud the cornea. This type of condition in generally inherited, and happens in otherwise healthy people. There are several types of dystrophies:

  • Keratoconus. Keratoconus is one of the most common corneal dystrophies in the America. Keratoconus causes the cornea to become thin and bulge outward in a cone-like shape. This results is blurry, distorted vision. Keratoconus progresses slowly, and may require corneal transplant.
  • Fuch’s Dystrophy. Fuch’s dystrophy is a condition which causes corneal swelling and vision loss. This condition is caused fluid leaking into the cornea as a result of weakened endothelial cells. The excess fluid causes the cornea to thicken, swell and become cloudy. This condition may also be treated with a corneal transplant.

Here at OCLI, our cornea specialists are here to help you through diagnosis and treatment of a wide array of corneal diseases. If you have questions or concerns regarding the cornea, make your appointment with OCLI today!

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