Cornea

The cornea is the transparent tissue that covers the front of your eye. It lies in front of the iris and the pupil and has a slightly convex shape, a bit like a watch glass. Because of this dome-like shape and the transition from air to the denser corneal tissue, the light entering your eye is already significantly refracted before it reaches the lens of the eye. Around two-thirds of the total refraction of light in your eye takes place in the cornea.

The cornea contains no blood vessels, as these would cloud vision. Instead, it is nourished by the tear film on the outside and by the aqueous humour on the inside. The cornea obtains most of its oxygen from the air via the tear film, although a small amount can also be supplied via the blood vessels in the surrounding conjunctiva. The cornea is made up of five layers, from the outside in: the epithelium, Bowman’s layer, the stroma, Descemet’s membrane and the endothelium. Each layer has its own function, ranging from protection to regulating fluid balance.

Because the cornea needs to remain clear and has no blood vessels, it is vulnerable to drying out, scratches and infections. It does, however, contain many nerve endings, making it very sensitive. That is why an eyelash or a speck of dust in your eye immediately causes pain and brings tears to your eyes: your eye is trying to protect and flush the cornea.

Cornea and contact lenses

If you wear contact lenses, the lens rests directly on the tear film covering your cornea or, in the case of hard lenses, floats on a thin layer of tears. As the cornea obtains most of its oxygen from the air, it is important that your lens allows sufficient oxygen to pass through. Modern soft lenses are made from materials with high oxygen permeability, so your cornea can remain healthy even whilst wearing a lens. If you wear your lenses for longer than recommended or sleep in them when you shouldn’t, your cornea may not receive enough oxygen. This can lead to redness, swelling or, in severe cases, damage to the tissue.

Tear fluid and lubrication

The cornea relies on a stable tear film. This film keeps the surface smooth and clear, flushes away waste products and supplies oxygen. If your tear film is unstable or evaporates too quickly, your cornea can dry out and become irritated. You may notice this as a burning or gritty sensation, blurred vision or red eyes. If you suffer from dry or irritated eyes, have them assessed by your optician, optometrist or ophthalmologist.

Scratches and damage

The outermost layer of the cornea, the epithelium, heals itself fairly quickly in the case of superficial damage. A small scratch can heal within a day or two. Deeper scratches or injuries to the stroma or other layers heal more slowly and can sometimes leave scars that affect your vision. If you suspect that your cornea is damaged, for example after a blow to the eye or a sharp object entering your eye, contact your ophthalmologist immediately.

Corneal conditions

There are various conditions that can affect the cornea, such as keratitis (an inflammation), keratoconus (a bulge), or dystrophies (hereditary abnormalities in the structure). These conditions require medical advice and, in some cases, treatment or an adjustment to your contact lenses. If you suspect you have a corneal problem, please have it examined by an ophthalmologist or optometrist.