Cornea

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What is the Cornea?

The cornea is the circular, transparent membrane that covers the pupil. Its curved shape makes it work like a strong lens, focussing onto the retina all the light that enters your eye. (The retina is the photographic 'film' of the eye). The curvature is smoothly graded to give the highest quality focus to the light entering your eye, and is rounded in a similar way to a football.

To see well the cornea must be perfectly clear; if it is not transparent then the quality of sight will be reduced; if the opacification is marked there will be no useful sight.

Exposed to the air, the cornea needs to be constantly protected from damage from scratches, foreign bodies or infection. Damage to the cornea would lead to scarring and loss of sight and a set of defensive measures are present to protect it.

• It is bathed in tears which are refreshed every time you blink;
• It has more sensitivity to pain and touch than any other part of the body,
• Any slight touch of the surface produces a reflex closing of the eyelids and profuse watering.
• The tears also contain a natural antibiotic.

Internationally diseases of the cornea are common and a major cause of blindness. In the United Kingdom however, problems are less frequent but are important and may require specialist attention.

Common Diseases Of the Cornea

Common corneal conditions include:

• Astigmatism
• Herpes simplex
• Corneal infection
• Acanthamoeba keratitis
• Keratoconus
• Corneal oedema

You can also read more about Corneal Graft (Penetrating Keratoplasty)

Astigmatism

Distortion of the lens- shape of the cornea causes poor quality of focus. If the corneal shape is ovalled into a rugby ball shape then the focus of the eye will be distorted and this is called regular astigmatism. Usually this is mild and does not progress or change much during life. It is easily treated with glasses or contact lenses.

Surgical treatments are available now for more severe forms of regular astigmatism; the treatments include laser eye surgery and lens implantation with astigmatic lenses.

If the cornea is scarred as a result of infection or injury then the disturbance to the shape will result in very poor sight but a contact lens can be tried. It acts as a new 'front surface' of the eye and evens out these irregularities, with improvement in sight. Can be treated with laser, IOLs or even simply a pair of glasses.

Herpes simplex

About 60% of adults in the UK have been infected with Herpes simplex virus at some stage of their lives. For most of us this virus continues to live in our body causing absolutely no problems and we are unaware of its presence; others get sores around their lips, usually at the time of a cold - hence the term "cold sores".

In some patients however, the virus becomes active in the cornea causes a painful ulcer called a dendritic ulcer. This is treated with antiviral ointment and usually resolves in about ten days, and usually without any later problems or scarring.

Corneal infection

Corneal infections are serious problems that need urgent specialist attention. They are usually caused by bacteria, which attack and penetrate the surface of the cornea particularly if there is scratch or damage to the corneal surface. This tends to occur more frequently in people who are contact lens wearers (soft contact lens users and especially extended wear lens users); patients who have (very) dry eyes or who have lost corneal sensation. These high-risk groups are more likely to develop an infection that may lead on to an abscess (suppurative keratitis) or deeper infection of the eye.

Symptoms include pain and watering of the eyes, blurred vision, sensitivity to light (photophobia).

Untreated, the ulcers may go on to cause a deep infection of the whole of the eye. Loss of sight, dense scarring or even globe perforation with loss of the eye can occur. Prompt treatment is required.

Acanthamoeba keratitis

Is a rare infection of the cornea found almost exclusively in soft contact lens wearers who contaminate their lenses by washing them in unclean water (usually using bathroom tap water and not mains water) or who have poor standards of contact lens hygiene. The amoebae have the ability to penetrate the normal ocular.

The condition is excruciatingly painful and vision is rapidly blurred. If untreated this could lead to abscess formation, Iritis, Glaucoma or Scleritis. Urgent attention is required from an Ophthalmologist if severe pain is felt in the eye, particularly if you wear contact lenses regularly.

Keratoconus

Is a progressive condition of the cornea where its strength is reduced and the shape progressively changes resulting in increasing astigmatism. The cornea begins to bulge forwards causing irregular astigmatism, i.e. astigmatism that cannot be corrected by spectacles alone.

The disease tends to show itself in teenage or in the 20's; there may be long periods of stability. Initially the astigmatism is corrected by specialist contact lens fitting with gas permeable contact lenses; in the majority of patients the bulging of the cornea will continue and contact lens fitting becomes difficult or impossible. When this point is reached then the patient will usually need a corneal transplant, though surgery is not needed in all cases.

Corneal oedema

The internal fluids of the eye (aqueous humour) have a tendency to flood into the cornea, which would make it opaque with an almost bluish colour. The cells that line the inner layer of the eye (CORNEAL ENDOTHELIUM) are specialised and have the special function of keeping the cornea transparent by actively pumping back this fluid and thus prevent the cornea from becoming waterlogged. If the cornea becomes waterlogged, it indicates that these cells are either not working properly or that they have been damaged.

Unlike most other cell types in the body, these corneal endothelial cells are unable to repair or replace themselves and if the oedema (swelling/ oedema) persists, cells will need to be replaced surgically by corneal grafting.

As this condition tends to occur in older patients, cataracts are frequently also seen. Cataract surgery could cause more damage to the dwindling population of the cells. Special care is needed to prevent further damage and corneal oedema, (corneal decompensatation). If this occurs then a corneal graft will be needed.

Corneal Graft (Penetrating Keratoplasty)

It is 100 years now since the first corneal graft was performed, the first transplantation of an organ. A corneal specialist usually undertakes this highly successful operation.

Common reasons for requiring a corneal graft

• Corneal scarring (after injury, infection etc.)
• Corneal astigmatism not treatable by contact lenses (including Keratoconus)
• Corneal oedema (including Fuch's Corneal Dystrophy)

Surgery may be undertaken with the patient either awake with a local block usually with light sedation. The patient may go home immediately after the operation. The operation usually takes about 45 minutes.

A small, central, circular disc of the cornea is removed from the affected eye and is replaced by another disc of cornea taken from a donor eye. Very fine nylon sutures, which are so fine as to be virtually invisible to the naked eye, then suture the graft into position. Furthermore, the superficial layer of cells rapidly covers them and the patient is completely unaware of their presence.

Discomfort either during or following the operation is minimal. The patient is able to be immediately ambulant; there is no period of extended bed rest like there used to be in the early days of grafting.