The cornea is the clear part at the front of the eye that allows light into the eye. The curvature of the cornea works with the eye’s lens to focus the light onto the retina, with the cornea contributing around two thirds of the eye’s optical power. The cornea comprises six layers.

The term corneal disease covers a range of conditions that affect the shape and clarity of the cornea, and includes:

• Infections.
• Genetic disorders.
• Degenerative conditions.
• Auto-immune disorders.
• Nutritional disorders.
• Inflammatory diseases.

Major corneal diseases

There are a number of common corneal diseases that cause problems with vision as well as pain and discomfort. The main corneal diseases are:

• Keratoconus – the central part of the cornea thins, causing it to bulge outwards in a cone shape. Keratoconus distorts vision, causing problems with glare, double vision, starbursts and light sensitivity. In advanced cases of keratoconus, the back of the cornea may rupture, allowing fluid into the cornea. This is called acute corneal hydrops and can cause blindness if not treated promptly.
• Corneal dystrophy – the cornea becomes cloudy, reducing vision. In Fuch’s dystrophy, this cloudiness may only appear in the morning and can wear off as the day progresses.
• Keratitis – the cornea becomes inflamed and painful and very itchy. Keratitis will impair sight while the cornea is inflamed.
• Corneal neovascularisation – small blood vessels grow into the cornea, reducing its clarity. This is often caused by older, cheaper contact lenses that starve the eye of oxygen.
• Corneal ulcer (ulcerative keratitis) – the epithelial layer of the cornea becomes inflamed. This condition is common in the tropics but rare in the UK.

Symptoms of corneal disease

While each of the major corneal diseases presents differently, there are several common symptoms of corneal disease, including:

• Blurred or cloudy vision.
• Pain or severe irritation.
• Light sensitivity.
• Problems adapting to changing light levels.

Diagnosing corneal disease

Your ophthalmologist will use a range of tests to diagnose corneal disease including:

• Standard eye test – to check for any drop in visual acuity.
• Glare test – checking how bright lights affect your vision.
• Direct examination – using a slit lamp to observe the cornea.
• Pressure test – to check the pressure within the cornea.
• Pachymetry – an ultrasound test to measure the thickness of the cornea.
• Cell count – to assess the number and size of cells in the endothelia layer of the cornea.

Treating corneal disease

Many corneal diseases can be treated with topical steroids, to reduce inflammation, and antibiotics or anti-fungals, to fight infections. These treatments are usually applied as eye drops.

In severe cases of corneal disease, it may be necessary to perform a corneal transplant, using donated tissue. Corneal transplant surgery can involve replacing the whole of the cornea – called full thickness transplants or penetrating keratoplasty – or just some layers of the cornea.

Four types of partial thickness corneal transplant are possible:

• Anterior lamellar keratoplasty (ALK) – which replaces the outermost layers of the cornea.
• Deep anterior lamellar keratoplasty (DALK) – which replaces the outermost layers plus the middle layers of the cornea.
• Descemet’s stripping endothelial keratoplasty (DSEK) – which replaces the membrane lining of the back of the cornea and some of the supporting tissue.
• Descemet’s membrane endothelial keratoplasty (DMEK) – which replaces only the back lining of the cornea.

The type of corneal replacement surgery you need depends on the type and severity of your corneal disease. It can take as long as two years to fully recover from a full thickness corneal transplant, whereas recovery from partial transplants can take as little as few days.

Corneal transplants may also be done as part of complex reconstruction of the eye, combined with lens replacement and iris reconstruction.