Corneal dystrophies

Corneal dystrophies are relatively rare eye conditions and most tend to occur in the first few decades of life. The exception is Fuchs' Dystrophy, which appears much later.

About corneal dystrophies

Most corneal dystrophies run in families and if a corneal dystrophy is discovered in one family member, then all other adult family members should be examined. While they can lead to serious sight loss, they only account for roughly 0.5 per cent (half of one percent) of all those registered as blind and partially sighted.

There are several different corneal dystrophies and this page will describe the eight most common types.

About the cornea

The cornea is the clear outer layer covering the front of the eye. Normally it is smooth and as transparent as glass but it is also very strong. It helps the eye in two ways:

Firstly it acts as a barrier between the eye and the outside world and helps to protect the eye from injury and infection. The cornea is extremely sensitive and can detect the presence of even the smallest piece of dirt or fluff.

And secondly, it is the eye's outermost lens, controlling and focusing the entry of light into the eye. The cornea bends the incoming light onto the lens. The lens further focuses the light onto the retina at the back of the eye, which converts the light into images which are then sent to the brain.

Therefore, the cornea is vital for maintaining both the health of the eye and good, clear vision. It is normally able to cope well with minor injuries.

Structure of the cornea

The cornea is unusual in that it has no blood vessels to provide its nourishment and oxygen. Instead, it receives its nourishment from the tears and the fluid (the aqueous humour) filling the space behind it. The cornea must remain completely transparent if it is to carry out its role as the eye's outermost lens and the presence of even the smallest imperfection can interfere with this process.

The cornea consists of five distinct layers, each of which plays an important role.

Epithelium

The cornea's outermost layer. It acts as a barrier to protect the rest of the cornea from foreign bodies and infections. It is filled with tiny nerve endings which means the cornea is very sensitive when touched. It also provides an extremely smooth surface, which is essential for good vision.

Bowman's Layer

Lying directly beneath the epithelium, this is a thin, transparent sheet of tissue. If damaged, Bowman's layer can form a scar when it heals.

Stroma

The stroma accounts for roughly 90 per cent of the cornea's thickness. It consists mainly of water (75 per cent) and fibres of a material called collagen, which is very tough and elastic. The regular size and arrangement of the collagen in this layer gives the cornea its strength and elasticity, and ensures it remains transparent.

Descemet's Membrane

Descemet's Membrane (pronounced Dess-e-mays), is a thin, strong layer of tissue which acts as another protective barrier.

Endothelium

This is the innermost layer of the cornea and is extremely thin. It is vital to the transparency of the cornea because it acts as a pump and maintains the exact level of water in the stroma. Without this pump, the cornea would swell with water and would become hazy. Damage to this layer cannot be repaired and so can, therefore, cause swelling of the cornea which may lead to serious sight loss (please see section on Fuchs' Endothelial Dystrophy below).

Effect of corneal dystrophies on the eye

Corneal dystrophies are rare conditions in which the cornea is altered without the presence of any inflammation, infection or other eye disease. The clearness (transparency) of the cornea is affected and vision may or may not be disturbed. Corneal dystrophies tend to run in families. They have been described in many different ways but because each dystrophy will start in a particular layer of the cornea, they are classified as epithelial dystrophies, stromal dystrophies or endothelial dystrophies.

Recurrent corneal erosion

A common symptom of some corneal dystrophies is a 'foreign body' sensation caused by breakdown of the surface epithelial layer. This is a similar feeling to when an eyelash or a piece of dirt gets into the eye, but in some cases can be a lot more severe. The cause is actually due to changes in the outermost epithelial layer, which expose the sensitive nerve endings. With treatment, this layer can heal quickly.

Inherited corneal dystrophies

There are two methods by which corneal dystrophies can be inherited. The first is called 'autosomal dominant' and the second 'autosomal recessive'.

The 'dominant' method means that one of the parents actually has the condition. Each time that parent has a child, there is a 50 per cent (one in two) chance of the child having the condition as well.

The 'recessive' method is slightly more complicated. Neither of the parents actually has the condition themselves but they will both be 'carriers' of the condition. If two 'carriers' have children, there is a 25 per cent (one in four) chance that each child will have the condition.

In either case, both men and women can be affected.

Types of corneal dystrophy

Epithelial dystrophies

There are three main epithelial dystrophies. They are called epithelial dystrophies because they affect the epithelial layer of the cornea.

Meesman's dystrophy

This is a very rare condition, which tends to affect both eyes equally and whose cause is unknown. It is inherited by the 'dominant' method and may become apparent as early as the first year of life. Symptoms may not occur until early adulthood or middle age. The main symptom will be an occasional 'foreign body' sensation and any treatment will be directed at relieving this. Most people retain good vision.

Epithelial basement membrane dystrophy

This is the most common epithelial dystrophy and is inherited by the 'dominant' method. Most people with this condition have no symptoms, but painful 'foreign body' sensations and temporary blurring of the vision can occur after the age of 30 years. Treatment is usually directed at relieving the 'foreign body' sensation and may include temporary eye patching while using antibiotic eye drops and the use of an eye ointment at night to maintain a lubricating layer between the eyelid and the cornea during sleep. In certain cases a 'bandage' soft contact lens is used. The contact lens serves to protect the cornea, allowing it to heal, and may be worn for up to two months or more. The long-term prospects for someone with this condition are good. This dystrophy used to be called map dot fingerprint dystrophy or Cogan's dystrophy.

Reis-Bücklers' dystrophy

This condition affects both eyes equally and tends to be located centrally in the cornea. It is inherited by the 'dominant' method and first appears in childhood. The symptoms appear fairly early on and include light sensitivity and 'foreign body' sensations. Vision is affected because the surface of the cornea becomes hazy and irregular. Advanced cases may require a corneal transplant, but the dystrophy can recur quickly, even if the graft is successful. For this reason, the graft may be delayed until it is absolutely necessary.

Stromal dystrophies

Granular dystrophy

This is inherited by the 'dominant' method and usually occurs before the age of 20 years. Early on the vision is not affected but grey-ish dots can be seen through a microscope. Slowly the dots become larger, and more appear. They gradually become visible to the naked eye. Light sensitivity and 'foreign body' sensation may be a problem. Vision is not usually severely affected under the age of 50 years. If there is serious sight loss, a corneal transplant may be required. However, the dystrophy may recur within five years of the transplant being carried out.

Macular dystrophy

This dystrophy is inherited by the 'recessive' method, which means that it may be more severe than other types of dystrophy. It may appear before the age of 20 years. Irregular, cloudy areas appear in both corneas which gradually merge together. Very occasionally, someone with this type of dystrophy may become light sensitive and need to wear tinted glasses. Often by the age of 20 to 30 years there will be such serious sight loss that a corneal transplant will be required. A transplant will have a favourable outcome although it is possible that the dystrophy will recur in the donor cornea. If this does happen it can take, on average, 20 years.

Lattice dystrophy

This dystrophy usually begins before the age of 20 years, and is inherited by the 'dominant' method. Early symptoms tend to be a 'foreign body' sensation and a slight deterioration in vision. Under a microscope, very fine, overlapping lines will be seen in the cornea. As the dystrophy progresses, these lines become thicker and may make the cornea become hazy. Treatment for this type of dystrophy is aimed at relieving the 'foreign body' sensation, and, if there is serious sight loss, a corneal graft is performed. The results of the graft are generally good, but it is possible that the dystrophy may recur in the donor cornea within five to 10 years

Endothelial dystrophies

Fuchs' endothelial dystrophy

Fuchs' Dystrophy is more common in women than in men. Most affected individuals have no family history and in these, the disorder is unlikely to have been inherited. It usually begins after the age of 40 years, at which stage there are no symptoms. With time there is a painless deterioration of vision and glare becomes a problem. Vision will be worse first thing in the morning. The next stage will involve some painful episodes due to tiny blisters on the cornea, which will gradually disappear as the vision gets worse. Finally, there will be serious loss of vision, and a corneal transplant will be performed at an appropriate time. Corneal grafts are extremely successful in treating Fuchs' Dystrophy, since this type of dystrophy does not recur in the transplant. Sometimes cataracts will be removed at the same time as the corneal transplant.

The endothelium is the innermost layer of the cornea and acts as a pump which maintains the correct amount of fluid in the cornea. In Fuchs' Dystrophy the endothelium becomes irregular and doesn't function normally. As a result, the cornea begins to take on too much fluid and starts to swell. The swelling means that the cornea becomes hazy and this is why the vision is affected. As the swelling increases, the cornea becomes slightly wrinkled, affecting the vision even more. Pain also becomes a problem because the epithelium, the outermost layer of the cornea filled with nerve endings, is also affected. At this stage it may be necessary to use a soft contact lens as a bandage to protect the epithelium.

Due to a risk of infection the contact lens cannot be used for long, and so when either the pain can no longer be treated or the sight loss is sufficient, a corneal transplant is carried out.

Keratoconus

The name literally means "conical or cone-shaped cornea" and rarely appears until puberty or older. In keratoconus the cornea becomes stretched and thins at its centre, and the thinned part of the cornea bulges making the vision more shortsighted and irregular. As a result the vision is distorted. The stretching of the cornea tends to progress but the rate varies. Both eyes are affected but the condition may not be at the same stage in both eyes. The majority of people with keratonconus have no family history of the condition and are very unlikely to have affected children.

Treatment usually begins with the use of rigid contact lenses, although soft contact lenses or spectacles may still be suitable in the early stages. The rigid contact lenses help to provide the best possible vision but they do not affect the rate of progression of the condition. As the condition progresses good vision may be difficult to maintain and contact lens tolerance may vary.

A condition called 'hydrops' may also occur. This may result in a sudden drop in vision and is due to a break in Descemet's membrane allowing fluid to leak into the stroma of the cornea. This can be uncomfortable and cause the eye to water. It may also lead to scarring.

In at least 20 per cent of people with keratoconus, the cornea may become extremely steep, thin and irregular, or the vision can no longer be improved sufficiently with contact lenses. At this stage the cornea may need to be replaced surgically with a corneal transplant. Over 90 per cent of corneal transplants done for keratoconus continue to allow good vision at five years post-transplant.

What next?

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Our pages on information for everyday living offer ideas for people with sight problems on making life easier and ways of getting the most out of life.

Useful contacts

Unfortunately there is not a general support group for people with corneal dystrophies.

There is a support group for people with Keratoconus which can be contacted at:

Keratoconus Self Help and Support Group
P O Box 26251
London
W3 9WQ

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