Corneal dystrophy

What is corneal dystrophy?

Corneal dystrophy is a group of rare eye conditions that affect the cornea – the clear front surface of the eye – and can cause sight loss. In people with corneal dystrophy, abnormal materials build up in the cornea, causing it to lose its transparency and become cloudy.

Corneal dystrophy is often inherited and usually affects both eyes. It is a type of corneal disease.

What are the different types of corneal dystrophy?

There are over 20 different types of corneal dystrophy. Each type can affect one or more of these layers, and can be classed into groups according to which layers of the cornea are affected. 

The main groups of corneal dystrophies are: 

Anterior or superficial corneal dystrophies

The corneal epithelium (the thin outermost layer of the cornea) is full of sensitive, tiny nerve endings. Corneal dystrophies affecting this layer tend to cause grittiness, discomfort, and pain. The breakdown of the corneal surface is known as corneal erosion. Corneal dystrophies affecting deeper layers do not usually cause these symptoms.

It happens when the epithelium loosens from the Bowman’s membrane (the thin, transparent layer of the cornea directly below the epithelium). As the epithelium can heal quickly, these symptoms may be short-lived but can keep coming back. 

This group includes:

• Epithelial basement membrane dystrophy (also called map-dot-fingerprint or Cogan’s dystrophy)
• Lisch corneal dystrophy
• Meesmann dystrophy
• Reis-Bücklers corneal dystrophy
• Thiel-Behnke corneal dystrophy

Stromal corneal dystrophies

The stroma is the middle and thickest layer of the cornea, mainly made up of water and a protein called collagen. It is very strong, flexible, and transparent. In these dystrophies, deposits build up in this layer, making the stroma less clear and affecting vision.

This group includes:

• Gelatinous drop-like corneal dystrophy
• Granular corneal dystrophy
• Lattice corneal dystrophy
• Macular corneal dystrophy
• Schnyder crystalline corneal dystrophy

Posterior corneal dystrophies

The endothelium is the innermost, very thin layer of the cornea. It's a single layer of cells that act as a pump, controlling movement of fluids in and out. Problems with the endothelium can cause the cornea to take on water and swell. As well as cloudy corneas, these changes can cause lifelong vision problems such as blurred vision, glare and halos and sensitivity to light.

This type includes:

• Congenital hereditary endothelial dystrophy
• Fuchs’ dystrophy
• Posterior polymorphous corneal dystrophy

What causes corneal dystrophy?

Most corneal dystrophies are inherited, often passing down through families. Genetic mutations affecting the cornea's structure are typically responsible, and in most cases, the mistake in the DNA is just one single letter of the coding sequence for the gene that causes the disease.

Most corneal dystrophies are inherited in an autosomal dominant pattern. This means someone only needs to inherit one copy of the faulty gene from either of their parents to develop the disease. So if one parent has the condition, their children have a 50% (one in two) chance of having it too.

Some types of corneal dystrophy are passed on through autosomal recessive inheritance. In this inheritance pattern, two faulty copies of the responsible gene are required to develop the condition. Both parents are often unaffected carriers, carrying one faulty copy of the responsible gene and one healthy copy. Each of their children will have a 25% (one in four) chance of having the condition by inheriting two faulty copies of the gene – one from each of their parents - and a 50% (one in two) chance of being an unaffected carrier by inheriting a faulty copy from either parent.

What are the signs and symptoms of corneal dystrophy?

The symptoms depend on the type of corneal dystrophy, and some corneal dystrophies don't cause any symptoms. Some of the main symptoms include:

• Pain or grittiness in the eye
• Blurred vision
• Progressive loss of vision
• Sensitivity to light
• Glare or halos in vision

Symptoms vary depending on the type of corneal dystrophy and often worsen slowly over years or decades.

Is corneal dystrophy serious?

Corneal dystrophy doesn't always cause symptoms, but can cause pain or discomfort and can lead to progressive sight loss, or may require surgery such as corneal transplant. Unfortunately, there is currently no cure for the underlying causes of these diseases.

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How is corneal dystrophy diagnosed?

Corneal dystrophies are usually diagnosed using a slit light microscope to shine a thin, bright light into the eye, enabling the ophthalmologist to study the front part of the eye. They may also ask about family history and may recommend genetic testing to identify the gene responsible for the condition.

What are the treatments for corneal dystrophy?

Treatment depends on the type and severity of the condition. Glasses or contact lenses can help to improve vision. If your eyelids are rubbing against the cornea, then lubricating eye drops or ointments, or specialised eye patches or contact lenses, can help. Antibiotics during acute flare-ups can help prevent infections. 

If you have severe symptoms, such as significant and permanent vision impairment or eye pain, surgery can help. The type of surgery depends on the corneal layer affected. The main types are:

• Alcohol epitheliectomy with mechanical debridement: this involves scraping off the deposits in the corneal epithelium.
• Excimer laser superficial phototherapeutic keratectomy (PTK): A type of laser treatment that removes deeper deposits up to the stromal layer.
• Corneal transplant (or keratoplasty): This major eye surgery involves removing and replacing the affected cornea with clear donor corneal tissue.
• Unfortunately, most treatments currently available do not give long-term solutions, and it's common for the problem to recur one to five years later.

What can help corneal dystrophy?

Managing symptoms through prescribed treatments, regular monitoring, and in severe cases surgery, can help. Genetic counselling can provide guidance for families with a history of corneal dystrophy.

What research is there into corneal dystrophy?

An important goal for corneal dystrophy research is to find the exact genetic cause behind each person’s condition. This means that affected patients and their relatives can receive clearer genetic counselling about the expected progression of their condition and how it might affect their family.

With the help of funding from Fight for Sight, researchers discovered a new genetic cause of a type of corneal dystrophy called posterior polymorphous corneal dystrophy (PPCD). Two years earlier, the team identified changes in another gene that can also cause PPCD, or another condition called congenital hereditary corneal dystrophy (CHCD).

A proof-of-concept study with funding from Fight for Sight highlighted the potential of a cutting-edge antisense oligonucleotide (ASO) therapy to treat Fuchs' endothelial corneal dystrophy patients with a fault in a gene called TCF4. And the results from another study will help pave the way for early diagnosis of this condition using an innovative method to sequence the gene fault responsible for more than three-quarters of cases. The technique could also help the development of new treatments targeted to this mutation.

Research initially funded by Fight for Sight to develop novel gene therapy approaches to treat Meesmann’s Epithelial corneal dystrophy and Avellino GCD2is still ongoing with a commercial partner USA company Avellino labs. It aims to deliver eye drops to the front of the eye and prevent the disease from progressing by specifically cutting and destroying the faulty gene message that causes symptoms.

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