Keratoconus

Keratoconus affects the cornea, the clear round-shaped front surface of the eye. This becomes progressively thinner and weaker over time, eventually causing a cone-shaped protrusion to develop.

The abnormal change of shape causes a person to have blurred and distorted sight as it prevents the light that enters their eye from being correctly focused.

Keratoconus is estimated to affect between one in 500 to 2,000 people worldwide. But it may be more common in certain populations.

Different types of keratoconus

There are four main types of keratoconus, in addition to rarer variations of the disease.  

• Round cone keratoconus: Round cone keratoconus affects a relatively small area of the cornea, however the steepness of the affected area can be extreme. 
• Oval keratoconus: Oval or sagging cone keratoconus affects a larger portion of the cornea. It can cause rupturing (or hydrops) of the cornea's internal membranes and scarring, as well as making it harder to fit contact lenses.
• Forme fruste keratoconus (FFK): The most common type of keratoconus. It is also the most mild. FFK is usually symptom-free and only diagnosed by mapping the surface of the eye. In FFK the disease starts, but for some reason it stops progressing without any need for treatment. 
• Keratoglobus keratoconus: Keratoglobus differs from keratoconus. Instead of a single point of thinning of the cornea (which results in a bulging and the development of a cone), the cornea in people with keratoglobus is thin everywhere.
• Posterior keratoconus: A rare, usually congenital corneal disorder where there is a small region with an increase in curvature at the back of the corneal surface. It is unrelated to the other forms of keratoconus. 

What causes keratoconus?

Scientists do not understand exactly what causes keratoconus. But it is thought that many different genetic and environmental factors are involved.

Several genes have so far been linked with keratoconus. Although these genetic variants can influence a person’s risk of developing the condition, whether they do so is determined by complex interactions between other genes and environmental triggers.

Understanding the biology of keratoconus

Studying the function of these known risk genes is helping scientists to improve our understanding of the biology of keratoconus – and will hopefully lead to new prevention or treatment strategies. They have so far identified that these genes are involved in many diverse activities – including in eye development, the formation and structure of the cornea, the lattice of materials that fill spaces between the cells of the cornea, inflammation and in the control of cell growth. 

Learning about the development of keratoconus

Scientists are also searching for environmental triggers that may contribute to the development of keratoconus. For example, around one-third of people with the condition also have an allergy. One theory is that allergies can cause itchy eyes – and this may lead to excessive eye rubbing over long periods of time, which could potentially further weaken the cornea.

Keratoconus is also thought to involve a defect in the structure of collagen, the tissue that makes up most of the cornea.

Is Keratoconus genetic? 

Most people who develop keratoconus have no family history of the condition, and they are otherwise well. But keratoconus can occasionally appear to run in families and also sometimes occur as part of another genetic eye condition, such as Leber congenital amaurosis or retinitis pigmentosa – or a broader inherited syndrome affecting many parts of the body such as Down’s, Marfan or Ehlers-Danlos syndromes.

• Dr Nasser Siabi, OBE, is one of five brothers. Of those, four had keratoconus. Listen to the podcast where he discusses his family history and talks about assistive tech. 

What are the signs and symptoms of keratoconus?

Keratoconus will typically begin in young people during their teens or early 20s, with the changes in the eye slowly getting worse over time. Their symptoms will stabilise when the shape of their cornea stops changing, which usually happens around twenty years later.

A person’s eyesight will often be unaffected in the very early stages of keratoconus, although it may be slightly blurred and distorted. But as the condition progresses, their sight may become more distorted due to further bulging of their cornea. This will eventually lead to further short-sightedness and astigmatism – and so they will often need frequent changes to their spectacle or contact lens prescription to correct this.

They may also have increased sensitivity to glare and might see halos around bright lights, which can affect their ability to drive at night.

When the condition is more advanced, a person may have scarring of their cornea that can blur the vision further.

Keratoconus can affect each eye differently. A person may initially only be affected in one eye but eventually, both of their corneas usually develop the condition – although this may not be with the same severity.

How is keratoconus treated?

When keratoconus is in its early stages, a person’s eyesight can usually be corrected with spectacles or soft contact lenses. But when the condition becomes more advanced, rigid gas permeable contact lenses are usually required to fully correct their sight.

Most people with keratoconus can achieve good sight wearing contact lenses. But in a few severe cases of keratoconus, a person may need a corneal transplant to correct their sight. This involves surgery to remove all or part of a damaged cornea and replace it with healthy tissue from a donor. Even after a corneal transplant, a person almost always still needs to wear spectacles or contact lenses to correct their vision.

Newer treatments have become available to stop the condition from getting worse. One of these is called ‘collagen cross-linking’, which involves removing a small area of the surface of the cornea, applying drops of vitamin B2 (riboflavin) followed by a burst of ultraviolet-A (UV-A) light. This is thought to cause the tiny collagen fibres in the cornea to join together, making it stronger. Another option is corneal implants – flexible plastic rings that are inserted into the edges of the cornea to try and flatten its shape.