Choroideremia

What is choroideremia?

Choroideremia – also known as choroidal sclerosis or progressive tapetochoroidal dystrophy (TCD) – is a rare genetic eye condition that almost always affects males. It causes progressive sight loss from childhood due to a gradual loss of cells in and around the retina, the light-sensitive layer at the back of the eye. 

This happens as a result of damage to the choroid – the network of blood vessels that supplies the retina with the vital nutrients and oxygen it needs to remain healthy.

Choroideremia is estimated to affect between 1 in every 50,000 to 100,000 people in the UK. However, the condition is likely to be underdiagnosed as symptoms are similar to several other eye conditions, such as retinitis pigmentosa.

What causes choroideremia?

Choroideremia is caused by faults in a single gene known as CHM, which provides instructions for making a protein called REP1, which plays an essential role in cells in the retina. Faults in the CHM gene mean that either no REP1 is made, or it doesn’t work properly. In turn, this leads to the eye tissue damage that’s associated with the condition. 

This CHM gene is located on the X chromosome. Females have two X chromosomes, while males have one X and one Y chromosome. In affected females, generally only one of the X chromosomes will carry a copy of the faulty CHM gene and the other X chromosome is able to compensate for this. In males, there is no second X chromosome, so having just one copy of the faulty gene is enough to result in symptoms. 

This explains why males are much more likely to develop choroideremia. Females can carry the faulty CHM gene, however they usually have no symptoms, and when they do get symptoms, they are often a lot milder than in males. It is very rare for women to develop severe choroideremia.

What are the signs and symptoms of choroideremia?

Choroideremia symptoms usually start in childhood in boys, typically around age five or six, becoming progressively worse over time. The rate at which this happens can vary from person to person – even between affected members of the same family. However, choroideremia eventually leads to severe sight loss and blindness in adulthood.

Early signs and symptoms often include difficulty seeing in the dark, or night blindness, as well as ‘tunnel vision’ as the condition progresses due to loss of peripheral vision. This often starts as an irregular ring that gradually expands both inwards and outwards.

Children may say they struggle to see things in dark rooms or can’t see stars in the sky. There may also be other vision changes such as loss of sharpness, depth and colour perception. Severe sight loss and blindness usually develops in adulthood, typically around 20 to 30 years after symptoms began.

Females who carry the faulty CHM gene will often not experience any sight loss at all, as having two X chromosomes means they are less likely to develop choroideremia symptoms. When they are affected, women will usually only notice more mild symptoms of choroideremia, such as night blindness and sensitivity to glare, starting later in adulthood.

Is choroideremia serious?

Choroideremia is a progressive condition that gets worse over many years. Tunnel vision can have a real impact on people’s lives as it means their peripheral field of vision is gradually lost. Eventually, choroideremia leads to blindness, although this can take decades to happen.

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How is choroideremia diagnosed?

An ophthalmologist (hospital eye doctor) will do tests to examine your visual field, as well as taking a close look at the retina to look for signs of distinctive changes associated with choroideremia. Genetic testing and a family history of the condition can help to confirm the clinical diagnosis.

An optician might pick up on possible signs during a routine eye test – or you might have booked an eye test if you've noticed changes to your vision. However, seeing a hospital ophthalmologist usually requires a referral via your GP. An optician might suggest this if they notice any cause for concern and think further investigations are required. 

If you have a family history of choroideremia and are concerned about your sight or your child’s, talk to your GP about being referred to a specialist.

What are the treatments for choroideremia?

Unfortunately, there are currently no treatments for choroideremia – but recent advancements in gene therapy research, initially funded by Fight for Sight, are fuelling the development of potential new treatments, providing hope for the future.

In the meantime, wearing protective sunglasses outdoors may help to protect vision by reducing exposure to ultraviolet (UV) light, which can theoretically cause further retinal damage.

What can help choroideremia?

Support is available for people living with choroideremia to help maximise their remaining vision. For instance, low vision aids such as telescopic and magnifying lenses may be helpful. And there is a wide range of assistive technologies that may help people in their daily lives, such as speech to text software.

Genetic testing along with counselling is also recommended for people with choroideremia and their family members.

Have your eyes tested every two years even if you think your vision is fine. An eye test can spot some eye conditions and, if caught early, treatment may prevent further deterioration.