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Description

Choroideremia (alternative spelling: Choroideraemia) is an inherited retinal degeneration that causes progressive vision loss, most commonly in men. It occurs as a result of the degeneration of the specialised light-sensing photoreceptor cells of the retina, the retinal pigment epithelium and the choroid (supporting layer of blood vessels) that line the back of the eye. The vision loss due to choroideremia gets worse over time, eventually leading to blindness. However, the rate of progression can vary between individuals.

Choroideremia is a rare disease, estimated to affect approximately one in 50,000 people, although the exact prevalence in Ireland is currently unknown.

Symptoms

As with many conditions, the presentation of symptoms and rate of progression can vary between individuals living with Choroideremia.

One of the first symptoms of the condition is difficulty seeing at night time and this can present as early as during childhood. It occurs as a result of the death of a specific type of light-sensing photoreceptor called the rod cell. This is as a result of damage to the network of blood vessels behind the retina that are known as the choroid, which supplies oxygen and nutrients to support and nourish the retinal pigment epithelial (RPE) cells and the light sensing photoreceptor (rod and cone) cells.

With the death of these cells over time, the visual field begins to narrow and eventually progresses to tunnel vision, where some central vision will still be present. The condition results in blindness, which commonly occurs in late adulthood.

Female carriers of the mutated gene for Choroideremia usually do not experience any significant changes in their vision, although detailed clinical examinations of the back of the eye may reveal some signs indicative of the condition. In rare cases, females may develop the disorder if the healthy copy of the gene is inactivated.

Causes

Choroideremia is a genetic condition. Unlike some other inherited retinal degenerations, such as retinitis pigmentosa, cases of choroideremia are due to mutations in just one gene, known as CHM. This gene makes an essential protein called REP-1, which is involved in escorting essential nutrients between cells in the back of the eye. Mutations in this gene lead to impaired cellular activities, causing cell death.

Choroideremia is genetically passed through families by an X-linked pattern of inheritance. The CHM gene is located on the X chromosome.

Males only have one X chromosome and will become affected by the condition if he receives a faulty copy of the gene. Affected males cannot pass on the disease to their sons, because they pass on their Y chromosome. Men with choroideremia must pass on the disease gene to all of their daughters, who then become carriers of the gene.

Females have two X chromosomes, but generally only one of the chromosomes will carry a faulty copy of the gene and the other functioning copy will compensate. Therefore, females are carriers of the condition, but do not generally display the severe symptoms of the disease. In women, one or other of the two X chromosomes is randomly inactivated in every cell. Usually, in female carriers of X-linked disease genes, including CHM, this results in 50% of the retinal cells working on the altered CHM gene and the other 50% working on the normal copy of the CHM gene.

These women will have very subtle, if any, symptoms of the disease. Inactivation, however, in some women may be skewed in favour of either the normal or the altered CHM gene copy. If more than 50% of the normal CHM copy is inactivated, the carrier female will have more symptoms. In rare, extreme cases of skewed inactivation the carrier female might be almost as severely affected as a male.

For more information about inheritance patterns, visit out genetic inheritance patterns information page.

Diagnosis

Choroideremia is diagnosed through a number of assessments which are important for providing the correct diagnosis.

The eye doctor will ask about a person’s medical history, including any family history of eye conditions. Individuals will receive a clinical eye examination where they may be asked to read letters off a chart (a Snellen chart). They may also check intraocular pressure and examine visual field and visual acuity.

An individual may receive a number of imaging tests. Information about the condition may be identified using colour or wide-field fundus photography (which essentially takes photos of the back of the eye). Fundus autofluorescence may be used to identify stress or damage to the retinal pigment epithelium. Optical coherence tomography (OCT) can also be used to assess the various layers at the back of the eye. An individual may also have an electroretinogram (known as an ERG), which is used to evaluate the functioning of the different types of photoreceptor cells (rods and cones).

A genetic test is also an important component of testing, to ensure the correct condition is diagnosed. For people living in Ireland, genetic testing for Choroideremia can be performed through the Target 5000 programme.

If a family member is diagnosed with Choroideremia, it is strongly advised that other members of the family also have an eye exam by an eye doctor (ophthalmologist) who is specially trained to detect retinal diseases.

Choroideremia is likely to be underdiagnosed as its symptoms are quite similar to a number of other retinal conditions such as retinitis pigmentosa. The distinctive appearance at the back of the eye and X-linked inheritance pattern help eye doctors to make the diagnosis.

Genetic Diagnosis

It’s often impossible to tell which type of retinal disease a person has just by looking into the eye. As such, a genetic test may be necessary to confirm the diagnosis.

The Target 5000 programme provides free genetic and clinical testing to all individuals living in Ireland with a genetic retinal degeneration, including Choroideremia. Through genetic screening of the person awaiting diagnosis and their family members, the Target 5000 programme will provide more detailed information about the nature and inheritance pattern of the condition.

As gene-specific clinical trials and treatments become available, knowing the genetic mutation associated with a genetic retinal degeneration will become even more important. Taking part in this programme ensures participants are included on a national registry from which participants can be identified for clinical trials and treatments and from which further information about each of the genetic retinal degenerations can be defined. For further information on the Target 5000 programme, please contact the Research Department on 01 6789004 or email research@fightingblindness.ie.

Treatment

There are no proven treatments for Choroideremia, although in recent years there have been momentous leaps made in clinical research and development. To find out more about these advances, please visit the ‘Latest Research’ section.

Maximising the remaining vision that an individual has is a crucial step to take, and there are many low vision aids such as telescopic and magnifying lenses which may be of benefit. The wide range of assistive technologies for people with visual impairments provides plenty of choice for users at all stages of sight loss and this technology has also removed many barriers to education and employment.

General eye check-ups are important for people living with Choroideremia, as these individuals may still be at risk of developing other kinds of eye problems that affect the general population, some of which may be treatable.

No matter what level of vision a person may have, it is important to look after the eyes. To find out more about what can be done to take care of the eyes on a daily basis, please visit our Tips for Good Eye Health.

For further information, please contact the Research Department on 01 6789004.

Research

Advances in research have yielded a greater understanding of the condition, with much progress made to develop therapies for this condition. These efforts have resulted in numerous clinical trials, some of which are ongoing.

Gene therapy is a therapeutic approach which has proven safe in other retinal conditions and is a major area of focus in treating Choroideremia. Using this technique in experimental models, researchers have engineered small, safe viruses to deliver the correct version of the CHM gene to the light sensing photoreceptor cells in the retina. There are now multiple gene therapy-based clinical trials on humans taking place at various sites across the world.

Early results have been largely positive and in some cases, gains in vision have been observed. However, it is still early days and long-term monitoring of the safety and effectiveness of these interventions will be a priority for the researchers.

Information about clinical trials can be found on their website and can be searched by condition and trial location.

Fighting Blindness currently support a number of research initiatives which aim to improve the diagnosis, care and quality of life of individuals living with Choroideremia.

Target 5000 research, largely funded by Fighting Blindness aims to determine the genetic cause and understand the nature of the condition over time. Through Target 5000, a national registry will also be developed which will greatly improve access to suitable clinical trials and treatments for people living in Ireland with Choroideremia. For further information, please visit our Target 5000 section.

Dr Sarah Doyle based in Trinity College Dublin is conducting a study to investigate if a protein known as SARM1 plays a role in retinal degeneration, a key component of Choroideremia. This work aims to further understand the underlying mechanisms of retinal degeneration and identify potentially new therapeutic targets.

A study conducted by Professor Breandán Kennedy is investigating a group of neuroprotective drugs, known as HDAC inhibitors, for their potential to delay vision loss in retinal degenerations. This work seeks to identify new therapeutics which could provide significant benefit to individuals.

For further information, please contact the Research Department on 01 6789004.

Support

Receiving a diagnosis can be overwhelming for anyone, but this is not a journey that you have to make alone. There are many groups and resources available to provide support for people living with Choroideremia.

Fighting Blindness offers a free and confidential counselling service (Insight Counselling). For further information please contact insight@fightingblindness.ie or call 01 6746496.

A mindfulness group is also available on every Wednesday at the Fighting Blindness office at 11am.

For technology support and guidance, the Dublin-based Technology Exchange Club meets every Monday at the Fighting Blindness office at 11am. Another Technology Exchange Club, based in Cork, meet every Saturday in the Cork City Library, Grand Parade, Cork City at 11am. The Cork-based club do not meet on Bank Holiday weekends or on the second Saturday of the month. For further information please contact insight@fightingblindness.ie or call 01 6746496.

Resources

Féach provides support for parents of children living with sight loss in Ireland.

ChildVision is the national education centre for children with sight loss in Ireland.

NCBI (National Council for the Blind in Ireland) provides support and services for people living with sight loss in Ireland.

Irish Guide Dogs for the blind helps individuals and their families to achieve improved mobility and independence.

 

Last Updated: 4th February 2019