Achromatopsia

Overview

Achromatopsia is a rare, inherited eye condition that means a person has severely reduced or absent colour vision. It is also often associated with other visual challenges, such as poor central vision, increased sensitivity to light (photophobia), and involuntary eye movements (nystagmus) [PMID:31536214]. It typically affects individuals from birth. The condition primarily impacts the cone photoreceptor cells in the retina, which are responsible for detecting colour and fine detail in bright light.

The prevalence of achromatopsia is estimated to be between 1 in 30,000 and 1 in 50,000 people in the general population. This makes it a relatively rare condition. It affects both sexes equally and symptoms are typically present from infancy or early childhood, often becoming noticeable when a child struggles with colour recognition or seems bothered by bright light.

Symptoms & clinical features

The primary symptom of achromatopsia is a significant reduction or complete absence of colour vision. People with this condition typically see the world in shades of grey, black, and white, similar to a black-and-white film. This can make activities like distinguishing traffic lights or identifying ripe fruit challenging.

Alongside the impaired colour vision, other key symptoms include photophobia, which is extreme sensitivity to bright light. This often causes individuals to squint or seek out dim environments. Nystagmus, a condition where the eyes make rapid, involuntary movements, is also a common feature. Additionally, individuals with achromatopsia usually experience reduced visual acuity, meaning their vision is less sharp than normal, particularly in bright conditions [PMID:17968846].

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Affected organs

Achromatopsia primarily affects the eyes, specifically the retina. The retina is the light-sensitive tissue at the back of the eye that converts light into electrical signals sent to the brain. Within the retina, cone photoreceptor cells are responsible for detecting colour and fine details, particularly in well-lit conditions.

In achromatopsia, these cone cells do not function correctly or are absent. Rod photoreceptor cells, which are responsible for vision in low light and peripheral vision, are generally unaffected, allowing for some vision in dim conditions, albeit without colour perception.

Multiple body systems

Systemic involvement

Cellular impact

Mechanism at cellular level

Risks & severity

Achromatopsia presents as a lifelong condition, with symptoms typically evident from early childhood. The severity of visual impairment can vary, but generally, individuals experience profound colour blindness and significantly reduced visual acuity, usually ranging from 20/70 to 20/200 (6/21 to 6/60 in UK measurements) [PMID:19018471]. There is no progressive loss of vision with age; the visual impairment typically remains stable throughout an individual's life.

There are no known increased risks of other major health conditions associated with achromatopsia. The primary impact is on visual function and its implications for daily activities, education, and career choices. While the condition itself is not life-threatening, adapting to its challenges can require significant support and visual aids.

Genetic causes

Achromatopsia is caused by genetic changes (pathogenic variants) in specific genes. The most commonly implicated genes are CNGA3 and CNGB3. These genes provide instructions for making protein subunits that form part of a channel within the cone photoreceptor cells of the retina. This channel is crucial for phototransduction, the process that converts light signals into electrical impulses that the brain interprets as vision.

Normal functioning of the proteins encoded by CNGA3 and CNGB3 is essential for the proper electrical response of cone cells to light. Pathogenic variants in these genes disrupt this critical process, leading to the malfunction or degeneration of cone photoreceptors. This impairment results in the characteristic symptoms of achromatopsia, including absent colour vision and reduced visual acuity.

• CNGA3
  cyclic nucleotide gated channel subunit alpha 3
  The CNGA3 gene provides instructions for a vital subunit of cyclic nucleotide-gated (CNG) channels, which are crucial for phototransduction in the cone photoreceptor cells of the retina, enabling colour vision and sight in bright light.
• CNGB3
  cyclic nucleotide gated channel subunit beta 3
  The CNGB3 gene provides instructions for a subunit of cyclic nucleotide-gated (CNG) channels, which are crucial for colour vision and vision in bright light.

Inheritance pattern

Achromatopsia follows an autosomal recessive inheritance pattern. This means that a person must inherit two copies of a pathogenic gene variant - one from each parent - to develop the condition. Individuals who inherit only one copy of a pathogenic variant are known as carriers. Carriers typically do not show any symptoms of achromatopsia themselves because they have one working copy of the gene.

If both parents are carriers of a pathogenic variant in the same gene associated with achromatopsia, there is a 25% (1 in 4) chance with each pregnancy that their child will inherit two copies of the variant and develop achromatopsia. There is a 50% (1 in 2) chance the child will be a carrier, and a 25% (1 in 4) chance the child will not inherit any pathogenic variants and will not be a carrier or have the condition.

Diagnosis & testing

Diagnosis of achromatopsia typically begins with a comprehensive eye examination by an ophthalmologist, often initiated after parents or carers notice symptoms such as poor vision, light sensitivity, or nystagmus in early childhood. Specialised tests are used to assess visual function, such as colour vision testing (though traditional tests may not be applicable), visual acuity measurements, and electroretinography (ERG).

An ERG measures the electrical responses of the retina to flashes of light and is particularly useful in diagnosing achromatopsia by demonstrating absent or severely reduced cone responses. Confirmation of the diagnosis is typically made through genetic testing, which can identify pathogenic variants in genes like CNGA3 and CNGB3. In the UK, genetic testing for retinal conditions like achromatopsia can be accessed via the NHS Genomic Medicine Service, often following referral from a consultant ophthalmologist or a clinical genetics service. This falls under specific R-codes for inherited retinal disorders.

Management & lifestyle

Currently, there is no cure for achromatopsia, and management focuses on supporting visual function and managing symptoms. Strategies often include wearing dark-tinted glasses or contact lenses to reduce glare and photophobia. Visual aids such as magnifiers and technologies that can convert colours into shades or provide audio descriptions may also be helpful.

Regular follow-up appointments with an ophthalmologist are important to monitor eye health and support adaptation strategies. Genetic counselling is often recommended for affected individuals and their families to understand the inheritance pattern and reproductive risks. Research into gene therapy approaches for achromatopsia is ongoing, offering potential future treatments.

UK care pathway

In the UK, individuals suspected of having achromatopsia would typically be referred to an ophthalmologist. If an inherited retinal condition is suspected, they may then be referred to a specialist centre or clinical genetics service for further assessment and genetic testing. This testing is part of the NHS Genomic Medicine Service, using specific R-codes for inherited ophthalmological conditions. Genetic counsellors play a vital role in explaining genetic test results, inheritance patterns, and reproductive implications, as well as providing support and signposting to relevant services.

Related conditions

Several other conditions can present with similar symptoms to achromatopsia, requiring careful differentiation. Cone-rod dystrophies, for instance, also affect cone cells, but often involve progressive vision loss. Blue cone monochromatism is another rare form of colour blindness, where only blue cones function, leading to a poorer colour perception than normal but typically not the complete absence seen in achromatopsia. Genetic testing is crucial for accurately distinguishing between these conditions and confirming a specific diagnosis.

Frequently asked questions

References

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8. Gottlob I. Nystagmus. Current opinion in ophthalmology. 2001. PMID: 11588502