Achromatopsia: inherited colour blindness, light sensitivity and support

The retina contains two main types of light-sensing cell. Rods work in dim light and give black-and-white vision. Cones work in bright light and are responsible for colour vision and fine detail. Moorfields describes achromatopsia as a condition that affects the cone cells in the retina. In achromatopsia the cones do not respond to light properly, so the person relies on rod vision, even in bright conditions.

The result is that colour vision is partly or completely lost, vision is less sharp, bright light is uncomfortable or dazzling, and the eyes tend to move involuntarily (nystagmus). It is distinct from the common red-green colour vision deficiency that RNIB says is usually inherited in an X-linked pattern and does not affect the ability to see fine detail.

Moorfields says symptoms appear early in childhood. Parents often first notice that their baby squints, screws up their eyes or turns away in bright light, or that the eyes seem to shake. Moorfields' low vision service lists problems seeing small details, blurry vision, partial or complete loss of colour vision, reduced contrast, issues with mobility due to poor central vision, sensitivity to bright light and nystagmus.

Moorfields also reports that researchers at Moorfields and UCL Institute of Ophthalmology were able to show that achromatopsia is a stable condition with progression over time being relatively slow and subtle, and that people with achromatopsia do not go completely blind from the condition.

• Colour vision that is absent or very limited, with confusion between colours of the same hue family.

• Discomfort or squinting in sunlight or bright indoor light.

• Better vision in dim light, dusk or shade.

• Nystagmus, which often settles somewhat in childhood.

• Reduced sharpness of vision that is not corrected fully with glasses.

Moorfields says six genes have been associated with achromatopsia: CNGB3 and CNGA3, which account for around 75% of all cases, as well as GNAT2, PDE6C, PDE6H and ATF6. Most forms are inherited in an autosomal recessive pattern, which means that both parents are usually unaffected carriers and each child has a one in four chance of being affected.

A clinical genetics service can arrange testing to identify the gene involved, which is increasingly important because it can determine eligibility for research trials and helps with family planning. Genetic testing and counselling are available through NHS regional genetics services.

Children with suspected achromatopsia are usually seen by a paediatric ophthalmologist, often with an orthoptist and optometrist, and in the UK many are referred to a specialist inherited retinal disease service. The assessment includes examination of vision, colour vision, eye movements and the retina, imaging such as optical coherence tomography and, importantly, electrical tests of the retina (electroretinography) which show absent cone responses with preserved rod responses.

Genetic testing confirms the diagnosis and the type. Because the retina can look normal on a simple eye examination, the diagnosis may be missed or delayed unless the combination of colour vision loss, light sensitivity and nystagmus is recognised.

RNIB says there is no cure for achromatopsia and cone monochromatism at present. Adjustments that can help include prescription glasses to correct any short or long sightedness, wraparound sunglasses to reduce light sensitivity and provide UV protection, low vision aids and assistive technology, and assessment at a low vision clinic. RNIB notes that many people find red tinted sunglasses help with light sensitivity.

Moorfields' low vision service explains that red, orange and yellow tints filter out high-energy blue and green light, typically in the 480 to 650 nm range, which can reduce over-saturation of the remaining rod cells, improve contrast and detail and make shapes and text easier to distinguish in classrooms and playgrounds. Tinted contact lenses may suit sport because they do not leave gaps where glare can enter, and suitability should be discussed with an optometrist.

Moorfields' low vision service notes that children with achromatopsia rely heavily on high contrast to differentiate between materials, and because they often confuse colours within the same hue family it is important to place objects against a highly contrasting background. Labelling colours with text or symbols can help children participate without feeling excluded, and focusing on texture or outline makes differentiation easier.

Children may ask what colours look like, and explaining a partial or total absence of colour vision can be challenging. A qualified teacher of children and young people with vision impairment can advise schools on seating away from windows, glare control, lighting, large print and exam access arrangements. Adults often find work and daily life manageable with the right adjustments, though driving may be restricted if vision falls below the DVLA standard.

RNIB says research is looking into potential gene therapy treatment to replace the faulty gene with a healthy copy. Gene therapy studies are at an early stage and are not available as established treatments, so speak to your specialist centre about the current position.

Registering with a genetic eye disease service and keeping your genetic result on file allows you to be considered for research as it becomes available. Be wary of clinics offering unproven stem cell or gene treatments for a fee.

On the NHS, the pathway runs from the GP, optometrist or health visitor to a paediatric ophthalmology or inherited retinal disease service, with genetic testing, a low vision assessment and support from a qualified teacher of children with vision impairment. Charities such as RNIB and Retina UK offer information and peer support.

Moorfields states that treatment is available at Moorfields Private for those who self-fund or use private medical insurance. The key tests and tinted lens fittings are similar, and most families combine specialist NHS care with private optometry for lenses and accessories.