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Achromatopsia
Achromatopisia is an inherited retinal degeneration (IRD) characterised by partial or total colour blindness, in addition to other visual symptoms.
The rod and cone photoreceptor cells are responsible for capturing the visual field. The rod cells perceive dim light (scotopic vision), are not sensitive to colour and are responsible for peripheral vision. The cone cells are located at the centre of the retina (the macula) and perceive bright light and colours (photopic vision) to provide sharp central vision.
Genetic changes or mutations in genes that function in cone cells are responsible for achromatopsia. Individuals with Achromatopsia have reduced visual acuity and are completely or almost fully colour blind; however the condition is not progressive and it does not lead to blindness.
What are the symptoms of achromatopsia?
Achromatopsia is different to colour blindness in that people living with colour blindness can see colour but have difficulty in differentiating between them, for example the colours red and green. Achromatopsia is a total absence of colour vision and causes other visual issues, such as increased sensitivity to light (photophobia) and involuntary eye movements (nystagmus). In total, Achromatopsia is estimated to affect 1 in 30,000 individuals worldwide1.
What Causes Achromatopsia?
Achromatopsia is a genetic retinal condition which is passed down through generations in an autosomal recessive pattern of inheritance. In this instance, two mutated gene copies from each parent are required for the condition to develop. It is caused by a mutation in one of a number of genes, including the CNGA3, CNGB3, GNAT2, PDE6C, or PDE6H genes. Mutations in these genes prevent cone photoreceptor cells from interacting with incoming light as normal and interferes with phototransduction, a process which describes the transmission of visual signals from the eye to the brain.
Achromatopsia is diagnosed through a variety of tests, including fundus autofluorescence and optical coherence tomography (OCT) tests which assess the integrity of the different retinal layers. An electroretinogram is also used to investigate the functioning capacity of the rod and cone photoreceptor cells and measure their electrical impulses produced in response to light.
What treatments are available?
At present, there is no treatment available for Achromatopsia. The vision of people with Achromatopsia decreases as the levels of light increase. Some people living with Achromatopsia adapt to their reduced level of visual function in bright indoor and outdoor settings by using visual strategies such as squinting, shielding their eyes or positioning themselves in favourable light. Others sometimes wear medium tinted lenses in such settings. However, in full sunlight outdoors, or in very bright indoor spaces, almost all people with Achromatopsia use very dark tinted lenses, as they do not possess the functioning cone photoreceptors required for high quality vision in these settings.
Although treatments for Achromatopsia don’t exist, it is still very important for people living with the condition to have regular eye check-ups because they may be at risk of developing other kinds of eye problems that affect the general population, some of which may be treatable.
References
1. National Institute of Health. Available at https://ghr.nlm.nih.gov/condition/achromatopsia#statistics. Accessed April 2020.