Achromatopsia
What is achromatopsia?
Achromatopsia is a rare inherited retinal disease (IRD) which leads to the cone photoreceptors not working well (incomplete achromatopsia) or at all (complete achromatopsia). This results in poor or no colour vision, as well as low vision.
It is known as a stationary IRD. This means that, unlike conditions like retinitis pigmentosa, the condition is not progressive. Most people with achromatopsia will maintain some level of vision through life.
What are the symptoms of achromatopsia?
The condition is often first noticed in a young child by their parents, as children with achromatopsia may dislike bright lights (known as photophobia).
Nystagmus is another symptom of the condition, where their eyes may involuntarily move and “dance” from side to side.
Other symptoms of achromatopsia include:
- colour blindness
- poor central vision and
- high levels of refractive error (requiring high-prescription spectacles or contact lenses).
What is the cause of achromatopsia and how is it inherited?
Achromatopsia is very rare and is thought to affect around 1 in 30,000 people around the world. 1In Australia, this is roughly 880 people.
Researchers have identified six genes which are known to cause achromatopsia. 2Two of the most common genes linked to the condition (CNGB3 and CNGA3) account for 75% of achromatopsia cases.3
The condition is inherited in an autosomal recessive manner. This means that each parent is a carrier, but is not affected by the disease themselves. Each of their children will have a 25% chance of being affected, with males and females having an equal chance of being affected. The children have a 50% chance of being a carrier (having one healthy and one mutated copy of the gene).
What treatments are available?
Due to the high rates of photophobia (glare intolerance) in people with achromatopsia, it is highly recommended for people with this condition to wear tinted sunglasses and/or brimmed hats to reduce brightness.
There are currently no specific treatments for achromatopsia, but a number of emerging treatments are being developed that may assist people with achromatopsia in the future.
Bassen-Kornzweig Syndrome
This syndrome is a form of Retinitis Pigmentosa with progressive neurological problems.
Fundus Flavimaculatus
Fundus Flavimaculatus is a condition similar to Stargardt Disease but varies in its age of onset and severity. When there is macular damage, vision can deteriorate to 20/200, although it usually remains between 20/50 and 20/80. Damage to the macula first appears in the adolescent and early adult years, and the area of damage may gradually spread.
Gyrate Atrophy
Gyrate Atrophy is an autosomal recessive dystrophy caused by mutations in the gene for ornithine aminotransferase (OAT), located on chromosome 10. Originally thought to be a subtype of choroideremia, the disorder is the result of tenfold elevations of plasma ornithine, which is toxic to the Retina Pigment Epithelium (RPE) and choroid. Patients with Gyrate Atrophy have hyperpigmented fundi, with lobular loss of the RPE and choroid.
The finding of generalised hyperpigmentation of the remaining RPE helps to clinically distinguish Gyrate Atrophy from choroideremia. In the early stages, patients have large, geographic peripheral paving-stone–like areas of Atrophy of the RPE and choriocapillaris, which gradually coalesce to form a characteristic scalloped border at the junction of normal and abnormal RPE. Affected patients usually develop night blindness during the first decade of life and experience progressive loss of visual field and visual acuity later in the disease course. The clinical diagnosis can be confirmed by measuring serum or plasma ornithine levels; molecular confirmation can be obtained by mutational analysis of the OATgene.
Occasionally, older patients present with an uncommon syndrome of peripheral chorioretinal Atrophy that closely mimics Gyrate Atrophy; normal plasma ornithine levels in such patients exclude the diagnosis.
What treatments are available?
Although dietary restriction of arginine has been used to treat some Gyrate Atrophy patients, the diet is very difficult to maintain and must be monitored by paediatricians with experience in metabolic disease. Vitamin B6 treatment lowers the plasma ornithine levels in a small percentage of Gyrate Atrophy patients.
Whether such a reduction improves the long-term visual outcome is unknown, but, unlike arginine restriction, vitamin supplementation is relatively easy to administer. Long-term vitamin therapy should be considered only for patients whose ornithine levels can be shown to drop in response to treatment.
Laurence Moon
Laurence Moon is a rarer, but similar condition to Bardet Biedel with retinal pigmental changes plus progressive neurological changes and learning difficulties. Polydactyly is not present. Laurence Moon and Bardet Biedel are both genetic and there are several genes that are currently being investigated by researchers around the world.
Peripapillary (Pericentral) Choroidal Dystrophy
Peripapillary (pericentral) Choroidal Dystrophy is a condition, which causes wasting of the blood vessels that surround the optic nerve. Patients first notice symptoms in the late adult years, when the macula is affected. Stationary cone disorders are present at birth and have symptoms decreased acuity, decreased colour vision, sensitivity to light – that do not worsen with age. There are several types of stationary cone disorders (for example, cone monochromatism, blue cone monochromatism, dichromatism, trichromatism) with differing prognoses for visual acuity.
Refsum Disease
Adult Refsum disease is a rare metabolic disorder that occurs in 1 in 1,000,000 people. It can be fatal if left untreated. A common symptom of Refsum Disease is Retinitis Pigmentosa – initial symptoms generally appear between the age of 10 to 20, with loss of night vision being common. Further information about other symptoms associated with Refsum Disease and current treatments can be found at the links below: