Other inherited retinal diseases

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. ¹ In Australia, this is roughly 880 people.

Researchers have identified six genes which are known to cause achromatopsia. ² Two of the most common genes linked to the condition (CNGB3 and CNGA3) account for 75% of achromatopsia cases. ³ ⁴

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.

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Bassen-Kornzweig syndrome

This syndrome, also known as abetalipoproteinemia, is a form of retinitis pigmentosa caused by a gene mutation (in the MTTP gene), which results in the impairment of the person’s inability to absorb dietary fats.

Symptoms include:

balance problems
developmental delays
failure to thrive in infancy
muscle weakness (particularly after the age of 10)
a protruding abdomen
slurred speech and
stool abnormalities.

In severe cases, patients can experience neurological damage that can be life-limiting. Dietary modifications and treatment with large doses of fat-soluble vitamins can assist with symptoms.

Batten disease

These group of diseases are severe neurological conditions, which can be life-limiting. There are different types of Batten disease, classified based on the age of onset, and systemic complications. In general, children present with retinal degeneration (similar to RP), seizures and progressive neurological damage.

Bietti’s crystalline dystrophy

This inherited retinal disease (IRD) has higher prevalence in Asia than in the USA and Europe. Onset of symptoms are usually in teenage years, with progressive retinal degeneration leading to significant vision loss by mid-life.

Bietti’s crystalline dystrophy is caused by gene mutations in the CYP4V2 gene and leads to characteristic yellow-white crystalline deposits in the retina and cornea.

Central areolar choroidal dystrophy (CACD)

Central areolar choroidal dystrophy (CACD) is a very rare disease of the macular area, with less than 50,000 persons affected in the USA. ⁵ It is an autosomal dominant condition which generally begins to cause symptoms in people between 30 and 60 years of age. The condition causes atrophy of the macula, with associated central vision loss.

Congenital stationary night blindness

Congenital stationary night blindness (CSNB) is, at its name suggests, an inherited retinal disease (IRD) where the main symptom is often poor night vision. Visual acuity is mildly to moderately reduced, and the loss is not progressive. CSNB symptoms start from birth, so sometimes children will not be aware that their night vision is abnormal. People with CSNB are also likely to have myopia (short-sightedness), strabismus (turned eye) and nystagmus (uncontrolled, rhythmic eye movements).

Familial exudative vitreoretinopathy

Familial exudative vitreoretinopathy, or FEVR, can be inherited through autosomal dominant (especially FZD4 or LRP5 genes), recessive (commonly LRP5) or X-linked (NDP) inheritance patterns. It is also known as Criswick-Schepens syndrome.

In FEVR, the outer regions of the retina do not have the normal blood vessels, leading to a lack of oxygen in these areas. In response, new blood vessels grow (neovascularisation), which can lead to bleeding and retinal detachments. The disease has variable penetrance – this means that some family members may have significant vision loss, whilst others won’t have any symptoms.

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 inherited retinal disease (IRD) caused by a mutation in the gene OAT.

Clinically, the retina shows large areas of damage, which look like paving stones. Affected individuals usually have night blindness as children, with a progressive loss of vision later in life.

Management includes dietary modifications, by avoiding foods high in a compound called arginine (such as nuts, seeds, cereal, dairy products, seafood, meat, chicken, watermelon, and chocolate). Vitamin supplementation can also be helpful.

Refsum disease

Adult Refsum disease is a rare metabolic disorder that occurs in 1 in 1,000,000 people. ⁶ It is characterised by peripheral nerve damage due to phytanic acid buildup from dietary sources. Individuals are asymptomatic at birth, and don’t develop symptoms until late childhood to teens.

The main syndromic manifestations of Refsum disease are retinitis pigmentosa (RP), ataxia, deafness, loss of smell, sensory neuropathy, and bony changes (particularly in the fingers and toes). Management includes dietary changes to limit the intake of phytanic acid, which is found in dairy, beef, lamb and some seafoods.

Stationary cone disorders

Stationary cone disorders are present at birth and have symptoms of decreased vision, decreased colour vision and sensitivity to light. The symptoms do not worsen with age due to the gene mutation, but patients may be at risk of other complications (for example, high short-sightedness may lead to a higher chance of a retinal tear or detachment).

There are several types of stationary cone disorders (for example, cone monochromatism, blue cone monochromatism, dichromatism, trichromatism), with differing prognoses for visual acuity.

Senior-Løken syndrome

This very rare disorder is characterised by the combination of a kidney disease called nephronophthisis and the IRD, Leber congenital amaurosis. There are 10 known genes that can cause this syndrome, which is inherited in an autosomal recessive manner.

Stickler syndrome

Stickler syndrome is the general term for a group of diseases which affect the connective tissue in our bodies, known as collagen. Stickler syndrome can result in a cleft palate, joint abnormalities and arthritis, and hearing loss. In terms of vision, people with Stickler syndrome can get a range of retinal complications, most notably retinal detachments.