Inherited retinal dystrophies: types, symptoms, genetic testing and treatment in the UK

The retina is the light-sensitive layer at the back of the eye. RNIB explains that light is focused on the retina, where photoreceptor cells called rods and cones convert it into electrical signals that travel along the optic nerve to the brain. Rods are mostly in the peripheral retina and work in dim light. Cones, concentrated in the central macula, give fine detail and colour.

In an inherited retinal dystrophy the faulty gene stops retinal cells working correctly, and the cells eventually fail over time. Retinitis pigmentosa (RP) is the most common group, but there are many others with different names and patterns of sight loss. Retina UK uses the term inherited retinal dystrophies for these conditions, which include RP and Stargardt disease. Moorfields Eye Hospital says there are over 400 different genetic causes of inherited retinal disorders, and that they are the second commonest cause of severe visual impairment in childhood and the commonest in the working-age population.

RNIB groups IRDs according to which cells are mainly affected. The pattern determines which part of vision goes first and how quickly sight changes.

• Rod-cone dystrophies, such as retinitis pigmentosa: rods are affected first, then cones, though the whole retina is ultimately affected.

• Cone-rod dystrophies: cones are affected earlier and more severely, so central vision is affected more than peripheral vision.

• Macular dystrophies, such as Stargardt disease and Best disease: these mainly affect central vision, usually leaving peripheral vision intact.

• Choroidal (RPE) dystrophies, such as choroideremia: these affect the layers beneath the retina as well as the photoreceptors.

• Leber congenital amaurosis (LCA): a group of more severe rod-cone dystrophies with greater sight loss in very early childhood, which RNIB describes as the most common cause of inherited sight loss in children.

• Syndromic forms, where other parts of the body are also affected in addition to sight loss.

RNIB says most forms of IRD affect night vision and peripheral vision first, though some affect central vision first. In retinitis pigmentosa the first symptom is usually poor vision in dim light, such as at dusk or at night, called nyctalopia or night blindness. People without a sight condition can usually adapt fully to dim light in 15 to 30 minutes, but with RP adaptation takes much longer or does not happen at all. People may also begin to miss things to either side and trip or bump into things.

As RP progresses, peripheral sight is gradually lost, leaving a narrow central field, often called tunnel vision. RNIB says central vision may be preserved into the 50s or beyond depending on the type, but advanced RP often affects it, making reading or recognising faces difficult. Glare, difficulty moving between light and dark rooms, and sometimes flashes or coloured lights in the vision can occur. Sight may be stable for a time and then get worse over a short period before stabilising again.

Some IRDs begin in early childhood, while others may not affect sight until adulthood. RNIB advises discussing any new symptoms with your ophthalmologist.

RNIB says about half of people with an IRD have a relative who also has the condition. The patterns it describes are autosomal dominant, autosomal recessive and X-linked.

With autosomal dominant inheritance one faulty copy of the gene is enough, and there is a 50% chance of passing it on with each pregnancy. With autosomal recessive inheritance both copies are faulty, so both parents are usually unaffected carriers, and each child has a 25% chance of inheriting the condition and a 50% chance of being a carrier. This type often appears in families with no history and tends to have a more severe effect on sight than dominant types. X-linked conditions predominantly affect males, and RNIB says vision can be very poor by the age of 30 to 40 in males with an X-linked condition. Mitochondrial inheritance is rarer.

A faulty gene can also arise for the first time in a child with no family history. If no relative is known to have the condition, RNIB says it may not be possible to find out how it was inherited without genetic testing.

RNIB advises seeing an optometrist if you have problems with any aspect of vision, including in dim light or at night. The optometrist examines the retina and can carry out a field-of-vision test, which may not be offered routinely, so it is worth asking if you are worried about peripheral vision. They can refer you to an ophthalmologist for more tests. Mention any family history and any problems moving from light to dark.

At hospital the ophthalmologist examines the retina after drops to dilate the pupils, and tests may include retinal photographs, optical coherence tomography (OCT), autofluorescence imaging, visual field testing, colour vision tests and electrodiagnostic tests that show how the retina responds electrically. RNIB says it is unlikely all will be done at a first visit and that none are painful. It is not safe to drive until the effect of the drops has worn off. The specific type may not be known without genetic testing.

RNIB states that there is currently no cure for IRDs. For most IRDs there is no evidence that vitamin supplements or a particular diet will prevent sight loss; RNIB notes that vitamin A supplements have not been found to protect sight in RP and that large doses can be harmful, so any supplement should be discussed with a GP and ophthalmologist. Exceptions include some rare conditions such as Refsum's syndrome, which needs specialist dietary management.

The first gene replacement therapy, voretigene neparvovec (Luxturna), is approved for IRDs caused by faults in the RPE65 gene. NICE's public information says it is available on the NHS for people with vision loss from confirmed biallelic RPE65 mutations who have enough viable retinal cells. RNIB describes other approaches under investigation, including gene editing, optogenetics, stem cell therapy and electrical stimulation, and notes that there were no approved stem cell treatments for IRDs at the time of writing.

Practical support includes low vision aids, registration as visually impaired where eligible, and advice from RNIB and Retina UK.

RNIB says that because IRD affects sight in both eyes, you are legally required to tell the DVLA, which will assess whether your vision meets driving standards. Your ophthalmologist and optometrist can advise.

Monitoring is usually long term: your ophthalmologist may suggest review every year or two, and RNIB says to continue seeing an optometrist at least every two years or as advised for general eye health and other eye conditions. Retina UK describes itself as the only UK charity dedicated to supporting people with inherited sight loss and funding research into treatments, and RNIB runs a helpline.

RNIB describes the usual route as an optometrist examination, then referral to a hospital ophthalmologist for tests and, where needed, genetic testing. Moorfields Eye Hospital describes research with UCL into causes, prognosis and new treatments. Whichever route is used, genetic testing is central to knowing the specific diagnosis.