Cone dystrophy: symptoms, diagnosis and living with it

The retina lines the back of the eye and contains two main types of light-sensing cell. Retina UK explains that cone cells are present throughout the retina but concentrated at the macula, in the central region, and are useful for central reading vision. Rod cells help with night vision. In cone dystrophy, the cones degenerate over time and eventually die, leading to sight loss.

Gene.Vision describes cone dystrophy and cone-rod dystrophy together as a group of inherited retinal dystrophies caused by genetic changes in one of 35 genes identified so far, and says combined they affect between 1 in 20,000 and 1 in 100,000 people worldwide. In practice the two conditions are closely related, which is why the charities discuss them together.

Because the central retina is affected first, people usually notice problems with sharpness and colour before anything else. Gene.Vision says the macula is usually the first site of involvement, matching the initial blurring of central vision seen in most patients.

• Blurred or reduced sharpness of vision that glasses cannot fully correct.

• Difficulty recognising colours.

• Increased sensitivity to light (photophobia).

• Sometimes involuntary eye movements (nystagmus) or marked short-sightedness.

• In cone-rod dystrophy, later night blindness and loss of side vision.

Gene.Vision says an ophthalmologist can diagnose cone or cone-rod dystrophy from the symptoms, a clinical examination and an electro-diagnostic test of the retina called an electroretinogram (ERG). The ERG assesses the overall function of the photoreceptor cells: cone function is severely reduced in both conditions, but rod function is preserved in cone dystrophy. Special cameras may be used to image the retina.

Genetic testing can confirm the diagnosis by identifying a change in one of the genes associated with the condition. Gene.Vision adds that some children are referred to a paediatrician for general assessment, because cone-rod dystrophy can occasionally be the first feature of syndromes such as Bardet-Biedl or Alstrom syndrome.

In the UK, a first step is often an optician or GP noticing that vision cannot be corrected with glasses and referring to an eye clinic. Specialist inherited retinal disease services then coordinate testing and genetic counselling.

Gene.Vision describes several patterns. Autosomal recessive inheritance is the most common, with changes in the ABCA4 gene most frequent; both parents are usually unaffected carriers. Autosomal dominant inheritance needs only one faulty copy, and X-linked inheritance, most often involving RPGR, mainly affects males. Up to 40% of people do not know of any other affected relative, and Gene.Vision says most such sporadic cases turn out to be recessive after genetic testing.

A genetic counsellor can explain what the result means for children, siblings and other relatives. This is generally offered through an NHS clinical genetics service.

There are no approved treatments at present, but Gene.Vision lists practical measures that help: regular monitoring of vision and glasses where needed, referral to low vision services, visual aids and assistive technology, tinted glasses or contact lenses for light sensitivity, and hats or UV-protective sunglasses. It also says vitamin A supplements should be avoided in people with ABCA4 changes, so check with the eye specialist before taking any.

Retina UK stresses that general eye check-ups remain important because people with cone-rod dystrophy can still develop other eye problems that may be treatable, and that cataracts tend to develop at an earlier age. Low vision aids, including telescopic and magnifying lenses, have also removed many barriers to education and employment.

Progression varies greatly, even within the same family and gene. Retina UK says it is very difficult to predict what an individual's vision will be like at a given time, and Gene.Vision says cone-rod dystrophy generally declines faster than cone dystrophy. Gene.Vision also says most affected individuals are certified blind by mid-adulthood, although progression varies and many people keep useful vision for a long time.

Retina UK runs a helpline and peer support groups and funds research. Participating in a genuine clinical trial should never require payment, Retina UK notes, and trials should be discussed with your ophthalmologist first.