- Post date:
- Wednesday, 23 November 2016
Stunning pictures shared in The Guardian last month gave an insight into how people who only see black and white might 'see' colour. Find out more about achromatopsia, the rare condition, and see incredible imagery.
Most people with visual impairments need as much light as possible in order to make use of what residual sight they do have. For people whose sight loss is the result of achromatopsia, however, putting the light on is the last thing they need. Dusk, on the other hand, is quite welcome; while other blind and partially sighted people are struggling as the light dims, people with achromatopsia are much more at ease.
Achromatopsia is a very rare inherited condition, which with one notable exception (see/link to boxout) affects only about one in about 30,000. It affects the cone receptors, which are the specialist light-sensing cells responsible for colour vision and vision in bright light. Some people have ‘incomplete achromatopsia’, where not all the cone cells are affected.
The effects on the vision
From early childhood on, the symptoms of achromatopsia are fairly easy to identify; but in practice it can take some time for the specialists to add them up and get the right answer, because this condition is so rare.
One is nystagmus, or ‘wobbly eyes’, in which the eye moves constantly from side to side, affecting the vision and also affecting the appearance. Another is photophobia, or acute reactions to light – bright light is very painful and a person’s pupils may constrict in the opposite way to other people’s eyes, when the light levels change. People also have very poor vision overall – though this can vary from minute to minute, depending on the light levels (achromatopsia is sometimes called ‘day blindness’).
“People with achromatopsia have no cone function from birth, which means they have no central vision,” says Professor Michel Michaelides of Moorfields Eye Hospital “That means they have a markedly reduced field of vision: with acuity, recognising faces, reading, and so on. Light sensitivity is very acute too – they’ve also got this very debilitating light sensitivity which further degrades vision and is very uncomfortable. Many describe that as the second thing they’d like to address. They’re born always preferring the dark. At home, parents have the lights off and turned down.”
The additional symptom that characterises achromatopsia is complete lack of colour vision. With occasional exceptions for very bright red, people with complete achromatopsia see the world in monochrome.
Very occasionally, people develop acquired or cerebral achromatopsia as the result of brain damage (often associated with a stroke). In most cases achromatopsia is caused by gene mutations which inhibit the function of the cone cells in the eye: in people with complete achromatopsia these cells do not work at all. Six genes associated with the condition have been identified – the most recent one last June, by an international research team including Michaelides and his colleague Professor Andrew Webster – all of which are inherited in an ‘autosomal recessive pattern’, which means that children inherit a copy of the mutated gene from both parents (who do not usually have the condition). The vast majority of cases (around 80 per cent) are caused by mutations in the CNGB3 or CNGA3 genes.
At the moment, there is no treatment for achromatopsia. “Tints are helpful for addressing the glare – if you’re screwing your eyes up all the time you can’t see at all – but they further degrade the vision, because the level of tint required is huge,” says Michaelides.
Most research into treatments have focused on gene therapy: replacing the faulty gene with a ‘normal’ one. The eye is a very good site for delivering gene therapy, especially for conditions which are caused by a single malfunctioning gene, and researchers have now also found a suitable ‘vector’ for transmitting the correcting gene, using a virus which has been ‘re-engineered’ so that with the gene replacing some of the original virus material. This is injected into the cells and delivers the gene right into the nucleus. Three clinical trials using a vector based on a virus called recombinant adeno-associated virus (AAV) have shown improvements in another retinal dystrophy (Leber congenital amaurosis) so researchers are now exploring the possibilities for achromatopsia using a similar vector. A team based at the UCL Institute of Ophthalmology is halfway through a clinical trial targeting mutations in the CNGB3 gene, while a team based at the University of Tübingen is conducting a trial for the CNGA3 gene.
“We plan to recruit the first patients in the last quarter of this year and the first quarter of next. It’ll be a two-site study, at Moorfields and the Kellogg Eye centre in Michigan,” says Michaelides. “There are about 100 patients already in our ongoing study for the past five years, looking at patients in detail, so the majority of patients will likely be from that group.” They have also established from their existing study that achromatopsia is almost always a stable condition – people do not get worse – which is also a good sign for the therapy’s success.
If the trial does restore cone function, there’s still the issue of how a brain adjusts to functioning cone cells for the first time. “It may take time to develop, and we think it may be easier, the younger the patient is,” says Michaelides. “The animal models so far have been very encouraging and there’s likelihood of benefit – otherwise we wouldn’t be exploring this – but there are so many factors that can’t be assessed. We already know there are changes in the brain when you have a lack of cone input, so even if normal cone function is restored we don’t know if – or how long - it’ll take for the brain to adapt. It may be easier the younger the patient is. But in terms of the condition we’re confident we’ve done everything we can at this stage and it may make a huge difference.”
He concludes: “This is the first gene therapy trial to tackle cone photoreceptors, so it has implications for all gene therapy in the eye, because restoring cone function is relevant for all retinal disease. It’s potentially transformational. It’s very exciting.”
In 1775, typhoon followed by famine reduced the population of the Micronesian island Pingelap to around 20 people – one of whom presumably carried the CNGB3 mutation. Today, around five per cent of the island’s population has achromatopsia, and the gene mutation was first identified here.
After a career in banking, sales, manufacturing and recruitment Tim Morton is now the Enquiries and Entry Officer at the Royal National College for the Blind. In his current role he has spoken to over 1,000 blind and partially sighted people – only six of whom have achromatopsia, as he does himself.
My parents were told initially that I was one of only five people in the world with achromatopsia. In fact I didn’t meet anyone else with the condition till about five years ago. I felt very emotional – we sat and talked about our shared experiences – and now I do the same for parents who come to us with their young people, who feel isolated and unsupported.
Having no colour vision is incredibly hard to explain. Again and again people ask ‘what colour is that’? Colour by definition is incredibly difficult to describe – shades of colour are impossible. If I have my glasses with the filter on, I can see bright red but otherwise I see things in contrast. It’s a whole vocabulary that we achromats don’t understand. We can still see things – we are completely colour-blind but we do have vision. However, too much light will blind me. I’ve got a lifelong aversion to having my picture taken because of the flash.
And the question other achromats ask me most often is ‘where did you get your glasses?’ I got them at a visual impairment exhibition and for the first time I could go outside, see in the sunshine and visualise a colour. I saw a postman’s red van for the first time. That was a very emotional moment – but the real epiphany for me was the ability to see in the sunshine. They are heavy UV filters with a red tint. Inside, I’ll wear a pair of shop-bought sunglasses. However, I still function much better at night.
I watched a documentary on Pingelap and if I had the funding, I’d go. I’d also like to take pairs of glasses like mine for the people there. They’re in a place that’s incredibly bright for most of the day and I’d like to find out how they cope with that. And I’d like just to talk to other achromats. I have coped with my visual impairment and developed my own set of life strategies; and it’s always useful and interesting to see how other people have done.
- NB Online