Although we’re not the first to suggest this idea, I think we are among the first to suggest it as a measure to improve amblyopia. The thinking was that if the cells were capable of recovering to normal size in the dark, maybe the visual abilities that the cells provide might also recover.
Professor Kevin Duffy
A straightforward, non-pharmacological intervention might be possible to prevent permanent loss of vision in children suffering from the relatively common visual impairment amblyopia, according to research from Dalhousie University
Amblyopia is a visual impairment, intractable without some sort of intervention, which is found in children and affects approximately four per cent of the general population. The study, which appeared in the journal Current Biology
this week, suggests that a period spent in darkness could return the brain to the state of plasticity it occupies during early development, allowing neurons in the thalamus to recuperate from the visual deprivation believed to cause the condition.
In an interview with ScienceOmega.com
, Associate Professor Kevin Duffy, lead author of the paper and member of the Dalhousie Psychology Vision Group
explained what we know about amblyopia. It is believed to develop because the two eyes do not see as well as each other during early life when the connections between the eyes and the brain are still being formed. The disparity might come as a result of opacity on one or both lenses – due to a cataract – or as a result of misalignment of the two eyes, which is also found in children (known as strabismus, or ‘lazy eye’).
"Early in life, brain development is guided by our sensory experience," stated Professor Duffy. "If there is any perturbation to sensory experience during this period – such as might occur as a result of congenital cataracts – it shapes the brain in a particular way that leaves the circuitry of the brain abnormal and consequently renders vision in the human or non-human animal severely impaired."
Even though a cataract, for instance, can be repaired, if the repair happens outside early development, the changes in the brain produced by the abnormal experience cannot be undone because the brain lacks its former plasticity. This is how scientists know that, although the condition begins as a problem with the eye, a neurological problem is ultimately created.
Along with his colleague at Dalhousie, Professor Emeritus Donald Mitchell, Professor Duffy examined the effect of a period of ten days in total darkness on kittens with amblyopia. As the graph shows, the amblyopic eye recovered quickly recovered its visual acuity after the time spent in darkness.
"There are a number of reasons that cats are typically used to study amblyopia, but it’s mainly because the cat visual system is very much similar to the human visual system," explained Professor Duffy. "The visual system of rodents – rats and mice – is quite different."
Dark-rearing has been done on research animals for decades and there is a lot of evidence to indicate that darkness (the removal of visually-driven activity, to be specific) has an influence on the development of the brain. Many studies using dark-rearing have led us to better understand how the visual system develops.
"In an experiment preceding this one we examined the recovery of neurons in the context of dark-rearing and found that neurons in the thalamus, which were rendered smaller by a period of monocular visual deprivation, ended up recovering to normal size when animals were put into complete darkness for a period of eight days," Professor Duffy expanded.
It was this previous study that precipitated the researchers’ interest in the idea that darkness might produce the recovery of visual acuity.
"Other people decades before us have provided evidence to show that darkness can have a salubrious effect on the deprived brain," said Professor Duffy. "Although we’re not the first to suggest this idea, I think we are among the first to suggest it as a measure to improve amblyopia. The idea is based on the finding that we showed a recovery in cell size in the dark. The thinking was that if the cells were capable of recovering to normal size in the dark, maybe the visual abilities that the cells provide might also recover."
This is not
something to try at home, the team insists. The original cause of the amblyopia must be dealt with before the technique can be effective, and immersion in darkness must not be attempted while it may still affect the vision of the good eye.
"Before we can even think of implementing this as a treatment one would need to know the darkness parameters required to produce optimal benefit," Professor Duffy continued.
Establishing these parameters will require researchers to answer the question of how long the subject must be kept in the dark to produce benefits, and the level of light deprivation necessary.
"The kind of deprivation I’m talking about is absolutely no light – not a single stray photon," emphasised Professor Duffy. "There is other sensory stimulation, but not light. Would blindfolding be capable of producing the same beneficial effect?"
As Professor Duffy pointed out, it would be useful if you could remove a patient from the dark for around 30 minutes a day, so that they could shower and so on, before putting them back in the dark. But there is uncertainty about whether such a move would reverse any beneficial effects. These are just some of the factors that must be weighed up before immersion in darkness can be instituted as a treatment for childhood amblyopia.
According to Professor Duffy, although the possibility that the same sort of remedy would not be suitable for adults, he and his colleagues are optimistic that it could work for them too.
"Elizabeth Quinlan at the University of Maryland
has done work using dark-reared rats and shown that they experience some level of recovery of vision as a result of dark exposure," he commented. "We’re currently running this experiment to see if the same is true in all animals."