But what if a lazy eye — or rather, the part of the brain responsible for interpreting vision in that eye — could be reprogrammed with the flip of a light switch?
In effect, that's what researchers at Dalhousie University in Halifax have done, restoring normal sight to kittens with an induced lazy eye by keeping them in the dark for a period of time.
The research, published Thursday in the journal Current Biology, is based on the theory that darkness can cause some parts of the visual system to revert to an early stage in development, when there is greater flexibility.
"The idea is that the brain early on is very plastic," said Kevin Duffy, a neurobiologist and lead author of the study.
"Immersion in total darkness seems to reset the visual brain to enable remarkable recovery."
A lazy eye is unable to see details in sharp focus. Known formally as amblyopia, the condition is the most common cause of vision problems in children, affecting about four per cent of the population. A number of conditions can lead to a child developing a lazy eye, including a congenital cataract or having misaligned eyes (being cross-eyed).
Amblyopia occurs when the nerve pathway between the eye and brain doesn't develop properly during childhood, usually because the eye sends a blurred image to the brain. The brain then learns to see only blurry images from that eye, even when glasses are used.
Left untreated, the imbalance in what the two eyes see can lead to permanent vision loss.
To conduct the research, Duffy and senior researcher Donald Mitchell induced amblyopia in seven kittens by closing the lid of one of their eyes for a week.
Three of the kittens were then immediately put in a totally dark room for 10 days, along with their mother and litter mates. The cats were monitored by infrared camera; lab workers fed, cleaned and checked on them multiple times throughout each day. A radio was also played for periods during each 24-hour period, providing stimulus that approximated the day-night cycle.
Duffy describes the dark room facility in the lab as "a dark room inside a dark room, inside of another dark room. So when we say dark, we mean dark — zero photons of light. So it's not like closing your curtains at night with the street lamp on."
After the kittens were removed from the darkened room, "the vision of both eyes improved slowly but in lockstep and eventually reached normal visual acuity, in about 70 days," he said.
"The amazing part of this particular study is that amblyopia doesn't develop (in the good eye). So you end up getting recovery of vision in both eyes up to normal levels."
The researchers then exposed the remaining four kittens to darkness for 10 days, but waited five to eight weeks after the animals had their lazy eye induced. They wanted to see if there was a difference with slightly older kittens, whose brains might have been less plastic at that point.
The results were "astonishing," said Duffy.
"When you put those animals in the dark for 10 days, when they come out, they recover — that is, their deprived (lazy) eye recovers to normal acuity levels within a week," Duffy said.
"And the curious result here is that firstly when the animal's older, the darkness doesn't reduce the visual acuity of the non-deprived eye as it did in the younger animals, which is very good because you don't want to compromise the vision of (that) eye.
"The second, probably more astonishing result is the acuity of the amblyopic eye completely recovered within one week."
So ... that's good for kittens. But what about kids?
Having a parent stay in total darkness with their child for more than a week hardly seems practical.
While Duffy agrees, he said if his son had a lazy eye — and his vision would be affected for life — he wouldn't discount the idea to prevent lifelong vision problems. Even though patching can restore visual acuity in a lazy eye, it usually leaves children with a depth-perception deficit.
"So they would have great difficulty, for instance, threading a needle, presumably hitting a baseball," he said. "Even with patching, vision isn't normal."
Still, Duffy said much more research needs to be done before a darkness therapy could translated for use in children.
Researchers, he said, have a lot of questions to answer: "How long does the darkness have to be to produce a beneficial effect? So could eight days work, rather than 10? Does the darkness have to be absolute? So is it possible you could take somebody out of the dark for half an hour a day?
"Is it possible that blindfolding might be just as effective as dark rooming?" he said. And at what age should a child have such a therapy?
One thing Duffy stressed is that parents who have a child with a lazy eye should not try this experimental procedure at home.
"The first thing is to try to understand this better in research animals, then try to translate that to the human condition."