Pregnancy and child

Could being kept in the dark cure lazy eye?

‘Keeping children with a lazy eye in the dark for ten days could help them see better', reports the Daily Mail, somewhat irresponsibly, following research actually involving kittens.

The Mail reports on a study looking at how keeping kittens in the dark affects the visual pathways in their brains. The researchers hoped this might lead to new treatments for the condition ‘lazy eye’ (amblyopia). ‘Lazy eye’ is a childhood condition caused by the brain ignoring the signals sent by the affected eye, and instead processing signals from the other eye only. This leads to poor vision, and if left untreated, blindness, in the affected eye.

Lazy eye is commonly treated using an eye patch over the ‘good eye’ to force the brain to rely on the lazy eye, strengthening the connection between eye and brain. Researchers were interested in exploring whether enforced darkness could reset this connection. In this study, researchers induced a form of amblyopia in kittens, and then subjected them to 10 days of complete darkness, hoping to effectively ‘rewire the brain’.

They found that some kittens emerged from the darkness blind in both eyes, but that this resolved over time and after nearly two months, the kittens had normal vision in both eyes. Other kittens emerged with normal vision in the unaffected eye, with the ‘lazy eye’ eventually catching up about a week after they were released from the dark room.

The obvious practical and ethical problem with applying the results of this research are that keeping a young child in total darkness for 10 days would be upsetting and could amount to child cruelty. Don’t try this at home.

Researchers will need to adapt this technique, if possible, to make it more child-friendly.

Where did the story come from?

The study was carried out by researchers from Dalhousie University in Canada and was funded by the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council.
The study was published in the peer-reviewed journal Cell.

Despite the Daily Mail’s inappropriate headline, the main reports in the Mail and The Daily Telegraph covered the study fairly well. The Mail’s reporters should also be commended for their suggestion that significant research is still needed, and that there are possible problems with submerging children in total darkness for a week and a half, including “‘huge’ ethical concerns”.

What kind of research was this?

This was an animal study examining whether an extended period of complete darkness would improve amblyopia by altering the brain’s visual pathways.

Researchers have previously developed a technique to induce lazy eye in animals, providing an animal model for a human clinical condition. This allows them to study the effect of potential treatments for the condition.

It is generally believed that the brain’s pathways are adaptable and flexible during infancy. The researchers carried out introductory studies suggesting a protein called ‘neurofilament-light’ (NF-L) plays a role in reducing such plasticity over an animal’s lifetime. This experiment found the levels of NF-L gradually increase from birth through adulthood, and form part of a ‘braking system’, constraining plasticity and stabilising cell pathways within the brain. 

Researchers thought that darkness might reset the brain’s visual pathways, reverting it to a more ‘plastic’ state, thus allowing the pathways to treat the signals sent from each eye equally.

What did the research involve?

Researchers first assessed the impact of darkness on NF-L levels in healthy animals. They placed kittens with normal vision in complete darkness for 5, 10 or 15 days. Animals kept in the dark for 10 or 15 days had NF-L at approximately half the levels normally seen at similar ages. Five days of darkness had no effect on protein levels.

The researchers then induced a modelled version of amblyopia in seven kittens, and placed the kittens in complete darkness for 10 days.

A group of three animals were put in the dark room immediately after amblyopia was induced, and a second group of kittens were kept in normal light conditions for five to eight weeks, and then placed in complete darkness after the delay.

After the 10 day period, the researchers tested one aspect of vision called acuity, which refers to the sharpness or clearness of vision.

There are other aspects of vision that were not assessed in this study, such as visual field – the ability to see at the edge of vision when looking down.

What were the basic results?

Immediately after the first group of kittens was removed from darkness, they were blind in both eyes, which the researchers described as a “profound immediate effect of only 10 days of darkness”. Vision in both eyes gradually improved over a seven-week period, and amblyopia never developed. The second group of kittens developed amblyopia during the five to eight week delay period.

After the 10 days of darkness, this group had no vision loss in the healthy eye, and the vision in the ‘lazy eye’ improved quickly, eventually matching that of the healthy eye after approximately a week.

How did the researchers interpret the results?

The researchers say that the restoration of vision in the kittens was remarkable, and that this has “possible application to amblyopic children in order to boost outcomes of existing therapeutic interventions”.


This study suggests that an extended period in complete darkness may result in the restoration of normal vision among kittens with induced vision problems. It should not be interpreted to mean that keeping children with a lazy eye in the dark for 10 days could help them see better – even if it did, such an action would be unethical and arguably illegal.

Treatment of amblyopia in children generally involves first treating the underlying condition. For example, strabismus (crossed eyes) or other visual problems such as short sightedness (myopia). After treating these causes, a patch can be used to obscure the vision of the normal eye, forcing the brain to use the signal being sent from the affected eye. Read more about the treatment of lazy eye.

The researchers suggest there may be potential clinical applications, but that before this can be considered for humans, several things would need to be considered, including the strictness of darkness required in kittens, and exact knowledge of the ‘critical period’ for the effects of darkness, that is, at what age would this approach apply?

While animal models can be helpful for testing theories and possibly drug applications, they cannot be blindly applied for testing in humans, nor assumed to work in the same manner. Whether the 10 days of darkness assessed in this study would result in similar effects and side effects in humans is not known. What the non-visual impact of a prolonged period in complete darkness would be is also unknown.

Given these concerns, the cautions published in both the Mail and the Telegraph warrant repeating: don’t try this at home.

NHS Attribution