A "lazy eye" is a childhood condition where the vision does not develop properly. It's known medically as amblyopia.
It happens because one or both eyes are unable to build a strong link to the brain. It usually only affects one eye, and means that the child can see less clearly out of the affected eye and relies more on the "good" eye.
It's estimated that 1 in 50 children develop a lazy eye.
A lazy eye does not usually cause symptoms. Younger children are often unaware that there's anything wrong with their vision and, if they are, they're usually unable to explain what's wrong.
Older children may complain that they cannot see as well through one eye and have problems with reading, writing and drawing.
In some cases, you may notice that one eye looks different from the other. However, this is usually a sign of another condition that could lead to a lazy eye, such as:
If your child is too young to tell you how good their vision is, you can check their eyes by covering each eye with your hand, one at a time. They might object to covering the good eye, but they might not mind if you cover the lazy eye.
If they try to push your hand away from one eye but not the other, it may be a sign they can see better out of one eye.
Lazy eye is often diagnosed during routine eye tests before parents realise there's a problem.
If you want to be reassured about your child's vision, they can have their eyes tested when they're old enough to attend a sight test at a high-street opticians, which is usually after they're 3 years old.
All newborn babies in the UK have an eye test in the first days of life, and then again at 2 to 3 months old, to look for eyesight problems such as cataracts. Problems like squint and short or long sight may not develop until the child is a few years old.
It's difficult to treat lazy eye after the age of 6, so it's recommended that all children have their vision tested after their fourth birthday. This is the responsibility of your local council, which should organise vision testing for all children between 4 and 5 years of age.
Read more about eye tests for children.
You can also visit your GP if you have any concerns about your child's eyesight. If necessary, they can refer your child to an eye specialist.
Read more about diagnosing a lazy eye.
The eyes work like a camera. Light passes through the lens of each eye and reaches a light-sensitive layer of tissue at the back of the eye called the retina.
The retina translates the image into nerve signals that are sent to the brain. The brain combines the signals from each eye into a three-dimensional image.
A lazy eye happens when the brain connections responsible for vision are not made properly. To build these connections, during the first 8 years of a child's life, the eye has to "show" the brain a clear image. This allows the brain to build strong pathways for information about vision.
A lazy eye can be caused by:
Left untreated, this can lead to the eye's central vision never reaching normal levels.
Read more about the causes of a lazy eye.
In most cases it is possible to treat a lazy eye, usually in 2 stages.
If there's a problem with the amount of light entering the eye, such as a cataract blocking the pathway of light, treatment will be needed to remove the blockage.
If there's an eyesight problem such as short or long sight or astigmatism, it will first be corrected using glasses to correct the focus of the eye. This often helps correct a squint as well.
The child is then encouraged to use the affected eye again. This can be done using an eye patch to cover the stronger eye, or eyedrops to temporarily blur the vision in the stronger eye.
Treatment is a gradual process that takes many months to work. If treatment is stopped too soon, any improvement may be lost.
Treatment for lazy eye is most effective for younger children. It's uncertain how helpful it is for children over 8 years of age.
Read more about treating a lazy eye.