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.
How to tell if your child has 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:
- a squint – where the weaker eye looks inwards, outwards, upwards or downwards, while the other eye looks forwards
- short-sightedness (myopia), long-sightedness (hyperopia) and astigmatism
- childhood cataracts – cloudy patches that develop in the lens, which sits behind the iris (the coloured part of the eye) and pupil
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.
When to get medical advice
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.
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.
Causes of 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:
- a reduced amount of light entering the eye
- a lack of focus in the eye
- confusion between the eyes – where the 2 images aren't the same (such as a squint)
Left untreated, this can lead to the eye's central vision never reaching normal levels.
Treatment for 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.
Causes
A lazy eye (amblyopia) is caused when something disrupts the normal development of vision.
How vision develops
It's often assumed that younger children have the same vision as adults, but this is not the case.
Children have to learn how to see – more specifically, their brains have to learn how to interpret the nerve signals sent from the eyes to the brain.
It usually takes around 3 to 5 years before children can see as clearly as adults, and up to 7 years before the vision pathways in the brain become fully developed.
If something affects one of the images the eye shows the brain as the brain develops, the signals become disrupted.
The brain starts to ignore the poor-quality images, resulting in a "lazy eye". For most children this happens because of a problem with the image in one eye. As a result, the brain becomes increasingly reliant on the stronger eye, which makes the weaker eye even lazier.
Underlying eyesight problems
Common eyesight problems that disrupt the development of vision and can cause lazy eye include:
- squint
- long sight, short sight and astigmatism
Squint
A squint is a common eye condition that affects around 1 in 20 children. If a child has a squint, one eye looks straight ahead but the other eye looks off to the left, right, up or down.
This causes the brain to receive two very different images that it cannot combine. In adults, this would result in double vision.
In children who are still developing, it can cause the brain to ignore images from the squinting eye, leading to a lazy eye.
Some babies are born with squints. Older children can develop a squint as a result of eyesight problems such as long sight, short sight or astigmatism
Long sight, short sight and astigmatism
Long sight, short sight and astigmatism are eyesight problems that are caused when the light rays entering the eye are not properly focused. This is because of problems with the structure of the eye.
- long-sightedness – is where distant objects appear normal but nearby objects are blurred
- short-sightedness – is where nearby objects appear normal but distant objects are blurred
- astigmatism – is where an irregular-shaped cornea or lens leads to blurred or distorted vision due to problems focusing
Many children with these eyesight problems develop normal vision in both eyes. However, in some children, they can cause the brain to ignore the signals from one or both eyes.
Less common eye problems
Less common eye problems that can cause a lazy eye include:
- a scar on the transparent layer at the front of the eye (cornea)
- childhood cataracts – clouding of the naturally clear lens of the eye
- a droopy eyelid.