A lazy eye (amblyopia) is caused when something disrupts the normal development of vision.
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.
Common eyesight problems that disrupt the development of vision and can cause lazy eye include:
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 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.
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 that can cause a lazy eye include: