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 isn't 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 three to five years before children can see as clearly as adults, and up to seven 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 conditions that disrupt the development of vision and can cause lazy eye are discussed below.
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 can't 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 a group of eye conditions called refractive errors.
Refractive errors are caused when the light rays entering the eye aren't properly focused. This is caused by problems with the structure of the eye.
Examples of refractive errors that could lead to a lazy eye are:
- long-sightedness – where distant objects appear normal but nearby objects are blurred
- short-sightedness – where nearby objects appear normal but distant objects are blurred
- astigmatism – where an irregular-shaped cornea or lens leads to blurred or distorted vision due to problems focusing
- anisometropia – where the right and left eye have very different focusing powers
Many children with refractive errors develop normal vision in both eyes. However, in some children, refractive errors can cause the brain to ignore the signals from one or both eyes.
Less common conditions
Less common conditions 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 (ptosis)