Causes of lazy eye 

A lazy eye (amblyopia) is caused when something disrupts the normal development of vision.

How vision develops

It is often assumed that younger children have the same vision as adults, but this is not the case. Children have to learn how to see, or, more specifically, their brains have to learn to how to interpret the nerve signals that are sent from the eyes to the brain.

It normally takes around three to five years before children can see as clearly as adults, and up to seven years before the eyes become fully developed.

If something affects one of the eyes as it develops, the quality of the signals becomes disrupted and this affects the images seen by the brain.

The brain then begins to ignore the poor quality images and becomes increasingly reliant on the stronger eye.

Underlying conditions

Common conditions that disrupt the development of vision and cause lazy eye are discussed below.


squint is a common eye condition, affecting around 1 in 20 children. In cases of 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 causes the brain to ignore images from the squinting eye, leading to a lazy eye.

Left untreated, the brain begins to ignore images from the squinty eye, leading to the development of 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

Refractive errors are caused when the light rays coming into the eye are not properly focused. This is caused by problems with the structure of the eye.

Some examples of refractive errors that could lead to a lazy eye include:

  • long-sightedness – where distant objects appear normal but nearby objects are blurred
  • astigmatism – where an irregular-shaped cornea or lens leads to blurred or distorted vision

Refractive errors can lead to the brain relying on the signals from one eye, meaning the other eye fails to develop properly.

Less common conditions

Less common conditions that can cause a lazy eye include:

  • an eye disease such as a corneal ulcer (a sore on the transparent layer at the front of the eye) or a scar
  • childhood cataracts – clouding of the lens of the eye that is present from birth
  • a droopy eyelid (ptosis)

Page last reviewed: 17/04/2014

Next review due: 17/04/2016