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 eyes become fully developed.

If something affects one of the eyes as it develops, the signals become disrupted, which affects the quality of the images interpreted by the brain.

The brain starts 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 can cause lazy eye are discussed below.


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 causes 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

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.

Two examples of refractive errors that could lead to a lazy eye are:

  • 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 cause the brain to rely 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 sore on the transparent layer at the front of the eye (corneal ulcer) or a scar
  • childhood cataracts – clouding of the lens of the eye that's present from birth
  • a droopy eyelid (ptosis)

Page last reviewed: 16/06/2016

Next review due: 16/06/2018