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 (see below), the quality of the signals becomes disrupted and this in turn affects the images seen by the brain.

The brain then begins ignoring these images and becomes increasingly reliant on the stronger eye. This can then trigger a vicious circle - the less the brain uses the eye, the worse the vision in that eye becomes; the worse the vision in that eye becomes, the less likely it is that the brain will use it, and so on.

Underlying conditions

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

Squint

A 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, resulting in blurred and double vision.

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 groups 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 common refractive errors are:

• short-sightedness (myopia) - where close objects appear normal, but objects that are further away are blurred
• long-sightedness - where distant objects appear normal but nearby objects are blurred

Refractive errors can lead to a lazy eye if they cause a squint or a condition called anisometropia - where one eye is short-sighted and the other is long-sighted.

The brain tends to then just rely on the signals from one eye (usually the short-sighted eye) and the other eye then becomes lazy.

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 scar
• a congenital cataract - clouding of the lens of the eye that is present from birth
• a droopy eyelid (ptosis)
• glaucoma - a group of eye conditions that affect vision