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A lazy eye, also known as amblyopia, is when one eye fails to work properly with the other. This causes reduced vision in an otherwise healthy eye that cannot be corrected completely by wearing glasses. It usually occurs in one eye although occasionally it can affect both eyes.

A lazy eye arises because the nerve connections from one eye to the brain do not develop properly. These visual pathways normally develop during the first few years of childhood; development before the age of 7-8 is critical.
Amblyopia is a common condition in children; it can be treated successfully but timing is a crucial factor.

What causes a lazy eye?

The most common cause of lazy eye is a pre-existing eye condition that reduces vision in one eye, which prevents the eyes working together as a pair.

Several things can either cause or contribute to the development of a lazy eye:

  • One eye may be long-sighted while the other is normal, resulting in blurred images being sent to the brain from one eye. Typically, the brain will ignore the image from this eye and only use the normal eye. 
  • Astigmatism in one or both eyes.
  • If the child has a squint (see below).
  • Damage to the retina or optic nerve in one eye.
  • A family history of the condition.
  • A cataract present from birth or that develops during early childhood, causing reduced vision in one eye.
  • A droopy eyelid prevents the child seeing out of one eye.

What symptoms does a child with a lazy eye show?

When a child is very young, it is difficult for them to explain the problems they have with their vision, so it is important to look for clues that could indicate that the most common symptoms of lazy eye are present. Most children do not really show symptoms but it is worth looking out for:

  • A decrease in or lack of depth perception: this leads to a child being unable to judge distances very well. They may have difficulty catching a ball but this is relatively common in all young children.
  • A noticeable squint where one eye turns in a different direction compared to the other eye. Some children only have a slight squint or one that becomes more obvious when they are tired.

If you have concerns that your child may have a squint or a lazy eye, you should consult an ophthalmologist, orthoptist or optometrist who can do some tests to find out if there is a problem.

The relationship between lazy eye and squint

A squint, also known medically as strabismus, is a condition where the eyes don’t focus in the same direction. One eye moves away from the normal central position:

  • A convergent squint is when one eye turns inwards.
  • A divergent squint is when one eye turns outwards.
  • A vertical squint is when one eye focuses higher or lower than the other.

Images are ignored from the squinting eye that is not facing in the same direction, so children who have a squint often also have a lazy eye.

Treatment for squints depends on the type of squint as well as its severity. Surgery can be used to adjust the muscles around the eye so that it takes up a normal central position. Eye exercises can be helpful in some cases of mild squint. Treating a squint early and successfully can prevent a lazy eye developing.

Adult squints can be treated successfully from a cosmetic point of view using Botox® injections or surgery to reposition the eye.

Diagnosing a lazy eye

It is possible for both eyes to become lazy, but this causes an obvious problem with vision. When only one eye is a lazy eye, the brain compensates by favouring the other eye, so the condition may go unnoticed for several years as the child’s eyesight appears fine. If the signs above are observed, or if the child has an eye test that indicates they might have a lazy eye, they will then need to see an ophthalmologist for more specific tests.

The eye specialist will conduct a full physical exam of the eye. He or she will test to see if the child needs glasses and will test the movement of the eye muscles by moving light and objects in front of the child’s eyes. By examining the eye’s response, the ophthalmologist will be able to make an accurate diagnosis even if the child is too young to be able to read from an eye chart.

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