What is a squint?
(Don’t forget to use our Glossary if you are stuck with a word)
Squint is the term used when the two eyes are not pointing in the same direction.
An eye may turn in (convergent squint) or turn out (divergent squint). Occasionally one eye may be higher or lower than the other (vertical squint). The squint may be constant (present at all times) or occur only intermittently. 'Strabismus' is another word for 'squint' - popular in the USA and being increasingly used in Europe.
This boy has a convergent squint.
The right eye is turning in.
This girl has a divergent squint.
The left eye is turning out.
This patient has a vertical squint.
The right eye is turning upwards.
Why do squints develop?
We don’t always know why children develop squints. It can run in families and there is a bigger chance of a child developing a squint if their Mum or Dad had a squint as a child. We know that premature babies are more likely to develop a squint and babies who have been very ill at an early age, especially if there has been damage to the brain. In the past, illnesses such as measles and mumps seemed to cause squints, but with vaccination these conditions are luckily very rare now.
There are a number of other reasons why a squint may develop:
• Poor vision in one eye can stop the brain being able to use the eyes together.
• The most common cause of poor vision in one eye in childhood is the the need for glasses. One eye may be more long-sighted (hypermetropic) or more short-sighted (myopic) or the front of the eye may be an abnormal shape (astigmatism)
• Rarely, something may be wrong with the eye itself such as a cataract (cloudy lens), or a scar on the front of the eye. Very rarely, one eye may not have developed properly and have a problem with the retina or optic nerve
• Being long-sighted (or, less commonly, short-sighted) in both eyes can cause the eyes to develop a squint. Generally, long-sightedness causes a convergent squint (eyes turning in). This is because long-sighted children have to make more effort to focus, and this causes the eyes to turn in.
• Abnormal development or damage to the nerves or muscles that control movement of the eye. There are six muscles attached to each eye that are responsible for controlling the position and movement of the eye. The brain controls these muscles by sending signals down wires called nerves. In some children the muscles or nerves do not develop properly and this leads to the eyes not moving properly or not being lined up as they should be. The nerves controlling eye movements are delicate and can be damaged particularly by severe head injury. In adults, a stroke can cause damage to the nerves or parts of the brain responsible for eye movements and very rarely a brain tumour can cause a squint and double vision.
Why is diagnosis important?
If a squint develops, it is important to find out what has caused the squint as it may be necessary to treat the underlying cause.
In children it is important to get a squint sorted out soon because having a squint can affect development of the vision permanently. If an adult develops a squint they get double vision because each eye is seeing a different picture. If a young child develops a squint they tend not to get double vision. This is because the brain is still developing and it can turn off the vision from the squinting eye. Although this is good because it means they don’t get double vision, if the squint is not sorted out the vision will be permanently switched off in that eye by the brain. The poor vision in a squinting eye is what we mean by the term lazy eye. Another name for the lazy vision in a squinting eye is amblyopia, a Greek word which literally means “blunted sight”.
Can squints be treated?
The short answer is “yes” and you can find out more about treatment in the sections on child and adult squint. However, treatment is not the same as cure. Most of the common problems with having a squint such as the cosmetic appearance and double vision can often be made better with various treatments. But it may not be possible to get rid of the squint completely, and it may not be possible to get rid of double vision. Sometimes, even after surgery, the eyes are not perfectly aligned, and there is a limit to how much surgery can be performed. Surprisingly, age is rarely a limiting factor and just because an adult has had a squint since childhood does not mean it can’t be treated.
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