Squint

Introduction

A squint is a condition where one eye turns inwards, outwards, upwards or downwards while the other eye looks forwards. The misalignment of the eye is caused by an incorrect balance of the eye muscles. The medical name is strabismus.

Squints are common and affect about one in 20 children. They usually develop during the first three years of life, but can appear later. A squint is often spotted in early childhood, sometimes within weeks of a baby being born.

Types of squint

There are four different types of squint. They are classified according to the direction in which the eye turns.

Esotropia: the eye turns inwards.
Exotropia: the eye turns outwards.
Hypertropia: the eye turns upwards.
Hypotropia: the eye turns downwards.

Hypertropia and hypotropia are less common than esotropia and exotropia.

Squints can also be:

constant, apparent at all times, or
intermittent, only apparent at certain times.

The cause, severity and direction of a squint vary from person to person.

How does a squint affect vision?

It is very important that a squint is picked up and treated as early as possible to avoid vision problems developing. If a squint is identified when a child is young, there is a good chance it will be successfully treated.

In young children, a squint can mean binocular vision fails to develop (see the box, left). This may result in a ‘lazy eye’ (amblyopia). To avoid double vision, the child’s brain ignores the signals from the eye with the squint and only recognises images from the normal eye. As the squinting eye is not being used, it eventually becomes ‘lazy’.

In older children, a squint may cause double vision but not result in a lazy eye. This is because their vision has fully developed and their brain is unable to ignore signals from the eye with the squint.

If the vision in a child’s squinting eye is poor, they may have to wear a patch over their other eye to encourage the vision in the squinting eye to develop.

Can adults develop a squint?

Occasionally, squints that have been corrected during childhood reappear in adulthood. Squints that affect adults may cause double vision as the brain has been trained to collect images from both eyes.

If you develop a new squint, visit your GP as soon as possible. They may refer you to an ophthalmologist (an eye care specialist) who will carry out an examination to identify the cause.

Squints may cause a cosmetic problem in adults whose squint was not treated when they were young. In such cases, the appearance of a squint my lead to low self esteem.

Causes

The cause of a squint is not always known. Squints sometimes run in families and a baby can be born with the condition (congenital squint).

Squints are also sometimes the result of childhood illnesses or other sight-related problems, such as long-sightedness (hypermetropia).

Congenital squints

A child is either born with a squint or develops one during the first six months of life.

Sometimes other family members have a squint, which suggests it can be genetic.

In most cases of congenital squint, the eye turns inwards (congenital esotropia). It is also possible (although rarer) for the eye to turn outwards (congenital exotropia).

Refractive errors

Squints are sometimes caused by the eye’s inability to focus the light that passes through the lens. This is known as a refractive error, and is also the cause of conditions such as short-sightedness (myopia), long-sightedness (hypermetropia) and astigmatism (where the cornea at the front of the eye is unevenly curved).

See Useful links for more information about these other conditions.

If a child has a refractive error, their eye may turn inwards as it attempts to focus. Squints caused by refractive errors usually develop in children who are two or older and tend to be most common in children who are long-sighted.

Other causes

Most squints are congenital or caused by refractive errors. Occasionally, squints can be the result of:

childhood illnesses, such as viral infections like measles,
some genetic conditions, such as Noonan syndrome, or
a brain condition, such as hydrocephalus where there is too much cerebrospinal fluid (CSF) in the ventricles (cavities) of the brain.

Diagnosis

A squint can cause vision problems, such as a lazy eye (amblyopia), double vision or blurred vision. The earlier a squint is identified and diagnosed, the more chance there is of successfully treating it and restoring vision to normal.

Eye care professionals

If you or your child has a squint, there are three types of eye care professional who may be involved with your or your child’s care.

Optometrists carry out sight tests and prescribe glasses. They can also detect eye conditions and, if necessary, refer your child to an ophthalmologist or orthoptist.
Orthoptists are specifically trained to assess children and adults with squints and lazy eyes (amblyopia).
Ophthalmologists will examine and assess your or your child’s eyes. They also carry out any surgery that may be needed.

Routine eye checks

Routine eye checks are carried out at birth and again six to eight weeks later. Children are also given a routine eye check just before starting primary school.

Your child will have a number of different tests which can help diagnose a squint and assess their level of vision. The type of tests that your child has depends on their age, but may include:

looking at a light,
matching letters and pictures,
reading a letter chart, and
looking at visual targets, such as a toy, at different distances, first with one eye covered and then the other.

If a squint is suspected, your GP or optician will refer your child to an ophthalmologist for further examination. Your child will have their eyes tested to determine whether they need glasses. Before the test is carried out, your child’s pupils will be dilated (expanded) using eye drops.

Their retina (the light-sensitive membrane at the back of the eye) and the optic nerve (the nerve that carries visual messages from the retina to the brain) will also be examined to make sure that there are no other problems with them.

Symptoms

The most obvious sign of a squint is one eye that does not look straight ahead but turns inwards, outwards, upwards or downwards.

Minor squints may be less obvious.

Babies and young children

It is quite normal for the eyes of newborn babies to 'cross' occasionally, particularly when they are tired. Speak to your GP if you notice this happening to your child after the age of three months.

If your child looks at you with one eye closed or with their head turned to one side, it may mean that they are experiencing double vision and could be a sign that they have a squint. See your GP as soon as possible if this happens repeatedly.

Lazy eye

If a squint is left untreated, lazy eye (amblyopia) can develop. The vision in the affected eye gradually deteriorates because the brain ignores the weaker message being sent from that eye. It is not possible to correct damage once it has occurred, which is why it is so important to treat a squint as soon as possible.

Treatment

It is very important that a squint is treated as soon as possible after being detected. If it is not treated, vision problems, such as those caused by a lazy eye (amblyopia), are likely to get worse or could become permanent.

Treatment is most effective in very young children.

Types of treatment

Several types of treatment are available for squints:

glasses,
eye patch,
botulinum toxin injection,
eye drops, and
corrective surgery.

Glasses

If your child is long-sighted (hypermetropia), they may be prescribed glasses which can often correct both the vision problem and the squint.

Eye patch

A patch may need to be worn over the ‘good eye’ to encourage the eye with the squint to work harder and train it to work properly.

Botulinum toxin injection

Botulinum toxin (botox) is injected into one of the muscles on the eye’s surface. This may be recommended if a squint develops suddenly and no underlying cause can be found.

In children, a botox injection will usually be given under general anaesthetic. The injection temporarily weakens the injected muscle, allowing the eyes to realign.

Eye drops and eye exercises

In some cases, it may be possible to treat a squint using special eye drops or eye exercises.

Surgery

If none of the above treatments work, surgery may be needed. Surgery has two main benefits:

it can improve the alignment of the eyes, and
it can get the eyes working together (binocular vision).

The procedure

Surgery to correct a squint involves moving the muscles attached to the outside of the eye to a new position. It may be necessary to operate on both eyes to 'balance' them effectively, even if the squint is only in one eye.

As corrective squint surgery usually takes less than an hour to perform, the procedure is often carried out as a day case. The operation is usually performed while the patient is under a general anaesthetic.

You may be able to accompany your child to the operating theatre and stay with them until they have been given the anaesthetic. A nurse will be with your child throughout the procedure.

During the operation, your child’s eye will be kept open using an instrument called a lid speculum. The ophthalmologist will detach one part of the muscle that is connected to your child’s eye and will either move it backwards to weaken the pulling effect or shorten it to increase the pulling effect. Once the correction has been made, the muscles will be sewn back into place using dissolvable stitches.

There is very little chance that your child's eyesight will be damaged during the operation because the part of the eye responsible for focusing is not touched. For a short time after the operation, your child will need to use eye drops. The nurse will show you how to use them before you leave the hospital.

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