The medical name for squint is strabismus. It is a condition where the eyes do not look in the same direction. Whilst one eye looks forwards to focus on an object, the other eye turns either inwards, outwards, upwards or downwards. Most squints occur in young children.
A child with a squint may stop using the affected eye to see with. This can lead to visual loss called amblyopia, which can become permanent unless treated early in childhood. This treatment usually involves patching the good eye, to force the use of the affected eye. Sometimes surgery is needed to correct the appearance of a squint.
What is a squint (strabismus)?
A squint is a condition where the eyes do not look together in the same direction. Whilst one eye looks straight ahead, the other eye turns to point inwards, outwards, upwards or downwards. Squints are common and affect about 1 in 20 children. You might even spot that your baby has a squint. Most squints develop before preschool age, usually by the time a child is 3 years old. Sometimes squints develop in older children, or in adults.
This leaflet only deals with childhood squints.
Understanding the eye muscles
The movement of each eye is controlled by six muscles that pull the eye in specific directions. The lateral rectus muscle pulls the eye outwards. The medial rectus muscle pulls the eye inwards. The superior rectus muscle is mainly responsible for upwards movements, whilst the inferior rectus muscle mostly pulls the eye downwards. Finally, the superior and inferior oblique muscles help to stabilise the eye movements - particularly for looking downwards and inwards, or for upward and outward movements.
For example, to look to the left, the lateral rectus muscle of the left eye pulls the left eye outwards and the medial rectus of the right eye pulls the right eye inwards towards the nose.
A squint develops when the eye muscles do not work together in a balanced way, so that the eyes do not move together correctly.
Are there different types of squint (strabismus)?
Yes, squints can be divided into different categories:
- By the direction of the squinting (turning) eye:
- An eye that turns inwards is called an esotropia.
- An eye that turns outwards is called an exotropia.
- An eye that turns upwards is called a hypertropia.
- An eye that turns downwards is called a hypotropia.
- Whether the squint is present all the time (constant), or comes and goes (intermittent).
- Whether the affected eye turns when the eyes are open and being used (manifest squint) or whether the eye turns only when it is covered or shut (latent squint) but looks fine when the eyes are open.
- Whether the severity (angle) of the squint is the same in all directions or not:
- A concomitant squint means that the angle (degree) of the squint is always the same in every direction that you look. That is, the two eyes move well, all the muscles are working but the two eyes are always out of alignment by the same amount, no matter which way you look.
- An incomitant squint means that the angle of squint can vary. For example, when you look to the left, there may be no squint and the eyes are aligned. However, when you look to the right, one eye may not move as far and the eyes are then not aligned.
- By age of onset. Most squints develop at some time in the first three years of life. Some develop in older children and adults. Squints that develop in children usually have different causes to those that develop in adults.
- By the cause:
- In many cases of childhood squint, the reason why a squint develops is not known.
- In some cases of childhood squint (and most cases of adult squint), the squint occurs because of a disorder of the eye, the eye muscles, the brain or the nerves.
What are the causes of squint (strabismus) in children?
About 5 in 100 children aged 5 years have a squint. It is quite common to notice a brief squint in children when they are tired or daydreaming. Babies sometimes cross their eyes - it is quite normal for this to happen occasionally, especially when they are tired.
Some squints are much more obvious than others. You might notice your child has an eye that does not look straight ahead. Another sign of squint is that your child might close one eye when looking at you, or turn his or her head on one side.
Congenital squints of unknown cause
Congenital squint means that the child is born with a squint, or it develops within the first six months of life. In most cases, the eye muscles are not balanced but the reason for this is not known.
In most cases one eye turns inward. This is called congenital esotropia (sometimes called infantile esotropia). This common type of squint tends to run in some families. However, many children with congenital esotropia have no other family members affected. In some cases the eye turns outwards (congenital exotropia). Less commonly, a squint of unknown cause may result in an upward or downward turn of the eye.
Squint related to refractive errors
Refractive errors include: short sight (myopia), long sight (hypermetropia) and astigmatism. This leads to problems with focusing. These are all conditions that are due to poor focusing of light through the lens in the eye. An astigmatism is a vision problem where the surface of the eye (the cornea) or the lens, is more oval-shaped, rather than round. When the child with a refractive error tries to focus to see clearly, an eye may turn. This type of squint tends to develop in children who are 2 years or older, in particular in children with long sight. The squint is most commonly inward looking (an esotropia).
Most children with a squint have one of the above types of squint and are otherwise healthy. However, in some cases, a squint is one feature of a more generalised genetic or brain condition. Squints can occur in some children with cerebral palsy, Noonan's syndrome, Down's syndrome, hydrocephalus, brain injury or tumour, a rare type of eye cancer (called retinoblastoma) and several other conditions.
What problems can be caused by a squint (strabismus) in a child?
Amblyopia is sometimes called a 'lazy' eye. It is a condition where the vision in an eye is poor and it is caused by lack of use of the eye in early childhood. The visual loss from amblyopia cannot be corrected by wearing glasses. However, it is usually treatable (see below).
If amblyopia is not treated before the age of about 7-8 years, the visual impairment usually remains permanent.
To understand how amblyopia occurs, it is helpful to understand how vision develops. Newborn babies can see. However, as they grow, the visual pathways continue to develop from the eye to the brain and within the brain. The brain learns how to interpret the signals that come from an eye. This visual development continues until about age 7-8 years. After this time, the visual pathways and the 'seeing' parts of the brain are fully formed and cannot change.
If, for any reason, a young child cannot use one or both eyes normally then vision is not learnt properly. This results in impaired vision (poor visual acuity) called amblyopia. The amblyopia develops in addition to whatever else is affecting the eye. In effect, amblyopia is a problem with the development of vision in the brain, rather than a problem within the eye itself. Even if the other eye problem is treated, the visual impairment from amblyopia usually remains permanent unless it is treated early enough.
A squint is the most common cause of amblyopia. In many cases of squint, one eye remains the one that sees (the dominant focusing eye). The other, turned eye (the squinting one) is not used to focusing and the brain ignores the signals from this eye. The turned eye then fails to develop the normal visual pathways in childhood and amblyopia develops in this eye. See separate leaflet called Amblyopia for more details.
How the squint looks
A squint can be a cosmetic problem. Many older children and adults who did not have their squint treated as a child have reduced self-esteem because of the way their squint looks to other people.
Impaired binocular vision
With normal eyes, both eyes look and focus on the same spot. This is called binocular vision (bi- means two and ocular means related to the eye). The brain combines the signals from the two eyes to form a three-dimensional image. If you have a squint, the two eyes focus on different spots. In children with squint this does not usually cause double vision. In children the brain quickly learns to ignore the signals and images coming from the turned (squinting) eye. The child then effectively only sees with one eye. This means the child does not have a good sense of depth when looking at objects. As a result, he or she cannot see properly in three dimensions.
(Adults who develop a squint often have double vision, as their developed brain cannot ignore the images from one eye.)
How is a squint (strabismus) diagnosed and assessed?
It is important to diagnose a squint (and amblyopia) as early as possible. Routine checks to detect eye problems in babies and children are usually done at the newborn examination and at the 6- to 8-week review. There is also a routine preschool or school-entry vision check.
Some newborn babies have a mild intermittent squint that soon goes, reducing by 2 months of age and gone by 4 months of age. However, fixed squints are usually permanent unless treated. So, as a guide:
- A squint seen in a newborn baby is likely to resolve if it is intermittent (comes and goes), reducing by 2 months of age and gone by 4 months of age.
- A baby with a constant fixed squint, or with an intermittent squint that is worsening from 2 months, should be referred for assessment.
Any baby or child with a suspected squint is usually referred to an orthoptist. An orthoptist is a health professional who is specially trained to assess and manage children with squint and 'lazy' eye (amblyopia). If necessary, an orthoptist will refer a child to an eye surgeon (ophthalmologist) for further assessment and treatment.
Various tests can be done to check a child's vision. Sight tests can even be done for babies. Tests to find a squint can involve covering and uncovering each eye in turn. This often shows which eye has the squint and how it moves. The pupils of the eye can be checked with a torch, to check they become smaller (constrict) with light and widen (dilate) when the light is removed. An ophthalmoscope is a special torch used to examine the back of the eye (the retina). Very occasionally, if another cause of squint is suspected (other than a congenital squint or one related to refractive errors), a scan of the eye or the brain may be needed.
What are the treatments for squint (strabismus)?
Treatment typically involves the following:
- Treating amblyopia (visual loss) if this is present.
- Wearing glasses to correct any refractive error, if this is present.
- Surgery is often needed to correct the appearance of the squint itself and may help to restore binocular vision in some cases.
Treating 'lazy' eye (amblyopia)
The main treatment for amblyopia is to restrict the use of the good eye. This then forces the affected eye to work. If this is done early enough in childhood, the vision will usually improve, often up to a normal level. In effect, the visual development of the affected eye catches up. The common way this is done is to put a patch over the good eye. This is called eye patching.
The length of treatment with an eye patch is dependent on the age of the child and the severity of the amblyopia. The patch may be worn for a few hours a week or for most of the day, every day. Treatment is continued until either the vision is normal or until no further improvement is found. It may take from several weeks to several months for eye patching to be successful.
Your child will be followed up, usually until about 8 years of age, to make sure that the treated eye is still being used properly and does not become amblyopic again. Sometimes, further patch treatment (maintenance treatment) is needed before the vision pathways in the brain are fixed and cannot be changed.
Occasionally, eye drops to blur the vision in the good eye, or glasses that prevent the good eye from seeing clearly, are used instead of an eye patch.
Vision therapy can be used as a treatment to maintain the good work achieved by eye patching. This involves playing visually demanding games with a child to work the affected eye even harder - like eye training.
Note: eye patching and other treatments for amblyopia aim to improve vision; they do not by themselves correct the appearance of a squint.
Correcting refractive errors
If a child has a refractive error (long or short sight, for example) then glasses will be prescribed. This corrects vision in the eye. It may also straighten the squinting eye, if the refractive error was the cause of the squint.
In many cases an operation is advised to make the eyes as straight as possible. The main aim of surgery is to improve the appearance of the eyes. In some cases, surgery may also improve or restore binocular vision (this means that the two eyes are working together).
The exact operation that is done depends on the type and severity of the squint. It may involve moving the place where a muscle attaches to the eyeball or, one of the muscles that moves an eye may be shortened. Sometimes a combination of these techniques is used.
Botulinum toxin (also know as Botox®) stops muscles from working (it paralyses them). It is used for a variety of conditions where it is helpful to weaken one or more muscles. In recent years, injections of botulinum toxin directly into eye muscles have been used as a treatment for certain types of squint, particularly for squints that turn inward (esotropia). This treatment is an alternative to surgery - but only for certain types of squint.
What is the outlook (prognosis)?
For 'lazy' eye (amblyopia), if it develops
As a rule, the younger the child is treated, the quicker the improvement in vision is likely to be and the better the chance of restoring full normal vision. If treatment is started before the age of about 7 years then it is often possible to restore normal vision. If treatment is started in older children then some improvement in vision may still occur but full normal vision is unlikely ever to be achieved.
For improving the appearance of the eyes
Squint (strabismus) surgery usually greatly improves the straightness of the eyes. Sometimes, even after an operation, the eyes are not perfectly straight. In some cases, two or more operations are needed to correct the squint. Sometimes a special stitch is put in place which can be adjusted later on if further correction is needed.
It is possible that several years after successful surgery, the squint may gradually return again. A further operation is an option to restraighten the eyes.
Further reading and references
; Royal College of Ophthalmologists (2012)
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