Squint (strabismus)

What is a squint (strabismus)?

A squint, medically called strabismus, is when your eyes don't point in the same direction. One eye may look straight ahead while the other turns inwards, outwards, upwards or downwards, and this can happen all the time or only sometimes.

What are the symptoms of a squint?

The main sign of a squint is one eye turning while the other looks straight ahead. This might only be noticeable when you're tired or concentrating hard, or it may be there all the time.

Other symptoms can include:

• double vision
• tired eyes
• difficulty judging distances or seeing in 3D
• turning or tilting your head to try to see more clearly
• in children, keeping one eye closed or regularly turning their head to one side.

In young children, a squint can also lead to reduced vision in the affected eye, sometimes called a ‘lazy eye’, because the brain starts to ignore signals from that eye.

What are the causes of squints and who is at risk?

In many cases, the exact cause of a squint isn't known. Some people are born with one, while others develop one later in life, and it can run in families.

In children, a squint often develops because the eye is trying to compensate for a vision problem, such as:

• long-sightedness (difficulty seeing things up close), which is the most common cause of an inward-turning squint
• short-sightedness (difficulty seeing things far away)
• astigmatism, where the front of the eye is unevenly curved.

Less commonly, a squint can be caused by a weak or abnormal eye muscle, or a problem with the nerves controlling eye movement. Your child is more likely to develop a squint if a family member has had one, if they were born prematurely or with a low birth weight, or if they have a condition such as cerebral palsy, Down's syndrome, or a general delay in development.

In adults, a squint may be one that was treated in childhood and has come back, a longstanding tendency that's become harder to control over time, or the result of an accident or illness affecting the eye muscles or nerves.

Are there different types of squint?

Squints are usually described by the direction the affected eye turns:

• Esotropia (convergent squint) – the eye turns inwards, towards the nose.
• Exotropia (divergent squint) – the eye turns outwards.
• Hypertropia – the eye turns upwards.
• Hypotropia – the eye turns downwards.

A squint can also be described by how often it happens. A constant squint is there all the time, while an intermittent squint comes and goes. In some children, the turn swaps between the two eyes rather than always affecting the same one – this is called an alternating squint, and it's often a sign that the child sees equally well with each eye.

How is a squint diagnosed?

If you or your child might have a squint, a GP, health visitor or optician can refer you to an eye specialist team for assessment.

This usually involves a full eye examination, including a vision test and an assessment by an orthoptist, a specialist in eye movement and alignment, who will measure the type and size of the squint. Eye drops are often used to widen the pupils so the back of the eyes can be examined closely. An ophthalmologist or optometrist will also check the health of the eyes and whether glasses are needed.

How is a squint treated?

Treatment depends on the type and cause of the squint, and you or your child may need one treatment or a combination.

Non-surgical treatment for a squint

Glasses are often the first treatment, especially if long-sightedness, short-sightedness or astigmatism is contributing to the squint. Wearing them consistently helps the eyes focus without straining, and can sometimes straighten the eyes completely, though the squint is likely to return, and may look more noticeable, if the glasses come off. Occasionally, temporary bifocal glasses are used to relax the focusing needed for close-up work.

If a squint has caused reduced vision in one eye (a ‘lazy eye’), patching the stronger eye for part of the day can help the weaker eye's vision develop. This treats the vision problem rather than the squint itself, though it can sometimes make the squint smaller as a result.

Eye exercises can help some squints by improving how well the eyes work together, and prism glasses can help control double vision or eye strain.

Surgical treatment for a squint

Surgery involves adjusting the muscles that move the eyes so they line up better. It's carried out under general anaesthetic, and sometimes an adjustable technique is used, where the eye position is checked once you're awake and fine-tuned further under local anaesthetic if needed.

Surgery can be done at any age, not only in childhood, and is used either to reduce the appearance of the squint, to help the eyes work together, or both, though the latter is only possible for some types of squint. If glasses or patching are needed to improve a child's vision, this is usually done first, since surgery realigns the eyes but doesn't improve eyesight itself.

When should I see a doctor?

See a GP, health visitor or optician if:

• your child's squint is there all the time
• your child is older than 3 months and has a squint that comes and goes (in babies younger than this, an occasional squint is common and usually nothing to worry about)
• you have any concerns about your child's vision, such as them regularly turning their head to one side or closing one eye
• you or your child develop a squint or double vision later in life.

A squint developing in a child older than 5 can cause sudden double vision and needs urgent referral. A squint can also, rarely, be a sign of a type of childhood eye cancer called retinoblastoma, so it's important to get any new squint checked.