Squint (strabismus)

A squint is when the eyes do not point at the same thing: one looks straight ahead while the other turns in, out, up or down. It can be there all the time, or only appear towards the end of the day, when the child is tired or unwell. It is found most often in young children, but it can begin at any age. The one thing to take away from this page is that a squint is not a cosmetic question but a question of sight, and time works against a child. In an adult, a squint that appears suddenly is a signal of a quite different kind — and in some cases an emergency.

What happens to sight when the eyes point different ways

The brain receives two pictures and normally merges them into a single three-dimensional one. If the eyes point differently the two pictures do not match, and the person ought to see double. An adult does. A child's brain, still plastic, does something else: it simply switches off the image from the turning eye so that it stops getting in the way.

The double vision disappears, the child complains of nothing, and the parents assume all is well. In fact the worst part has just begun. The eye whose signal the brain has stopped accepting is perfectly healthy as an organ, but it is not learning to see: the visual pathways in the brain only develop when they are used. Little by little that eye becomes the weaker one — a state called amblyopia, or a lazy eye.

Everything then turns on age. The visual system is built in the first years of life, and that is precisely the window in which it can be re-taught, in which the brain can be made to accept the switched-off eye again. Results in a preschool child are markedly better than in a schoolchild, and once visual development is complete only part of what was lost can be recovered. Hence the conclusion this whole page exists for: what matters is not so much the treatment as when it starts. A year of waiting in a three-year-old costs more than the entire course of treatment that follows.

Why "they'll grow out of it" is a dangerous phrase

A newborn's eyes really can drift apart: the muscles work out of step and the gaze has not settled yet. That is normal in the first weeks and passes by itself. By a few months of age the eyes should be working together — and from then on any deviation stops being a stage of development.

What genuinely needs an examination:

• a squint that is present all the time, at any age, including the first months of life;
• a squint that appears after the gaze has already settled;
• a squint that comes and goes. This is a separate point and the one most often missed. An intermittent deviation matters no less than a constant one: during the minutes the eye drifts, the brain switches it off, and the habit of switching off takes hold. It is also how a squint that later becomes constant frequently begins. A child with an intermittent squint needs their vision tested exactly as much as a child with a constant one.

There is a mirror-image situation too. In babies with a broad nasal bridge and folds of skin at the inner corners of the eyes, the gaze often looks converged although the eyes are perfectly straight — this is called a pseudosquint, and it disappears as the face grows. It cannot be told from the real thing in a photograph, which is exactly why an examination is needed. So the right answer to a doubt is not "let's watch it" but "let's have them looked at".

And one more thing: a child's vision should be checked even with no complaints at all, as part of routine health checks. A child does not know that they see worse than other people and will never say so — they assume everyone sees the way they do.

How to spot it at home

The deviation itself is easiest to catch when a child is tired, unwell or distracted, because that is when control over the eye slips. But there are indirect signs by which parents often notice something earlier:

• the child constantly turns or tilts their head to look at something — they are hunting for the position in which the double vision goes away;
• screws up or covers one eye, particularly in bright light;
• holds books and toys very close to the face, sits right up against a screen;
• rubs the eyes, complains of headache and tiredness after close work;
• judges distance badly: misses a step, drops things, is clumsy in running games;
• says they can see two of something — from a child this comes out as "two grannies" or "the letters are doubled";
• the eyes reflect a camera flash differently: a red glow in one and a white one, or none at all, in the other.

A white pupil in photographs

Normally a flash produces the same red reflection in both eyes. If the reflection in one eye is white, yellowish or absent, and the same thing shows up in other photographs, the child needs to be seen by an eye specialist immediately, not put off even by a week.

Behind this sign there may be a cataract, and there may be a retinoblastoma, a rare malignant tumour of the retina in young children. Caught early it is curable and often allows the eye and its sight to be saved; missed, it threatens life. Do not wait for a routine appointment and do not try to judge the photograph yourself: this sign has either to be confirmed or ruled out, and that is done by a doctor examining the back of the eye.

When it is an emergency

A squint that comes on suddenly, in a child or an adult, needs to be assessed the same day. Particularly if it comes with any one of the following:

• double vision that appeared along with the squint;
• headache, especially one that is worst in the morning or wakes the person at night;
• vomiting, especially in the morning and without nausea or any sign of a stomach bug;
• a drooping eyelid;
• pupils of different sizes, or a pupil that reacts sluggishly to light;
• a sudden drop in vision in one eye;
• unsteadiness on the feet, weakness or numbness in an arm or leg, slurred speech, a drooping face;
• in a baby: a head circumference growing fast, a bulging fontanelle, unusual drowsiness, or the eyes rolling downwards.

The reason for the urgency is straightforward: the eye muscles are driven by cranial nerves, and those nerves suffer when pressure inside the skull rises, when there is a mass in the brain, an aneurysm or a stroke. A sudden squint is sometimes the first and only visible sign of such a process.

If a squint or double vision appears together with weakness, slurred speech, a drooping face or a sudden, severe headache, call an ambulance — across Europe the single emergency number is 112. A squint after a head injury also needs urgent assessment.

Where a squint comes from

The exact cause is not always found, but the range is well known.

• Uncorrected long-sightedness. The commonest cause in children. To bring an object into focus the eye strains its focusing muscle, and that reflexively pulls the eyes inwards. An important consequence follows: this kind of squint often disappears completely with nothing more than the right pair of glasses.
• Other refractive problems — short-sightedness, astigmatism, a large difference between the two eyes.
• Family history: squint and amblyopia often run in families.
• Prematurity and low birth weight.
• Differences in how the nervous system developed: cerebral palsy, Down's syndrome, the after-effects of infections and injuries.
• Anything that sharply reduces the sight of one eye: a congenital cataract, clouding of the cornea, a tumour of the retina. An eye that cannot see stops holding its position.
• In adults, damage to the nerves that drive the eye muscles, and diseases of the muscles themselves.

What the eye specialist does

The examination requires a child neither to read nor to speak: there are methods for every age. The doctor will watch how the eyes follow an object, measure the sight in each eye separately, measure the angle of the deviation, put in drops that relax focusing so the need for glasses can be judged accurately, and will always examine the back of the eye — that is where a cataract or a tumour shows.

Treatment is built in stages, and the order matters.

• Glasses. First and most effective. If the squint is driven by long-sightedness, glasses often straighten the eyes on their own and nothing further is needed. They have to be worn all the time, not just for lessons.
• Treating the amblyopia. The eye that has fallen behind is trained by limiting the stronger one: a patch over the good eye for a few hours a day, or drops that blur its vision for a while. This is the hardest part for a family — the child resists, because it sees worse — and the most important, because this is what restores sight.
• Eye exercises for working the two eyes together help in some forms; prisms in the glasses shift the image and cancel double vision, and are used mostly in adults.
• Surgery. The muscles that turn the eye are weakened or tightened by moving where they attach. It is done under general anaesthetic, usually with discharge the same day. The eye is sore for a few days and stays red for weeks, one operation is sometimes not enough, and glasses are usually still needed afterwards.
• Botulinum toxin injections weaken a muscle temporarily; the effect lasts a few months, so this is an option for particular situations.

Surgery straightens the eyes but does not treat amblyopia: the sight of the weaker eye is restored beforehand and the work continues afterwards. Pain relief is chosen by the doctor; aspirin is not given to children or teenagers.

Squint and double vision in adults

An adult brain can no longer switch off the surplus picture, so a squint in an adult nearly always causes double vision — and nearly always means a new cause has appeared that has to be looked for.

Most often it is a fault in one of the nerves that drive the eye muscles. In people with diabetes and high blood pressure this can happen because a small vessel supplying the nerve has been damaged; the double vision comes on suddenly, lasts weeks and usually clears by itself over a few months — but it is a doctor, not the patient, who has to establish that this is indeed the cause. Other causes include thyroid disease affecting the eye muscles, myasthenia (in which double vision and a drooping lid get worse towards evening and after exertion), multiple sclerosis, the after-effects of injury, stroke, an aneurysm, a tumour.

There is a separate and quite harmless story as well: a squint that was present in childhood, was held in check for years and then broke down in adult life, usually on a background of tiredness or illness. It also needs assessment, but is generally sorted out with prism glasses or surgery. The general rule is that double vision and a squint appearing in an adult are not watched at home — they go to a doctor, and the same day if any one of the signs in the emergency section is there too.

Online consultation

A remote appointment answers the question "should I be worried, and how quickly" rather well. The doctor will ask when you first noticed the deviation and whether it is constant or comes with tiredness, will look at your photographs and videos — these often show both the angle itself and the character of the pupil reflections — and will say whether this looks like a pseudosquint or needs an examination in person, and how soon. They can go through an eye specialist's report if you already have one: what the numbers mean, why the glasses, why the patch and how many hours it needs to be worn. And they can talk through what to do when a child refuses the glasses or the patch, which is the single commonest reason treatment fails. The signs in the sections on the white pupil and on emergencies are not for an online appointment: with those you go to a doctor immediately.

This material is for information only and does not replace medical advice.