Why do we have two eyes, and what happens when one turns?

Almost every week a parent repeats to me a question their child asked them first. Why two eyes? You only get one nose and one mouth. It sounds like a nursery question, and it is very close to the question I spend my clinic days on, because nearly everything I check in a child whose eye turns comes back to it.

Two eyes are not one eye plus a spare. They are meant to work as a pair, and a young visual system spends its first months learning to use them that way. That is why an eye that turns is not really a question about how a face looks. It is a question about a pair.

Why do we have two eyes?

Because they are built to be used as a pair, and a great deal of your child's sight is assembled around that pair. The word we use for it is binocular vision, which simply means the two eyes working together. It is not an ornament laid over ordinary sight. Untreated early crossing can stop vision developing properly, and the two things named as consequences are a lazy eye and poor depth perception. That second one is the plainest statement I know of what the pair is for.

You can watch a piece of it happening at home, and children love this one. Hold a finger up at arm's length, line it up with something across the room, then close one eye and the other in turn. The finger jumps sideways. Two eyes, two positions, and a child who has just discovered that will do it fifteen more times before dinner.

One qualification, because parents ask me this after something serious has been found in one eye. Depth does not come only from two eyes. Where a child has one working eye, the brain adapts to judging depth with one eye, and it does that well. Two working eyes give the fullest version of it, and while a child is small, that fullest version is the one we work to protect.

How do eyes work together?

By being aimed at the same thing at the same moment. That is the whole of it. When the eyes are oriented in different directions, the brain receives two different visual images, and every difficulty that follows a squint grows out of that one fact.

Aiming is also why so much of a children's eye examination looks like nothing at all. A small light held in front of the face, with the reflection expected in the centre of each pupil at the same time. One eye covered and uncovered while I watch the other for movement. A toy carried slowly across a room. Each of those asks the same question, which is whether the two halves are aimed together, and testing a child who cannot yet read is built almost entirely around it. No letters required, and no answers required from your child either.

When do babies get depth perception?

Not all at once, and not fully at birth. At three months old a baby's eyes should work together to focus and track objects. Adult-like binocular vision, the mature version of the two eyes working as a pair, emerges between about twelve and sixteen weeks of age. By around five months a baby is more completely seeing the world in three dimensions.

Before three months, eyes that drift or cross now and then are ordinary, and I am shown that photograph in clinic constantly, usually taken at bedtime by a mother who has not slept properly in weeks. Crossed eyes in a newborn is its own topic with its own thresholds, and the rest of the first year, month by month, sits in how babies learn to see.

What does a squint do to vision?

It takes the pair apart. Two eyes aimed at two different places are no longer working as one instrument, and the NHS is blunt about what can follow a squint left alone: persistent blurred or double vision, a lazy eye in which the brain starts to ignore signals coming from the affected eye, and vision problems that may be permanent if they are not treated at a young age.

I say that across the desk, and then I say the other half in the same breath, which is that this is a problem with a window in it, and in a small child the window is wide open. Turns have directions and we name them. Esotropia is an eye crossing inward toward the nose, exotropia is an eye drifting outward toward the ear, and hypertropia is one eye pointing higher than the other. The direction tells me useful things about the cause. It makes no difference at all to what has been lost, which is the same every time.

Why would a young brain switch off a healthy eye?

Because it has been handed two pictures and dropping one is the quickest way to end the argument. The brain may ignore the image from the misaligned eye to avoid double vision, so no double vision arrives and your child is comfortable. The price is paid quietly, by the eye that was set aside, in which vision then does not develop as it should.

That is amblyopia, what we call a lazy eye, and when a turn is the cause we call it strabismic amblyopia. Amblyopia is the commonest cause of reduced vision in children. It is also why a turn is worth checking rather than watching, whatever its size looks like from across a room, and why the noticing falls to the adults. Spotting a drift at home covers the silence and what a drift looks like across an ordinary week.

If we catch it early, can it be treated?

Yes. A turn, and the lazy eye that can follow it, can be treated, and treatment works far better when it starts young, while the pair is still being put together. Amblyopia should be treated before a child is seven or eight years old for the strongest result. Starting in the early teens can still help, though it helps less, which is an argument for this month rather than next year.

Where the turn comes out of focusing effort, glasses are often the first move rather than the last resort. A child who is more long-sighted than normal has to work harder to see clearly, that effort can pull an eye inward, and glasses that take the effort away will often straighten the eye. That surprises most families, since the news that a small child needs glasses usually arrives as bad news. Where one eye has already been set aside, patching the stronger eye or blurring it with drops gives the weaker eye work to do. Both are well studied and effective, and an eye doctor chooses between them. There are other treatments an eye doctor may discuss, depending on the type of turn and what is causing it.

Timing, plainly, because that is what a parent actually wants at midnight. An eye that turns all the time, at any age, is a this week appointment. A turn that comes and goes and is still doing so after three months should be booked, and a few weeks is fine. A squint that appears suddenly in an older child whose eyes were straight before is a same day one, and is this urgent sets all three out in a line.

So the child's question turns out to be a good one. We have two eyes because they are built to be used as a pair, that pair is assembled in the first months of life by being used, and when one eye stops joining in, the pair is what is at stake. Caught while it is still being built, a turn is a problem with well studied treatment behind it, and the treatment is far less dramatic than the worrying.

Common questions

References

1. AAPOS · Strabismus · aapos.org
2. American Academy of Ophthalmology · Strabismus in children · www.aao.org
3. NHS · Squint · www.nhs.uk
4. AAPOS · Accommodative esotropia · aapos.org
5. American Academy of Ophthalmology · Typical and atypical development of ocular alignment and binocularity · www.aao.org
6. American Academy of Ophthalmology · Baby vision development in the first year · www.aao.org
7. AAPOS · Amblyopia (lazy eye) · aapos.org
8. American Academy of Ophthalmology · Amblyopia (lazy eye) · www.aao.org
9. AAPOS · Pseudostrabismus · aapos.org

By Dr Chan Li Yen. General information only. It does not replace a consultation. If you are worried about your child's eyes or vision, please see an eye doctor.