Your first child wears glasses. What about the second?

There is a moment I see in clinic most weeks. The older child is on the chair having a new prescription checked, and the younger one is on a parent's lap or spinning the stool round, entirely uninterested. Somewhere near the end of the appointment the parent looks down at that second child and asks, half apologetically, whether they should bring this one in too, or whether that would be over-reacting.

It is not over-reacting. It is one of the more sensible questions a parent asks me, and the answer is yes. But the reason is not the one most parents assume. It is not that the younger sibling is destined for glasses. It is that the problems worth finding in a small child are silent, and you now have a good reason to look.

My eldest needs glasses. Should I get the younger one checked too?

Yes, and you do not need to wait for a symptom, a school letter or a birthday. The clearest statement of this comes from the NHS, which says an eyesight test is particularly important where there is a history of childhood eye problems such as squint or lazy eye in the family. Your family now has that history. That single fact moves your second child from "no particular reason" to "a reason".

The practical objection I hear next is always about age. She is two, she cannot read, she will not sit still. None of that stops a test. Children can have their sight tested at any age, and they do not need to be able to read or even speak for the measurement to be accurate. How we test eyes before a child can read goes through exactly what happens with a child that small.

We are both short-sighted. What are the actual chances for the children?

For short-sightedness, the numbers exist and they are worth knowing, though they describe parents rather than siblings. In an Australian population study of 2,353 twelve year olds, short-sightedness was present in 7.6% of children with no short-sighted parent, 14.9% of those with one, and 43.6% of those with two.

Two things about that gradient matter here. The first is that the same study measured the eyes themselves and found the difference was real rather than an artefact of reporting: with more short-sighted parents, the prescription was more myopic and the eyeball was longer. The second matters in this part of the world. When the researchers looked at ethnicity, the effect of having short-sighted parents was significantly stronger in the East Asian children than in the European ones.

What I will not do is convert any of that into a number for your second child specifically. Those figures are about parents and children, not about brothers and sisters, and nobody has published a sibling figure I would be willing to quote at you. Higher risk is not the same as certainty, and a great many children of two short-sighted parents never need a prescription at all.

Does a squint or a lazy eye run in families as well?

Almost certainly, and here I have to be honest that I cannot hand you a percentage. There is no figure for squint or lazy eye in siblings that I am prepared to print, because the good published numbers are for short-sightedness. What exists instead is the instruction, and it is the same one: a family history of squint or lazy eye is a specific reason to have the other children's eyes tested.

One rare condition does come with a family figure, and it is worth naming because it is the one where hours matter. In primary congenital glaucoma, about 10 to 40% of cases run in families. If a sibling has had that diagnosis, the other children need looking at by an eye doctor, not a screening.

The reason I push on squint and lazy eye rather than glasses is that they are the ones with a clock on them. A child whose two eyes send different quality pictures to the brain can quietly stop using one of them, and by school age that is much harder to undo. How to spot the drift of a lazy eye is the article I would read next if your eldest has one.

At what age should I bring the second one in?

Now, whatever age that is. There is no waiting list of birthdays to work through: a first check is possible from the newborn period and useful at every stage after it. If your eldest was diagnosed at four, do not wait until the second child is four. Diagnoses in siblings do not politely arrive at the same age.

What I would not do is rely on the school screening to catch it. In Malaysia the school health team checks sight in Year 1, Year 6 and Form 3, in government and government-aided schools. That is three points across thirteen years, it is a quick pass-or-refer check rather than a full examination, and a child in a private school or kindergarten may not be covered at all. What is actually checked and when, in Malaysia sets out the real schedule. A normal screening result does not rule out a squint or a refractive error, and it is not a reason to cancel an appointment you had good grounds to make.

She has no symptoms at all. Is a check still worth it?

Especially then. This is the part that catches out careful parents. A child using one good eye behaves exactly like a child using two. She will run, catch, colour, recognise you across a room and pass every test a loving parent can devise at home, while one eye quietly does very little of the work. There is nothing for her to complain about, because she has never seen anything different.

The things that are worth reporting, if you do see them, are unglamorous: the eyes not pointing in the same direction, headaches or eye strain, holding a book very close to the face, losing her place regularly while reading, clumsiness that seems out of step with her age, poor hand-eye coordination, constant eye rubbing, and sitting very close to the television. Any one of those in the younger sibling of a child who already wears glasses is worth a phone call rather than a wait-and-see.

If I keep her outdoors more, will she avoid glasses?

It shifts the odds, and it does not decide the outcome, and I would rather you had that than a promise. An American study followed 514 children and 111 of them became short-sighted. Two things separated them back in third grade: how many of their parents were short-sighted, and how many hours a week they spent on sport and outdoor activity. The children who stayed clear averaged about 11.7 hours a week outdoors; those who went on to need glasses averaged about 8. Low outdoor time raised the odds most in the children who already had two short-sighted parents.

Then comes the caveat that keeps me honest. That whole model, parental history plus outdoor hours together, reached a predictive accuracy of 0.73, where 1.0 would be perfect. It sorts groups of children well. It does not tell you what will happen to yours. The outdoor time evidence, sized honestly covers what that habit can and cannot promise.

One more finding from the Australian study, because it lands on the guilt most parents in this position are carrying. The odds of short-sightedness in those children did not rise with more near work. Reading, homework and close-up play were not what separated the children who became short-sighted from those who did not. If you have been quietly blaming yourself for the books, the worksheets or the tuition that produced your first child's glasses, that particular verdict is not supported by the study.

So bring the second one. Not because you have done something wrong, and not because she is doomed to match her sister, but because you have been handed a genuine reason to look, and looking early is the entire trick in this specialty. If the check finds nothing, you have spent an afternoon confirming that your younger child sees well, which is not a wasted afternoon. If it finds something, you have found it at the age when it is still straightforward to treat. If a prescription does come out of it, that first fortnight goes better than parents expect.

Common questions

References

1. Investigative Ophthalmology and Visual Science 2007 · Ethnic differences in the impact of parental myopia, Sydney Myopia Study · pubmed.ncbi.nlm.nih.gov
2. Investigative Ophthalmology and Visual Science 2007 · Parental history of myopia, sports and outdoor activities, and future myopia · pubmed.ncbi.nlm.nih.gov
3. NHS · Eye tests for children · www.nhs.uk
4. AAPOS · Glaucoma in children · aapos.org
5. AAPOS · Amblyopia (lazy eye) · 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.