MYOPIA CLINIC · AXIAL LENGTH

The measurement that matters most

Axial length tells us how your child's eye is actually changing — to a tenth of a millimetre. It's more accurate than the glasses prescription alone.

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Reviewed by Dr Mark Joung, B.Optom (Hons) UNSW · Last updated June 2026

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Axial Length Monitoring

AXIAL LENGTH

Why the length of your child's eye matters more than their prescription

Axial length is the physical measurement of your child's eye from front to back, measured in millimetres. It's the single most accurate way to track whether myopia is getting worse — because an eye can be quietly growing longer even when the glasses prescription looks stable.

A normal adult eye is about 23 to 23.5mm long. In myopia, the eye grows too long — and every additional millimetre of length adds roughly -2.50 to -3.00 dioptres to the prescription. That's a big change from a tiny measurement.

More importantly, longer eyes carry a higher lifetime risk of serious conditions like retinal detachment, glaucoma, and myopic maculopathy. Research shows that once axial length exceeds 26mm, these risks increase sharply. The goal of myopia control is to keep that number as low as possible while your child's eyes are still growing.

"In our experience, once a child's axial length approaches 24mm — around 24.1mm in boys and 23.7mm in girls — we start monitoring more closely and discussing treatment options, regardless of what the prescription says."

REFRACTION VS AXIAL LENGTH

Why the glasses prescription doesn't tell the whole story

A standard eye test measures refraction — the prescription needed to correct your child's vision. But refraction fluctuates with focusing effort and accommodation, especially in children. Axial length doesn't. It's a fixed physical measurement that shows exactly how the eye is changing over time.

This disconnect matters most in the early stages. A child can have perfect 6/6 vision while their eye is already growing faster than it should. By the time the prescription catches up, progression is already well underway.

These are the children we call "pre-myopes" — they don't need glasses yet, but their axial length is tracking above age-matched norms. Identifying them early is one of the most valuable things axial length measurement can do, because it gives us a window to intervene before significant myopia develops.

"We sometimes see children with perfect vision whose axial length is already growing at 0.2mm per year or more. These are the children most likely to develop significant myopia — and they're the ones who benefit most from early monitoring."

How We Measure Axial Length

30 seconds, no drops, no contact

We measure axial length using the Zeiss IOLMaster 500 — an optical biometer that uses a brief flash of infrared light to measure the eye's length to a hundredth of a millimetre. The test takes about 30 seconds per eye, requires no eye drops, and involves no contact with the eye at all.

The IOLMaster 500 is the same instrument ophthalmologists use worldwide for cataract surgery pre-operative measurements. It's the gold standard in optical biometry — up to ten times more accurate than the older ultrasound methods some practices still use.

For children, this matters. There's nothing touching the eye, no drops to sting, and it's over quickly. Most children find it easier than the standard eye chart test. Your child just looks at a small light inside the instrument, and the measurement is done.

Most optometry practices don't have a biometer — they rely on the glasses prescription alone to track myopia. That's like weighing a child without measuring their height. Both numbers matter.

TRACKING GROWTH OVER TIME

How we use axial length to guide your child's treatment

A single axial length measurement tells us where your child stands today. But the real value comes from tracking it over time — typically every six months — and comparing against age-matched growth curves from international research. This shows us whether your child's eye growth is normal, accelerating, or responding to treatment.

Normal eye growth in school-aged children is about 0.1mm per year, slowing down after age 10 and stopping by the early teens. Growth above 0.2mm per year is a warning sign. In fast progressors, we see 0.4 to 0.5mm per year — that's the equivalent of roughly -1.00 to -1.50 dioptres of myopia added every year.

We compare each measurement against published growth curves from the SCORM study and other international datasets, adjusted for age, gender, and ethnicity. This tells us whether your child's eyes are growing within expected limits — or whether we need to start or adjust treatment.

We offer all four first-line myopia control treatments in-house: Ortho-K overnight lenses, MiyoSmart spectacle lenses, MiSight daily contact lenses, and low-dose atropine eye drops. Axial length data helps us choose the right option — and tells us when to change course.

"When an Ortho-K patient has good visual acuity but their axial length is still progressing at 0.3mm per year or more, we add low-dose atropine as combination therapy. The prescription alone wouldn't have told us the treatment needed adjusting — the axial length data did."

WHAT TO ASK YOUR OPTOMETRIST

The one question that tells you whether myopia is being properly monitored

If your child has myopia or is at risk of developing it, ask your optometrist one question: "Do you measure axial length, or just the prescription?" If the answer is prescription only, your child's myopia progression may not be fully tracked — because the prescription can lag behind what the eye is actually doing.

The International Myopia Institute (IMI) recommends axial length measurement as the gold standard for monitoring myopia management outcomes. It's increasingly considered standard of care in dedicated myopia clinics worldwide.

At Concord Eyecare, we measure axial length at every myopia review using the Zeiss IOLMaster 500. You'll see the data yourself at each visit — tracked over time so you can see exactly how your child's eyes are responding to treatment.

FREQUENTLY ASKED QUESTIONS

What parents ask us about axial length

What is axial length of the eye?

Axial length is the distance from the front surface of the cornea to the back of the eye (the retina), measured in millimetres. It's the key biological marker for myopia — a longer eye means more short-sightedness.

What is a normal axial length for a child?

A normal mature eye is about 23mm in girls and 23.5mm in boys. But context matters — a 24mm eye at age 8 is more concerning than 24mm at age 14, because the younger child has more years of potential growth ahead.

Does axial length measurement hurt?

Not at all. The Zeiss IOLMaster 500 is completely non-contact — nothing touches the eye and no drops are needed. It takes about 30 seconds per eye. Most children find it easier than a standard eye test.

How often should axial length be measured?

Every six months during active myopia management. For at-risk children not yet on treatment, at least annually. We measure at every myopia review appointment.

Do all optometrists measure axial length?

No. Most optometry practices rely on refraction (the glasses prescription) alone, which is less sensitive to early changes. Axial length measurement requires a dedicated biometer — an instrument not all practices have.

Can axial length go back to normal?

No. Once an eye has grown longer, it doesn't shrink. That's why early intervention matters — the goal of myopia control is to slow the rate of growth, not reverse it. Every millimetre prevented is permanent protection.

Clinical references

Yam JC, Jiang Y, Tang SM, et al. Low-Concentration Atropine for Myopia Progression (LAMP) Study: a randomized, double-blinded, placebo-controlled trial of 0.05%, 0.025%, and 0.01% atropine eye drops in myopia control. Ophthalmology. 2019;126(1):113-124.
Chia A, Chua WH, Cheung YB, et al. Atropine for the treatment of childhood myopia: safety and efficacy of 0.5%, 0.1%, and 0.01% doses (ATOM2). Ophthalmology. 2012;119(2):347-354.
Aspen Pharmacare Australia. EIKANCE Product Information. Available from the Therapeutic Goods Administration ARTG.
Pharmacy Board of Australia. Guidelines on Compounding of Medicines. August 2024.

The measurement that matters most

AXIAL LENGTH

Why the length of your child's eye matters more than their prescription

REFRACTION VS AXIAL LENGTH

Why the glasses prescription doesn't tell the whole story

How We Measure Axial Length

30 seconds, no drops, no contact

TRACKING GROWTH OVER TIME

How we use axial length to guide your child's treatment

WHAT TO ASK YOUR OPTOMETRIST

The one question that tells you whether myopia is being properly monitored

FREQUENTLY ASKED QUESTIONS

What parents ask us about axial length

What is axial length of the eye?

What is a normal axial length for a child?

Does axial length measurement hurt?

How often should axial length be measured?

Do all optometrists measure axial length?

Can axial length go back to normal?

Clinical references

Your child's eyes are still growing

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