What Is Axial Length and Why Does My Child's Optometrist Measure It?

If your child has myopia — or if their prescription has been increasing — you may have heard your optometrist mention axial length. It's not a term that comes up in most general health conversations, but for children with progressive myopia, it's one of the most important numbers in their eye health record.

What axial length actually is

The eye is essentially a sphere, and like any sphere, it has a diameter. Axial length is the distance from the very front of the eye (the cornea) to the very back (the retina) — typically measured in millimetres.

In a newborn, the eye is about 17 mm long. It grows rapidly in early childhood and more slowly through adolescence, reaching its adult size (typically 22–24 mm) sometime in the late teens or early twenties.

In myopic (nearsighted) eyes, the eye grows longer than it should. This extra length shifts the focal point of incoming light in front of the retina rather than onto it — which is why distant objects appear blurry without correction.

Why prescription alone isn't enough information

A glasses prescription tells you how much optical correction is needed today. It doesn't tell you whether the underlying problem is getting worse.

Two children can have the same prescription (say, –2.00 D) but very different axial lengths — and very different prognoses. One child may have an eye that has stabilized; the other may be elongating rapidly. The prescription looks the same, but the clinical picture is completely different.

Axial length measurement closes this gap. By measuring the physical length of the eye at each visit, your optometrist can see whether the eye is continuing to grow, how fast it's growing, and whether a myopia control intervention is working.

What the measurement looks like

Axial length is measured with an optical biometer — a tabletop device that looks similar to a standard eye exam instrument. Your child sits at the device, rests their chin on a chin rest, and looks at a small light inside the instrument while the measurement is taken.

The process takes about 30 seconds per eye. There are no drops, no contact with the eye, and no discomfort. Most children find it unremarkable.

How to interpret the results

Your optometrist will track axial length measurements over time and compare them to age-based reference data. The most important metric is the rate of change between visits — specifically, how much the eye is elongating per year.

As a rough reference:

• Less than 0.10 mm/year of growth in a school-age child is relatively slow
• 0.10–0.20 mm/year is moderate and typically warrants monitoring and discussion of myopia control
• More than 0.20 mm/year is fast and generally indicates that a myopia control intervention should be considered or adjusted

Why slowing elongation matters beyond the prescription

Parents often focus on the prescription number, which is understandable — it's the number on the lens box and the number that determines whether your child can see the board at school. But the prescription is a symptom of a structural change. It's the axial elongation that carries the long-term risk.

Myopia is already a significant public health issue: approximately 30% of the world's population is currently myopic, with projections reaching 40% by 2050. It is the leading cause of visual impairment in children.

An eye that has elongated to 26 mm or longer — or a prescription of –6.00 D or higher — is classified as high myopia and carries meaningfully higher lifetime risks of:

• Retinal detachment — the retina stretches as the eye elongates and becomes more prone to tears
• Glaucoma — elongated eyes are at higher risk
• Cataracts — high myopia is an independent risk factor
• Myopic macular degeneration — a leading cause of irreversible vision loss in adults with high myopia

These risks are not hypothetical — they are well-documented in population studies, particularly in East Asian populations where myopia rates are very high. The goal of myopia control is not just to reduce a child's glasses prescription: it's to limit lifetime axial elongation and the risks that come with it.

Current myopia control interventions — including orthokeratology, low-dose atropine, and specialty soft lenses — have been shown to slow myopia progression by an average of approximately 60% and reduce axial elongation by a similar margin. Starting early, while there is more growth ahead, maximizes the benefit.

What to ask at your child's next appointment

If your child has myopia and axial length hasn't been mentioned, it's worth asking:

• Has my child's axial length been measured?
• What is the current measurement, and how does it compare to last visit?
• Is the rate of change within normal range for their age?
• Should we be considering any myopia control options?

The earlier a myopia control program is started — when there is still more growth ahead of the child — the more of that growth it can influence.