Axial Length Explained: The Measurement That Matters Most in Myopia Management

If your child has been diagnosed with myopia, you have probably focused on their glasses prescription as the main number to track. But there is another measurement that eye care professionals consider even more important for understanding myopia risk: axial length. Knowing what axial length is and why it matters can help you have more informed conversations with your child’s optometrist about their myopia management plan.

What Is Axial Length?

Axial length is the distance from the front of the eye to the back of the eye, measured in millimetres. In a child with normal vision, the axial length is typically around 22 to 24 millimetres, depending on their age. In myopia, the eye has grown longer than it should, and this excess length is what causes distant objects to appear blurry.

Think of the eye like a camera. If the camera body is too long, the lens focuses the image in front of the film plane instead of on it. That is exactly what happens in a myopic eye. The longer the eye, the more myopic the person tends to be.

Why Axial Length Matters More Than the Prescription

A glasses prescription tells you how blurry your child’s distance vision is and how strong the lenses need to be to correct it. But it does not directly tell you about the structural health risk to the eye. Two children with the same prescription can have different axial lengths, and the child with the longer eye faces greater long-term risks.

The reason is that as the eye elongates, the retinal tissue at the back of the eye stretches and thins. This stretching increases the risk of serious conditions later in life, including:

• Myopic macular degeneration: thinning and damage to the central retina
• Retinal detachment: the retina pulling away from the back of the eye
• Glaucoma: increased risk of optic nerve damage
• Early cataracts: clouding of the lens at a younger age

Every additional millimetre of axial length growth substantially increases these risks. That is why controlling axial length growth, not just correcting the blur, is the primary goal of modern myopia management.

How Is Axial Length Measured?

Axial length is measured using a device called an optical biometer. The test is completely painless and non-invasive. Your child simply looks at a small target inside the device while it takes a precise measurement of the eye’s length using light. The whole process takes less than a minute per eye.

Not all optometry practices have a biometer, but its availability is increasing as myopia management becomes a more recognised part of children’s eye care. If your optometrist does not currently offer axial length measurement, they can refer you to a practice that does.

How Often Should It Be Measured?

For children undergoing myopia management, axial length is typically measured every six months. This allows the optometrist to track the rate of eye growth over time and assess whether the management strategy is working effectively. A slowing of axial length growth is the clearest sign that treatment is having the desired effect.

What Is a Normal Rate of Eye Growth?

All children’s eyes grow during childhood as part of normal development. The eye typically reaches its adult length by the mid-teenage years. A normal rate of growth in a school-age child is roughly 0.1 millimetres per year or less. In a child with progressing myopia, growth rates of 0.3 millimetres per year or more are common.

The goal of myopia management is to bring the rate of axial length growth as close to the normal range as possible. Even modest reductions in growth rate can make a meaningful difference to long-term eye health outcomes.

How Axial Length Informs Treatment Decisions

Your optometrist uses axial length data alongside the glasses prescription and other measurements to decide which myopia management approach is most appropriate for your child. Treatment options may include special spectacle lenses, contact lenses or low-dose atropine eye drops, and the choice often depends on how fast the eye is growing.

If a particular treatment is not slowing growth adequately, axial length measurements provide the evidence needed to adjust the approach. Without this measurement, it is much harder to know whether management is on track until the prescription has already worsened significantly.

What Parents Can Do

Ask your child’s optometrist about axial length measurement at their next visit. If your child has myopia or is at risk of developing it, this measurement provides the most precise picture of how their eyes are changing over time. Combined with adequate daily outdoor time and a management plan tailored to their needs, tracking axial length gives you and your optometrist the best tools to protect your child’s vision for the long term.