Axial length measurement: what to know

• Axial length is the measurement of an eyeball from front to back. It plays an important role in understanding vision health.
• Axial length measurement helps monitor myopia progression, identify individuals at risk of future eye diseases, and assist in planning cataract surgery.
• Axial length measurement is safe, quick, and painless for both children and adults.

Axial length refers to the length of the eyeball, measured from the cornea (the clear surface at the front of the eye) to the retina (the light-sensing tissue at the back of the eye).

As the eye develops from birth to adulthood, the length of the eye increases. Rapid elongation occurs in the first few years of life, followed by slower, controlled growth through childhood and adolescence.1,2 

• In birth/infancy the average length is 16.8mm
• In toddlerhood (age of 3) the average axial length is 21.8mm
• In childhood (age of 6) the average length is 22.36mm
• In teenagers (age of 15) the average length is 23.41mm
• In adults (age of 18) the average length is 23.67mm

Apart from age, the length of the eye can vary by sex, ethnicity, and prescription (myopia vs hyperopia).

Measuring axial length is an important way to monitor myopia progression.3,4  It is about 10 times more accurate than tracking changes in prescription. Monitoring changes in axial length helps to assess whether your child’s eyes are growing normally for their age, and whether their myopia control treatment is working effectively.

By measuring axial length, the optometrist can better identify individuals at a higher risk for eye disease.5  When the eye elongates, its tissues are stretched and become mechanically weaker, which raises the likelihood of developing eye diseases such as retinal detachment or myopic maculopathy. An axial length greater than 26 mm significantly increases the risk of serious eye diseases and potential visual impairment,

• 26–28 mm: About 2× higher risk of visual impairment by age 60
• 28–30 mm: About 11× higher risk
• 30 mm or more: About 25× higher risk

Axial length measurement is also important for planning cataract surgery. The data helps the ophthalmologist calculate the correct power for the intraocular lens (IOL, the artificial lens that replaces the natural one during surgery) and ensuring clearer vision and minimizing refractive errors afterward. In addition, knowing the axial length helps surgeons adjust their surgical approach for patients with longer eyeballs.

The length of the eye can be measured in one of two ways.

• Optical biometry: Optical biometry is one of the most accurate methods of measuring eye length. During the procedure, the patient fixates on a small light inside the machine, while the instrument automatically takes several precise measurements.

• A-scan ultrasound: The A-scan method works effectively in situations where it may be difficult to take high-quality measurements, such as eyes with significant cataract. To ensure maximum comfort, numbing eye drops are applied to the eye. The optometrist or eye doctor then gently places a small probe on the surface of the cornea to take a reading. It is not as accurate as optical biometry.

Axial length measurement is safe for children. Both testing methods are completely safe, quick, and painless. No special preparation is required – your child simply needs to stay relaxed and look at a small target for a brief moment.

For myopia management, axial length should be measured every 6 months.6  However, children who show fast myopia progression may need more frequent measurements, such as every 3 months.

For cataract surgery, axial length may only need to be measured before the surgery.