Measuring myopia progression using axial eye length

• The axial length of the eye is a measured from front-to-back, and it grows too long in myopia
• Normal axial eye growth occurs in childhood until around age 12, but in myopia it grows too quickly and continues until the late teens
• A key goal of myopia control is to keep axial length below 26mm if possible, to protect long term eye health.

Childhood eye growth occurs quickly in the first few years and then is meant to continue more slowly until around age 12 when it is meant to then stop. This process results in a normally sighted eye.¹ In myopia, childhood eye growth occurs too quickly and can continue until the late teenage years and even beyond.^1,2

A concern with increasing childhood myopia is that as the eye grows longer and longer, this brings increased risks of that person suffering eye diseases and vision impairment in later life.³ In myopia control, the goal is to reduce myopia progression so that a person’s final level of myopia is lower than what it would have been without treatment.⁴

Myopia control treatments include special types of glasses and contact lenses, and atropine eye drops. To calculate myopia control efficacy, research studies compare these treatments to wearing single vision spectacles or contact lenses, which are considered a non-treatment.

Progression of myopia over time is measured by assessing change to refraction and/or axial length of the eye, which is a physical measurement of the depth of the eye. 

Refraction is the power of spectacles or contact lenses required by the child for clear vision. This is measured at each eye examination. 

Axial length measurement can be up to 10 times more sensitive to detect changes in myopia than refraction, when measured with an optical biometer instrument.⁵ Axial length is also more closely linked with increased risk of eye diseases due to myopia than is refraction.³ 

For these reasons, slowing down axial eye growth is the key goal of myopia control, and the gold standard measurement in research studies of myopia control treatments.⁵

In clinical practice, measuring axial length requires special equipment, and is not considered a standard requirement at this stage,⁶ but its use is increasing with time. 

The axial length of the eye is the measure from front to back – measured from the front surface of the cornea (the clear window over the coloured iris of the eye) right through to the back curve of the eye underlying the retina (the light sensitive layer at the back of the eye). The most accurate way to measure axial eye length is using an instrument called an optical biometer.⁵

A child’s eye is meant to have some normal growth, from birth until around age 12.¹ In school-aged children, this growth is around 0.1 to 0.2mm per year, slowing down after age 10 to around 0.1mm per year and ceasing by the early teens.^7-9 Eye growth tends to stop at a final measure of around 23mm in females and 23.5mm in males.⁸ 

In myopia, eye growth starts to accelerate in the year before the child becomes myopic.⁷ This eye growth can be around 0.2mm per year, with the average axial length at onset of myopia being around 23.7mm in girls and 24.1mm in boys.⁹

When a child has myopia, this eye growth continues at an accelerated rate of more than 0.3mm per year until age 10-11. After age 12, kids and teens with progressive myopia still show eye growth but at a slower rate than younger children, of around 0.2mm per year.^1,7-9

The earlier a child becomes myopic, the more years they have for their myopia to progress and their axial length to continue to grow too quickly. Having myopia at a younger age is the main risk factor for ending up with more severe myopia.² More severe myopia and longer axial length is linked with increased risk of eye disease and vision impairment in adulthood.³ 

If the eye grows to more than 26mm, this dramatically increases the risk of eye problems in adulthood. If the eye grows to more than 30mm, there is a 90% chance that that person will suffer vision impairment in their lifetime.³

In a person with normal sight and no myopia, their eye growth typically stops around age 12.¹ In myopia, around half will show no more eye growth after age 16, while half will continue to grow. At this point, average axial length at stabilization was around 25mm in females and 25.5mm in males.² 

Research has shown that eye growth can even continue into the 20s, with almost 40% of young adults with myopia showing myopia progression. This is more likely to occur in females and those who have parents with myopia.¹⁰ 

The ideal outcome of myopia control is to slow axial eye growth down in children and teenagers with progressive myopia.⁴

No treatment can promise to stop eye growth. Considering that normal eye growth is around 0.1mm per year in children with normally sighted eyes, seeing this amount of growth in a child undergoing myopia control treatment would likely indicate a very good outcome. 

A normally sighted eye typically has an axial length of less than 24mm. Eyes longer than this typically have myopia, but there can be large variation between individuals. Keeping axial eye length less than 26mm will reduce the risk to eye health across a person’s lifetime.³

1. Jones LA, Mitchell GL, Mutti DO, Hayes JR, Moeschberger ML, Zadnik K. Comparison of ocular component growth curves among refractive error groups in children. Invest Ophthalmol Vis Sci. 2005. (link)
2. Hou W, Norton TT, Hyman L, Gwiazda J; COMET Group. Axial Elongation in Myopic Children and its Association With Myopia Progression in the Correction of Myopia Evaluation Trial. Eye Contact Lens. 2018 Jul;44(4):248-259. (link)
3. Tideman JW, Snabel MC, Tedja MS, van Rijn GA, Wong KT, Kuijpers RW, Vingerling JR, Hofman A, Buitendijk GH, Keunen JE, Boon CJ, Geerards AJ, Luyten GP, Verhoeven VJ, Klaver CC. Association of Axial Length With Risk of Uncorrectable Visual Impairment for Europeans With Myopia. JAMA Ophthalmol. 2016 Dec 1;134(12):1355-1363. (link)
4. Brennan NA, Toubouti YM, Cheng X, Bullimore MA. Efficacy in myopia control. Prog Retin Eye Res. 2021 Jul;83:100923. (link)
5. Wolffsohn JS, Kollbaum PS, Berntsen DA et al. IMI - Clinical myopia control trials and instrumentation report. Invest Ophthalmol Vis Sci. 2019; 60. (link)
6. Gifford KL, Richdale K, Kang P, Aller TA, Lam CS, Liu YM, Michaud L, Mulder J, Orr JB, Rose KA, Saunders KJ, Seidel D, Tideman JWL, Sankaridurg P. IMI - Clinical Management Guidelines Report. Invest Ophthalmol Vis Sci. 2019 Feb 28;60(3):M184-M203. (link)
7. Mutti DO, Hayes JR, Mitchell GL, Jones LA, Moeschberger ML, Cotter SA, Kleinstein RN, Manny RE, Twelker JD, Zadnik K; CLEERE Study Group. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Invest Ophthalmol Vis Sci. 2007. (link)
8. Fledelius HC, Christensen AS, Fledelius C. Juvenile eye growth, when completed? An evaluation based on IOL-Master axial length data, cross-sectional and longitudinal. Acta Ophthalmol. 2014. (link)
9. Rozema J, Dankert S, Iribarren R, Lanca C, Saw S-M. Axial Growth and Lens Power Loss at Myopia Onset in Singaporean Children. Invest Ophthalmol Vis Sci. 2019;60(8):3091-3099. (link)
10. Lee SS, Lingham G, Sanfilippo PG, Hammond CJ, Saw SM, Guggenheim JA, Yazar S, Mackey DA. Incidence and Progression of Myopia in Early Adulthood. JAMA Ophthalmol. 2022 Feb 1;140(2):162-169. (link)