How much axial length growth is normal?

By definition, axial myopia is "a myopic refractive state that can be attributed to excessive axial elongation" above and beyond the normal amount of axial length growth we expect to see as a child grows.¹ But what is the normal amount of axial length growth? Should that still occur during myopia management?

When defining myopia, the International Myopia Institute suggests this is done using refraction as the benchmark (-0.50D). This is also the case for high myopia (>-5.00D) as eyes with axial lengths smaller than 26.5mm can still have pathological myopia signs.¹

After birth, the eye begins the complex multifactorial process of emmetropization, which includes genetic factors and visual feedback regulating eye growth. There is evidence for this based on genetic, animal and human population studies.² In the human eye, the majority of ocular growth occurs in the first few years of life. After this, in the child destined for emmetropia, the corneal curvature is quite stable while the growth of axial length is balanced by flattening of the crystalline lens. In myopia, this compensation is lost as axial length grows too quickly.³

The large-scale Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study found that children aged 6 to 14 who remained emmetropic showed an average of 0.1mm per year axial growth. More specifically by age, this was 0.16 mm per year for age 6–9 years, 0.08 mm per year for age 9–12 and 0.02 mm annually for age group 11–14 years.⁴ Other authors found this same average growth of 0.10mm per year until age 13, with a small amount of growth thereafter in teenage emmetropes declared to be "minute and without practical implications". These authors found that in European children, the average final axial length for females was around 23mm and males 23.5mm, with this gender size difference emerging around age 5 and persisting thereafter.⁵ Authors agree that faster growth tends to occur before age 10, with Asian 8-year-old emmetropes showing 0.12±0.24mm annual growth⁶ and European 9-year-old emmetropes showing an annual growth of 0.19±0.05mm with a range of 0.12 to 0.29mm.⁷

The answer is yes. The CLEERE study found that the fastest axial growth occurred in the year before myopia onset, where future myopes grew by by 0.33mm, slowing to 0.20-0.27 per year after onset and throughout their myopia progression. This was not a myopia control study, so these were all single vision corrected children.⁴

Further analysis of the CLEERE data found that an axial length change of 0.22mm per year was a way of identifying fast myopia progressors, and this was independent of the prior year's axial or refractive progression history.⁸

Another large-scale refractive development study, the Singapore Cohort Study of the Risk Factors for Myopia (SCORM), found that regardless of age of onset, myopia onset tended to occur at a similar axial length of 24.08±0.67mm in boys and 23.69±0.69mm in girls.⁶

Axial length growth starts to accelerate before myopia onset and continues thereafter. Myopia progression observed in large-scale non-myopia control studies (single vision correction) indicates growth of around 0.3mm per year in younger children and 0.2mm per year in older children.

• Tideman et al found that 9-year-old European myopes experienced 0.34mm of axial elongation in a year, with a range of 0.17 to 0.53mm.⁷
• Similarly, Rozema et al found that 7- to 9-year-old progressing myopes of Asian ethnicity experienced at least 0.3mm axial growth per year, slowing to around 0.2mm per year until the end of their measurement period of age 12-13.⁶
• An ethnically diverse group in the Correction of Myopia Evaluation Trial (COMET) Study demonstrated myopic eye growth of 1.0±0.4mm from ages 8-11 years, giving an annual average of just over 0.3mm per year. Myopes who were still progressing at ages 13 to 16 years showed around half of this axial progression, with 0.5±0.1mm over three years or around 0.17mm per year.⁹

The COMET study found the average age of axial length stabilization was 16.3±2.4 years and the average axial length at that time was 25.2±0.9mm. Ethnicity, gender and number of myopic parents were interestingly not associated with age of myopia stabilization.

Male eyes stabilized almost 0.5mm longer than female eyes (25.5 versus 25mm respectively), even though they had progressed similarly, stabilized at a similar age and had similar levels of myopic refractive error - male eyes were simply longer throughout all visits. Having two myopic parents made for a longer final eye length (25.05mm with no myopic parents and 25.45 with two myopic parents), but ethnicity did not have an influence.⁹

An important point is that stabilization by age 16.3, on average, means that half were stable before this age, and half continued to progress after this age. Research has shown that around 20% of myopes in their 20s progress by at least one dioptre in that decade,^10,11 we don't yet know much about axial length growth in late teenage or young adult myopia progression.

Here are the key points as illuminated by the literature, and a handy reference graphic, below.

1. Males tend to have around 0.5mm longer eyes than females, despite similar refractive error and axial growth rates. This is observed in emmetropia and myopia - remember this when comparing children of a similar age but different genders.
2. Children's eyes grow fastest before age 10. Emmetropic eyes grow by around 0.1-0.2mm per year before age 10 whilst myopic eyes grow more than 0.3mm per year. Accelerated growth of more than 0.2mm per year could indicate pre-myopia.
3. Axial eye growth slows in the pre-teen and teen years. Emmetropic eyes grow by only 0.1mm per year and stabilize in the pre-teen years. Myopic eyes grow faster and for a longer duration, still growing by around 0.17 in teenage myopia progressors.

Let's firstly consider myopic children under age 10, for whom their early onset and faster progression of myopia puts them at greater risk of high myopia. If 'untreated' myopia progression is typically greater than 0.3mm per year, then slowing annual progression to less than this is the ideal outcome. Since emmetropes of that same age progress by 0.1-0.2mm per year, slowing progression to this rate is an excellent outcome.

For pre-teen and teenage progressing myopes, slowing their axial elongation to less than the 'untreated' average of 0.2mm per year is an ideal outcome. Slowing to less than 0.1mm per year is an excellent outcome, considering that emmetropes of the same age show axial elongation of around this amount, ceasing typically by age 12-13.

The main downside of using averages, as in these examples, is that you simply don't know if the child in your chair is an 'average' or not. Instead, using percentile growth charts is ideal to firstly understand how that child's axial length compares to their peers, and secondly to gauge the success of a myopia control strategy by seeing a reduction in their percentile rank. Read more about this in How to use axial length growth charts. New axial length measurement instruments specifically designed for myopia management, such as the OCULUS Myopia Master, incorporate axial length growth charts into the measurement software to support these clinical judgements.

• Case study: A mismatch between myopia and axial length

• Case study: Is axial length regression possible?

• How to use axial length growth charts

• Measuring the whole eye in myopia