Myopia is a refractive error characterized by the focusing of light in front of the retina, mainly caused by excessive elongation of the eyeball. This process, called axial elongation, is considered the main morphological factor in the development and progression of myopia. Understanding how the axial length of the eye normally grows during childhood is essential for distinguishing physiological development from pathological growth, which is associated with a risk of progressive myopia and retinal complications.

Physiological context: emmetropization and normal ocular growth

After birth, the human eye undergoes a complex process of emmetropization, which aligns the dioptric power of the optical components with the axial length. During this period, ocular growth is regulated both genetically and by visual feedback mechanisms. In the eye that becomes emmetropic, corneal curvature stabilizes early, and the crystalline lens gradually flattens to compensate for axial elongation.

According to the CLEERE study (Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error), children between 6 and 14 years who remained emmetropic showed an average axial length increase of 0.1 mm per year, with variations by age:6–9 years:

• 0.16 mm/year
• 9–12 years: 0.08 mm/year
• 11–14 years: 0.02 mm/year

Legend: the turquoise line indicates the mean values observed in emmetropic children; a progressive reduction in the rate of axial growth can be seen.

These values confirm that, under physiological conditions, axial elongation gradually slows with age and stabilizes by late adolescence, around 13–14 years. Gender differences are consistent: boys show an average final axial length about 0.5 mm longer than girls (23.5 mm vs. 23 mm).

Identifying the risk of myopia onset

Longitudinal studies show that accelerated axial growth precedes the onset of myopia. In the year before myopia onset, children who later became myopic had an average annual axial elongation of 0.33 mm—more than twice the physiological rate. After onset, the growth rate stabilizes between 0.20 and 0.27 mm per year.

A clinically relevant marker is the 0.22 mm per year threshold, identified by the CLEERE analysis as an indicator of rapid myopia progression, independent of previous refractive status. In addition, an absolute axial length of 24.1 mm in boys and 23.7 mm in girls indicates a higher risk for myopia onset.

Myopia evolution and axial growth rate

After onset, the myopic eye continues to elongate at a faster rate compared with the emmetropic eye. Data from the COMET, SCORM, and Tideman et al. studies show the following trends:

• Myopic children <10 years: mean growth rate 0.3 mm/year
• Myopic children >11 years: mean growth rate 0.2 mm/year
• Adolescents 13–16 years: ≈0.17 mm/year, with gradual stabilization

Legend: the turquoise line represents the axial growth rate in emmetropic eyes; the yellow bars indicate the higher average values observed in myopic eyes.

Axial elongation in myopic children remains significantly higher throughout the 6–14-year interval. For a more detailed visual representation of the distribution of mean values, see the next figure.

The COMET study determined that myopia stabilization occurs, on average, at 16.3 ± 2.4 years of age, when the axial length reaches 25.2 ± 0.9 mm. Interestingly, neither ethnicity, sex, nor the number of myopic parents influences the age of stabilization—only the final eye length: 25.5 mm in boys and 25.0 mm in girls.

Clinical implications and practical interpretation

In practice, monitoring axial length is an objective indicator for assessing the effectiveness of myopia control interventions.
The following ranges are considered:

• Physiological (normal) progression: <0.1–0.2 mm/year
• Rapid progression / pre-myopia risk: >0.2 mm/year
• Uncontrolled progression: >0.3 mm/year in children under 10 years

Legend: color thresholds differentiate clinical risk levels according to the axial growth rate.

For children under 10, reducing annual elongation below 0.3 mm is considered a good result; for preadolescents and adolescents, a rate below 0.2 mm/year indicates effective myopia control (orthokeratology, multifocal lenses, pharmacological treatment, etc.).

Conclusions

Normal axial elongation of the eye follows a predictable pattern, with gradual slowing until stabilization at the end of adolescence. Exceeding the 0.2 mm per year threshold indicates a risk of developing or progressing myopia, while values above 0.3 mm per year signal the need for control interventions.

In modern clinical practice, regular measurement and monitoring of axial length are essential tools for evaluating treatment effectiveness and preventing pathological myopia.

Based on the CLEERE, SCORM, and COMET studies, as summarized by Gifford (2021).