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Axial Length: An Essential for Myopia Management

Dr. Nicole Poon, OD, FAAO
Optometrist and BCDO Member

Myopia is more than a refractive error requiring optical correction. It is an ocular disease of eye length elongation.

With global prevalence projected to reach 50% by 2050 and 10% expected to have high myopia, there is no safe level of myopia1,2. The potential visually debilitating consequences of myopia are due to the increased risk of pathological complications, such as myopic macular degeneration, retinal detachment, cataracts, and glaucoma.

Studies show that the benefits of myopia management outweigh the risks and reveal that the Number Needed to Treat (NNT) to prevent five years of visual impairment is approximately five3. This means that for every five myopic patients that is treated, one five-year visual impairment is averted. Eye care providers should initiate myopia management as early as possible to reduce the risk of complications. This will improve a patient’s quality of life and reduce the burden on our public health system.

The Shift from Dioptres to Millimeters

Traditionally, risk assessment in myopia management relied on monitoring refractive error. However, there is a paradigm shift towards incorporating axial length measurements because axial length monitoring is becoming the new standard of care. Axial length measurements are ten times more sensitive than refraction4. For instance, consider two seemingly similar patients, matched in age, race, genetic, and environmental risks. Both have a refractive error of -2.00D, but one has an axial length of 23mm, while the other has an axial length of 25mm. The management of these two patients is very different. In myopia management, axial length should be considered essential as visual fields and OCT are in glaucoma management. Just as eye care practitioners would not monitor glaucoma based solely on IOP, they should not track myopia progression based exclusively on refractive error.

The IMI white papers define pathological myopia as posterior structural changes that lead to reduced best corrected visual acuity, often associated with an axial length greater than 26mm5. Hence, the goal of myopia control is to keep the axial length under 26mm. Studies have demonstrated that an axial length greater than 26mm is associated with a 25% chance of visual impairment after seventy-five years old, and this risk increases to 90% if the axial length exceeds 30mm6.

Measuring Axial Length

Axial length is an objective measurement easily obtained in clinical practice. It is not influenced by pupil size or accommodation and can be measured via ultrasound or optical biometry.

  • Ultrasound Biometry

    This contact method requires a topical anesthetic, making it less suitable for the pediatric population. Due to user variability, the measurements can be less consistent and accurate. A 0.1mm difference is significant when monitoring changes in tenths to hundredths of a millimeter. The devices, however, have a lower price point and a smaller office footprint.

  • Optical Biometry

    This method utilizes light, which has a shorter wavelength than ultrasound, allowing for precise measurements. It is non-contact and provides repeatable results across different users. Several devices are on the market, some of which offer additional functions such as topography and autorefraction. Some also incorporate software that enables practitioners to track progression and analyze a patient’s risk.

Assessing Risk with Axial Length and the AL/C Ratio

Axial length can provide a broader perspective on risk assessment and when to initiate treatment. Changes in axial length occur before the onset of measurable myopia, with the fastest changes happening the year before a child becomes myopic7. On average, children under ten can experience an axial length increase of 0.1mm to 0.2mm per year, which reduces to 0.1mm per year if they are ten or older7. However, this rate varies based on age, sex, race, and baseline refractive error.

Younger children show more rapid shifts in axial length compared to their older peers7. Males typically have longer eyes than females, reaching approximately 25.5mm versus 25mm, likely due to height differences6,8,9,10. Axial length accelerates faster in Asians than Caucasians and in myopes than emmetropes or hyperopes11,12. Myopic children experience significant axial elongation up to three years before onset and through to five years after onset7. For myopes under ten years old who are not on myopia management, axial length growth can exceed 0.3mm per year, decreasing to 0.2mm per year during their preteens and teens10. If a child has an axial length progression greater than 0.2mm per year or their axial length approaches 23.7mm in females and 24.1 mm in males, they have a higher predisposition to myopia13.

Several studies also evaluate axial length with corneal curvature when assessing the risk for myopia. This is because axial length is synchronized with the refractive component of the eye, specifically corneal curvature. The ratio of axial length to corneal curvature (AL/C), both in millimeters, has proven more predictive than axial length alone8,11,14,15. Patients with an AL/C ratio of greater than three are likely premyopic and should be considered for treatment15. The power of the cornea can be calculated by averaging the flat and steep radii and can be obtained with topography or optical biometry.

Utilizing Axial Length Growth Charts

Instead of using average axial length changes for the entire population, specific growth charts that match a patient’s gender and ethnicity should be used. Parents are very familiar with growth charts, which are great tools for communication and education. See Figure 1 for growth charts for Asians and Figure 2 for Caucasians8,11.

To utilize these charts, plot the patient’s axial length as a function of age to determine their percentile. A percentile greater than the 50th indicates a higher risk of developing myopia, while a percentile greater than the 75th suggests a higher risk of high myopia11. A group in the Netherlands customizes their treatment based on axial length percentiles. Patients in the 75th percentile or higher require aggressive treatment, such as combination therapy or higher concentrations of atropine. Those below the 75th percentile can be managed with a less aggressive approach16. As further research is conducted, eye care providers will have access to growth charts for various patients.

Figure 1. Diez’s axial length growth charts for Chinese children, showing females (top left) and males (bottom left). The right part of the figures illustrates the prevalence of myopia associated with axial length percentiles in females (top) and males (bottom)8.
Figure 2. Tideman’s axial length growth charts for European patients, males (left) and females (right)11.
Monitoring Axial Length Overtime and Determining Treatment Success

Axial length should be monitored at least every six months, and management should be adjusted according to axial length and percentile changes17. Treatment is successful if a patient’s axial length acceleration matches that of their emmetropic peers. This would mean an axial length change of 0.1mm to 0.2mm per year in those under ten and 0.2mm in those older than ten. Treatment can also be deemed effective if there is a reduction in percentile on the growth charts. If there is no reduction in percentile, then therapy needs to be adjusted.

Summary and Key Points

The importance of axial length in assessing the risk of myopia and monitoring myopia is indisputable. Eye care practitioners should incorporate axial length into their clinical practice to manage myopia more effectively.

  • Increased Sensitivity

    Axial length measurements are ten times more sensitive than refraction.

  • Control Goal

    The aim of myopia control is to maintain the axial length below 26mm.

  • Reliable Measurement

    Optical biometry provides more consistent and repeatable results across users.

  • Early Changes

    Axial length changes typically precede measurable myopia, with the most rapid changes occurring the year before onset. Children with an axial length acceleration greater than 0.2mm per year are at a higher risk of developing myopia.

  • Growth Charts

    Use age- and gender-specific axial growth charts to assess a patient’s risk for myopia. Those above the 50th percentile are at increased risk of becoming myopic, and those above the 75th percentile should receive more aggressive management.

  • Predictive Ratio

    Axial length (mm) to corneal curvature (mm), AL/C, can be more predictive than axial length alone.

  • Regular Monitoring

    To determine treatment efficacy, axial length measurements should be conducted at least every six months.

References

  1. Holden BA, Fricke TR, Wilson DA, Wong TY, Naduvilath TJ, Resnikoff S. Global prevalence of myopia and high myopia and temporal trends from 2000 through 2050. Ophthalmology. 2016;123(5):1036-1042.
  2. Bullimore MA, Brennan NA. Juvenile-onset myopia—who to treat and how to evaluate success. Nature. 2023;38(3):450-454.
  3. Bullimore MA, Ritchey ER, Shah S, Leveziel N, Bourne RRA, Flitcroft DI. The Risks and Benefits of Myopia Control. Ophthalmology. 2021;128(11):1561-1579. doi:10.1016/j.ophtha.2021.04.032
  4. Wolffsohn JS, Kollbaum PS, Berntsen DA, et al. IMI – Clinical myopia control trials and instrumentation report. Investig Ophthalmol Vis Sci. 2019;60(3):M132-M160.
  5. Flitcroft DI, He M, Jonas JB, et al. IMI – Defining and classifying myopia: A proposed set of standards for clinical and epidemiologic studies. Investig Ophthalmol Vis Sci. 2019;60(3):M20-M30. doi:10.1167/iovs.18-25957
  6. Tideman JWL, Snabel MCC, Tedja MS, et al. Association of Axial Length With Risk of Uncorrectable Visual Impairment for Europeans With Myopia. JAMA Ophthalmol. 2016;134(12):1355-1363.
  7. Mutti DO, Hayes JR, Mitchell GL, et al. Refractive error, axial length, and relative peripheral refractive error before and after the onset of myopia. Investig Ophthalmol Vis Sci. 2007;48(6):2510-2519.
  8. Diez PS, Yang LH, Lu MX, Wahl S, Ohlendorf A. Growth curves of myopia-related parameters to clinically monitor the refractive development in Chinese schoolchildren. Graefe’s Arch Clin Exp Ophthalmol. 2019;257(5):1045-1053. doi:10.1007/s00417-019-04290-6
  9. Twelker JD, Mitchell GL, Messer DH, et al. Children’s ocular components and age, gender, and ethnicity. Optom Vis Sci. 2009;86(8):918-935.
  10. Hou W, Norton TT, Hyman L, Gwiazda J, Group C. Axial elongation in myopic children and its association with myopia progression in the Correction of Myopia Evaluation Trial (COMET). Eye Contact Lens. 2018;44(4):248-259.
  11. Tideman JWL, Polling JR, Vingerling JR, et al. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018;96(3):301-309.
  12. Jones LA, Mitchell GL, Mutti DO, Hayes JR, Moeschberger ML, Zadnik K. Comparison of ocular component growth curves among refractive error groups in children. Investig Ophthalmol Vis Sci. 2005;46(7):2317-2327. doi:10.1167/iovs.04-0945
  13. Jos R, Sebastian D, Iribarren R, Lanca C, Saw SM. Axial growth and lens power loss at myopia onset in Singaporean children. Investig Ophthalmol Vis Sci. 2019;60(8):3091-3099. doi:10.1167/iovs.18-26247
  14. He X, Sankaridurg P, Naduvilath TJ, et al. Normative data and percentile curves for axial length and axial length/corneal curvature in Chinese children and adolescents aged 4–18 years. Br J Ophthalmol. 2023;107(2):167-175.
  15. Liu L, Li R, Huang D, et al. Prediction of premyopia and myopia in Chinese preschool children: a longitudinal cohort. BMC Ophthalmol. 2021;21:1-10.
  16. Klaver CC, Polling JR, Group EMR. Myopia management in the Netherlands. Ophthalmic Physiol Opt. 2020;40(2):230-240.
  17. Gifford KL, Richdale K, Kang P, et al. IMI – Clinical management guidelines report. Investig Ophthalmol Vis Sci. 2019;60(3):M184-M203. doi:10.1167/iovs.18-25977

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