The ability to measure axial length is now widely regarded as necessary for effective myopia management. Unfortunately, many eye care professionals (ECPs) do not have the ability to take this vital measurement. Some advocates of myopia management, seeking to reduce this barrier argue that automated refraction-based methods to estimate axial length (AL) are adequate. Recent research, however, suggests that generally these methods are less reliable compared to direct measurement methods such as optical biometry.
Several reasons contribute to this reduced reliability:
Simplified Mathematical Models: Automated refraction-based methods typically rely on simplified mathematical models that use refractive error and keratometry to estimate AL (axial length). These models often fail to account for individual variations in ocular anatomy, particularly in eyes with abnormal refractive errors or irregular corneal shapes. As a result, the estimated AL can be inaccurate, leading to errors in clinical decision-making, particularly in high myopia cases (Mehta et al., 2020).1
Susceptibility to Refractive Variations: The accuracy of AL estimation using automated refraction is highly dependent on the accuracy of the refractive error measurement. Factors such as accommodation, astigmatism, and malingering can significantly influence the refractive error, leading to incorrect AL estimates. This is particularly problematic in paediatric populations and individuals with high refractive errors, where the correlation between refractive error and AL is less predictable (Savini et al., 2021)2.
Limited Precision: Automated methods often lack the precision of direct measurement techniques. Optical biometry, for example, directly measures the distance from the cornea to the retina with high accuracy using interferometry. In contrast, automated refraction-based methods infer AL indirectly, leading to greater variability in the results. Studies have shown that optical biometry provides more consistent and repeatable AL measurements, which are crucial for accurate intraocular lens power calculations and myopia management (Chamarty & Verkicharla, 2023)3.
Increased Prediction Error in Clinical Outcomes: The use of automated refraction-based methods can result in increased prediction errors, particularly in eyes with extreme axial lengths. This has been observed in the context of intraocular lens (IOL) power calculations, where the errors in AL estimation can lead to suboptimal postoperative refractive outcomes. Direct measurement methods like optical biometry consistently outperform automated refraction-based methods in terms of accuracy, especially in cases of high myopia (Abulafia et al., 2015)4.
Conclusion:
In summary, automated refraction-based methods to estimate AL are less reliable due to their dependence on simplified models, susceptibility to refractive variations, limited precision, and increased prediction errors in clinical outcomes. Optical biometry remains the gold standard for accurate and repeatable measurement of AL, particularly in the context of myopia management and IOL power calculations.
At Occuity, we're developing the AX1 Axiometer to solve this need for taking axial length measurements of children quickly and easily. With its handheld utility and non-contact performance, it's no wonder that the Occuity AX1 is creating great excitement in the market.
References
Mehta, N., Kansara, S., & Desai, R., 2020. MYOPIA INDIAN EYES: COMPARING CALCULATED AND MEASURED AXIAL LENGTH IN YOUNG PATIENTS. International journal of scientific research, pp. 80-82. https://doi.org/10.36106/ijsr/7111932.
Savini, G., Hoffer, K., Carballo, L., Taroni, L., & Schiano-Lomoriello, D., 2021. Comparison of different methods to calculate the axial length measured by optical biometry. Journal of Cataract & Refractive Surgery, 48, pp. 685 - 689. https://doi.org/10.1097/j.jcrs.0000000000000821.
Chamarty, S., & Verkicharla, P., 2023. Accuracy and Precision of New Optical Biometer Designed for Myopia Management in Measurement of Ocular Biometry. Optometry and Vision Science, 100, pp. 745 - 750. https://doi.org/10.1097/OPX.0000000000002078.
Abulafia, A., Barrett, G., Rotenberg, M., Kleinmann, G., Levy, A., Reitblat, O., Koch, D., Wang, L., & Assia, E., 2015. Intraocular lens power calculation for eyes with an axial length greater than 26.0 mm: Comparison of formulas and methods. Journal of Cataract and Refractive Surgery, 41, pp. 548–556. https://doi.org/10.1016/j.jcrs.2014.06.033.