Juvenile myopia is a result of axial length (AL) growing too fast and too long to match the refractive components (e.g., corneal curvature and the power of the crystalline lens) of the eye, resulting in a myopic refractive error (RE). It has been well established from cross-sectional studies^1-4 that AL is significantly associated with the refractive error of the eyes, in which the higher the myopia, the longer the eyes. However, despite this correlation, it is much more clinically meaningful to keep track of the rate of AL growth and refractive development of each patient, rather than making assumptions based on their current axial length. Here, we will discuss why.

HOW AL CAN BE MISREAD

Many practitioners misinterpret the apparent association of axial length and refractive error as evidence for interpatient comparisons, and use it to make assumptions about the risk of myopia-related complications. For example, between two patients with different ALs, the one with a longer AL is assumed to be more myopic and “must” carry a higher risk of myopia complications, regardless of the severity of myopia. Such oversimplified utilization of the cross-sectional correlation between AL and RE carries little clinical value and fails to consider the other critical refractive components, such as the curvature of the cornea and the power of the crystalline lens of the eye.

During the early stage of emmetropization, the growth of the AL is highly coordinated with the development of the refractive components, so that the physical and focal lengths of the eyes are best matched to form a clear image on the retina. As a result of this highly synchronized growth, a steeper cornea is coupled with a shorter AL and a flatter cornea with a longer AL. Consequently, an eye with a very steep cornea tends to be much shorter at birth but does not necessarily carry less risk of complications if the postnatal AL elongation is substantial. It is the excessive stretching of the scleral tissue as a consequence of abnormal AL growth, rather than the length of the axis itself, that drives most of the pathological cascades of myopia complications.

WHEN TO START MEASURING AL

If the change in axial length better predicts the onset, progression, and stabilization of myopia, how early is sufficient to establish a good baseline and growth pattern for such prediction? The answer is as early as possible, for two reasons.

First, multiple measurements spanning several years would be necessary to track the change in AL and identify the pattern of growth over time. The earlier the baseline is established, the more accurately the ocular development associated with normal growth and an increase in body height can be tracked.

Secondly, it has been clearly demonstrated that the accelerated AL growth far precedes the refractive onset of myopia, so the detection of the premyopic AL elongation is essential.^2,5,6 This not only allows timely initiation of preventative interventions, but also facilitates accurate quantification of the AL-inhibiting efficacy of myopia-controlling treatments, enabling the physician to identify the exact difference between pretreatment and posttreatment AL growth.

CURRENT DATA LIMITATIONS

With recent development and enrichment of the normative data for ocular growth, it is possible to predict the onset of myopia, the risk for fast progression, and the likelihood of high myopia as well as related complications, at least to certain extent. However, because various research groups have developed the normative growth curves independently, their sample populations were either heavily biased toward northern Europeans or Asians/East Asians. This limits the broader utility of this data, because the growth patterns of children growing up in Asia many not correspond to the patterns of children raised in North America.

Moreover, it is well established that both physiological AL growth and myopia-associated AL growth show a significant gender-specific difference,^2,5,6 so it is critical to carefully consider a patient’s gender when applying growth curves.

REMEMBER, PATIENTS ARE INDIVIDUALS

Last but not least, the development of myopia is heavily regulated by a patient’s own visual experience. Although risk factors, such as history of parental myopia, age-specific refraction, and AL in early childhood, explain a significant portion of the data variance, it is impossible to account for the ongoing changes of each patient’s lifestyle and visual demands in the ocular growth charts, so practitioners need to take caution in rigidly applying such a tool. Regularly communicate with patients and their parents to stay informed of changes to the patient’s visual needs. OM

REFERENCES

1. Atchison DA, Jones CE, Schmid KL, et al. Eye shape in emmetropia and myopia. Invest Ophthalmol Vis Sci. 2004 Oct;45(10):3380-6.
2. Mutti DO, Mitchell GL, Sinnott LT, et al. Corneal and crystalline lens dimensions before and after myopia onset. Optom Vis Sci. 2012 Mar;89(3):251-262.
3. Blanco FG, Fernandez JC, Sanz MA. Axial length, corneal radius, and age of myopia onset. Optom Vis Sci. 2008 Feb;85(2):89-96.
4. Ishii K, Yamanari M, Iwata H, Yasuno Y, Oshika T. Relationship between changes in crystalline lens shape and axial elongation in young children. Invest Ophthalmol Vis Sci. 2013 Jan;54(1):771-7.
5. McCullough S, Adamson G, Breslin KMM, McClelland JF, Doyle L, Saunders KJ. Axial growth and refractive change in white European children and young adults: predictive factors for myopia. Sci Rep. 2020 Sept 16;10(1):15189.
6. 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 Jul 1;60(80):3091–3099.

DR. LIU is an associate professor at Pacific University College of Optometry and is the founder of UC Berkeley’s Myopia Control Clinic. She is a world-recognized clinical researcher in myopia. Dr. Liu is a consultant for CooperVision and Essilor International.