When you have low myopia and high axial length

Axial length and refractive error are typically correlated. This interesting post discusses a case where there is a mismatch between axial length (AXL) and refractive error whereby the patient with low myopia had unexpectedly high axial length.

Measurement of axial length is a gold standard for a research study investigating myopia control interventions, but is not routinely measured in clinical practice. Given that refraction is always measured in clinical practice, it has traditionally been used as the marker for myopia pathology risk. Historically, low and moderate myopia up to 5-6D was termed 'physiological' but the International Myopia Institute Defining and Classifying Myopia Paper (link) did away with this erroneous terminology. We now understand that any level of myopia increases risk of pathology compared to emmetropia - there is nothing 'physiological' or 'normal' about this increased risk.¹

We understand that axial length is a useful disease risk indicator for adult myopes, and a useful indicator for urgency of a myopia control strategy in kids, with 26mm being the apparent delineation from the literature.

This child's axial length is already very close to this 'line in the sand' for where the lifelong risk of vision impairment jumps from around 3% for eyes 24mm to less than 26mm long, to around 25% for eyes 26mm to less than 30mm long.²

Typically a longer eye is more myopic, but this is not always the case as shown here. Rozema et al³ found that in Singaporean children, myopia onset occurred at 24.08±0.67mm in boys and 23.69±0.69mm in girls. The three-year MiSight study found a ratio of 1mm axial to 2.40D refractive change in their treatment and control groups.⁴ It’s spurious to merge these studies given variable ethnicities but for the sake of demonstration, we might expect eyes of 25.5mm axial length, as in this case, to equate to around -3.25-3.50D, rather than the -1.25-1.50D best spheres described. In such cases, the crystalline lens power may play a larger role in masking axial elongation from having a greater influence on refraction.⁴

From the research, the relationship between corneal radius and refractive error is varied. Myopes generally have steeper corneas than emmetropes,.^7,8 although corneal power doesn’t seem to alter much after onset of myopia in children.⁵ Research has also shown that the contribution of corneal power to total eye power reduces with longer axial lengths.⁹

The original poster KHT also mention that the patient had grown much taller in the last year. Research has shown that body growth and axial length are correlated.¹⁰ Besides, myope has greater axial elongation than emmetrope. For persistent emmetropes, a 1cm increase in body height was correlated with a 0.03mm increase in axial length. For persistent myopes, every 1cm increase in body height was correlated with a 0.15mm increase in axial length. It is logical that a myope would exhibit greater increase in axial length compared to an emmetrope, though, due to myopia progression.

Due to this relationship between axial length and body height, it's worth keeping in mind that the peak height velocity (PHV) - the fastest annual rate of height change - was shown in an American study of primarily white children to be 12.1 years for girls and 13.7 years for boys.¹¹

1. Axial length measurement is a very useful metric for assessing disease risk, and potentially casts a different light on management of the long-eyeballed low myope.
2. An axial length of 26mm or greater appears to be a key delineator for lifelong risk of vision impairment due to myopia-associated pathology.
3. If axial length measuring equipment is not available in your practice, you can consider co-managing with a local optometrist colleague or ophthalmologist with the required equipment.

• Axial length measurement - a clinical necessity?

• Six questions on axial length measurement in myopia management

• Axial length measurement in myopia management – how often and how much change is normal? A clinical case study.