A low myope with long axial length

Myopia progression can be monitored by all eye care practitioners through measurement of refractive error, but sometimes axial length can tell a different story of both progression and the necessity for treatment. With instrumentation to measure axial length, treatment can be better targeted, especially when faced with a patient who seems to have 'less risky' myopia due to her age and refraction. Here is a noteworthy case from AV of a low myope who has a longer axial length than expected.

Typically, we see minimal myopic progression in the mid-to-late teenage years as axial elongation slows with age.¹ Based on the refractive error, AV's patient seems to be in the 'safe' zone for future risk of ocular disease and has a lower likelihood for progression. However, there is more to this case than what meets the eye.

The average emmetropic eye length is around 23mm. A 2D increase in myopia correlates with an approximate 1mm increase in axial length; or put differently, 1D of myopia progression equates to around 0.50mm change.² Hence, we can estimate based on this patient's degree of refractive error that her axial length should roughly be around 24mm. Her axial length of 26mm and 25.87mm far exceeds the expected 'normal'. Moreover, an axial length of 26mm or greater is associated with a greater risk of vision impairment due to myopia-associated pathology.³

Myopic refractive error and axial length are usually highly correlated.³ However, as the manifest refractive error also depends on the corneal curvature and lens power, myopes with flat corneas can turn out to have long axial lengths.

How much axial length growth is normal? The Correction of Myopia Evaluation Trial (COMET) Study found that myopes who were still progressing at ages 13 to 16 years showed around axial progression of 0.5±0.1mm over three years or around 0.17mm per year. The axial elongation of this patient fits along similar lines for her age and the fact that myopia control treatment has not yet been instigated.⁴

Without the axial length measurement one would likely miss the opportunity to treat her myopia more proactively, so as to avoid further axial length elongation given she already has progressed into the 'higher risk' category for future pathology risk.³

As the patient appears to be declining contact lens options, the remaining options include myopia controlling spectacle lens designs and low-dose atropine. For both of these treatments, the participants included in randomized controlled trials are only aged up to around 15-16 at the end of the study,^5-7 so commencing treatment at this age should be discussed with the patient and their parents or carers. A key message here is that efficacy is likely an extrapolation from the available data and cannot be guaranteed, but that pursuing treatment is more worthy than doing nothing,⁸ considering the patient's myopia risk factors.

1. A low myope is not always a 'safe' myope once axial length data is added to the analysis of risk factors.
2. For as long as axial length demonstrates progression, myopia management strategies should be considered for children and teenagers with myopia.

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