A mismatch between myopia and axial length

The ultimate goal of myopia control is to reduce axial length elongation. The longer the axial length, the higher risk for ocular disease. Usually, one would tend to pay more attention to higher myopes as they would typically have longer axial lengths. However, this is not necessarily always the case. KG has illustrated in comparing two cases of myopia, whereby there is a mismatch between myopia and axial length - the lower myope actually has a longer axial length of the two.

Atchison et al showed that the average emmetropic eye is 23mm long, with every 1D of myopia equivalent to a 0.35mm increase in axial length in adults aged between 18-36.¹ If we were to make the calculations based on these assumptions, one would estimate the female patient's axial length to be roughly 24.49mm and the male patient's about 23.88mm. Both patients have axial lengths longer than their estimated average - however, it is surprising that the male patient has a significantly longer axial length than the female, despite 1.50-1.75D less myopic.

The refractive components of an eye include the cornea, crystalline lens and its axial length. Of all these components, axial length change has the highest correlation with myopic progression.² The crystalline lens typically thins and flattens prior to myopia onset but then becomes stable, while the corneal curvature also remains quite stable.³ Therefore, higher myopia is typically due to longer axial length.

The cornea flattens slightly in early childhood then remains relatively stable from then on.³ Corneal curvature has also been shown to have minimal contribution in refractive error especially in a long eyeball.⁴

On the other hand, the crystalline lens plays a big role during emmetropisation. The thinning, flattening and loss of power in the crystalline lens happens in tandem with axial length elongation when a child is undergoing emmetropization.^3,5 The CLEERE study group results show that the lens stops thinning and flattening earlier in a myope than an emmetrope. As axial length continues to progress, the used-to-be-emmetrope becomes a myope having lost the compensating effect of thinning crystalline lens.

In the case of the male patient where there is a mismatch between myopia and axial length, his cornea (e.g. flat cornea) or crystalline lens may have played a role in masking the minus power contributed by the long axial length. One could consider that his lens has done an excellent job at emmetropization (whilst pulling a fast one on all his eye care practitioners).

There is further complexity with this patient as he has an optic disc pit and form fruste keratoconus. Optic disc pits can be congenital or found in high myope with significant high axial length.^6,7 It is a rare condition with incidence of 1 in 11,000 people and is usually asymptomatic, although carries risk of macular oedema.⁶ Both the form fruste keratoconus and the optic disc pit will require regular eye examinations to monitor this patient's ocular health.

While it's been shown that every dioptre of myopia increases eye disease risk, it appears that axial length is more closely correlated with ocular pathology and vision impairment likelihood, especially in those with axial lengths exceeding 26mm.⁸

Take this case study for example - if we were to only consider refractive error, we would be more concerned about the female patient because she has a higher degree of myopia. Without axial length measurements, we would have missed the chance to properly identify the higher risk to the male patient.

Since many primary eye care practitioners (ECPs) do not currently have access to measurement of axial length, consideration of working with a local ECP colleague and/or ophthalmologist can enable determination of this risk. A single measure in adult myopes, as shown in this case, can make the difference in how you formulate your management plan for that patient.

1. Reducing axial elongation is the key motivation for myopia control, and as a single measure in adult myopes it provides a useful indicator of ocular disease risk.
2. Low myopia doesn't always equate to shorter axial length. Having axial length data can potentially change a practitioner's management plan.
3. For those without the means to measure axial length, co-managing with a practitioner in the local area who has appropriate instrumentation is a great way to get started with understanding and utilizing this important clinical data.

• Case study: When you have low myopia and high axial length

• Axial length measurement - a clinical necessity?

• Measuring the whole eye in myopia

• Six questions on axial length measurement in myopia management