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Editors Selection IGR 17-4

Refractive Errors: Amylysing the RNFL in Myopic Eyes

Comment by Ki Ho Park on:

69879 Evaluation of a Myopic Normative Database for Analysis of Retinal Nerve Fiber Layer Thickness, Biswas S; Lin C; Leung CK, JAMA ophthalmology, 2016; 134: 1032-1039

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Myopia is one of the risk factors of primary open-angle glaucoma. Its severity is associated with higher risk, as shown by the Tajimi Study, in which the odds ratio of low myopia for correlation with primary open-angle glaucoma was 1.85, while that of moderate-to-high myopia was 2.60.¹ Further, myopia and high myopia estimates and projections for the years 2000 to 2050 suggest significant increases in prevalences globally: by 2050, half of the global population (five billion people) will be myopic, and one-fifth of those (one billion) will be considered highly myopic (≥5 D).²

Therefore, it is obvious that the diagnosis and monitoring of glaucoma in myopic patients is essential and will become even more important. As emphasized by Leung et al., the interpretation of OCT scan results for highly myopic eyes is difficult, and normative data sometimes return a false positive called 'red disease'. Thus, the authors' attempt to set up a normative database of highly myopic eyes is timely and necessary in the present clinical environment.

As shown in the figures, the conventional normative database provides false positives on the deviation map of superior and inferior retinal nerve fiber layer (RNFL) thickness. This is related to the temporal convergence of the superior and inferior RNFL thickness peaks associated with the elongation of the posterior pole and temporal tilting of the optic disc. However, it should be noted that posterior elongation does not always occur in the posterior pole but also, sometimes, anywhere in the posterior part of the sclera.³ If the elongation occurs inferiorly to the disc, the disc will be tilted inferiorly, and if the elongation occurs nasally to the disc, the disc will be tilted nasally, inducing RNFL thickness variations case by case.

By 2050, half of the global population will be myopic, and one-fifth of those will be considered highly myopic (>?5 D)

So, the highly myopic normative database should be large enough to cover the frequencies of all possible cases. Another limitation of the present study is the relatively young age distribution. Also, even though there is no difference in RNFL thickness between the sexes, the maleto- female ratio in the current highly myopic database (49 to 131) is quite different from that in the normal population. Further study including a larger number of highly myopic eyes with older ages, a more balanced sex distribution, and different types of myopic variations of the posterior sclera might enhance the effectiveness of OCT for subjects with extreme refractive errors.

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