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Editors Selection IGR 7-2

Optic Nerve Head: ONH and RNFL in myopes

Comment by Joseph Caprioli on:

24791 Confocal scanning laser ophthalmoscopy in high myopic eyes in a population-based setting, Tsutsumi T; Tomidokoro A; Saito H et al., Investigative Ophthalmology and Visual Science, 2009; 50: 5281-5287

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This prospective, population-based study by Tsutsumi et al. (1534) aims to determine the differences in structural characteristics of the optic nerve and the surrounding nerve fiber layer in high myopes compared to emmetropes. The underlying hypothesis is that the normal architecture is significantly different in high myopes from emmetropes. Such differences would affect the ability of imaging instruments, in this case, confocal scanning laser ophthalmoscopy, to determine early glaucomatous damage. The difficulty in evaluating tilted discs of high myopes has been well known to clinicians for many years. It often presents a diagnostic dilemma to determine whether such patients demonstrate signs of early or even moderate optic nerve damage. Confounding the problem are the frequent visual field defects that are found in highly myopic eyes with oblique optic nerve insertions. The authors propose that a separate database of 'non-glaucomatous myopes' should be compared to enhance the early detection of glaucoma in these patients.

Not surprisingly, the authors found significant differences in the structure of the optic nerve heads and peripapillary retina in non glaucomatous high myopes compared to non glaucomatous emmetropes. High myopia was associated with greater ovality, smaller cup volumes, and differences in height variation contour and nerve fiber layer thickness. High myopes were found to have more vertically oval discs which were smaller, and had a smaller cup volume than those of emmetropes. The mean nerve fiber layer thickness was measured to be thicker compared with emmetropes. Whether this latter finding is an artifact of the imaging methods is not clear to this reviewer. A notable negative: there was no difference with respect to parameter 'cup shape measure' between the two groups. This suggests that the latter parameter may still be useful in identifying glaucomatous damage in high myopes without a reference to separate comparative database. The study was well planned, executed, and reported.

A recent study by Law et al in Arch. Ophthalmol (2010; 128: 141-142) demonstrated that OCT measurements of nerve fiber layer thickness had a clearly different pattern in non glaucomatous patients with tilted discs; the nerve fiber layer thicknesses temporally and inferiorly was significantly thinner than in the control group. Both studies highlight the need for quantitative imaging techniques to use a distinct comparative database in patients with high myopia to enhance our ability to detect glaucomatous damage.

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