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Editors Selection IGR 9-4
Imaging: OCT - high myopia
Comment by Joel Schuman & Gadi Wollstein on:
15249 Comparison of optic disk and retinal nerve fiber layer thickness in nonglaucomatous and glaucomatous patients with high myopia, Melo GB; Libera RD; Barbosa AS et al., American Journal of Ophthalmology, 2006; 142: 858-860
Melo et al.(1150) investigated the retinal nerve fiber layer (RNFL) thickness in eyes with myopia. RNFL thickness was determined by optical coherence tomography (Stratus OCT, Carl Zeiss Meditec, Inc., Dublin, CA, USA), GDx with variable corneal ., Dublin, CA, USA), and Heidelberg retina tomography (HRT, Heidelberg Engineering, Germany). As most myopic eyes have longer axial length, it has been hypothesized that the retina is stretched over a larger area in the bigger eyeball, and as such, these eyes have a thinner RNFL. The investigators found no difference between glaucomatous and healthy eyes in the presence of high myopia. Several fundamental topics are not addressed in the manuscript, whichs limit the ability to verify the validity of the reported findings: The authors defined glaucoma status based on visual field (VF) findings, but no criteria were listed, nor was the reproducibility of the findings indicated.
No information was provided regarding the mean and range of the VF abnormalities in the glaucoma group.
As RNFL thickness is associated with age, it is mandatory to verify that there was no age difference between healthy and glaucomatous eyes. However, there was no information on age in either group.
Most of the imaging devices assume a fixed axial length that corresponds to an emmetropic eye. In high myopic eye the scan is projected further away from the disc margin. As RNFL thickness is inversely related to the distance from the disc margin, this might affect the measurements. The authors did not mention if axial length information was incorporated in the imaging device measurements, thus correcting for this confounder; therefore, there is no way to assess the effect of axial length in their findings. The authors reported that overall the quality of the scans was poor (e.g., only four eyes had GDx VCC scans adequate for analysis according to the manufacturer's quality criteria). This calls into question the validity of the measurements.
The standard deviation (SD) of the measurements was large. This might be due to a wide range of tissue damage (glaucomatous or myopic) or due to the poor scan quality. In the presence of such a big SD, a larger sample size would be required in order to ascertain that there is no difference between the groups.
In light of the above mentioned limitations, the clinically important question of the effect of myopia on RNFL thickness still remains to be answered.
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