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Editors Selection IGR 17-4

Clinical Examination Methods: RNFL and macular thickness II

Comment by Tae-Woo Kim on:

54788 Macular ganglion cell/inner plexiform layer measurements by spectral domain optical coherence tomography for detection of early glaucoma and comparison to retinal nerve fiber layer measurements, Nouri-Mahdavi K; Nowroozizadeh S; Nassiri N et al., American Journal of Ophthalmology, 2013; 156: 1297-1307

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Recent studies have suggested that evidence of glaucomatous damage can be observed in the inner retina or ganglion cell complex (GCC) in the macular area by means of spectral domain optical coherence tomography (SD-OCT) even in eyes with preperimetric glaucoma. Nouri-Mahdavi et al. compared the diagnostic performance of the GC/IPL thickness measurement as determined by Cirrus HD-OCT and that of circumpapillary RNFL thickness measurements for discrimination between eyes with early glaucoma from normal eyes. They found that average RNFL thickness performed better than average GC/IPL measurements for detection of glaucoma. However, the minimum and inferotemporal GC/IPL thickness, which was the best performing regional GC/IPL measures, showed similar performance with the inferior quadrant RNFL thickness, the best performing regional RNFL parameter. They additionally found that combining the two best GC/IPL and RNFL parameters improved the performance of the SD-OCT.

The macular imaging is measuring an area encompassing only half of the retinal ganglion cells (RGCs) in the eye, whereas RNFL imaging evaluates the entire complement of the RGC axons. Therefore, one may expect that the diagnostic performance of the GC/IPL is inferior to that of the circumpapillary RNFL thickness parameters. The better performance of global RNFL measurements than the average GC/IPL measurements of their study was consistent with this idea. Nonetheless, it is encouraging that adding the information derived from the macular GC/IPL measurement to the RNFL parameter improved discrimination of early glaucomatous from normal eyes.

In this study, they also explored the utility of combining outer retinal thickness, but failed to find improved prediction of glaucoma when either the absolute outer retinal thickness or the ratio of the GC/IPL to outer retinal thickness as covariates in multivariate logistic regression models. This finding is in contrast to the recent finding by Kita et al.¹ that GCC/total retinal thickness ratio outperformed the corresponding GCC parameters. Further study appears to be needed regarding this issue.

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