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Editors Selection IGR 20-2

Structure and function measurements: Optic disc topography in OAG versus ACG

Comment by Linda Zangwill on:

20395 Comparison of optic nerve head topography and visual field in eyes with open-angle and angle-closure glaucoma, Boland MV; Zhang L; Broman AT et al., Ophthalmology, 2008; 115: 239-245e2

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The identification of differences in optic nerve head damage between primary open angle glaucoma (POAG) and primary angle closure glaucoma (PACG) could help improve our understanding of the pathophysiology of the two diseases. In this comprehensive report of an observational case series, Boland et al. (181) improve on previous work comparing optic disc topography of POAG and PACG by controlling for the severity of glaucoma in the analysis. They completed two analyses, one on controlling for functional (visual field MD) and the other controlling for structural (optical coherence tomography retinal nerve fiber layer thickness) measures of severity. When glaucoma severity was included with disc area, axial length, age and gender in the model, the PACG patients tended to have significantly smaller cup area, larger rim area, and smaller cup to disc area ratios than the POAG patients. The authors emphasize that these differences do not remain statistically significant after correction for the seven multiple comparisons in their analysis. Given that there are differing opinions on whether to correct for multiple comparisons, the relatively small sample number of PACG patients (n = 36), and the power calculations indicating that relatively large differences between the groups are needed to reach statistical significance, the conclusion that no differences exist may be overstated, or considered conservative. Although reporting statistical significance is important, it is essential to consider the magnitude of the observed differences between the groups. Unfortunately, this information is not available, as the topographic measurements of each group are not reported in the manuscript. The authors also include interesting analyses showing a quadratic relationship between visual field MD and PSD, and no relationship between OCT RNFL difference from the mean, and standard deviation difference from the mean. Additional well-designed studies that control for severity of glaucoma are needed to determine whether the differences in optic disc topography between POAG and PACG are replicated in studies with larger numbers of PACG patients.

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