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Editors Selection IGR 15-4

Clinical Examination Methods: Morphological changes and Visual Field Progression

Comment by Jost Jonas on:

73046 β-Zone Parapapillary Atrophy and Rates of Glaucomatous Visual Field Progression: African Descent and Glaucoma Evaluation Study, De Moraes CG; Murphy JT; Kaplan CM et al., JAMA ophthalmology, 2017; 135: 617-623

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In the study by de Moraes and colleagues, larger parapapillary beta zone at baseline (but not the enlargement of beta zone during the study period) was correlated with faster progression of glaucomatous perimetric defects.¹ This process was more evident for patients of European descent than for patients of African descent. In a parallel manner, in a group of individuals with ocular hypertension, larger beta zone at baseline (but not the enlargement of beta zone during the study period) was associated with faster development of perimetric defects, without marked inter-ethnic differences. The findings show the importance of parapapillary beta zone as parameter (of second ranking order) for the diagnosis of glaucomatous optic neuropathy. It confirms previous hospital-based and population-based, cross-sectional and longitudinal, studies and adds to the current knowledge the information about potential inter-ethnic differences in the association between beta zone and glaucoma progression.

The findings show the importance of parapapillary beta zone as parameter (of second ranking order) for the diagnosis of glaucomatous optic neuropathy

As any well conducted study, the investigation by de Moraes et al. has some limitations. The European descent group and the African descent group differed in some parameters at baseline. Beta zone at baseline was larger in the African descent group than in the European descent group, corresponding to the (statistically?) larger perimetric defect in the African descent group (-3.29 ± 5.2dB versus -2.25 ± 3.7 dB). It is in agreement with previous cross-sectional studies reporting on a correlation between larger size of beta zone and more marked glaucomatous optic nerve damage. The question arises whether the inter-ethnic difference in beta zone (and perimetric defect) at baseline might have influenced the inter-ethnic difference in the association of baseline beta zone and glaucoma progression. Also, both ethnic groups likely differed in disc size since the optic disc is larger (and central corneal is thinner) in African descendants than in European descendants.² It leads to the question whether the detection of beta zone at baseline and the detection of beta enlargement in the follow-up might have been influenced by differences in disc size between both ethnic groups. Another limitation of the study was the assessment of beta zone on optic disc photographs instead on optical coherence tomographic (OCT) images. Since the background pigmentation of the fundus may influence the ophthalmoscopical delineation of beta zone from alpha zone, the inter-ethnic difference in the fundus background pigmentation might have influenced the assessment of differences in beta zone between both ethnic groups. This potential bias could have been avoided by using OCT images. More importantly, as also pointed out by the authors, OCT images would have allowed the differentiation between a newly defined beta zone characterized by parapapillary Bruch's membrane denuded of retinal pigment epithelium, and a gamma zone without Bruch's membrane.⁴ Although both zones, the newly defined beta zone and gamma zone, share similarities in their ophthalmoscopical appearance with a visible sclera and visible choroidal vessels, the difference between both zones is that in the newly defined beta zone, Haller's layer and Sattler's layer of the choroid are still more or less present and the choriocapillaris is occluded. In gamma zone however, in association with the lack of Bruch's membrane, the major elements of the choroid are absent, except for some large feeder vessels. Recent studies have suggested that the newly defined beta zone was associated mainly with glaucoma and not, or only to a minor degree, with axial myopic elongation.² In contrast, gamma zone was correlated mainly with axial elongation and not, or only to a minor degree, with glaucomatous optic neuropathy. Differentiating the old beta zone into the newly defined beta zone and gamma zone could therefore have increased the specificity of measured beta zone for glaucoma and could have augmented the diagnostic precision of the measured beta zone for the diagnosis of glaucoma.

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