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Editors Selection IGR 20-3

Anatomical Structures: Optic disc rim and parapapillary zone

Comment by Jost Jonas on:

56348 Microstructure of β-Zone Parapapillary Atrophy and Rate of Retinal Nerve Fiber Layer Thinning in Primary Open-Angle Glaucoma, Kim YW; Lee EJ; Kim TW et al., Ophthalmology, 2014; 121: 1341-1349

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Recent histomorphometric and clinical studies have suggested that the conventional parapapillary beta zone which has so far been defined as the area with visible sclera and visible large choroidal vessels can be subdivided.¹ In the new classification, alpha zone in the periphery of the parapapillary region is ophthalmoscopically characterized by irregular pigmentation in the level of the retinal pigment epithelium (RPE). Upon optical coherence tomography (OCT) and upon histology, it is featured by the presence of Bruch's membrane covered with irregularly structured RPE. 'Beta zone' in the new classification is the region following centrally to the alpha zone and is characterized by the presence of Bruch's membrane and absence of RPE; Bruch's membrane is denuded from the RPE.^2,3 Upon ophthalmoscopy, it is characterized by visible large choroidal vessels and visible sclera, since the RPE is absent and thus the view onto the choroidal and scleral structure is facilitated. A new gamma zone (or called peripapillary atrophy minus Bruch's membrane in the article by Kim and colleagues⁴) is located between the new beta zone and the optic disc border and is characterized by the absence of Bruch's membrane.^2-4 Since there is no Bruch's membrane, there is neither RPE or choriocapillaris. The development of the gamma zone may be due to the myopic enlargement of globe which takes place asymmetrically towards the posterior pole. If at birth, all three layers of the optic nerve head (Bruch's membrane opening; choroidal opening; scleral opening) may be aligned to each other, the myopic stretching taking place predominantly closer to the posterior pole may lead to a shift of the inner opening (i.e., Bruchs' membrane opening) in the direction of the fovea. It leads to a misalignment of Bruch's membrane opening with the scleral opening, so that Bruch's membrane may overhang into the optic nerve head on the nasal side, while at the temporal disc border, a region develops which is no longer covered by Bruch's membrane. This would be the gamma zone (peripapillary atrophy minus Bruch's membrane in Kim's et al. Study⁴).

Differentiating the old beta zone into a new beta zone (associated with glaucoma) and a gamma zone (associated with myopia) may increase the precision of the beta zone for the diagnosis of glaucoma

Typically, the gamma zone exists in medium myopic or highly myopic globes. In the study by Kim and colleagues on patients with open-angle glaucoma, the rate of retinal nerve fiber layer thinning was faster for eyes with 'beta zone with intact Bruch's membrane' ('new beta zone', see above) than for eyes without beta zone or eyes with beta zone without Bruch's membrane (new 'gamma zone', see above). It fully agrees with recent histologic and clinical studies, in which the new beta zone (Bruch's membrane present, RPE absent) was associated with glaucoma, while the gamma zone (Bruch's membrane absent, RPE absent) was strongly associated with axial myopia.^2,3 All studies mentioned suggest that differentiating the old beta zone into a new beta zone (associated with glaucoma) and a gamma zone (associated with myopia) may increase the precision of the beta zone for the diagnosis of glaucoma.

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