The optic nerve head canal consists anatomically of three layers: the Bruch’s membrane opening (BMO), the opening in the choroidal layer delineated by the peripapillary border tissue of the choroid (Jacoby), and the scleral flange opening, covered by the lamina cribrosa and delineated by the peripapillary border tissue of the scleral flange (Elschnig).
1,2 Recent studies have revealed that, with axial elongation in moderately myopic eyes, the BMO shifts from its original position backward, usually into the temporal direction toward the fovea.
3 This leads to an overhanging of Bruch’s membrane (BM) into the intrapapillary compartment at the nasal disc border, as well as a lack of BM in the temporal parapapillary region, referred to as a gamma zone.
3–5 Other studies have shown that the axial elongation-associated enlargement of the distance between the fovea and the optic disc is caused by the development and enlargement of a temporal gamma zone.
6 In contrast, the distance between the fovea and the border of BM at the peripheral margin of a gamma zone on the disc–fovea line is independent of axial length.
6 In a similar manner, the distance between the temporal superior arterial arcade and the temporal inferior arterial arcade as measured on a vertical line through the fovea is independent of axial length.
7 This observation led to the notion that BM at the posterior pole does not markedly enlarge in axially elongated eyes, unless macular BM defects have developed.
6,8,9 Correspondingly, the thickness of BM, in contrast to the thickness of the choroid and retina, does not decrease with longer axial length.
10–12 The concept of a shifting BM in axially elongating eyes can explain the dependence of the horizontal fovea position on the presence and size of a temporal gamma zone, with the horizontal fovea position measured as horizontal distance between the fovea and the optic disc.
8,9 Here we examined whether the vertical position of the fovea, measured as the distance between the fovea and a horizontal line drawn through the optic disc center (fovea vertical distance) is also dependent on the location and size of the parapapillary gamma zone. The results may provide further information regarding the validity of the notion of a shifting of BM during the process of axial elongation.