Fig. 2

The distance between the fovea and the optic disc, which generates sloping of the optic disc. a T2-weighted MRI image of eyeball. Please note the actual distance from the fovea (blue line) to the optic disc (red line). The x and y axes are drawn to simulate axial growth in the rectangular coordinate system. b The axial growth is simulated by the c times increase of a unit sphere along the y axis. In order to simplify the calculation, the optic disc locations before and after the axial elongation are designated as d₁ and d₂, respectively. The optic nerve head (ONH) tilt angle can be defined as the angular deviation of the tangential lines at the disc points (d₁ and d₂) from the x axis (α and β). From the equations of a sphere and an ellipsoid, the equations of tangential lines at d₁ and d₂ could be calculated (Appendix). Since the ONH is very close to the fovea relative to the horizontal diameter of the eye, \( \frac{\beta }{\alpha}\approx \frac{\tan \beta }{\tan \alpha }=c \) (Appendix). This means that, for the initial ONH tilt angle of α to be 2 α, the additional ONH tilt of angle α requires a 2-fold increase of the eyeball’s diameter along the y axis (c = 2). This degree of tilt would not be acquired until the axial length has increased from 24 to 36 mm, even if we assume that axial growth occurs only in the posterior half of the eyeball. We could hardly observe either significant sloping of the emmetropic optic disc or high myopia with an axial length of 36 mm without any posterior staphyloma. c, d Shifting of the temporal disc margin, which imitates progressive tilt despite the same posterior polar curvature. c Let us imagine a best fit sphere along the posterior polar curvature. F is the fovea, and D is the nasal optic disc border. These two points are within 12° in the external view, when we considering that the Humphrey visual field test 24–2 can capture the blind spot in nearly all cases. The tangential lines at point F (green line) and at point D (red line) are drawn. d Magnified view between the fovea and the ONH. If the temporal optic disc border is shifted from A to B, the angle becomes more oblique (red dotted line and blue dotted line). Please note that the shifting is exaggerated to visualize the angular difference. Not only is the angular difference small, but also, the angle cannot exceed the angle of the tangential line at point D (red line), regardless of how large the shift that occurs is. The only exception is the posterior staphyloma (orange dotted line), which has a focally steeper change of curvature in the posterior pole. In this case, the ONH can be truly tilted