Parapapillary Gamma Zone and Axial Elongation-Associated Optic Disc Rotation: The Beijing Eye Study

PURPOSE. The parapapillary gamma zone has been defined as the parapapillary region without Bruch's membrane. We examined which morphologic parameters are associated with the presence and size of the parapapillary gamma zone. METHODS. Using fundus photographs and spectral-domain optical coherence tomographic images of the optic nerve head in the population-based Beijing Eye Study, we determined parapapillary gamma zone width, macular Bruch's membrane length, optic disc-fovea distance, the angle of horizontal optic disc rotation, the angle of vertical optic disc rotation, and the ratio of the vertical-to-horizontal disc diameter. RESULTS. The study included 2068 individuals with a mean age of 63.0 +/- 9.0 years (range, 50-91 years), and mean axial length was 23.2 +/- 1.0 mm (range, 18.96-28.87 mm). In multivariate analysis, larger width of parapapillary gamma zone was associated with more marked vertical optic disc rotation (P < 0.001; standardized correlation coefficient beta, 0.15; nonstandardized correlation coefficient B, 0.02; 95% confidence interval [CI], 0.02, 0.03), more marked horizontal optic disc rotation (P = 0.02; beta, 0.05; B, 0.01; 95% CI, 0.001, 0.01), longer axial length (P = 0.01; beta, 0.07; B, 0.02; 95% CI, 0.01, 0.04), longer horizontal optic disc diameter (P = 0.02; beta, 0.05; B, 0.06; 95% CI, 0.01, 0.12), longer disc-fovea distance (P < 0.001; beta, 0.25; B, 0.22; 95% CI, 0.18, 0.27), higher degree of fundus tessellation (P = 0.03; beta, 0.17; B, 0.04; 95% CI, 0.03, 0.0), and thinner subfoveal choroidal thickness (P < 0.001; beta, -0.13; B, 0.000; 95% CI, -0.001, 0.000). Parapapillary gamma zone width was not significantly associated with macular Bruch's membrane length (P = 0.72), disc-fovea angle (P = 0.62), age (P = 0.62), or sex (P = 0.46). CONCLUSIONS. The parapapillary gamma zone was associated with an axial elongation-induced rotation of the optic disc mainly around the vertical disc axis, leading to a stretching of the temporal peripapillary scleral flange. Because macular Bruch's membrane length was independent of axial elongation, it leaves the temporal parapapillary region with an uncovered Bruch's membrane (i.e., parapapillary gamma zone develops).