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The grid from the semiautomated vessel grading software siva as placed on an optic disc centred 45° fundus image. Arterioles and venules drawn as red and blue lines. Zones A, B and C are located 0.0–0.5, 0.5–1.0 and 1.0–2.0 disc diameters from the edge of the optic disc, respectively. All geometric variables are measured in Zone C.
