Calculation of venous tortuosity index.

Measurements of superior and inferior venous arcades were obtained starting from the optic disc margin to the crossing point of a circle (A) whose diameter is the distance from the center of optic disc to the fovea. The course of the veins were traced using Photoshop (Adobe Systems, Inc. Ca, USA). NIH ImageJ software was used to measure the lengths of the vein (A) and chord (B and C) of the vessels. The venous tortuosity index was calculated by dividing the length of the retinal veins by the chord length of the same segment (B/ A and C/A). The average of the venous tortuosity ((B/A + C/A)/2) was calculated to obtain the venous tortuosity index.