Method:
(1) Photographs of each eye are taken.
(2) The field to be measured is marked.
(3) The branches for the retinal artery and retinal vein in each field are identified and the diameters measured.
parent arteriole width in microns =
= SQRT((0.87 * ((width of smaller branch in microns)^2)) + (1.01 * ((width of larger branch in microns)^2)) - (0.22 * (width of smaller branch in microns) * (width of larger branch in microns)) – 10.76)
parent venule width in microns =
= SQRT((0.72 * ((width of smaller branch in microns)^2)) + (0.91 * ((width of larger branch in microns)^2)) + 450.05)
Starting at the periphery, the branches are combined until the central retinal vessel is reached.
(1) The calculated diameter of the central retinal artery is termed the central retinal artery equivalent (CRAE).
(2) The calculated diameter of the central retinal vein is termed the central retinal vein equivalent (CRVE).
arterial-to-venous ratio (AVR) =
= (arteriole diameter in microns) / (venule diameter in microns)
An AVR of 1.0 indicates that arteriolar and venular diameters are about the same, while an AVR < 1 indicates that the arterioles are narrower.
