All images were evaluated by a certified ocular disease evaluator (grader) using Topcon IMAGEnet software for display and a handheld stereoscope (Screen-Vu, Portland, OR). Images were graded longitudinally, that is, with access to images and evaluation data from previous visits. Images from all visits were grouped in the same patient folder. Based on iris color and pattern, the grader confirmed that all visits belonged to the same patient.
The status of the lens was classified as phakic, pseudophakic, or aphakic. The visibility of lens capsule, a haptic, or a positioning hole was clear evidence of pseudophakia. In the absence of such evidence, the distinction between a clear lens and an intraocular lens was based upon the central light reflex. A single central light reflex usually indicates a phakic eye and presence of more than one reflex indicates pseudophakia. The light reflex also helps identify right–left stereo orientation. With appropriately placed stereoscopic photograph pairs, the reflex is expected to be in the posterior region of the lens. Further detailed evaluation was performed in phakic eyes only.
In phakic eyes, a digital grid was overlaid on the left stereoscopic photograph pair. The grid had been modified from AREDS by the addition of a fourth outermost circle used to align the grid with the limbus. The diameters of the 4 circles were 2, 5, 8, and 12 mm. The area within the 2 mm circle was called the central subfield (or circle), between the 2 and 5 mm rings was the inner zone, and between 5 and 8 mm was the outer zone. The central subfield and inner zone together were referred to as the central zone, the primary area of evaluation in AREDS2. The space between the 8 and 12 mm rings was not evaluated. Eight equally spaced radial lines divided the inner and outer zones at the following clock-hour positions 10:30, 12:00, 1:30, 3:00, 4:30, 6:00, 7:30, and 9:00 creating 16 subfields. Each meridian had a 0.5 mm hatch mark. A lens grid drawing tool was used to draw a horizontal line from nasal to temporal across the horizontally visible iris diameter to calibrate the grid and center it. Using the horizontally visible iris diameter rather than the pupillary margin for grid centering is advantageous in asymmetric dilation. Horizontal and vertical pupil diameters were measured in mm using line tools from the inner edge of the pupillary margin. In case of lid obstruction, the horizontal measurement was duplicated for vertical diameter also.
Areas of cortical and PSC opacity involvement were evaluated using percentage assessment in each of the 16 subfields and the central circle. Typical cortical opacities are linear or wedge-shaped radially oriented spokes, originating in the periphery of the lens and pointing towards the center. The appearance can vary as a string of vacuoles or dense black opacities. Anterior subcapsular cataracts, although rare, were graded as cortical opacities. The grader estimated and recorded the percentage, to the nearest whole number, of area covered by cortical opacities in each subfield. In estimating area involved by stippling, the grader mentally swept the opacities together and estimated the area they would cover if contiguous. Image quality and pupillary dilation can have a significant effect on identification of cortical opacities. For the outer subfields, presence of significant lens opacity, that is >50% of visible subfield involvement, was documented. The outer zone was considered ungradable if presence or absence of opacity could not be documented in at least half of the zone.
Longitudinal grading can help reduce noise due to variable image quality between visits. The graders had access to previous and baseline visit measurements in the electronic forms. For ease of data entry, the measurements for previous and baseline visits were displayed alongside pertinent fields. Since images of all visits also were grouped together, the grader could review the images to confirm changes at any time point. If definite cortical opacity was seen at a visit, but disappeared at a follow-up visit due to issues with focus, the grader labeled the subfield as ungradable. A subfield also was considered to be ungradable if less than two-thirds of that subfield was visible, for example, if a subfield was limited in size by the extent of pupillary dilation or ptosis.
Identification of PSC opacities was based on their location in the posterior central zone of the lens. The peripheral extent of opacity was used for the area measurement without any attempt to subtract clear spaces, even if PSC was lacy with small open areas. The light reflex was included within the estimation if the PSC surrounded it. If a lens had cortical and PSC opacities with overlapping borders, the grader attempted to distinguish the boundaries, so that the area covered by each type of opacity was estimated separately. Other opacities, such as white anterior cortical opacities (WACO), Mittendorf's dot, and pseudoexfoliation of lens, also were documented, but not included in lens opacity measurements.