Circularity index as a risk factor for progression of geographic atrophy

To develop a parameter that can assess the relative rate of progression of geographic atrophy (GA) based on the hypothesis that noncircular configuration of the atrophic lesion may be a risk factor for enlargement.

Cohort study.

Digitized color photographs of 593 eyes with GA from the Age-Related Eye Disease Study (AREDS).

A novel parameter called the "Geographic Atrophy Circularity Index" (GACI) was developed on the basis of area and perimeter measurements to categorize the irregularity of the shape of GA. The GACI ranges from 0.0 to 1.0 and is categorized into 3 groups: 0.25 (very irregular), 0.25 to <0.75 (partly irregular), and ≥ 0.75 (circular).

Growth rate of GA.

The mean growth rate in the 3 categories was 0.40 (± 0.18), 0.36 (± 0.30), and 0.21 (± 0.22) mm/year, respectively (P < 0.001). By adjusting for known confounders, baseline area, duration of GA, and configuration, GACI categories were significantly associated with increased growth rate of GA (P < 0.001).

The GACI was associated with the progression rate of GA and may be a useful measure for clinical trial eligibility. The association also suggests that enlargement of GA may be related to the extent of the junctional zone of damaged retinal pigment epithelium, which increases with noncircularity for a given GA area.