To correlate the area of geographic atrophy (GA) and residual foveal sparing (FS), and to identify the minimum FS and maximum GA area allowing sufficient visual acuity (VA) for daily tasks.

Prospective cohort study.

Thirty-six eyes of 25 patients with GA and FS were followed for 18 months using spectral-domain optical coherence tomography and VA tests. Volume scans were imported into software enabling grading of areas in B-scans and computing of planimetric measurements in complete volume scans. Correlation of areas 1 (complete atrophy), 2 (FS in the central millimeter), and 3 (FS in the central 3 mm) with each other and with best-corrected VA (BCVA) were evaluated.

Baseline means of areas 1, 2, and 3 were 6.15 mm², 0.49 mm², and 3.08 mm², respectively. At 1 year, area 1 increased by a mean of 1.33 mm², while areas 2 and 3 were decreased by 0.12 mm² and 0.65 mm², respectively. From baseline to 18 months and from visit to visit, all areas and BCVA changed progressively (P < .001). Significant thresholds in GA size and FS for achieving a BCVA ≥ 70 ETDRS letters were detected (area 1: ≤6 mm²; area 2: ≥0.48 mm²; and area 3: ≥3.28 mm²).

GA and FS changed inversely over time. In general, FS highly correlated with BCVA, while GA progression correlated with the central 3-mm FS regression, but not with BCVA. A threshold in GA and FS area could be determined for BCVA necessary for daily activity.