Spatiotemporal Summation of Perimetric Stimuli in Early Glaucoma

To investigate achromatic temporal summation under the conditions of standard automated perimetry (SAP), using a Goldmann III (GIII) stimulus and a stimulus scaled to the local area of complete spatial summation (Ricco's area) in open-angle glaucoma (OAG) patients and healthy age-similar control participants. Twenty patients with OAG (mean age, 63 years; mean MD, -3.3 dB) and 15 healthy controls (mean age, 64 years) were recruited. Contrast thresholds were measured for seven stimulus durations (1-24 frames, 1.8-191.9 ms) using a near-GIII stimulus (0.48° diameter) and stimuli scaled to the local Ricco's area, in four oblique meridians at 8.8° eccentricity in the visual field. The upper limit of complete temporal summation (critical duration) was estimated using iterative two-phase regression analysis. Median critical duration values were significantly longer (P < 0.05) in the OAG group for the near-GIII (107.2 ms; interquartile range [IQR], 38.0-190.5) and Ricco's area-scaled (83.2 ms, 41.7-151.4) stimuli, compared to those in healthy subjects (near-GIII, 34.7 ms; 18.2-47.9; Ricco's area-scaled, 49.0 ms; 25.1-64.6). The greatest difference in contrast thresholds between healthy and OAG subjects (i.e., disease signal) was found when stimuli were scaled to Ricco's area and shorter than or equal to the critical duration in healthy observers. Temporal summation is altered in glaucoma. The stimulus duration and area of conventional SAP may be suboptimal for identifying early functional damage. Simultaneously modulating stimulus duration, area, and luminance during the examination may improve the diagnostic capability of SAP and expand the dynamic range of current instruments.