Effect of stimulus size on static visual fields in patients with retinitis pigmentosa

Abstract

Purpose To determine the effect of stimulus size on sensitivity of patients with retinitis pigmentosa (RP) as measured by automated static perimetry. Design Comparative case series. Participants Thirty-nine patients with RP and a control group of 10 healthy volunteers. Methods Automated static perimetry (full threshold programs 24-2 or 30-2) was performed twice on one eye of each participant using stimulus sizes III (0.43° diameter) and V (1.72° diameter). Data from the same 50 test locations were used from each field. Main outcome measures At each location, for each participant, the size effect was computed as the difference (in decibels) in sensitivities for sizes V and III, and the average sensitivity was computed as the mean of sensitivities for the two sizes. Results For both patient and control groups, the size effect was negatively correlated with average sensitivity ( r 2 > 0.124; P < 0.001). The mean size effect was significantly greater for the patient group than for the control group: 8.6 (± 3.6) dB versus 5.4 (± 2.2) dB ( t = 18.0; P < 0.001). The percentage of abnormal locations (more than 8 dB below mean normal) tended to be lower for size V than for size III, with a mean of 67% for size V versus 95% for size III. The percentage of absolute defects was also lower for size V than for size III, with a mean of 35% for size V versus 54% for size III. Conclusions In damaged regions of the visual fields of patients with RP, increase in stimulus size from III to V can produce abnormally large increases in perimetric sensitivity. Size III may be more useful than size V for detection of field abnormality, whereas size V may be more useful than size III for observing progression of advanced RP.