PURPOSE

Visual acuity (VA) is a primary outcome measure that defines the success of clinical interventions for retinal diseases such as age-related macular degeneration (AMD) or diabetic macular oedema (DME). These conditions can lead to the presence of subretinal fluid, causing substantial photoreceptor layer elevation. Hyperopic defocus then occurs, affecting the VA measurements. In this study, we simulated the induced hyperopic shift for real-world values of photoreceptor layer elevation and measured the effect on VA measurements.

METHODS

To simulate the hyperopic shift, we used a four-surface eye model. To measure the effect of defocus on VA, normally sighted adults (N = 44, mean [SD] age = 32 [13.0] year, range: 21-71 year) performed four test conditions, that is, defocus of 0.00, +0.75, +1.50 and +2.25 D. For each subject, mean VA and SD obtained from a cumulative normal fit to the VA data provided the coefficient of variation (CV) and 95% confidence interval (CI).

RESULTS

Refractive error induced by photoreceptor layer elevation was maximum for hyperopic error conditions, followed by emmetropic and myopic refractive error conditions. The 76% threshold VA worsened with increasing defocus conditions. The 95% CI was significantly larger for +0.75, +1.50 and +2.25 D defocus compared to no defocus (p = 0.04, 0.02 and 0.01, respectively). The CI for the +2.25 D defocus condition was larger (3-10 letters) compared with no defocus (3-6 letters).

CONCLUSIONS

Photoreceptor layer elevation causes a hyperopic shift sufficient for clinically meaningful changes: worse VA and more variable measurements.