Scotoma size reduction as an adaptive strategy in age-related macular degeneration

Retinal areas with reduced sensitivity to light stimuli represent the true scotoma size in patients with age-related macular degeneration (AMD), whereas the perceived visual field defect area that covers a specific target of regard may represent an effective size of the same scotoma. This study was designed to highlight the conceptual difference between the "true scotoma size" and its "effective scotoma size" counterpart.

Prospective nonrandomized observational case series.

Ten adults with documented AMD, low vision, and best-corrected visual acuity of 20/50-20/200 in the better eye.

Effective scotoma size and true scotoma size were calculated from measurements with the macular grid test performed with automated perimetry and from microperimetry performed with the Nidek MP-1, respectively.

Ten patients aged 70-92 years (mean 81 years) met the inclusion criteria. Mean effective scotoma size measured with the macular grid test was 40.19 (SD 34.88) deg2. Mean true scotoma size measured with microperimetry was 75.17 (SD 56.08) deg2 (p < or = 0.003). The log unit change in scotoma size, defined as scotoma utility score, was -55.91%. The effect size observed for the scotoma utility score was 0.74.

Effective scotoma size experienced by patients with AMD is significantly smaller than true scotoma size. This reduction may be explained by adaptive variability in eye positions during any single fixation stability attempt, which ultimately results in enhanced visual field perception.