Low-luminance visual acuity and low-luminance deficit: optimising measurement and analysis.

CLINICAL RELEVANCE: Low-luminance visual acuity and low-luminance deficit (standard visual acuity minus low-luminance visual acuity) are gaining popularity as outcome measures in clinical trials for retinal disease, demonstrating capability to detect central visual function changes earlier than standard visual acuity. BACKGROUND: The aim of this study is to explore suspected sources of low-luminance visual acuity variability, standardise the method of measurement of low-luminance visual acuity, and define a 'normal' low-luminance deficit upper limit for young adults (<55 years). METHODS: Data from three separate studies were collated. Standard visual acuity was obtained using ETDRS charts (Precision Vision, Bloomington, IL, USA) and low-luminance visual acuity was obtained with the addition of a 2.0-log neutral density filter. The effects of dark adaptation and different background luminance levels on low-luminance visual acuity results were explored. The Electronic Visual Acuity chart (M&S Technologies, Niles, IL, USA) for low-luminance visual acuity testing was also assessed. RESULTS: Three-minutes of dark adaptation and different background luminance levels (1.6 and 0.85 cd/m2) did not demonstrate clinically significant changes in low-luminance visual acuity and low-luminance deficit. Bland-Altman analyses revealed significant variability between the ETDRS physical charts and the electronic chart indicating the two cannot be used interchangeably in the presence of a luminance difference. An upper low-luminance deficit limit of 11 ETDRS letters for younger individuals was also identified. CONCLUSION: Formal dark adaptation does not improve low-luminance visual acuity results since any increased sensitivity is nullified by extremely quick cone light adaptation times. Small reductions in background luminance levels are not a clinically significant source of variability. However, for consistency, the same luminance level should be maintained throughout testing. Results from electronic and physical charts are not transferrable without proper luminance calibration. A low-luminance deficit greater than 11 ETDRS letters, in younger individuals, should prompt further investigation.