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

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.

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).

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.

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.

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.