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

Area of Science:

• Ophthalmology and visual science research.
• Clinical trial methodology for retinal diseases.

Background:

• Low-luminance visual acuity (LLVA) and low-luminance deficit (LLD) are increasingly used in clinical trials for retinal diseases.
• These measures can detect early changes in central visual function compared to standard visual acuity.

Purpose of the Study:

• To investigate sources of variability in LLVA measurements.
• To standardize LLVA measurement protocols.
• To establish a normal upper limit for LLD in young adults (<55 years).

Main Methods:

• Collated data from three studies using ETDRS charts for standard visual acuity and LLVA with a 2.0-log neutral density filter.
• Assessed the impact of dark adaptation (3 minutes) and background luminance levels (1.6 and 0.85 cd/m²).
• Evaluated the Electronic Visual Acuity (EVA) chart for LLVA testing and compared it with physical charts.

Main Results:

• Dark adaptation and varying background luminance levels did not cause clinically significant changes in LLVA or LLD.
• Significant variability was found between ETDRS physical charts and the EVA chart, indicating they are not interchangeable without calibration.
• An upper limit for LLD was identified as 11 ETDRS letters for younger individuals.

Conclusions:

• Formal dark adaptation does not enhance LLVA due to rapid cone light adaptation.
• Minor reductions in background luminance are not a significant source of variability, but consistent luminance levels are recommended.
• Electronic and physical charts require luminance calibration for interchangeable use; an LLD >11 letters in young adults warrants further investigation.