MISCELLANEOUS 4 Repeatability of mesopic visual acuity measurements 5 using high - and low - contrast ETDRS letter charts

12 Abstract 13 Purpose To determine the repeatability of mesopic high14 contrast (HC) and low-contrast (LC) visual acuity (VA) mea15 surements made at distance and near in healthy young indi16 viduals. While the repeatability of photopic VA is well17 known, there is a lack of information with regard to the 18 repeatability of VA measured under low luminance 19 conditions. 20 Methods In two different sessions 1 week apart, best21 corrected monocular VA was determined using HC (96 %) 22 and LC (10 %) ETDRS charts under mesopic luminance 23 conditions (0.75 cd/m) at distance (HCD, LCD) and near 24 (HCN, LCN) in 47 healthy subjects aged 22.9±6.8 years. 25 Repeatability was estimated by the Bland and Altmanmethod, 26 whereby the mean difference (MD) and the 95 % limits of 27 agreement were determined as the coefficient of repeatability 28 (COR). 29 Results Mean logMARVA values were HCD = 0.09, LCD = 30 0.44, HCN = 0.21, and LCN = 0.57. Mean differences in 31 measurements between sessions 1 and 2 were not significant, 32 and low in clinical terms (≤1 letter). Repeatability was better 33 for the distance measurements at both high and lowcontrast 34 (COR HCD ±0.11 and COR LCD ±0.11 logMAR vs COR HCN 35 ±0.15 and COR LCN ±0.16 logMAR), and MDs were also 36 slightly closer to zero for the distance measurements. Similar 37 repeatability was observed between HC and LC VA, both at 38 distance and near. 39 Conclusions In mesopic conditions, ETDRS charts offer re40 peatable best-corrected monocular VA measurements. The 41 criterion for a significant change in logMAR VA was 1 line 42 at distance and 1.5 lines at near. 43