Mesopic Functional Visual Acuity in Normal Subjects

To evaluate mesopic functional visual acuity (FVA) with a newly developed system in normal subjects and to compare the results with photopic FVA, sixty-eight healthy volunteers (24.03 +/- 4.42 [mean +/- standard deviation] years) were enrolled in this study. A commercially available FVA measurement system (AS-28; Kowa, Aichi, Japan) was modified to measure FVA under mesopic conditions as well as photopic conditions. Measurements were performed monocularly in photopic conditions during 60 seconds. After dark adaptation for 15 minutes, the same measurements were repeated in mesopic conditions. Outcomes included starting visual acuity (VA), FVA (the average of VAs), visual maintenance ratio (VMR), maximum VA, minimum VA, and numbers of blinks during the 60-second measurement session, and were compared between mesopic and photopic conditions. Starting VA was -0.11 +/- 0.08 and 0.39 +/- 0.12 logarithm of the minimum angle of resolution (logMAR) in photopic and mesopic conditions, respectively. FVA was -0.06 +/- 0.09 and 0.52 +/- 0.14 logMAR, VMR was 0.98 +/- 0.02 and 0.94 +/- 0.04, maximum VA was -0.15 +/- 0.06 and 0.33 +/- 0.12 logMAR, the minimum VA was 0.05 +/- 0.12 and 0.78 +/- 0.20 logMAR, and the number of blinks was 8.23 +/- 7.54 and 7.23 +/- 6.20, respectively. All these parameters except the number of blinks were significantly different between the two conditions (P < 0.001). Besides, the difference between maximum and minimum VAs and standard deviation of VA were significantly larger in mesopic than in photopic conditions (P < 0.001). This study revealed that not only overall visual function decline but also instability of vision under mesopic conditions even in healthy subjects.