Positive/negative binocular fusional C-optotypes and 2D planar C-optotypes on transient accommodation and stability in adult eyes: implications for myopia prevention and control

AIM: To investigate the effects of binocular fusional C-optotypes (positive/negative) and 2D planar C-optotypes on the amplitude and stability of transient accommodation (TAC) in adults, and to provide a basis for non-contact myopia intervention. METHODS: This was a self-controlled study. Using red-blue 3D technology, four experimental stages were set up: Test A [fixating on the 1 m negative fusional C-optotypes, 8△ base-in (BI)], Test B (fixating on the 5 m planar C-optotypes), Test C (fixating on the 1 m planar C-optotypes), and Test D [fixating on the 1 m positive fusional C-optotypes, 20△ base-out (BO)]. A WAM-5500 open-field autorefractor was used to measure TAC and accommodative microfluctuations [evaluated via interquartile range (IQR) and median-based coefficient of variation (CVmed)]. Additionally, the convergence accommodation to convergence (CA/C) ratio was calculated, and a visual fatigue questionnaire was administered to assess participants' subjective visual comfort. RESULTS: A total of 21 subjects (7 males, 14 females; aged 23-41y) with normal binocular visual function were enrolled. The results showed that the TAC increased gradually across the four stages, and these values were Test A (-0.35±0.26 D) 0.05), but the fluctuation stability of Test D showed a significant difference between the first 20s and the second 20s (P=0.017). The CA/C ratio was significantly higher in Test D (0.05±0.02 D/△) than in Test A (0.03±0.02 D/△, P=0.007), indicating stronger accommodation-convergence linkage during positive fusional fixation. The visual fatigue scores of all stages were low (median 0-1), with Test D slightly higher than Test B and Test C (P