Purpose: To quantify changes in visual acuity (VA) with soft toric contact lenses as a result of lens movement and/or rotational instability caused by versional eye movements.
Methods: A novel chart for vision assessment at near (40 cm) for soft toric contact lenses (VANT chart),consisting of a central, color-coded logMAR panel and eight peripheral letter targets set on a white background measuring 60 × 40 cm was constructed. In the developmental phase of the work, 10 subjects (20 eyes) wore 2 toric lenses in random order, and the impact of rapid and delayed eye versions in 8 directions of gaze on VANT acuity was investigated. In phase 2, 35 subjects (68 eyes) wore 4 toric lenses in random order, and a streamlined clinical protocol using the VANT chart was implemented. Standard assessments of toric lens fit and distance VA were also performed.
Results: Testing in the first phase showed no difference for change in VA for rapid vs. delayed version movements, (p = 0.17) but acuity reduction was greater for diagonal compared with horizontal/vertical versions (p = 0.06). As such, testing in phase 2 proceeded using rapid, diagonal versions only. In this second phase, there were differences for low-contrast distance VA measures between lens types (p = 0.02) and for both VANT baseline acuity (p = 0.03) and postversion acuity (p = 0.04), but no differences were found between lenses for magnitude of vision loss (p = 0.91), which was about one line. No relationship was established between the magnitude of vision loss and measured rotational stability (p = 0.75).
Conclusions: This work has demonstrated that conventional approaches to measuring VA do not fully replicate the "real world" experience of soft toric lens wearers. The VANT chart has shown that VA is reduced immediately after versional eye movements and suggests that more dynamic methods of assessing visual performance should be considered for soft toric contact lens wearers, especially given the apparent inability of lens stability measurements to predict visual performance.