Effect of cylinder power and axis changes on vision in astigmatic participants

Abstract

Purpose

To ascertain the impact of altering cylinder (cyl) power and axis on vision in astigmatism.

Methods

In a prospective, randomized, participant-masked, crossover clinical trial, 28 astigmatic participants were tested for the following conditions on different days: full sphero-cyl correction and undercorrection by 0.25, 0.50, and 0.75 DC while maintaining spherical equivalence. Axis was also misaligned between −30° and +30°, in 10° steps. For each configuration, monocular high- and low-contrast visual acuities (HCVA, LCVA) were measured at 6 m, and participants rated vision clarity (1–10), vision satisfaction (1–10), and vision acceptability (yes/no). Linear mixed models were used to compare visual performance in the overall group and in low, medium, and high cyl subgroups.

Results

Undercorrecting cyl power affected all groups equally (P≥0.073). Undercorrection by 0.75 DC was significantly different to full cyl power for all variables (P≤0.007), while 0.25 DC undercorrection did not cause any significant decreases (P>0.05). Undercorrection by 0.50 DC was significantly different to full cyl power for HCVA (P=0.006, however not clinically significant) and vision acceptability (P=0.034). Axis misalignment affected the cyl groups differently (P<0.001), with the greatest impact in the high cyl group, followed by the medium then the low-cyl group. Misalignment by ±30° caused significant decreases in almost all cases (P≤0.003), while misalignments by ±10° or ±20° caused significant decreases for some cyl groups and test variables.

Conclusion

Undercorrection of cyl by ≤0.50 DC while maintaining spherical equivalence has no significant effect on HCVA, LCVA, vision clarity, and vision satisfaction, while the amount of axis misalignment that can be tolerated is dependent on the cyl power. These results may have practical ophthalmic applications, such as reducing the total number of stock keeping units of toric contact lenses.

Keywords:

• astigmatism
• toric
• sensitivity
• misalignment
• visual performance

Acknowledgments

The authors would like to acknowledge the clinical team (Ms E Robertson and Ms B Ludlow) and the database management team (Dr T Naduvilath and Ms K Laarakkers) for their invaluable support to run this trial at the Clinical Research Trials Center, Brien Holden Vision Institute, Sydney, Australia. This project was entirely funded by the Brien Holden Vision Institute. Interim findings from this paper were presented at the American Academy of Optometry Conference in Chicago (2017) as poster presentations. The abstracts were published in Optometry & Vision Science: 1) Fedtke et al. Effect of cylinder power and axis changes on vision in participants with low, medium and high astigmatism. Optom Vis Sci. 2017;94:E-abstract 175395. 2) Tilia et al. Sensitivity of astigmats to changes in cylinder power and axis. Optom Vis Sci. 2017;94:E-abstract 175397.

Disclosure

The authors report no conflicts of interest in this work.