Spatial Vision and Astigmatism Correction

Uncorrected astigmatism can cause visual quality to deteriorate significantly (Figure 1). Spectacle lenses typically correct defocus and astigmatism with an accuracy of ±0.25 D; however, contact lenses with a toric component lack rotational stability, thereby reducing the overall efficacy of astigmatism correction. Today, toric contact lenses are typically available in cylindrical powers of 0.75 D or higher, in 0.50 D steps. On the other hand, in laser refractive surgery, errors in astigmatism correction are approximately 0.50 D or higher. Therefore, the correction of cylinder errors of 0.75 D or less poses a dilemma for surgeons.

In cataract surgery, toric IOLs are an option for pseudophakic patients with astigmatic corneas. But the effects of surgically induced astigmatism from the corneal incision and rotational and tilt errors during lens positioning limit the efficacy of these lenses for correcting small amounts of astigmatism. For this reason, the lowest cylindrical powers available in modern IOL designs are still greater than 1.00 D.

SPATIAL VISION

One common option to minimize the visual impact of residual astigmatism is to leave residual defocus in order to reach a null value of the spherical equivalent—that is, to place the so-called circle of least confusion on the retinal plane.

One practical question that has not been completely answered is this: What is the minimum amount of astigmatism that has a significant impact on spatial vision? If this could be determined, a lower limit for astigmatism correction could be set—a limit that obviously would also be affected by the particular accuracy of each correcting procedure.

In this context, we recently studied how small amounts of natural astigmatism (below 0.50 D) and their correction affect visual acuity (VA). We measured astigmatism and higher-order aberrations with a custom wavefront sensor in 54 healthy, young eyes with astigmatism ranging from 0.00 to 0.50 D. Astigmatism was corrected for natural pupil diameters using an experimental crossed-cylinder device. VA was measured for high and low contrast stimuli, at best subjective focus with the natural and corrected astigmatism.

There was no significant correlation between the amount of astigmatism and VA. That is to say, when astigmatism was less than 0.50 D in these normal eyes, VA did not depend on the precise value of astigmatism. Additionally, despite some individual variability, the correction of anything less than 0.30 D of astigmatism did not produce any improvement in VA. Some patients even experienced a mild reduction in VA after correction (Figure 2).

OPTIMAL CORRECTION

The results of our study have practical implications for determining the optimal correction of astigmatism equal to or less than 0.50 D with optical or surgical approaches. According to our results, it is safe to expect a relative improvement in VA when correcting amounts of astigmatism higher than 0.30 D, as long as the axis orientation of the correction is perfect.

However, an axis error of 10º leaves residual astigmatism of 35% of the cylinder correction with a 40º change in orientation and an additional defocus equal to 50% of the remaining astigmatism. For example, a 10º error for a correction of -0.50 D of cylinder leaves 0.17 D of residual astigmatism with the axis rotated 40º and induced sphere of 0.09 D.

Such errors in the correction of small amounts of astigmatism could adversely affect VA in two ways: (1) due to potential breakdown in the neural adaptation process² to the original orientation of astigmatism, and (2) due to the addition of positive spherical power, thereby inducing myopia.

CONCLUSION

Our results provide an argument to leave small values of natural astigmatism, typically below 0.50 D, uncorrected in refractive and cataract surgery procedures.

Pablo Artal, PhD, is a Professor at the Laboratorio de Optica, Universidad de Murcia, Campus de Espinardo, Spain. Professor Artal states that he has no financial interest in the material presented in this article. He may be reached at tel: +34 968357224; e-mail: pablo@um.es.

Eloy A. Villegas, PhD, is a researcher at the Laboratorio de Optica, Universidad de Murcia, Campus de Espinardo, Spain. Dr. Villegas states that he has no financial interest in the material presented in this article.

1. EA Villegas, E Alcón, P Artal. Minimum amount of astigmatism that should be corrected. J Cataract Refract Surg. 2014;40(1):13-19.
2. Artal P, Chen L, Fernández EJ, et al. Neural adaptation for the eye’s optical aberrations. J Vis. 2004;4:281-287.