Optimizing Vision in Patients Who Have Astigmatism
BY DAVID A. BERNTSEN, OD, PHD, FAAO
In the February 2013 “Refractive Focus” column, Jason Marsack, PhD, discussed predicting visual performance using metrics of image quality that are calculated from the eye’s aberrations. As discussed in his article, research is currently being done to determine which patients (such as those who have keratoconus) might benefit from custom, higher-order-aberration-correcting contact lenses.
While current technology has made research into optimizing retinal image quality using custom contact lenses possible, a large number of patients who have normal refractive errors do not have their astigmatism (a lower-order aberration) corrected by their contact lenses. While low amounts of astigmatism are typically prescribed in spectacles, this is not always the case in contact lenses. It is estimated that roughly 47 percent of patients have 0.75D or more of astigmatism in at least one eye (Young et al, 2011), yet soft toric contact lenses make up about 25 percent of all soft contact lenses prescribed, according to the January 2013 “International Contact Lens Prescribing in 2012” article by Morgan et al. Though lower-order aberrations (sphere and cylinder) typically dominate refractive errors, patients who have low astigmatism are often still fitted with spherical contact lenses using the spherical equivalent refractive error. While these patients may see 20/20 on a high-contrast visual acuity chart in the office, studies (described below) have demonstrated that uncorrected astigmatism has a negative effect on vision.
Toric Lenses and Improved VA
Several studies have demonstrated that even patients who have low astigmatism benefit from wearing soft toric lenses as opposed to spherical lenses. Richdale et al (2007) conducted a study in which patients were fitted with both spherical and toric soft contact lenses manufactured in the same material. High- and low-contrast visual acuity was measured under both photopic and mesopic conditions. Even for patients who had between 0.75DC and 1.00DC of astigmatism, the toric lenses resulted in up to a 5.5 letter (one-line) improvement in visual acuity compared to spherical lenses. Not surprisingly, improvements in visual acuity when wearing toric lenses were even greater for patients who had 1.25DC to 2.00DC of astigmatism (up to two lines better compared to a spherical lens). Even though prism-ballasted toric designs were found to increase vertical coma in the same study, the toric lenses improved patients’ visual acuity because the astigmatism was corrected (Berntsen et al, 2009).
Similarly, another study of myopic patients who had astigmatism of 0.75DC to 1.25DC also found visual acuity improvements of up to 5.5 letters (one line) in low-contrast visual acuity when wearing a toric versus a spherical lens (Cho et al, 2012). Not surprisingly, the same study reported that patients rated their vision better with the toric lenses compared to the spherical lenses.
Though we typically evaluate our patients’ acuity using a high-contrast acuity chart, both of these studies demonstrate that correcting even low amounts of astigmatism with toric contact lenses yields clinically meaningful gains in low-contrast acuity. Given that many things in life are low-contrast and that uncorrected aberrations have a greater effect on image quality as pupil size increases under dim lighting, it is not surprising that the improvement in low-contrast VA experienced when correcting astigmatism of less than 1.25DC in the study by Cho et al resulted in a significant improvement in subjective vision.
Success Fitting Soft Torics
Knowing that soft toric lenses improve acuity for even low astigmats, the success of fitting patients who currently do not wear soft toric contact lenses is important to consider. Though early toric lenses first developed in the 1970s were plagued by instability, poor vision, and poor comfort, today’s toric lenses are very different.
A recent study by Sulley et al (2013) examined the success rate of fitting toric soft contact lenses in astigmatic patients who fell into one of three groups: contact lens dropout, spherical soft contact lens wearer, or had never worn contact lenses. They found that 88 percent of toric lenses were fitted on the first attempt. After one month, a fit was considered a success if the toric lens fit well, had stable rotation, VA was within one line of spectacle VA, and subjective vision and comfort ratings were “good” or better. After one month, 70 percent to 80 percent of patients in each group had been successfully fit.
Though some patients in the study were not considered successful toric wearers after one month, the study utilized only two different toric lenses made by the same manufacturer. Eyecare practitioners have multiple lens brands and stabilization designs available if a particular toric lens does not fit well, which may increase success in instances in which fit or stability were an issue. Of course, GP lenses continue to be an option, and high success with toric orthokeratology designs has also been reported (Chen et al, 2012).
Aspheric Soft Lenses and VA
When patients have low amounts of astigmatism, soft aspheric contact lenses have been suggested as an alternative to fitting a soft toric contact lens with the expectation that correcting spherical aberration would provide improved vision even in the presence of uncorrected astigmatism. Clinical studies have not borne this out.
A study by Morgan et al (2005) examined the effect of a soft toric lens and an aspheric soft lens on high- and low-contrast visual acuity in patients who had 0.75DC to 1.00DC of astigmatism. Similar to the studies described above, the soft toric lens provided better low-contrast visual acuity with larger pupil sizes, with the soft toric lens visual acuity being up to one line better than that with the aspheric lens.
In another study by Efron et al (2008), aspheric soft contact lenses were compared to spherical soft lenses in subjects who had less than 1.00DC of astigmatism. No differences in high- or low-contrast visual acuity were found between the two lens types, and patients did not report subjective differences in vision when wearing each of the lenses. Both lens types reduced the amount of spherical aberration compared to wearing no lenses; this result is similar to another study comparing the spherical and toric designs of four different lens brands in which spherical aberration was decreased by both spherical and toric soft lenses compared to wearing no lens (Berntsen et al, 2009). Even in a study in which statistically significant differences in spherical aberration were measured between spherical and aspheric soft contact lens designs, meaningful differences in visual acuity were not found (Lindskoog Pettersson et al, 2011). At least with commercially available aspheric contact lens designs, which are not customized to the patient’s individual amount of spherical aberration, there is no evidence that these lenses consistently provide meaningful VA improvements.
In summary, patients who have uncorrected astigmatism have reduced visual acuity, especially under low-contrast conditions and with larger pupil sizes. Even in patients who have astigmatism as low as 0.75DC to 1.00DC, soft toric contact lenses produced clinically meaningful improvements in visual acuity and subjectively reported quality of vision. While patients who have corneal ectasias such as keratoconus may one day benefit from custom contact lenses that correct their specific higher-order aberrations, normal contact lens patients who have low amounts of astigmatism can benefit today from having their astigmatism corrected. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #210.
Dr. Berntsen is an assistant professor at the University of Houston College of Optometry. He has received research funding from the Johnson & Johnson Vision Care Institute and Alcon.
Contact Lens Spectrum, Volume: 28 , Issue: May 2013, page(s): 15 17