Prescribing for Astigmatism
Astigmatism in Early Presbyopia
BY THOMAS G. QUINN, OD, MS, FAAO
I am a believer in correcting even low (0.75DC) amounts of astigmatism in contact lens patients, and science supports the visual benefits of doing so (Richdale, 2007). However, when working with an incipient presbyope, it might get you into trouble. Perplexed? Let me introduce you to “Jim.”
Jim is a 42-year-old TV cable technician who presented for a comprehensive ocular examination and contact lens check. He had no visual complaints.
I found him to be a moderate myope with 0.75D of astigmatism in each eye. He was wearing spherical soft contact lenses.
I showed Jim how his vision could be improved at distance by correcting his astigmatism. I recommended that we refit him from a spherical soft lens to a toric soft lens in each eye. He agreed and we began that transition.
Jim returned one week later. He was furious. He reported that distance vision was a bit better, but he couldn’t see details at near, a vital ability in his line of work.
Does Amount of Uncorrected Astigmatism Matter?
A recent study of pseudophakic individuals (Sing et al, 2013) found that uncorrected astigmatism up to 1.00D improved near vision. Amounts above 1.00D showed no additional improvement in near vision and led to greater degradation of distance vision. The visual benefits at near from uncorrected astigmatism are limited to those who have low (1.00D or less) amounts of astigmatic error.
Does Axis Matter?
Sing et al (2013) also found that distance vision suffered with uncorrected astigmatism at any axis. Gracia et al (2011) found that the greatest compromise in vision occurs when the axis of the uncorrected astigmatism is oblique.
Astigmatic Neural Adaptation
Gracia et al (2011) found that inducing 0.50D of astigmatism in those whose astigmatic error was habitually corrected resulted in a decrease in visual acuity by 21 percent, whereas this same amount of induced astigmatism decreased visual acuity in habitually non-corrected astigmats by only 5 percent. They concluded that neural adaptation can take place in eyes normally exposed to astigmatic blur.
What About Pupil Size?
Watanabe et al (2013), studying pseudophakic individuals, found that pupil size impacted distance visual acuity in all patients who have uncorrected against-the-rule astigmatism—the larger the pupil, the more compromised the distance vision. Distance vision of with-the-rule astigmats was not significantly impacted until pupil size was 2.9mm or greater, at which time vision performance began to drop.
On the other hand, Sing et al (2013) found that small pupils (1.5mm), while enhancing visual performance at distance and near, muted the beneficial effects of uncorrected myopic astigmatism on near vision.
So, small pupils help uncorrected low astigmats with distance vision, but diminish the beneficial effects of the residual astigmatism on enhancing near vision performance.
Correcting astigmatism has never been easier (Bayer and Young, 2005). It can make a significant difference in visual performance. While full visual correction is an admirable goal, it can occasionally lead to trouble, as occurred with Jim. While striving to perfect distance vision, I created a problem at near. We returned to a simple spherical lens on each eye.
Remember Jim Croce’s hit song “You Don’t Mess around with Jim?” I finally learned my lesson. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #213.
Dr. Quinn is in group practice in Athens, Ohio. He is an advisor to the GP Lens Institute and an area manager for Vision Source. He is an advisor or consultant to Alcon and B+L, has received research funding from Alcon, AMO, Allergan, and B+L, and has received lecture or authorship honoraria from Alcon, B+L, CooperVision, GPLI, SynergEyes, and STAPLE program. You can reach him at firstname.lastname@example.org.