Effects of astigmatism greater on multifocals than monofocals IOLs

October 1, 2014

The effect of astigmatism varies with different factors and can affect patient satisfaction with multifocal or monofocal IOLs.

 

Take-Home

The effect of astigmatism varies with different factors and can affect patient satisfaction with multifocal or monofocal IOLs.

 

Dr. MacRae

By Lynda Charters; Reviewed by Scott M. MacRae, MD

Rochester, NY-The effect of astigmatism varies with different factors, such as magnitude, manifestation, light level, and dominant or non-dominant eye.

“These factors all affect the patient satisfaction with multifocal or monofocal IOLs,” said Scott M. MacRae, MD, professor of ophthalmology, and professor of visual science, University of Rochester, Rochester, NY.

How pupil size plays role

The use of multifocal and accommodating IOLs raised a number of questions for Dr. MacRae and colleagues, Len Zheleznyak, MS, and Geunyoung Yoon, PhD. They began studying the effect of astigmatism and higher-order aberrations on visual performance and found that the size of the pupil has a great impact on astigmatic magnitudes.

For example, when going from a 2- to 6-mm pupil, the amount of the reduction in image quality resulting from the astigmatism is about 50% as the pupil dilates, Dr. MacRae explained.

“This becomes even more profound with higher degrees of astigmatism, that is, from over 0.75 to 1.5 D. This may be mildly or very disabling for a patient who is not corrected for the astigmatic effect,” he said.

These patients can function while driving in bright light, for example, but not at night in dim light, he noted.

 

Axis of astigmatism

Another important factor to consider is the axis of the astigmatism. The investigators found that the neural system is highly tuned to horizontal and vertical gratings, but not as much to oblique gratings.

“We perform better in interpreting images with horizontal and vertical images than oblique images, resulting in about a 50% reduction in image quality,” Dr. MacRae said.

Against-the-rule or with-the-rule

Studies of myopic against-the-rule astigmatism have shown that patients can see better during near vision tasks compared with patients who have with-the-rule astigmatism.

“The phenomenon that occurs is that patients tend to ‘cue off’ of vertical letters, such as p, q, or t,” he said.

Dr. MacRae and colleagues found that patients who had myopic against-the-rule astigmatism obtained a better image of a poster with the word “Rochester” printed on it than those with with-the-rule astigmatism because the vertical letters stood out.

With distance tasks, the opposite is true. Patients with with-the-rule myopic astigmatism obtained slightly better images because of the vertical letters being presented.

 

Dominant versus non-dominant eye

The effects of the astigmatism are greater in the dominant eye than the non-dominant eye, Dr. MacRae noted.

“In the non-dominant eye, for near vision tasks, patients are not affected by myopic against-the-rule astigmatism,” he said.

Correction of the astigmatism in the dominant eye will result in good binocular function at distance.

The investigators found that with increasing astigmatism, the image quality decreases- especially in multifocal wearers compared with monofocal and pseudoaccommodating IOL users.

“This finding indicates that we need to be more attuned to subtle levels of astigmatism for multifocal patients,” he said.

“Less than 0.5 D of astigmatism has a negligible effect on visual acuity and visual performance, while 0.75 D definitely affects the image quality,” Dr. MacRae said. “The visual system is tuned to horizontal and vertical directions; there is worse visual performance with oblique astigmatism and better visual performance with horizontal and vertical astigmatism.

“Astigmatism has greater effect on multifocals than monofocals IOLs, resulting in reduced depth of field and image quality,” he said. “Finally, astigmatism reduces the image quality more with larger pupils, with multifocal IOLs [being] particularly susceptible.”

 

Scott M. MacRae, MD

E: Scott_Macrae@urmc.rochester.edu

Dr. MacRae is a consultant to AcuFocus, Bausch + Lomb, and Ziemer.