MONOVISION

Adaptive optics may one day be used to optimise monovision strategies in clinical practice, Scott M MacRae MD, of the University of Rochester Medical School, New York, US, told a session of the American Academy of Ophthalmology annual meeting in
New Orleans.

“In 1965, Campbell and Green noted that two eyes are better than one. As clinicians we assume this, but it is actually a more powerful mechanism than I understood until we looked into it in a little more detail,” he said.

Dr MacRae and his colleagues used a binocular adaptive optics system that independently detects and corrects aberrations in each of the test subjects’ eyes using wavefront sensors and deformable mirrors. The device measures visual performance, including visual acuity and contrast sensitivity for each eye and binocularly, as well as stereo acuity, under various correction scenarios.

A model of binocular summation developed by Gordon Legge in 1984 showed that two eyes with full distance correction have 40 per cent better binocular contrast sensitivity compared to monocular vision. This improvement in contrast sensitivity is called binocular summation. If the two eyes are in focus at different focal points and the focal point difference is too large, then binocular inhibition can occur, that is the two eyes perform worse than the eye that has the best visual quality at that point.

Tipping Point

As the focal point difference between the two eyes increases, the binocular summation is reduced and it becomes more difficult for the brain to reconcile the two images. The tipping point for monovision where binocular summation no longer occurs is at 1.5 D of near if there is minimal astigmatism and higher order aberrations. Thus at this point, “two eyes can be worse than one” and the patient may prefer to close the near eye, Dr MacRae noted.

The reason? Binocular visual performance follows the visual performance of the best eye’s image quality and suppresses the image of the most blurred eye. When the images are close, the brain can fuse the image easily and contrast is improved. When the images are more different, the brain has to work harder to reconcile the differences of the images and contrast sensitivity and vision is reduced, Dr MacRae said.

This also explains why traditional monovision is generally limited to a difference of 1.5 D, Dr MacRae said. Without aberrations, there is a sharp peak in image quality for distance in the distance eye and a sharp peak in image quality for near in the near eye but the intermediate vision image quality is not as good. If we add a modest amount of spherical aberration (positive or negative) in the near eye, this will increase that eye’s depth of focus improving the intermediate vision and even extending the near vision image range to see even closer objects. It essentially increases the range of vision in this eye. It also brings the near eye’s image closer to the distance eye image and improves binocular summation, contrast sensitivity and stereo acuity.

Correcting aberrations for both eyes, making the near eye -1.5 D and adding 0.2 microns of positive spherical aberration in the non-dominant eye, slightly flattens the binocular visual acuity curve decreasing the distance vision only slightly while improving the intermediate and near vision range. Modified monovision only minimally decreases distance vision but the advantage is that the patient has a wider range of vision for intermediate and better near vision beyond the 1.5 D near range of traditional monovision, Dr MacRae reported.

Scott M MacRae:
scott_macrae@urmc.rochester.edu