Monovision
Author: Sherman Winston Reeves, M.D., M.P.H.
Definition:
Monovision is the optical status of having one eye focus at distance and the other eye focused at near. This optical condition allows the relieve of presbyopia symptoms.
How Monovision work?
To understand how Monovision helps a presbyopic person, it is helpful to understand the zoom function of the eye, which is called accommodation. When accommodation is fully relaxed in an eye, the eye can see an object clearly at distance. In order to see a close object, like a book or sewing needle, the eye increases its accommodative power by changing the shape of its crystalline lens, zooming in onto the near object and allowing it to be seen clearly too.
In childhood, the eye has tremendous power to accommodate, such that objects can be held extremely close to the eye and be seen very clearly. As a person ages, though, the eye steadily looses its ability to accommodate. When a person reaches their mid 40s to early 50s, their eye has usually lost enough of its accommodative zoom power such that they can no longer zoom in much closer than an arm’s length. The loss of the zoom function of the eye is called presbyopia. This presents difficulty in seeing small objects, such as newsprint, that could usually easily be seen by simply holding them closer to the eye when the person was younger. that small objects such as newsprint which requires no accommodation, but also to zoom in onto a near object or to change its focus from seeing distant objects clearly. Reading glasses, which artificially set the focus point of the eye closer without requiring accommodative effort by the eye, then become increasingly needed. As person reaches around 70 years of age, their eyes have typically lost all accommodative zoom power, and reading glasses are constantly needed to see things up close.
Monovision, however, is an optical trick to get around the problems of presbyopia. In Monovision, one eye is focused at distance, while the other is focused at near. The person’s brain, then, learns to use the distance focused eye for distance viewing and the near focused eye for near viewing. This process of adaptation to Monovision usually takes about 1 to 2 weeks, after which the brain begins to seamlessly use the eyes in a Monovision manner, and a person becomes mostly unaware which eye is focused at near and which is focused at distance. Reading glasses may occasionally still be required for some near activities, but Monovision can help a presbyopic person achieve a good amount of independence from reading glasses.
How is monovison obtained?
The most common method of achieving Monovision is through the use of contact lenses. Glasses can sometimes be used for Monovision, but typically the difference in the thickness of the glass between the two eyes can cause bothersome symptoms when a person looks through the edges of the glass. This problem, however, is avoided in contact lenses.
Monovision can also be obtained by surgical means. Excimer laser refractive surgery, such as LASIK or PRK, can be used to induce Monovision by correcting one eye for distance vision and the other for near vision. Conductive keratoplasty can also be used to induce Monovision by focusing one eye at near. Monovision can also be achieved during cataract surgery.by implanting an intraocular lens in one eye that is focused for near, which implanting a distance focused lens in the other.
Are there downsides of Monovision?
While most people who try Monovision can adapt to it well, about 10 to 15% of people who try do not adapt. These individuals may feel somewhat “off balanced” when using Monovision, or may have eyestrain or headaches with it. Also, Monovision can cause a mild loss of stereo vision. Typically, this loss is not noticeable to a person and is inconsequential to their daily activities. However, if a large amount of near correction is used in the near eye, this loss of stereo vision may become more noticeable. As such, most doctors prescribe the Monovision eye with enough correction to allow good intermediate distance viewing, such as looking at cans on a grocery store isle, and for reading larger print, such as dinner menus. For very fine print or viewing very small objects, however, Monovision patients may still need the help of reading glasses.
