MonoVision
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 relief of presbyopia symptoms.
How does monovision work?
To understand how monovision helps a presbyopic person it's helpful to understand the zoom function of the eye, or 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 the eye steadily looses its ability to accommodate. When a person reaches their mid 40s to early 50s their eyes have usually lost enough of their accommodative zoom power 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 that could usually easily be seen by simply holding them closer to the eye when the person was younger. Reading glasses, which artificially set the focus point of the eye closer without requiring accommodative effort by the eye, then become increasingly needed. As a 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 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 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 with 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.
Monovision can also be obtained by surgical means. Excimer laser refractive surgery (such as LASIKor PRK) can be used to induce monovision by correcting one eye for distance vision and the other for near vision. Another option, Conductive Keratoplasty (CK) focuses one eye at near. And finally monovision may be achieved during cataract surgery by implanting an intraocular lens in one eye that is focused for near, while implanting a distance focused lens in the other.
Are there downsides of monovision?
Most people who try monovision adapt to it well, however about 10 to 15% of people cannot adapt. These individuals may feel "off balanced" when using monovision, or may experience eyestrain or headaches with it.
Also, monovision can cause a mild loss of stereovision. 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 stereovision 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 monovision patients may still need the help of reading glasses.
