Monday, January 30, 2012

Cataract Surgery Monovision & Crossed Dominance

Question: I have a question about Monovision cataract surgery. I had cataracts in both eyes and, together with my cataract surgeon, decided to operate on the eye with the more advanced cataract first. So we scheduled surgery to put in a monofocal IOL. The night before the surgery, I spoke with my brother, who'd had cataract surgery for one eye a few years ago. He told me about Monovision and the possibility of not needing reading glasses. This was the first I'd heard of this option.

The next morning, before the surgery, I brought this up with my surgeon. He suggested that we go ahead and do the prepped eye and discuss Monovision after the surgery. So we did.

Afterwards, I learned that it's customary to put the distance lens in the dominant eye. Furthermore, from my medical records I learned that the eye with the monofocal IOL was NOT my dominant eye. So, if I decided on Monovision now, the arrangement would not be optimal.

To evaluate my options, over the past three months I have been wearing contact lenses in the eye that still has a cataract. (The cataract is much less advanced, so my vision in the eye is good.) One lens corrects for distance, and the other gives me Monovision. I have had no trouble with Monovision, other than mildly less sharp distance vision. And, I have decided I like Monovision better than the alternative of both eyes having distance implants and using reading glasses for close-up and computer work (which I do a great deal). So now I want to plan the surgery for the other eye.

In light of the fact that the distance lens is already in my non-dominant eye, what things should I take into consideration now? Is there anything I can do to adjust for the fact that the monofocal lenses will be "backwards" from the usual Monovision arrangement? Or, am I being overly concerned, with the difference between having the distance lens in the dominant eye vs in the non-dominant eye having inconsequential implications?

Answer: Monovision Cataract Surgery is becoming more and more popular as patients become aware that they can have the option of being "glasses free" for the vast majority, if not all activities, after a Cataract operation. Monovision vision correction has been used for almost 50 years in contact lens vision correction and then for almost 20 years with Laser Vision Correction. So the concept of modifying the optical correction so that one eye is fully corrected for distance and one eye is corrected for a mild near correction is not at all new-and it has in fact been researched and studied for may years regarding success factors and patient acceptance. The "gold standard" for predicting patient success with Monovision of any type of optical correction is estimating the patient's "tolerance to defocus".

You are correct that typically the dominant eye is corrected for distance vision and the non-dominant eye is corrected for near. This is based on the research that demonstrates that +/-50% of patients have a better tolerance to defocus when the dominant eye is corrected for distance, 20-30% have the same acceptable tolerance regardless of which eye is corrected for far or near and 20-30% cannot tolerate defocus whatsoever. The patient response is thought to be based on the "strength of dominance" which is a learned neurological pattern. Thus, for almost 1/3 of patients it makes no difference which eye is corrected for far or near-they do well.

NOW-the important thing to know is that the best predictor of "tolerance to defocus" is a trial with contact lenses-exactly as you have done. If you are comfortable with your vision over a 3 month period of Monovision correction with a "crossed dominance", then there is every reason to predict that you will do well with you Monovision Cataract Surgery as long as the amount of attempted correction difference between the two eyes approximates what you have been wearing. The only additional consideration that you might wish to discuss with your Cataract Surgeon is if in fact you have had a basic spherical monofocal lens implant, the possibility of using a monofocal aspheric lens implant for the near vision corrected eye. IF-BIG IF-the Cataract Surgeon measures the aberrations in your eye and finds them to be of a particular type and magnitude, it is sometimes possible to select an aspheric lens implant that actually enhances the optical depth of focus and adds a little additional "range" to the Monovision correction. Your cataract Surgeon may already be doing this as a matter of course or in fact they may find that it would be counter productive to use this type of Lens Implant based on the measurements and calculations. It might be worth inquiring.

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