Which Lens Do I Use in Delivering Spectacle Independence?
I recently had a fellow surgeon ask me to explain why I choose mono vision as my favored methodology of delivering spectacle independence. I will try to summarize some of my thoughts about this:
Rather than just saying “post operative spectacle independence is the result of amazing neuroadaptation,” I wanted to try to discover from the point of “how we see” what is truly going on when we choose one of the many methods available. I owe much of this understanding to Dr. Randolph Blake of Vanderbilt University and Dr. Martin Mainster of the University of Kansas Medical Center.
Additionally, depending on the pupil size of the patient with the diffractive lens, there is an optical signal loss as well. These two factors do produce a challenge to the visual cortex in its construction of the perceived image. In most people with normal retina and conductive integrity, this loss of contrast goes unnoticed. However, in some patients who are aware of their high level of visual “sharpness,” they complain of a “loss of vision” even though they are 20/20. These are the small but significant number of patients who end up having lens exchanges for monofocal lenses.
There is another group of patients where this decreased contrast gain causes a problem: those who in living out their lives lose retinal receptor proficiency. At some point of their loss, the addition of the signal loss from the multifocal lens thus tips them over the edge and they become aware that their vision is no longer working properly. Dr. Martin Mainster, a true scholar in the world of ophthalomology who pioneered the addition of a chromophore for the UV filter in our intraocular lenses, showed us just how vision embarrassing a multifocal lens can be. He put this into quantitative terms.
As seen in the figure, it takes a 20 decibel loss of vision before we become symptomatic. If we choose to insert a multifocal lens, the patient loses 10 decibels; half of the patient’s visual reserve is lost. In most patients this 10 decibel loss goes by unnoticed. However a few visually sensitive patients almost immediately observe that they have had a drop in contrast sensitivity manifested by their loss of edge sharpness. This is a reminder that decibels are a log rhythmic not linear. These patients are not satisfied until the lens is removed and replaced with a monofocal lens. If we perform an IOL exchange, what we are doing is giving the patient his/her 10 decibels of contrast sensitivity back.
If we utilize monovision, or as we prefer to call it blended vision, there is a loss of bilateral contrast sensitivity but not monocular contrast sensitivity. The binocular contrast sensitivity is immediately restored with a pair of spectacles. This is not an option with patients who have multifocal lenses. Again, this small loss of contrast sensitivity is not noticed by the patient. This is especially minimized if we use aspheric lenses and we are only separating the eyes 1.25 to 1.50 diopters.
However, as we age, or if one suddenly has a visual compromising event, with monovision none of our monocular reserve or binocular reserve has been irrevocably discarded. A simple pair of spectacles can restore all but the pathologic loss.
To me, this is a compelling fact that, as the “steward” of my patients’ visual life, I currently choose monofocal spectacle independence. This can be accomplished using a crystalens or any of the available aspheric monofocal lenses.
