This article is more than 5 years old. Information may no longer be current.
Presbyopia correction: Multifocality at odds with neurophysiology
Some patients implanted with multifocal lenses lose confidence in the procedure before the neuroadaptive process is complete.
Click Here to Manage Email Alerts
 William F. Maloney |
The inherent limitation of multifocal implants is suddenly coming to the forefront. For anyone who took the time to look beyond industry’s marketing slogans, this comes as no surprise.
Even the briefest study of the neurophysiology of the visual cortex gives cause for concern about multifocality. In a recent column titled “Monovision in name only” (Oct. 1, 2006) I wrote:
“Multifocality’s intra-ocular image competition has no physiologic precedent. Without a neural template to single out and convey the winning precept into awareness, a prolonged neuroadaptation period is needed for the brain to put in place the necessary neural tracks. As that work goes on, the haloed vision emblematic of truncated image distinction typically slowly moderates and often — but not always — gradually disappears. It is a testament to the astonishing plasticity of the visual cortex that this is ever accomplished at all, let alone within [12] months.”
I will admit to seriously wondering, “What on earth are they thinking?” when the marketing of multifocals began in earnest. Twelve months is a long time to ask a patient to endure the significant visual disturbance that can accompany truncated image disparity. We are now seeing that some lose confidence in both the procedure and the surgeon before the neuroadaptive process is complete, and many surgeons are now understandably reassessing their use of multifocal implants.
Pseudophakic monovision: Physiologic binocular rivalry
Alternatively, pseudophakic monovision makes strategic use of a specific class of binocular rivalry, which the visual cortex is hardwired to process. From the same article:
“Each interocular defocus limit is precisely calculated to generate a specific range of uncorrected vision, according to a candidate’s ocular dominance, suppression capacity, defocus tolerance and particular reading goals. … Neuronal gates evaluate and instantaneously select the better image … to obtain the most effective … percept in accordance with the task at hand. Suddenly, one begins to appreciate how misleading the misnomer ‘monovision’ has been — and not only for patients. It obscures the reality that pseudophakic monovision is binocular vision, operating just as it has been for 20 million years.”
With the benefit of this full complement of preop assessments, each patient’s anisometropic “sweet spot” can be clearly identified. As a result, there is virtually no neuroadaptation period with authentic pseudophakic monovision.
ASCRS course
I had originally intended to outline each of these preop tests in this column, but then decided they would be better illustrated visually. For those interested, I plan to describe the details of these tests during my course on pseudophakic monovision, scheduled for Saturday, April 28, from 1 to 2:30 p.m., during the American Society of Cataract and Refractive Surgery meeting.
This year’s course will focus more specifically on the preop assessments that make pseudophakic monovision work for a wide range of candidates. As the limitations of multifocality are increasingly apparent, I am convinced that pseudophakic monovision is the most effective approach for presbyopia correction presently available.
That is not to say that there is no compromise with pseudophakic monovision. However, the compromise — typically encountered while night driving — is such a small segment of the full visual experience that it is often not an issue for most cataract patients. Younger candidates, who typically do more night driving, can be advised to expect to use glasses while night driving.
The agenda will include the details of ocular dominance testing, including target dominance, oculomotor dominance and sensory dominance assessments. We will describe how to determine each patient’s anisometropic sweet spot and use that to customize the visual result to each patient’s particular reading goals.
I will detail our use of the preop questionnaire as well as the nature of our preop discussion with these candidates, which rarely takes more than 5 minutes. I have become convinced that surgeons spend too much time detailing the different lens technologies when patients really want to know which approach will work best for them and why. The whole issue of preop additional “chair time” has become moot in my practice.
Postoperatively, too, pseudophakic monovision candidates typically require no more time than a routine cataract patient because with the right anisometropic sweet spot in place, they instantly adapt to their customized uncorrected vision.
As regular readers know, I have been writing about my 20-year experience with pseudophakic monovision because I have felt that it deserved equal time and consideration along with the three other alternatives. More and more, however, I am writing about it because I firmly believe that, when properly performed, pseudophakic monovision will consistently deliver the best results.
For more information:
- William F. Maloney, MD, is head of Maloney Eye Center of Vista, Calif., and a well-known teacher of cataract and lens-based refractive surgery techniques. He can be reached at 2023 West Vista Way, Suite A, Vista, CA 92083; e-mail: maloneyeye@yahoo.com. In the interest of objectivity, Dr. Maloney has no financial interest in any ophthalmic product and has no financial relationship with any ophthalmic company.
- Lens-based Refractive Surgery Column Mission Statement: To educate readers on all aspects of lens implant refractive surgery including presbyopia correction, refractive cataract surgery, refractive lens exchange and phakic IOLs.