Surgical monovision is not optometric monovision

For the surgeon, preop quantitative analysis replaces trial and error.

Issue: September 1, 2006

ByWilliam F. Maloney, MD

William F. Maloney

In a previous column, I described my earliest clinical experience with pseudophakic monovision (20 years of developing conventional IOL presbyopia correction, Jan. 1, 2006). To expand the range of uncorrected vision, I began incrementally targeting increased amounts of myopic anisometropia. The effect gradually carried to intermediate and then eventually full reading focus. I moved closer to 1.5 D with increasing concern because this interocular difference was considered to be the symptom threshold for most patients using contact lens monovision — my clinical model at the time.

It is not my model today. Among the most important things I have learned in my 20 years of correcting presbyopia is one thing that pseudophakic monovision is not. Without question, it is not contact lens monovision.

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Optometric monovision

I refer to contact lens monovision as “optometric monovision” because optometry deserves credit for its clinical development, but also to categorically differentiate it from surgical monovision.

Optometric monovision (Figure 1) typically determines ocular sighting dominance using the “hole in card” test or one of its variations. Full distance correction is then typically assigned to the sighting-dominant eye. Anisometropia and the resulting correction assigned to the non-dominant near eye are determined by trial and error.

A patient not adapting relatively early in this process is advised that “monovision will not work for you” – thus its stigma in the minds of many patients.

This is hardly a suitable prototype for a surgical procedure, yet it remains just that. The predominant core conception of surgical monovision, especially pseudophakic monovision, is that of a fixed, less forgiving form of contact lens monovision.

The common use of a contact lens trial to help predict surgical monovision success is emblematic of this holdover of the optometric model. As a result, we have been unable to see the fundamental differences between it and surgical monovision. As is typical of such a paradigm-induced scotoma, we have not seen because we have not looked.

Figure 1: Optometric monovision

Optometric monovision typically determines ocular sighting dominance using the “hole in card” test or one of its variations. Full distance correction is then typically assigned to the sighting-dominant eye. Anisometropia and thus the correction assigned to the non-dominant near eye are determined by trial and error.

Dominance

Sighting

Anisometropia

Trial and Error

Source: Maloney WF

Comprehensive preop analysis is available

I have been studying the optometric literature with some regularity for more than 20 years. It details a variety of tests, measurements and other assessments that, taken together, comprise a surprisingly comprehensive evaluation of the monovision candidate.

As just one example, more than 25 different ocular dominance tests are described within three separate classifications: sighting dominance, sensory dominance and oculomotor dominance.

Some reports outline which tests are most likely to ascertain monovision success and why. Others detail the measurement of suppression capabilities and describe how best to establish the limit of anisometropia that falls comfortably below each candidate’s suppression capability.

These are just a fraction of the measurements comprising the comprehensive quantitative analysis that I have referred to as preop “presbyometry.”

With all of these tools at its disposal, why did clinical optometry continue with such a bare-bones approach to contact lens monovision? Why did the full complement of this comprehensive analysis not make it out of the optometric ivory tower?

I want to be clear that I do not mean to disparage optometry; far from it, since I learned almost everything I know about the principles and practice of monovision from the publications of our optometric colleagues.

The answer can be found within the “time value of testing” equation. In short, for the practitioners of contact lens monovision, trial and error is simply the fastest and most cost-effective means to evaluate these candidates. In most cases, there is just no practical value to these additional tests.

Surgical monovision

But for a surgeon utilizing monovision, the balance of this equation shifts dramatically. For us, the information available from presbyometry is indispensable. In fact, success or failure often hinges on the decisions based on these data.

Preop presbyometry is important for conductive keratoplasty and LASIK, too. Corneal monovision procedures are no place to “wing it” when these measurements are so accessible. But it is absolutely essential with pseudophakic monovision, which must provide a workable permanent remedy to the complete presbyopia that automatically accompanies lens extraction.

Presbyometry for pseudophakic monovision is outlined in Figure 2. Ocular dominance testing is methodical. Particular emphasis is placed on the results of sensory dominance testing, which measures the relative overall impact of monocular blur. Ideal candidates manifest sensory distance dominance in one eye and sensory near dominance in the fellow eye – a frequent finding.

Figure 2: Pseudophakic monovision

The preop presbyometry that we typically employ in planning pseudophakic monovision is shown here. Ocular dominance testing is extensive. Particular emphasis is placed on the results of sensory dominance testing, which measures the overall subjective impact of monocular blur. The ideal candidates demonstrate sensory distance dominance in one eye and sensory near dominance in the fellow eye – a common occurrence.

Dominance

Sighting
Sensory: Near and distance
Oculomotor

Anisometropia

Focus zone chart
Interocular defocus tolerance
Suppression capacity
Pupillometry
Stereopsis

Vergence, Version, Other

Source: Maloney WF

The myopic anisometropia employed varies both in amount and placement according to the results of these tests. I have described this process in a previous column (Conventional IOL presbyopia correction: six steps to success, March 1, 2006).

Briefly, each patient’s desired reading vision goal is determined using the focus zone chart. The required near point is thereby fixed. The amount of anisometropia is selected in accordance with the suppression capacity and defocus tolerance. The exact range of uncorrected focus along with the far point then falls into place.

Cataracts can potentially influence these assessments. In practice, however, this is rarely a concern because it seems to require a significant amount of asymmetric lens opacity to skew these results enough to alter the final outcome.

Pseudophakic monovision is much more than permanent contact lens monovision. Properly performed, the end result is highly predictable and provides a level of reading versatility not readily achieved with the current alternatives. But there is another aspect to what this procedure is not. Pseudophakic monovision is not monovision at all, and this is no idle semantic quibble. It has important neuroadaptive implications for each approach to pseudophakic presbyopia correction that we will consider in the next column.

In the Oct. 1 issue

Monovision in name only: The new neurophysiology of binocular rivalry.

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.