Our offering this month is another back-to-basics kind of discussion, indended to give you pause and help you step back for a moment to think about how you come to the endpoints of your refractions. Over the years, we have found that the specific questions asked of patients often result in different endpoints of refraction.
We have found that many assumptions are made regarding the terms used to describe refraction, so it will be helpful to provide some definitional clarity of the relevant endpoint terms before moving on:
• Binocular balance. The most plus or least minus to the first good 20/20 as measured binocularly. For the purposes of this article, we will assume we are not dealing with unequal visual acuities as a result of amblyopia, strabismus or pathology.
• Manifest. The most plus or least minus that gives the patient their best binocular visual acuity. Routinely, once the binocular balance has been identified, we reduce plus or increase minus binocularly until some endpoint is reached, then take a visual acuity and record this as the manifest.
A number of years ago, while involved in an Optometric Extension Program (OEP) study group in the Baltimore area, I (Dr. Harris) participated in exercises that led to some interesting insights into how different questions contribute so significntly to a differential in endpoints.
The topic of endpoint questions and their impact on refraction endpoint measurements came up as a result of having attended a two-day seminar by Bruce Wolff, OD, known for developing the Wolff wand. A private practitioner from Cincinnati, he was a noted lecturer and researcher involved with the Skeffington/Alexander National Optometry & Education Learning Research Center in Lancaster, Ohio.
|The standard 20/20 line in two different sizes. Although here the size difference is exaggerated, this is the kind of change we look for patients to report when asked, “Which of these looks bigger?”|
The endpoint discussion began by having each member present some of the questions they asked patients at this critical point in the refraction. A few examples included:
• Which of these looks blackest or clearest?
• Which of these looks the clearest?
• Which of these look the boldest or blackest?
• Which of these do you see better with?
• Which of these looks larger?
• Which of these feels the most comfortable to look at?
Empirics: Finding the Best Question
The sheer number of questions that emerged from just 12 study group participants was shocking. We realized that, at various times, we had each tried several of these approaches, yet had no real rationale for why we chose one over the other. So, we decided to do some “quasi-research.” We all agreed to try one of the questions for a month and report back on our experiences.
Over several months we cycled through each of these questions. We then narrowed the list of questions down to those that seemed to yield the cleanest endpoints.
Nearly a year later we emerged with the most utilitarian question. Originally, this question didn’t appear to be useful or easily explained:
“Which of these looks larger to you?”
Specifically, we continued decreasing plus or increasing minus binocularly from the binocular balance until the person reported that the next new lens altered their perception of what they were seeing (e.g., making the letters appear to shrink in size). We recorded that as our second endpoint, making certain to record visual acuities with this lens as well.
So you might ask, how could less plus in the case of patients with hyperopia, or more minus for myopic patients, lead to the perception of increased letter size? And that is a wonderful question, which—if answered purely on the basis of optics—would lead to the opposite answer than what is observed in patients. Less plus or more minus should minimize the image, and there is no question about what the optics do; however, this is not what the majority of patients report seeing.
In general, the vast majority of patients indicate that the endpoint of 0.25 or 0.50 less plus or more minus looks the largest. About 10% or so see no size change or, at the first click of the lenses, report that the letters get smaller. About 15% show a 0.75 or more difference between the two endpointsWe hope this gets you thinking about how you investigate different endpoints of your refraction. We would love to hear from you about your experiences with this. Many in the profession believe that one answer—a single refraction—is the patient’s prescription. The value of having two distinct endpoints, and how we use them to derive a patient’s final prescription, is a topic that we are building to. We have found great use in having two endpoints for our patients: (1) The endpoint with the most plus or least minus to the first good 20/20, and (2) the endpoint through which we get the perceptual response of seeing the letters on the chart the largest.