A public thank you to Dr. Paul Freeman who, as Editor of Optometric Clinical Practice, encouraged me to organize my thoughts into an article The Art of Prescribing Low Amounts of Prism for the new “Expanding the Box” feature of his outstanding journal.
The article was predicated on my clinical experiences honed through previous Vision Help Group blogs, continuing education lectures to our local optometric society in Monmouth and Ocean County New Jersey, and most recently through the iHeart VT 2020 online program. My intent in sharing the information in the article was to bring the concept to a wider audience, and it has been very gratifying to receive positive feedback about it not only from primary care optometric practitioners but from those who specialize in vision therapy and rehabilitation as well.
Aside from the “VT List-serve” and several Facebook platforms, I received a great question after presenting of this information as part of our recent annual Vision Help Group Think Tank, held this year as a Zoom Meeting.
The question was what I used as an initial probe value of prism, particularly in the vertical direction, and my response grew into a long one that serves as an addendum to the article. I’d suggest reading the article first, and coming back to what follows below after you start to use this approach clinically.
I typically begin with a probe prism value of 1.50^, and the target is an isolated horizontal row of 20/40 Snellen letters. The normal supra/infravergence is 4/2, so facility should be something inside that range (mirroring what we do with horizontal). Ideally the patient fuses that symmetrically. This may include brief diplopia, but the time taken to recovery is identical in the BU and BD directions. I will go up to 3^ max. What I’m looking for in patients who have symptoms, or generalized concern about visual performance, is that one direction is clearly easier to fuse than the other.
If the patient can fuse neither 1.50^ BU or BD, I then drop to 1.0^ and repeat, going down to 0.5^ if necessary. But invariably, with those cases that are clinically significant for Rx-ing, the relative ease of BU as compared to BD can be demonstrated with a 1.50 loose prism vertical jump.
The next step is just as important, which is probing symmetry. If, for example, 1.50^ BU OD is noticeably easier to fuse than 1.50^ BD, then when we switch to the opposite eye we should get confirmation that 1.50^ BD OS is easier to fuse than 1.50 BU. Let’s say that is not the case – and the patient fuses both directions of 1.50^ similarly, either with equal ease or equal difficulty – then I’ll go back and repeat with OD to see if it’s consistent. After all, the patient might just have a slow engine getting started (if fused easily OS) or the opposite (if diplopia on both) – indicating infacility or ill-sustained binocularity.
If it is repeatable that the prism asymmetry is present OD but not OS, then the vertical prism is Rxed only in the R lens. For example, associated phoria and/or Maddox rod free space phoria measures 1^ BU OD … if facility confirms the preference for fusing BU OD, then I’m contemplating Rxing ^ only in the R lens.
However, let’s say it is asymmetrical but not complete – meaning that
I use the word contemplating, because I’m setting up the trial as a tentative Rx to be confirmed. If the patient’s primary symptomology is reading/near performance, I have them look at text or my laptop screen with representative font size/spacing of their habitual demand and confirm that with the tentative ^ Rx it looks and feel more comfy than without.
If symptomology or performance relates more to ambulating or driving, then I want them to walk around and/or look at signs and/or cars through the large front office windows and confirm that it looks and feel better. Occasionally (but not often) the functional cross-check doesn’t result in improved comfort/performance. Then despite associated, dissociated, and facility findings pointing to asymmetry, I’ll go back to splitting it evenly between the two eyes. If they report that’s preferred, I’ll go with that.