In Part 1 we introduced a patient self-diagnosed as having agoraphobia who wrote two paperback memoirs available through Amazon touting the benefit of a small amount of vertical prism in her glasses. In Part 2 we reviewed the logic of prescribing small amounts of vertical prism. In those blogs I included a link to a 2017 interview on Gary Gerber’s Power Hour with the wife/husband team of Dr. Debby Feinberg (an optometrist) and Dr. Mark Rosner (an ER physician). I’d encourage you to listen to that, as well as to their initial interview which was conducted in 2015. It details the patient types which include those experiencing unresolved headaches, motion sickness, visual-vetibular disintegration, nausea, gait/balance/postural issues, head tilt/neck pain, anxiety, agoraphobia, photophobia, and reading disabilities. They estimate that VH (Vertical Hetereophoria)/SOP (Superior Oblique Palsy) as a subset of BVD (Binocular Vision Disorder) occurs in up to 10% of the population, and that small amounts of vertical prism that sets both eyes in synchrony results in 80% treatment success.
Of note, a caller during this interview asked specifically about the measurement device used for Neurolenses (formerly manufactured by Stereo Optical and now bundled as part of eyeBrain Medical which this year formed a collaborative with IDOC). Dr. Feinberg replied that she tried the device in her office, but found that because at least 60% of her patients had some form of brain injury, they required out-of-instrument testing often through trial framing that didn’t lend itself to testing with the device. As an aside, Neurolens has a fixed progressive or contoured horizontal prism of 0.375^ base-in for each lens maximal in the reading area added to the distance power (total of 0.75^ BI OU). My personal approach is to be sensitive to small amounts of either horizontal or vertical prism, sometimes using both, and sometimes utilizing yoked prism effects. This is due to the significance of subtle cyclovertical asynchrony (at the cortical level)/misalignment (at the motoric level).
Here is the test protocol that I have evolved, done entirely out of the phoropter:
- Measure dissociated phoria with Red Maddox Rod/Risely Prism measure at distance and near, comparing vertical phoria with measuring prism over right eye vs. left eye. Near testing is done at the habitual reading or work distance and angle of gaze (typically below eye level).
- Probe vertical fixation disparity distance and near, taking note of unilateral vs. bilateral slip, and asymmetry. Again, near testing is below eye level.
- Present a horizontal line of letters and measure vertical vergence range at distance and near. Look for asymmetry (for example, a right hyper of 1^ should be confirmed with diplopia occurring on BU^ sooner than BD^, and/or recovery BU^ poorer than BD^.
- If there is habitual head tilt, do the Worth 4 Dot test looking for diplopia that occurs in the field opposite to the habitual tilt. See if the tentative vertical prism in primary gaze helps extends the range of single vision.
- Do Wirt Stereo Circles without prism and then repeat with tentative vertical prism to note any improvement.
- Have patient look at a paragraph of print and gain a sense of comfort or performance, then repeat with tentative vertical prism to note any improvement.
- If there is significant horizontal or cyclophoria, repeat with prism in place and see if it lessens considerably. If not, repeat the vertical phoria measure with horizontal prism as derived through horizontal associated phoria or disparity.
- With tentative prism in place, take the patient out into an open space and note any change in head posture or gait, and elicit their sense of comfort in space as they look around as well as out into the open as through a window across the street with cars passing by.
- Based on history and your testing, decide with the patient where the area of greatest concern is, distance or near. Many times the patient will be able to absorb the same prism in low amounts for all distance. But that may not be the case.
- When distance and near differ based on angle of gaze, consider yoked prism. You can factor the net prism by Ring different yoked values to preserve the net vertical difference between the two eyes.
- Consider the influence of vertical prism on accommodative balance, as well as the effect of plus lenses on vertical heterophoria and the indications for asymmetrical plus power at near. You would expect lateral interactions through the CA/C and AC/A relationships, but don’t assume that vertical prism has no impact on accommodation and vice-versa. You can probe accommodative balance at near under binocular conditions with Bernell’s polarized duochrome test slide.
(Dr. Curt Baxstrom alluded to this in his comments at the end of the previous blog, writing: “Skeffington suggests in some of his writings that the earliest finding of an accommodative issue is a vertical finding. Thus these patients should probably have accommodation tested regarding amplitudes, flexibility and sustainability.”)
A general tip. I much prefer to use high quality large diameter round prisms for probing and trial. I use the set available through Optomat in Spain. In many instances I’m able to do the steps above using loose prism over one eye or the other. Because of the lightness and large diameter of the lenses, they are very easy to maneuver even without the accompanying trial frame. And in the low powers that we use, the etched prism amount and base direction is very helpful.
For reference, here are some additional useful links discussing the application of prism to binocular vision disorders: