One of my favorite VT procedures, elegant in its simplicity, is pointer-in-straw. When the patient has normal spatial localization without a significant phoria, it is relatively easy to guide the pointer into the straw when touching down from above, or along the “Y” axis as illustrated here by our Resident, Dr. Kristen Vincent.
A few days ago I was scratching my head, trying to come up with an easy demo for parents enabling them to experience what it’s like to be a child with poor visual localization due to binocular imbalance. Then it hit me that the pointer-in-straw would be the perfect procedure if we could put them into either an eso or exo posture. The way to do that would be to use a prism bar and take them slowly up to diplopia, and then slowly reduce the prism until they just recover single vision. Sure enough it worked like a charm!
Here is Dr. Vincent doing the procedure through base in prism, just inside her recovery point. This induces an exophoric posture, and she mislocalizes the pointer beyond the plane of the straw.
When doing the procedure through base out prism, just inside her recovery point, eso projection is induced and she mislocalizes the pointer ahead of the plane of the straw.
So to review, you have the parent do the procedure first without prism. If they’re off initially, do it a couple of times until they get it. Then use either base-in or base-out prism just inside their recovery point to demonstrate how their child comparatively misperceives where objects are in space. Even with another try or two, they will find it much harder to adjust than without the prism.
As we discussed using this as a demo, it also occurred to us that this would also be a useful therapy procedure, aiding the patient through feedback on how to recalibrate to make the appropriate bincoular perceptual-motor match. Can’t wait to work through this with the therapy staff at our next case conference team meeting!