Pointer in Straw with a Twist


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.

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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.

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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.

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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!

9 thoughts on “Pointer in Straw with a Twist

  1. Len, Nice idea and execution, will try it. I sometimes have parents try it monocularly and then binoc, to get a sense of how BV helps.

    • Thanks, Mike. I think you (and your staff) will enjoy it. Sometimes we forget how “lost in space” it can feel to be fused but living on the outskirts of the horopter. When I tried it myself first, I had a visceral sense of feeling like I was visually groping to localize. Then when I did it as a demo with a mom & dad that evening, whose primary concern about their child was depth judgment and avoiding obstacles, it made an immediate and profound impression. When I shared it with Dr. Vincent, she had the same “aha moment”. And as Keith Miller presciently noted below, you can observe her subtle postural adjustments.

  2. Behavioral clinicians often speak about a misjudgement of position causing a phoria, “A esophore perceives the fixation target to be closer than it really is. But here your prism causes a “phora” which alters localization. So which comes first. And maybe I’ve got this wrong, but base in prism causes an artificial esophoria–a base in prism on the string causes the patient to see the strings cross “closer” than the bead. A base out prism, on the string, causes exo projection. Similarly, a base-in prism increases esophoria on a cover test. Thus the projection is opposite the induced phoria. So which comes first, the chicken or the prism? All of this is moot, however, because the demonstration is excellent and the suggested therapy technique is ingenius–and, like all therapy procedures, forces the patient to move (her eyes at least) in a novel environment. Great post.

  3. Thanks, David. I haven’t tried it yet, but you gave me another idea to play with. There’s no doubt so far on the people I’ve tried it with, who have BV that is considered “within normal range”, that when we take them just inside their limit of fusion, base-in prism results in localization beyond the plane or simulated exo projection when used as I described. I have a hunch that the reason why base-in prism might cause artificial esophoria on the Brock String is that the patient is holding the string, and the string is tethered to the nose, so there is strong feedback connecting the patient to the target. Notice here, however, that there is no kinesthetic sense of where the target is in space since the straw is not being held by the patient. Therefore base-in prism moves the appearance of the target further outward in space and patient is diverging from the physical plane to maintain fusion. It may be that if we have the patient hold the straw, and touch provides accurate feedback that the plane of the target has not changed, we get the opposite mis-localization with base-in prism (the esophoric over-ride), as you describe for the Brock String.

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