BLOG: A methodology for initiating, adjusting lens and prism corrections
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by Nancy M. Mackowsky, OD, FCOVD, FNORA
For most of us, what we see accurately represents reality. When I’m walking in the woods and I perceive a tree to be 5 feet straight ahead of me, that’s actually where the tree is. I can turn left or right to avoid it.
But for patients with a brain injury, perception may not match reality due to abnormal egocentric localization, also known as visual midline shift syndrome. The tree that is directly ahead appears to be off to one side, and in attempting to walk past it, they run into the tree.
The ability to focus on and track an object in space while you are moving is a complex task. It involves multiple sensory modalities: the inner ear and vestibular system; head, neck and posture; the eyes, working independently and together; and visual processing pathways in the brain. A stroke or traumatic brain injury can disrupt any part of this system.
To address processing mismatches and restore function, we can prescribe a lens/prism correction. The challenge is that, even with two patients who present similarly, the same prescription may have a positive impact on one but not on the other. Does that mean it is just a guessing game? Not at all; there is plenty of science to the art. By following a stepwise process, it is possible to accurately determine the type, amount and direction of prism that will create a better match between the patient’s subjective and objective egocentric localization.
I start by using photos and video to evaluate the patient’s gait and posture when looking at different points in space. They are tested on a spatial location board and undergo a full sensory-motor evaluation of focusing, eye teaming and eye tracking. Based on those results, I put a starting prescription in a trial frame and I videotape the patient walking again with this correction to see if there is an improvement in localization. This process may be repeated several times as adjustments are made to support how that patient is responding to a particular lens-prism prescription. I also perform visual evoked potential (VEP) testing to objectively determine whether the prescription improves binocular summation and visual processing.
Patients will wear the resulting prescription for at least 2 months (and sometimes much longer), often in conjunction with vision therapy, photobiomodulation and other rehabilitation techniques. The ultimate goal is for the visual processing pathways to recalibrate and for the patient’s VEP and symptoms to be the same with or without the prism, so the prism correction can be removed.
This can be a long process, but it is a rewarding one for me and certainly for my patients, who can see the trees where they belong in the forest.
For more information:
Nancy M. Mackowsky, OD, FCOVD, FNORA, is the founder and owner of the Mackowsky Visual Learning and Rehabilitation Clinic in Raleigh, N.C. Dr. Mackowsky’s most recent publication, “The role of gait analysis, egocenter and yoked prism in Parkinson’s disease,” can be found in the April 2018 edition of the Optometry & Visual Performance journal. She has served on the College of Optometrists in Vision Development International Certification and Examination Board and is currently a member of the Neuro-Optometric Rehabilitation Association Fellowship Committee.
Mackowsky will teach a 4-hour course on her systematic method for prescribing lens/prism corrections at the NORA annual conference, Sept. 11-13, 2020, in Columbus, Ohio. For schedule and registration, visit https://noravisionrehab.org/about-nora/2020-annual-conference.
Disclaimer: The views and opinions expressed in this blog are those of the authors and do not necessarily reflect the official policy or position of the Neuro-Optometric Rehabilitation Association unless otherwise noted. This blog is for informational purposes only and is not a substitute for the professional medical advice of a physician. NORA does not recommend or endorse any specific tests, physicians, products or procedures. For more on our website and online content, click here.
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