Acquired Brain Injury and the Treatment of Perceptual Midline Shift
Two thirds of all sensory or afferent neurons originate from the eyes. It is therefore easy to see that brain injury can cause disruption in the perceived visual field, attention to the visual field, control of the extra-ocular muscles of the eyes, and to the perception of visual space.
A stream of neurons that leave the eyes and enter various lobes of the brain called the ventral stream establish a spatial construct inside of which we become conscious of the visual space around us. The disruption of this pathway can interfere with our perception of space, and can cause significant interference to visually guided movement and balance.
The spatial construct precedes conscious awareness of what we see. The ventral stream of neurons is primarily made up of nerves with large diameter axons that conduct information very quickly called magnocellular neurons; they provide movement predictions surrounding the space we are in. From this information, the body prepares itself by establishing the proper posture for potential movement through the visual and vestibular space we are immersed in. The Parvocellular neurons are responsible for allowing conscious awareness of what is in the space around us. Parvocellular nerves leave the eye at the same time the magnocellular nerves do. The information from this pathway, with smaller, slower conducting axons arrives later than the magnocellular pathway.
When the synchrony of these two visual pathways is disrupted, it can result in misjudging the spatial location of elements in one’s visual field in relationship to one’s body. This is often referred to as a Visual Perceptual Midline Shift Syndrome or Abnormal Egocentric Localization.
An individual with Visual Perceptual Midline Shift Syndrome may perceive objects that are in line with the vertical center of the body (mid-sagittal plane), to be located off to one side or the other. For example, if one judges a hallway to be directly in front of them, when it is actually to their left, they will walk along the right side of the hall instead of the middle of it. They will often bump into objects on their right side, and will often put more weight on their right foot, and tilt their body to their right while walking. This results in a very dysfunctional and potentially dangerous interaction in the space surrounding the patient. A Visual Perceptual Midline Shift Syndrome may exist with behaviors associated with dizziness, vertigo, visual field cuts, and visual neglect.
A powerful and immediate intervention is often helpful: yoked prism. Yoked prisms are optical lenses that redirect visual space by shifting light laterally so that visual space is re-aligned to the patient’s anatomical midline. The patient then experiences coherence between the anatomical midline of the body and the perceived visual space around them, allowing for much more functional and reliable movement through space. Yoked prism can be prescribed to be incorporated in an individual’s spectacles to be worn on an everyday basis, and are also used clinically during neuro-rehabilitative therapy.
Neuro-rehabilitative optometrists are well versed in the use of yoked prism. This treatment can provide an invaluable improvement in the quality of life of our patients recovering from brain injury. A clinician who uses yoked prism in treatment can be found through the Neuro-Optometric Rehabilitation Association and the College of Optometrists in Vision Development.
- Vision Rehabilitation: Multidisciplinary Care of the Patient Following Brain Injury. Suter, P.S, and Harvey, (Ed.) L.S. CRC Press 2011.
- Padula. W. V., Nelson, C. A., Benabib, R., Yilmaz, T., Krevisky, S. (2009). Modifying postural adaptation following CVA through prismatic shift of visuo-spatial egocenter. Brain Injury, pp. 23, 566- 576.
This article was contributed by Carl G. Hillier, O.D., FCOVD, San Diego Center for Vision Care.