What is Visual Midline Shift Syndrome?
Visual midline shift syndrome (VMSS) is a dysfunction of the ambient visual process commonly found in congenital or acquired brain injuries whereby the perception of the egocentric visual midline is shifted to the side opposite the neurological impairment, resulting in a postural imbalance. It is often related to, and may be from, neglect and/or hemianopia.
The theory is that the preconscious feed-forward inputs of the ambient system supply information about the spatial orientation that is necessary for balance, locomotion, coordination and postural control. When it fails to match its spatial component with the incoming sensorimotor information, including proprioception, the ambient visual process will try to create balance by expanding the visuospatial information on one side and compressing it on the other side to produce a midline shift. Instead of being proactive, vision becomes reactionary and the focal visual process takes over from the ambient process to become too dominant, this is called over-focalization. This focal binding does not allow the ambient process to release and/or to adjust for its disassociation from proprioception and it generates a spatial confusion that gets exacerbated when the person or the environment moves.
Traumatic Brain Injur & Visual Midline Shift Syndrome
In a retrospective study, researchers found an alarming 93 percent of patients with a TBI have VMSS as compared to 13 percent of patients without a TBI. Another study showed VMSS was present in over 50 percent of the post-stroke subjects.
The mismatch of the ambient process with sensorimotor information can cause a variety of signs and symptoms. The signs may include posture modifications such as weight shift toward the affected side of the hemiparesis immediately following the insult and then weight shift away from the affected side within the first week, head and body as a unit turning relative to the y-axis or weight shifting relative to the z-axis, anatomical structures are not levelled with the ground, unstable gait, uneven motor output and eye-hand coordination problems. The symptoms may include photosensitivity, headaches, difficulty scrolling text on the computer screen or on digital devices, floors/walls appearing tilted, lack of confidence when walking, feeling off-balance, avoiding crowds (social outing, shopping malls), clumsiness, and general difficulty navigating the environment.
The specific procedure to test for VMSS has not been standardized yet. It is, at present, done with a pen or a Wolff wand placed about 40 cm away, that is moved about 5 inches per second by the examiner who is standing off to the side, from the right and then from the left side of the patient across the visual field. The patient tracks with their eyes, not with their head to determine the correct alignment of the target at their perceived horizontal visual midline. The test is repeated a few times to check for consistency in the results. The whole sequence is then duplicated across the superior to the inferior visual field for a vertical visual midline. Additionally, observations in posture, gait, balance and behaviour during locomotion as the individual is walking toward the exam room or into the exam chair can provide signs of physical posture shifts that might indicate the individual has VMSS.
Visual Midline Shift Syndrome Treatment
Treating VMSS involves the use of yoked prisms, and/or with balance training activities in what is called neuro-visual postural therapy (NVPT). The goal is to help the patient put more weight-bearing onto one side to modify their body posture and, their spatial perception. Yoked prisms are two prisms with their bases oriented in the same direction. They can affect postural orientation by shifting the person’s centre of gravity. When placed with the base in the direction opposite to the perceived shift in the midline and in the right prismatic power, they can effectively realign the person’s ego center to influence posture and balance during locomotion with the aim of re-establishing the visual volume to before the brain injury.5 The amount of prism prescribed can be the same in both eyes or asymmetric and the final quantity required is subjective and depends on the compromise between the ambient visual process and the degree of focalization engaged and the patient’s individual responses to the yoked prisms. Hence, it is done by a skilled neuro-optometrist who can repeatedly probe using trial yoked prisms in different prism powers usually up to 10 prism diopters and evaluating the patient’s uprighting response. Full-time wear of the yoked prismatic lenses is commonly prescribed with close follow-up to assess and to monitor their effects while the patient is in ambulation.
Neuro Optometry for Visual Midline Shift Syndrome
The goal is to reduce the amount of yoked prism, if possible and, as needed, with concurrent NVPT. NVPT is a method oriented towards improving postural tone using visually guided spatial localization activities. The facilitation to an upright posture is frequently done by a physiotherapist or an occupational therapist and/or by the neuro-optometrist if he/she is comfortable facilitating. That is why a multidisciplinary approach involving other disciplines to assess posture is very important. Together with the performance of specific visual tasks while wearing the yoked prisms, facilitation can help develop and promote the motor planning/learning and movement necessary to advance the person toward the more normal, graceful and effortless movement in all stances and in all environments (home, work, play).