Post Trauma Vision Syndrome: Part II
By William V. Padula, OD and Stephanie Argyris, OD
Visual Midline Shift Syndrome
An unusual phenomenon that often occurs following a neurological event, such as hemiparesis or hemiplegia, is that the ambient visual process changes its orientation and concept of midline (the ambient visual process lets you know where you are in space, and provides general information needed for balance, movement, coordination, and posture). To understand this more completely, let us think for a moment about the toddler who begins to gain orientation to a standing posture. The toddler must have organized, at various developmental levels, concepts of visual midline that were established through vestibular, kinesthetic, proprioceptive, and ambient visual processing. These midlines include, but are not limited to, a lateral midline, and a transverse midline. The toddler must gain orientation to the midline in order to develop weight transfer and position sense.
Information from the two sides of the body must be matched through kinesthetic and proprioceptive systems with ambient and vestibular information. This information develops experience and creates a set by which the child continues to process information throughout the developmental years. Given a neurological event such as a CVA causing hemiparesis or hemiplegia, information from one side of the body becomes interfered with. The ambient visual process is a relative processing system. It attempts to create a relative balance based on the information established. With interference of information from one side of the body compared to the other, the ambient visual process attempts to create balance by expanding its concept of space on one side of the body compared to the other. In so doing, a perceived amplification of space occurs internally on one side and a perceived compression of space occurs on the other side. This phenomenon causes a shift in the person's concept of midline, which usually shifts away from the neurologically affected side.
The authors have developed a simple test whereby a wand is passed before the person laterally and the person is asked to state when the wand appears to be directly in front of his nose. A high correlation has been found with a shift in midline away from the neurologically affected side. In other words, the person would frequently report that the object appears to be directly in front of his nose when in fact it may be to the right. This individual will frequently have left-side hemiparesis or hemiplegia. This shift in concept of midline can also occur in an anterior posterior axis, causing the individual to experience a midline shift anteriorly or posteriorly. The result is that posture will be affected by either emphasizing flexion, as in the former, or extension in the case of the latter. Combinations of anterior, posterior, and lateral shift are quite common. While these individuals do have a neurological problem such as a paresis to one side, it has been the author's experience that frequently persons involved in physical therapy will not be able to increase weight bearing on their affected side and/or stand erect without constant reminders from the physical therapist. The therapist will frequently tell the person to stand straight and the person will follow these directions. However, after the physical rehabilitation therapy session is over a shift away from the neurologically affected side continually occurs, frequently causing the person to experience a plateau and therapy to be discontinued.
The authors are stating that there is a visual relationship that occurs through the ambient visual process. However, it must be understood that there are many individuals who do benefit from physical therapy programs and can improve function of their paretic side. Perhaps this is not simply through a strengthening of muscles but really a total change in neurological processing that occurs at the midbrain level in relationship to organization of internal space thereby affecting even the ambient visual process.
Persons with a VMSS will walk as if the plane of the floor is tilted. In fact a number of individuals with VMSS have reported to the authors that the floor appears to be tilted. A neuro-optometric treatment approach that works effectively is to utilize prisms before both eyes positioned in yoked fashion. A prism is a wedge of glass or plastic. The thick end is called the base. When the base ends of the prism are positioned in the same direction for each eye; for example to the right or to the left for both eyes, this is termed yoked prisms. The effect of the prism is to counter the expansion and compression of space that is occurring in the ambient visual process. In turn, this causes the midline to shift to a more centered position.
The authors have noted that clinically persons will frequently shift their weight almost immediately and increase weight bearing on the affected side. The use of these yoked prisms is for short durations each day. The reason for this is that prisms provide a profound change in the ambient visual process and develop a level of experience of weight bearing in relationship to the ambient system as part of the sensory-motor system. The effect will be maintained for longer periods of time throughout the course of rehabilitation. Frequently, the yoked prisms are prescribed to be used in conjunction with physical and/or certain approaches of occupational therapy. These yoked prisms are prescribed following a neuro-optometric evaluation.
Visual Field Loss Affecting VMSS
Visual field loss following a CVA or TBI can often influence a shift in visual midline causing a VMSS. A homonymous hemianopsia frequently occurs following a CVA. The bilateral field loss causes the visual concept of the midline to become centered in the remaining portion of the visual field. In turn, weight bearing will be shifted away from the side of the homonymous hemianopsia producing the effect of the VMSS. Yoked prisms are effective in re-centering the concept of the visual midline and thereby increasing weight bearing on the affected side. Eventually, expanded field prisms can be mounted into the lenses that increase the visual awareness into the side affected by the homonomous hemianopsia.
Persons who have suffered a neurological event have often had visual problems that have been misinterpreted as well as misdiagnosed. Recent advances in research enabling clinicians to gain a better understanding of vision as a process have uncovered more than one visual processing system. The ambient visual process is essentially a silent process. We cannot think in this process. Instead we somehow develop a level of feeling through this process that establishes one's organization of space for balance, movement, and coordination, while also providing a spatial net by which the higher focal process delivers information about detail and identification.
Following a TBI, CVA, or other neurological event frequently individuals lose this ambient visual process and instead are left with a focal processing system that breaks up the visual world into isolated parts. This causes individuals extreme difficulty, not only with balance and movement, but also affects the person in other ways such as in the person's tendency to compress and limit their spatial world. This creates experiences such as an inability to find an object on a shelf in a store. The compression of space causes a focalization process to function both centrally as well as peripherally. This has greater meaning when one thinks of what the experience must be like when all the bottles, cans, and boxes on the shelf is suddenly experienced as massive amounts of detail causing the person to be unable to isolate one detail from another.
Movement in a crowded environment also becomes quite disturbing because the ambient visual process is supposed to assist in stabilizing the image of the peripheral retina. Without this system the person internalizes the movement that he or she is experiencing in the peripheral vision. This become extremely disturbing and causes vertigo, and severe dysfunction. The authors have found that a combination of low amount of base-in prisms and binasal occlusion have been extremely effective in almost immediately offering increased stability to the ambient visual process, thereby reducing the symptoms and enabling the person to re-establish levels of independence that were otherwise not achieved.
Interference caused by Post Trauma Vision Syndrome can also affect higher cognitive functioning. The focal process is very much related to higher perceptual and cognitive function. However, the focal process cannot function properly unless it is grounded by the ambient visual system. In turn, this loss of grounding following PTVS will cause a slowing of responses in general, and interference with higher perceptual cognitive function. Therefore, cognitive therapy should be supported by neuro-optometric rehabilitation.
Neurological problems affecting states of motor function may be diagnosed and even related to specific cortical lesions. However, the total impact of function and performance concerning balance, posture, and movement may be interfered with further by shifts in concepts of the visual midline. The midline with VMSS affects not only persons who are attempting to ambulate, but also those individuals who are wheelchair bound, causing them to lean to the side, forward, or backward. Yoked prism therapeutic lenses can make significant changes in concept of midline, thereby affecting posture, balance, and movement. The use of these yoked prisms can be developed in a transdisciplinary approach, so that they may be incorporated into existing physical and/or occupational therapy programs. The authors again emphasize that neuro-optometric rehabilitation should support the overall rehabilitation of the individual prior to and/or concurrent with physical and/or occupational therapy.
The authors emphasize that these type of neuro-optometric approaches are rehabilitative in nature and should not be thought of as a cure. As with all rehabilitation, progress depends on many factors.
The profound affects of dysfunction in the ambient visual system can greatly interfere with function and performance at all levels for persons with neurological insults. Neuro-optometric rehabilitation has effectively delivered new approaches toward treatment regiments in conjunction with treatment already being conducted in hospitals and clinics. The optometrist who has developed an understanding of neuro-optometric rehabilitation can be an important member of the multi-disciplinary team who is serving these special populations.
For more information please contact NORA at noravisionrehab.org