NeuroRehabilitation 6 (1996) 165-171
By William V. Padula, OD, SFNAP, FAAO, FNORA*, Stephanie Argyris
Following a neurological event such as a traumatic brain injury (TBI), cerebrovascular accident (CVA), Multiple Sclerosis (MS), etc. Vision imbalances can occur between the focal and ambient visual process that can affect balance, posture, ambulation, reading, attention, concentration and cognitive function in general. Post Trauma Vision Syndrome (PTVS) and Visual Midline Shift Syndrome (VMSS) can be the cause of these difficulties. This paper discusses the symptoms and characteristics of these syndromes as well as methods of treatment.
Keywords: Post trauma vision syndrome; Visual midline shift syndrome; Vision; Traumatic brain injury; Cerebrovascular accident
Following a neurological event such as a traumatic brain injury, cerebrovascular accident, multiple sclerosis, cerebral palsy, etc., it has been noted by clinicians that persons frequently will report visual problems such as seeing objects appearing to move that are known to be stationary; seeing words and print run together; and experiencing intermittent blurring. More interesting symptoms are sometimes reported such as: attempting to walk on a floor that appears tilted and having significant difficulties with balance and spatial orientation when in crowded, moving environments.
These types of symptoms are not uncommon. Frequently, persons reporting these symptoms to eyecare professionals (optometrists and ophthalmologists) have been told that their problems are ‘not in their eyes’ and that their eyes appear to be healthy. In many instances persons experiencing these difficulties also experience anxiety with these symptoms and are often referred to psychologists or psychiatrists in an attempt to treat their anxiety. The referral for psychological or psychiatric care is sometimes made based on a diagnosis of hysteria without recognizing that many of these individuals are suffering from syndromes affecting the visual process in the brain. These syndromes have been called Post Trauma Vision Syndrome (PTVS) and Visual Midline Shift Syndrome (VMSS) .
Recent research has documented PTVS utilizing Visual Evoked Potentials (VEP) . This documentation concludes that the ambient visual process frequently becomes dysfunctional after a neurological event such as a TBI or CVA. Persons suffering from a neurological event begin to experience a wide variety of symptoms as a result of dysfunction in this portion of the visual system. Yet persons who are experiencing these visual difficulties may frequently have healthy eyes and relatively normal visual acuity.
2. Vision: the process
The visual system is composed of two separate processes. The process that we are most familiar with has been called the focal process [3,4]. This process neurologically is related to the central visual function. The eye represents central vision primarily through an area called the macula located in the retina. Aiming your eye directly at an object causes focalization by the brain through the macula.
As noted by Leibowitz and Post , the focal process does not have to be delivered directly by the macula. For example, you can aim your eye at a particular object such as a doorknob on a door across the room. Fixating on the doorknob represents a central focalization. However, you can also focalize with your peripheral vision. While you are aiming your eye at the doorknob, you can use your peripheral vision to focalize on objects about the room, such as a picture or a chair. The focalization process, however is most easily delivered through the macula. While you can focalize in any portion of the visual field, the peripheral vision is primarily used as a general spatial orientation system. The reason for this is that peripheral vision is mostly a function of a second visual process called the ambient process.
The ambient process lets you know where you are in space and provides general information used for balance, movement, coordination and posture. Neurologically, nerve fibers from the peripheral retina that are part of the ambient visual process provide axons that are delivered to a level of midbrain where they become part of the sensory-motor feedback loop. The importance of this system is that it is less sensorially involved and more motoric in function. It must match information with kinesthetic, proprioceptive, vestibular, and even tactile systems for the purpose of orienting and acting as a master organizer of these other processes. Once this is accomplished, a feed-forward mechanism enables this information to be directed to higher cortical areas, including the occipital cortex, as well as 99% of the cortex.
The ambient visual process must let you know where you are in space and essentially where you are looking before you process information about what you are looking at.
Given a neurological event such as a traumatic brain injury (this includes a mild whiplash) multiple sclerosis, cerebrovascular accident, etc., the ambient visual process can lose its ability to match information with other components of the sensory-motor feedback loop. Even a whiplash, as mentioned, can cause significant dysfunction at the level of midbrain. Thomas  has calculated that at the level of the foramen magnum, as much as 14,000 lbs. of inertial force is exerted on the spinal cord with a minimal 10 m.p.h. rear-end collision. This can cause a dysfunction in the sensory-motor feedback loop and more specifically in the ambient visual process. Although this type of an injury cannot be seen in most cases on a CT scan or MRI, injured individuals will frequently experience the types of symptoms explained in the introduction of this paper.
3. Post trauma vision syndrome
Research has been conducted by the authors utilizing Visual Evoked Potentials to capture this state of dysfunction at the level of midbrain. Subjects were given binocular visual evoked cross-pattern reversal P-100 evaluation with their best distance correction.
An experimental group was used in this study. Immediately following phase 1 of the VEP testing, binasal occlusion and base-in prisms were introduced before both eyes. In the experimental group there was an increase in the amplitude(N1P1) of the VEP.
This indicates that the ambient process became more organized and provided appropriate feed-forward spatial information for the higher binocular cortical cells at the level of the occipital cortex to improve upon states of binocularity and fusion. This same prism and binasal occlusion for the control group statistically caused a decrease in the amplitude. In addition for persons without a neurological problem affecting the ambient visual system, binasal occlusion and base-in prism interferes with visual processing.
The results of the study along with clinical findings, have led the authors to document a new syndrome called Post Trauma Vision Syndrome. This syndrome is caused by a dysfunction of the ambient visual process and has the characteristics, as well as symptoms, presented in Table 1.
Person’s with PTVS will frequently have characteristics of exophoria or exotropia (a tendency for the eyes to turn out or an actual eye turned outward), accommodative or focusing dysfunction, oculomotor dysfunction, convergence insufficiency (a difficulty converging the eyes and sustaining convergence at a near plane), as well as increased myopia. The common symptoms frequently include diplopia (double vision), perceived movement of print or stationary objects, headaches and photophobia (light sensitivity).
Persons who are not treated for PTVS can experience this syndrome for many years following a neurological event. Treatment of this syndrome may include binasal occlusion in conjunction with low amounts of base-in prism and
Characteristics and symptoms of Post-Trauma Vision Syndrome
|Common Characteristics||Common Symptoms|
|Exophoria||Blurred near vision|
|Convergence Insufficiency||Perceived movement of print or stationary objects|
4. Visual midline shift syndrome
An unusual phenomenon that often occurs following a neurological event, such as a hemiparesis or hemiplegia, is that the ambient visual process changes its orientation to concept of the midline. 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 midline that were established through vestibular, kinesthetic, proprioceptive, and ambient visual processing. These midlines include but are not limited to a lateral midline, as well as a transverse midline. The toddler must gain orientation of 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 a 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 an 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 phenomena causes a shift in concept of midline that usually shifts away from the neurologically affected side.
The authors have developed a simple test (see Fig. 1) 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 the person’s 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 their nose when in fact it may be shifted to the right (see Fig. 2 and 3). This individual will frequently have a left hemiparesis or hemiplegia. This shift in concept of midline also can occur in an anterior posterior axis, causing the individual to experience a midline shift anteriorly or posteriorly (see Figs. 4-7).
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 authors’ 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.
Fig. 1 – Visual midline shift test normal response.
Fig. 2 – Visual midline shift to the right
Fig. 3 – Visual midline shift to the right affecting posture.
Fig. 4 – Posterior visual midline shift
Fig. 5 – Posterior visual midline shift affecting posture
Fig. 6 – Anterior visual midline shift
Fig. 7 – Anterior visual midline shift affecting posture
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 prisms 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.
5. Visual field loss affecting VMSS
Visual field loss following a CVA or TBI often can 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 to increase the visual awareness into the side affected by the homonymous hemianopsia can be mounted into the lenses.
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, has uncovered more then 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 obtains information about detail and identification.
Following a TBI, CVA, or other neurological event individuals frequently 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 his 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 and 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 suddenly begin to be experienced as massive amounts of detail causing the persons 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 becomes extremely disturbing and causes experiences of vertigo, and severe dysfunction. The authors’ have found that the combination of a 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 affect higher cognitive levels of function as well. The focal process is very much related to higher perceptual and cognitive function. However, the focal process can not 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 that 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 concurrently being given physical and or occupational therapy.
The authors emphasize that these types 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 regiment 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.
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