Neuro Rehabilitation 6 (1996) 165-171

 

W. V. PADULA, S. ARGYRIS

Abstract

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

 

1. Introduction

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) [11].

 

Recent research has documented PTVS utilizing Visual Evoked Potentials (VEP) [2]. 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 [4], 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 [51] 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

 

 

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.