After a TBI the prevalence of visual dysfunction is high . Traditionally, vision problems post-TBI have been classified or diagnosed according to the oculomotor difficulty such as, exotropial third nerve palsy, convergence insufficiency and accommodative insufficiency. While these diagnoses help in understanding the ocular dysfunction they often do not support understanding the greater visual problems and symptoms that the person with a TBI may present. These diagnoses are specific, and do not take into account the complex relationships between the visual process and the sensory-motor systems.
The results of this study suggest that many persons who have suffered a TBI do have significant visual problems which cause a reduction in acuity, binocular dysfunction and accommodative dysfunction, as well as oculomotor dysfunction. However, the study indicates that the symptoms presented may be due to a disturbance of the ambient visual process, which in turn interferes with binocularity. The VEP of the experimental group showed a statistically significant increase in the amplitude when base-in prisms and bi-nasal occluders were introduced before both eyes. This indicates that the binocular cortical cells had to some extent been compromised. These binocular cortical cells function primarily as part of the focal process. Post-TBI there appears to be an interference in spatially organizing the visual field which in turn affects focalization [ 18]. Thus, the amplitude will reduce when tested binocularly on a VEP because the focal process has been compromised. The focal process of the visual system relates to attention and concentration, and thus cognitive functioning. Any interference with the focal process will affect binocularity, accommodation, convergence and oculomotor function.
The ambient visual process is a spatial orienting process that is part of the sensory-motor feedback loop. When used properly it supports the focal process by orienting this system spatially. In order for the focal process to function effectively the ambient process must initially organize and stabilize the field.
The increase in amplitude of the binocular VEP for the experimental group when using base-in prisms and bi-nasal occluders suggests that by affecting the ambient visual process through structure from the bi-nasal occluders and field expansion from the base-in prisms, the binocular cortical cells increase in effectiveness of function. This increase in binocular cortical function is also correlated with the verbal responses from the subjects. Frequently, the subjects reported that the perceived movement of the letters on the chart or of the floor stabilized. They also reported that it was easier to fixate with two eyes and, for some, the diplopia was eliminated.
This study further indicates that the ocular conditions diagnosed after a TBI may be due to a dysfunction of the ambient visual process in its ability to organize spatial information with other sensory-motor systems. This in turn causes a compromise of the focal process.
This disturbance in the ambient system appears to cause the dysfunction of binocularity for the persons in this study and, for many affected persons, may actually lead to strabismus, convergence insufficiency, accommodative insufficiency and oculomotor dysfunction. Thus, it is suggested that, following a TBI, ambient processing dysfunction may be the cause of what the authors have termed post-trauma vision syndrome (PTVS). Understood in this way, the specific binocular dysfunctions are actually characteristic of PTVS (see Table 9).
While the present study does demonstrate that VEP amplitude changes are possible post-TBI by affecting the ambient visual process, there is a need for a more extensive study with an increased number of subjects. Also, specific analysis of bi-nasal occluders separate from base-in prisms is needed, to determine if one of these regimens is more effective than the other.
Fig. 1-a Visual evoked potentials with no correction.
Fig. 1-b Visual evoked potentials with bi-nasal occluders and base-in prisms.