Research in Developmental Disabilities 2015;43-44:61-71.
By: Kristen Vincent (Class of 2017)
Pennsylvania College of Optometry, Salus University
Prism adaptation (PA) is an efficient therapeutic mediation to shift the visual field horizontally. The horizontal movement of the visual field reveals a clear directional influence on visuo-motor and sensory-motor communications. To offset the visual field deviation, the individual has to reestablish his mobility, leading to a modification of sensorimotor coordinates. Removal of the prism glasses creates the egocentric coordinates of the sensory-motor reference frame to be altered, thus generating immediate visual and proprioceptive adjustments in the direction of pointing and reaching, named after-effects. After-effects are the critical component in determining the value of PA.
PA has been a successful intervention for performance adaptation in a variety of spatial task in adults: both in healthy individuals and in patients with unilateral neglect following stroke. Due to its abiding influence, PA is considered an efficient method for rehabilitating unilateral spatial neglect by shifting the neglected hemisphere into the “seeing” field.
PA is also utilized for correcting strabismus in children, and research shows that even infants at 6-9 months of age exhibit adaptive responses to PA. The visuo-motor relationship is an attractive focus for therapeutic mediation in children since perceptuo-motor procedural learning fundamentally occurs at school age. While unilateral neglect, described as a deficit in attention and appreciation of stimuli on the paretic side of the body, has been characterized in children with cerebral damage, the achievability of PA in these children had not been explored prior to this study.
While wearing prismatic glasses, PA involves the simultaneous performance of a sensorimotor task within the visual space. Repetition of pointing movements toward visual targets is regarded as the standard procedure, both in healthy individuals and in unilateral neglect patients. As an alternative procedure to the repetitive and tedious pointing tasks, a series of visuomotor activities performed with daily life objects i.e. ecological tasks, has been recommended as more varied and appealing for long-term management. Therefore, the present study investigated the beneficial effects of PA supplemented with selected game tasks as a feasible, child-friendly alternative to the aforementioned ecological task procedures proposed for adults. It also aimed at determining whether one session of prism adaptation could generate substantial visuomotor after-effects in children with unilateral brain lesion compared with neutral glasses.
The participants of the study included 21 children with unilateral brain lesion resulting in hemiparesis. The children were divided into two study arms: one group was randomized to prism treatment, the other to neutral glasses. The intervention for all participants involved one daily task session on two consecutive days, one day consisting of ecological tasks and the other of game tasks. Children were encouraged by verbal instruction to perform all tasks bimanually for a duration of 20 minutes. All procedures were conducted while children wore glasses: prismatic glasses in the prism group and neutral glasses in the control group. Prismatic glasses deviated the visual field 20 prism diopters (approximately 11 degrees) towards the non-paretic side.
During the ecological task session, children completed an assortment of 10 visuomotor activities based on the handling of common daily life objects. During the game task session, participants completed an assortment of 10 common children games selected for promoting lateral investigation of space. To measure visuomotor adaptations provoked by the intervention, children executed two open-loop pointing tests just before and immediately after each intervention: (1) the visual open-loop pointing test, i.e. a visuoproprioceptive test with opened eyes, and (2) the subjective straight ahead pointing test, i.e. a proprioceptive test with closed eyes. Instantaneous effects of PA have been correlated with changes of these pointing assessments.
After intervention with ecological tasks in the prism group, visuoproprioceptive pointing errors of 9 (out of 11) children had drifted towards the hemiparetic side. In this same group, after intervention with game tasks, visuoproprioceptive pointing errors of 9 (out of 11) children had drifted towards the hemiparetic side. In contrast, the control group presented these findings: visuoproprioceptive pointing errors of only 1 (out of 10) child displayed a shift toward the hemiparetic side after intervention with ecological tasks, and visuoproprioceptive pointing errors of 4 (out of 10) children displayed a shift towards the hemiparetic side after intervention with game tasks.
Results were similar for the proprioceptive test. After intervention with ecological tasks for the prism group, proprioceptive pointing errors of 10 (out of 11) children had drifted towards the hemiparetic side and after intervention with game tasks, proprioceptive pointing errors of 9 (out of 11) children had drifted towards the hemiparetic side. In contrast, in the control group, proprioceptive pointing errors of 7 (out of 10) children displayed a shift towards the hemiparetic side after intervention with ecological tasks, and proprioceptive pointing errors of 4 (out of 10) children displayed a shift towards the hemiparetic side after intervention with game tasks.
This was the first study to justify the feasibility and effectiveness of prism adaptation in a pediatric population with unilateral brain lesion. It was proven that one single session of PA therapy was successful in stimulating after-effects, measured by subjective straight ahead pointing and visual open-loop pointing. As anticipated, PA in hemiparetic children provoked visuoproprioceptive after-effects towards the paretic hemispace, i.e. in a direction contralateral to the optical shift created by PA. On an interesting note, these after-effects were more distinct in children with right hemispheric lesion. A greater proprioceptive pointing error towards the paretic hemispace in the prism group compared to the control group was also observed. Moreover, game tasks presented to be as effective as ecological tasks at provoking prism adaptation-related after-effects. The children were more engaged and interested while playing common games than while conducting ecological tasks, and therefore games may promise longer interventions in children with visuospatial impairments.
After-effects are the clinical correlate of modified cortical networks involved in spatial attention and awareness while performing neglect recovery. Consequently, the manifestation of after-effects in children with unilateral brain lesion proposes the presence of short-term plasticity of cortical areas within the visual reference frame.
The prismatic glasses (20 prism diopters) utilized in this study induced a shift of the visual field by about 11 degrees during the performance of tasks and resulted in after-effects of 4 degrees on average. A larger magnitude of prisms would have afforded a larger after-effect, but this could have produced other disadvantages including distortion and unmeasurable displacement while pointing.
Limitations of the study included small sample size and it comprised primarily of children with right hemiparesis (i.e. left hemispheric lesion). The results proposed that PA effects could be larger in children with left hemiparesis (i.e. right hemispheric lesion), yet a larger and more homogenous sample is indicated to corroborate the reliability of this finding.
Since PA therapy requires repetitive intervention to produce long-lasting effects, further research is necessary to establish the efficiency of a long-term intervention in the rehabilitation of brain-damaged children with visuospatial deficits and/or neglect. Future studies should also assess different intervention durations to authenticate the most effective therapeutic dosage scheme in children. Lastly, exploration of acquiring differential improvement of extensive cognitive, motor, and sensory effects in children, as has been presented in adults, is needed. This study lays the groundwork for future research on the subject and unravels new possibilities for the rehabilitation of children with neurological dysfunctions.