How does lighting and visual task affect postural stability in patients with severe peripheral field loss? A preliminary study

Abstract

Purpose : Retinitis pigmentosa (RP) is a genetic eye disease causing progressive loss of peripheral vision. They often complain of glare due to exposure to bright light. Because of the important role of visual input on postural control, patients with peripheral field loss have poorer balance. However, it remains unclear whether glare imposes further challenge to RP patients’ postural control, especially during visual search - a common visual activity. Methods : Six RP participants aged above 50 with binocular visual field < 10 deg and six age-matched healthy controls (HC) were recruited. All subjects were required to stand on a force platform (SMART EquiTest) and perform 2 visual tasks at 3 different illumination levels (100, 520 and 2100 lux). They were asked to undergo either 1) a pure fixation task for 18 s, or 2) a combined task with visual search in the first 6 s followed by fixation for 12 s. To simulate daily visual search, subjects had to identify a target on one of 6 monitors located at 3 m away in an arc. Postural sway area (mm2), maximum sway (mm) in anterior-posterior and medio-lateral directions were analyzed across 3 time-points (6 s each) to study the effects of illumination, visual task and group. Results : Results from generalized estimating equations showed that postural sway significantly reduced across time (p<0.01), with larger sway area and displacement in the first time-point compared with others. Postural sway was not significantly affected by illumination (p>0.05), but by nature of visual tasks (p<0.01). Subjects swayed significantly more in the first 6 s during visual search, but this reduced during fixation (p<0.01). Interestingly, RP patients swayed less than HC, but their differences did not reach statistical significance. No interaction effect between group and illumination or group and visual task was found (p>0.05). Conclusions : Our preliminary findings indicated that visual search significantly disrupted the postural stability in older adults and RP patients, so longer time was required to regain stability. This might outweigh the impact of illumination. The lack of group differences in the postural stability might be due to small sample size. However, it is possible that RP patients have adopted a sensori-motor strategy to maintain their postural stability, minimizing the impact of the environmental changes on postural stability.