Physiologic alterations associated with visual height intolerance: balance control and muscle activity

Question: Visual height intolerance (vHI) occurs when a visual stimulus causes the apprehension of loosing balance and falling [1]. Although vHI affects almost one-third of the general population [2] and has relevant consequences on the quality of life [3], a quantitative assessment of physiological alterations that may trigger postural imbalance in vHI is missing.

Methods: In order to evaluate vHI-related changes in postural control, we assessed center-of-pressure displacements and electromyographic data of selected leg, arm and neck muscles in sixteen susceptibles to vHI while standing on the balcony and while standing in the laboratory at ground level. Stabilogram diffusion analysis was applied to examine the characteristics and modes of interaction of open- and closed-loop processes that make up the postural control scheme in vHI under height exposure.

Results: Under height exposure, (1) open-loop control was disturbed by a higher diffusion activity (p < 0.001) and (2) the interaction of open- and closed-loop processes was altered in that the sensory feedback threshold of the system was lowered (p = 0.001). Altered postural control under height exposure was predominantly associated with increased co-contraction activity of leg muscles. By providing nearby stationary contrasts in the visual surround or while standing with eyes closed, alterations in postural control mainly disappeared. Performing a cognitive dual task also diminished degraded balance under height exposure.

Conclusions: These data indicate that postural imbalance in vHI is associated with characteristic changes in the coordination of open- and closed-loop postural control and increased leg muscle co-contraction. A twofold effect of visual environment and anxiety may evoke inadequate control of posture in vHI.

References:

1. Brandt, T., J. Benson, and D. Huppert, What to call “non-phobic” fear of heights. Br J Psychiatry. 2012a, 2012. 190: p. 81.

2. Huppert, D., E. Grill, and T. Brandt, Down on heights? One in three has visual height intolerance. Journal of Neurology, 2013. 260(2): p. 597 – 604.

3. Schaffler, F., et al., Consequences of visual height intolerance for quality of life: a qualitative study. Quality of Life Research, 2013.