Effects of a two-diopter vertical prism on posture.

Postural control in upright stance requires the central integration of visual, vestibular, somatosensory (as cutaneous receptors) and proprioceptive (as joint receptors) inputs. Clinical studies seem to indicate an association between vertical heterophoria (VH) and balance control. The purpose of the study was to simulate a VH and examine its influence on body stabilisation in quiet stance. We studied 15 healthy subjects (25.6+/-3.0 years). The postural stability was measured with a platform under the following conditions: normal viewing, with a two-diopter prism base down placed on the dominant eye (PDE) or the non-dominant eye (PNDE). Both eyes were open in all conditions. All conditions were run at two distances: 200 and 40 cm. The results showed: (i) PNDE increased the antero-posterior body sway for both distances; this result could be related to sensory processing of disparity and/or to inappropriate eye movement response to the disparity induced by the prism; (ii) PDE improved the postural stability only at far distance (reduction of the center-of-pressure excursion area and of the lateral body sway). Such positive result could be due to appropriate sensory processing of disparity and/or eye movement response; the latter would reduce vertical disparity and modify the dynamic and tonic eye muscle activity relative to normal viewing at far distance. We conclude that: (i) experimentally induced vertical phoria does indeed influence postural control; (ii) vertical prisms, even of small power, can have complex effects, positive or negative, depending on the eye wearing it and at the distance fixated.