Influence of extero- and proprioceptive afferents of the plantar surface in determining subjective visual vertical in patients with unilateral vestibular dysfunction.

Subjective visual vertical refers to an individual's ability to indicate what, in his or her opinion, is a perfectly vertical line in specific experimental conditions. Although the otolith organs play a key role in the perception of verticality, the contribution of other sensory systems, e.g. the visual and proprioceptive systems, cannot be overlooked. The aim of this study was to test the hypothesis that extero- and proprioceptive afferent signals, particularly from the plantar surface of the foot, can influence the temporal evolution of altered subjective visual vertical following unilateral acute vestibular dysfunction. Subjective visual vertical was studied in 40 consecutive patients: 19 females and 21 males (mean age 46.4 years). It was first measured at diagnosis (1-2 days after onset of symptoms). For this measurement, a baseline test was performed (patient standing in direct contact with the floor), followed by a provocation test with a soft support between the patient's feet and the floor. Based on a comparison between the baseline and provocation tests, the patients were divided into three groups: Group A--patients showing a significant increase (p < 0.05) in subjective visual vertical(0 )values in the provocation test compared to baseline values; Group B--patients showing a significant decrease (p < 0.05) in subjective visual vertical(0 )values in the provocation test compared to baseline values; Group C--patients showing no significant changes (p < 0.05) in subjective visual vertical(0 )values in the provocation test compared to baseline values. The baseline test was repeated at 30, 90 and 180 days. At the end of the follow-up, a persistent change in subjective visual vertical was noted in 87% of the patients from Group B, 31% of the patients from Group C but none of the patients from Group A, all of whom were able to correct the perception error during the second examination. The study demonstrates that normalisation of subjective visual vertical in subjects with unilateral vestibular lesions seems to be influenced by the possibility of exploiting extra-vestibular sensory information, particularly extero- and proprioceptive information from the plantar surface.