OBJECTIVES AND METHOD The relation between body sway recorded through a stabilometric platform and the subjective report of steadiness was studied in 20 young and 20 elderly subjects and 20 neuropathic and 20 parkinsonian patients standing upright. The trials were performed under two stances (feet apart, feet together) and two visual conditions (eyes open, eyes closed). At the end of each trial, subjects scored their performance on a scale from 10 (complete steadiness) to 0 (fall). RESULTS In all subjects, independently of the stance conditions, the larger the body sway the smaller the reported score. The function best fitting this relation was linear when sway was expressed on a logarithmic scale. The scoring reproducibility proved high both within and across subjects. Despite the different body sways and scores recorded under the different visual and postural conditions (eyes closed >eyes open, feet together>feet apart) in all groups of subjects and patients, the slopes of the relations between sway and score were broadly superimposable. In the normal subjects, the scores were slightly higher during eyes open than eyes closed trials for corresponding body sways. This was interpreted as a sign of perception of greater stability when vision was allowed. Parkinsonian patients swayed to a similar extent as normal subjects, and their scores were accordingly similar, both with eyes open and eyes closed. Neuropathic patients swayed to a larger extent than normal subjects, and their scores were matched appropriately. Although the slope of their relation with eyes closed was not different from that of normal subjects, with eyes open it was steeper and similar to that with eyes closed, suggesting that these patients did not feel more stable when they could take advantage of vision. CONCLUSIONS The subjective evaluation of body sway, irrespective of stance condition, age, neuropathy, and basal ganglia disease, reflects the actual sway, and is inversely proportional to the logarithm of the sway value. The remarkable similarity of the relation between score and sway across the various groups of subjects with eyes closed indicates a common mode of sway evaluation, possibly based on integration of several sensory inputs. All groups except neuropathic patients seem to take advantage of the redundancy of the inputs. Basal ganglia integrity does not seem to have a role in the evaluation of sway.

The effect of hyperventilation on postural balance was investigated. Voluntary hyperventilation increased body sway in normal subjects, particularly in the sagittal plane. The possibility that this hyperventilation-induced unsteadiness is due to interference with lower limb somatosensory input, vestibular reflexes or cerebellar function was assessed. (i) The effect of hyperventilation on peripheral compound sensory action potentials (SAPs) and somatosensory evoked potentials (SEPs) (recorded centrally, from the scalp) elicited by electrical stimulation of the sural nerve was measured in six normal adults. A reduction in the scalp SEP amplitude and an increase in the peripheral SAP amplitude were observed during hyperventilation, which reversed during the recovery period. These changes indicate increased peripheral neural excitability which could lead to a higher level of ectopic activity; the latter would interfere with central reception of peripheral input. (ii) The click-evoked vestibulo-collic reflex was recorded to study the effect of hyperventilation on vestibulo-spinal activity. EMG recordings from both sternocleidomastoid muscles of six healthy subjects were made in response to loud clicks presented to either ear. Neither the amplitude nor the latency of the response were altered significantly by hyperventilation. (iii) Eye-movement recordings were obtained in the six normal subjects to assess the effect of hyperventilation on the vestibulo-ocular reflex and its visual suppression, the latter being a function largely mediated by the cerebellum; no changes were detected. (iv) Three-dimensional eye-movement recordings and body-sway measurements were obtained in six patients with longstanding unilateral vestibular loss in order to evaluate if hyperventilation disrupts vestibular compensation. In all patients, a horizontal nystagmus either appeared or was significantly enhanced for > or = 60 s after voluntary hyperventilation. Sway was also enhanced by hyperventilation in these patients, particularly in the frontal plane. This study suggests that hyperventilation disrupts mechanisms mediating vestibular compensation. The increase in sway may be, at least partly, mediated by deranged peripheral and central somatosensory signals from the lower limbs. Hyperventilation seems to spare vestibular reflex activity and cerebellar-mediated eye movements.