THE EFFECTS OF HYPERVENTILATION ON POSTURAL CONTROL MECHANISMS

The effect of hyperventilation on postural balance was investigated. V oluntary hyperventilation increased body sway in normal subjects, part icularly in the sagittal plane. The possibility that this hyperventila tion-induced unsteadiness is due to interference with lower limb somat osensory input, vestibular reflexes or cerebellar function was assesse d. (i) The effect of hyperventilation on peripheral compound sensory a ction potentials (SAPs) and somatosensory evoked potentials (SEPs) (re corded 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 w ere observed during hyperventilation, which reversed during the recove ry period. These changes indicate increased peripheral neural excitabi lity which could lead to a higher level of ectopic activity; the latte r would interfere with central reception of peripheral input. (ii) The click-evoked vestibulo-collic reflex was recorded to study the effect of hyperventilation on vestibule-spinal activity. EMG recordings from both sternocleidomastoid muscles of six healthy subjects were made ib response to loud clicks presented to either ear: Neither the amplitud e nor the latency of the response were altered significantly by hyperv entilation. (iii) Eye-movement recordings were obtained in the six nor mal subjects to assess the effect of hyperventilation on the vestibule -ocular reflex and its visual suppression, the latter being a function largely mediated by the cerebellum; no changes were detected (iv) Thr ee-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 compensa tion. In all patients, a horizontal nystagmus either appeared or was s ignificantly enhanced for greater than or equal to 60 s after voluntar y hyperventilation. Sway was also enhanced by hyperventilation in thes e patients, particularly in the frontal plane. This study suggests tha t hyperventilation disrupts mechanisms mediating vestibular compensati on. The increase in sway may be, at least partly, mediated by deranged peripheral and central somatosensory signals from the lower limbs. Hy perventilation seems to spare vestibular reflex activity and cerebella r-mediated eye movements.