Vestibular effects on cerebral blood flow

Humans demonstrate a number of unique adaptations that allow the maintenance of blood pressure and brain blood flow after transition to the upright position. While these adaptations maintain heart-level mean arterial pressure similar to supine values, the brain remains ~30 cm above the heart, resulting in a ~25% decrease in perfusion pressure. To maintain brain blood flow, the cerebral vessels must dilate in response to this change in position. While several physiological systems are involved in adaptation to the upright posture, including cerebral autoregulation, the unique role that the vestibular system plays in helping to maintain brain blood flow is just beginning to be elucidated. Since the vestibular system not only assists in balance control and locomotion but provides direct information about the body's position relative to gravity, it can, within milliseconds, detect a change in posture. Thus it is possible that a vestibular signal indicating upright could assist in this necessary cerebral vasodilation. In this work we demonstrate a direct effect of vestibular activation on cerebral blood flow regulation. By stimulating the otoliths, the organs that sense gravity, using sinusoidal translation or tilt in the dark at five frequencies, we found that cerebral blood flow was modulated according to the frequency of stimulation. In addition, changes in cerebral blood flow were in opposition to blood pressure changes, likely indicating a direct effect of otolith activation on cerebral blood flow regulation. We anticipate these findings may lead to new treatment modalities for cerebral hypoperfusion under a variety of circumstances. For example, with aging there is well documented vestibular loss that might contribute to a general age-associated reduction in global cerebral blood flow. Similarly, patients with orthostatic intolerance could have vestibular impairment that exacerbates cerebral hypoperfusion when upright