Movements and position changes are followed by a series of physiological changes in the human body. The maintenance of stable blood pressure during a position change is crucial to keep a sufficient blood flow to the brain at all times. The inability to do so is called Orthostatic Intolerance (OI), which causes a drop in blood pressure when moving from sitting to standing. Orthostatic hypotension, POTS, Vasal-vagal, and syncope are conditions that fall under the category of dysautonomia meaning a disruption in the Autonomic Nervous System which controls homeostasis and vital function. The main cause of orthostatic hypotension is an excessive decrease in cardiac output or a failure to signal vasoconstriction is blood vessels. Orthostatic intolerance can lead to dizziness and syncope (fainting). A blood pressure drop upon standing is typically linked to the pooling of blood to the arteries in the lower limbs, as well as a failure of the arteries to constrict upon standing. The physiological changes accompanying OI can be described as a temporal mismatch between cardiac output and vascular resistance. When humans change positions from sitting to standing, it is the job of the vestibular system to detect changes in gravity and signal to the sympathetic system to successful constriction blood vessel, redistributing blood to the brain and increasing blood pressure.
The vestibular system contributes to the regulation of blood distribution in the body and blood pressure during movement and changes in posture. The autonomic nervous system includes two very important brainstem areas, the nucleus tractus solitarius and the rostral ventrolateral medulla (RVLM) which regulate the cardiovascular system. Both brainstem areas receive direct input from the vestibular nuclei. The sympathetic nervous system (SNS), controls cardiac output and regulates blood pressure. During postural changes, a range of sensory feedback mechanisms serves to increase activation of sympathetic nerves, which, among other effects, leads to an increased blood pressure. A very important reflex that contributes to these mechanisms originates from the vestibular system
The vestibular organs, located in the inner ears, senses rotation by means of three semi-circular canals, and linear accelerations, including gravity, by means of the otoliths. The Vestibular system detects body position changes relative to the force of gravity and together with information provided by the visual and proprioceptive system, these somatosensory systems ensure gaze stabilization and balance as well as orientation and navigation. This task is so fundamental to life on Earth that most animals have a highly specialized vestibular system.
The vestibular system plays an important role in the activation of the SNS. The vestibulo-sympathetic reflex (VSR) is the connect between signals from the vestibular system activating the sympathetic nerves which regulate the cardiovascular system. The VSR is essential for preventing a drop in blood pressure and symptoms of orthostatic Intolerance because the VSR is activated at the onset of sensing motion. The otoliths detect a position change with respect to gravity within milliseconds, thus play a key role in the activation of the VSR.
A decreased ability to maintain blood pressure upon standing is correlated with a decreased otolith (vestibular) response suggests that orthostatic intolerance can be treated by specifically activating the vestibular system. Vestibular stimulation or recalibration can, in turn, remap proper brain integrity and resolve symptoms of POTS and Dysautonomia