Using magnetic resonance imaging (MRI) techniques, to show that patients with EHTN have a statistically significant increased incidence of vascular compression of the surface of the ventrolateral medulla at or near the entry zone of CNs IX and X, compared to age-matched normotensive individuals. Among patients with EHTN, to show an association between increased sympathetic outflow from the central nervous system and vascular compression of the medulla. To show the exact nature of this relationship: Left versus right medulla; the vessel involved; the type of compression (touching or indentation); the site of compression (along the IXth and Xth CNs)and/or their root entry zone, upper or lower medulla.