Brief Review Assessment of Human Sympathetic Nervous System Activity from Measurements of Norepinephrine Turnover

SINCE the sympathetic nervous system has such a central place in homeostasis in general, and in circulatory adaptation in particular, it is paradoxical that so little is known about the possible contribution of disturbed sympathetic nervous function to the development of human diseases. Clinical tests of sympathetic nervous system activity have, by and large, been routinely applied in one setting only: the detection of sympathetic nervous failure, from autonomic insufficiency syndromes, in patients with postural hypotension. A list of "candidate" diseases for sympathetic nervous system dysfunction might include, for example, essential hypertension, cardiac failure, coronary artery spasm, cirrhosis, mitral valve prolapse, and Raynaud's syndrome. Until very recently, the picture of sympathetic nervous pathophysiology for conditions such as these was particularly sketchy, mainly because of the rudimentary nature of the tests of sympathetic nervous system function available to investigative clinical medicine. Pertinent questions have gone incompletely answered at best — questions such as: Is the sympathetic nervous system directly involved in the early pathogenesis of essential hypertension? Is increased renal sympathetic activity a common cause of sodium retention in edematous states? Is increased cardiac sympathetic nerve firing an important element in the development of cardiac arrhythmias in humans?