Postural syncope occurs under a wide variety of clinical conditions: (1) in otherwise normal individuals who, in certain circumstances, experience an excess centrally mediated sympathetic discharge, as described earlier under vasodepressor syncope, or the simple faint; (2) as part of a chronic, probably degenerative central nervous system syndrome known as idiopathic orthostatic hypotension or primary autonomic insufficiency and with a variety of central nervous system degenerations that have autonomic failure as an accompanying feature (multiple system atrophy, Parkinson disease, Lewy-body disease); (3) after a period of prolonged illness with recumbency, especially in elderly individuals with poor muscle tone; (4) in association with diseases of the peripheral nerves that involve autonomic nerve fibers—diabetes, tabes dorsalis, amyloidosis, Guillain-Barré syndrome, a primary idiopathic autonomic neuropathy, pandysautonomia, and several other polyneuropathies, all of which interrupt vasomotor reflexes; (5) in patients receiving L-dopa, dopamine agonists, antihypertensive agents, and certain sedative and antidepressant drugs; (6) in spinal cord transection above the T6 level, particularly in the acute stage; (7) in patients with hypovolemia, (8) in pheochromocytoma, in which repeated exposure to catecholamines leads to desensitization of α-receptors on resistance blood vessels.
The diagnosis of orthostatic hypotension from autonomic failure is established by measuring the blood pressure in the supine and then in the standing position and noting a substantial drop accompanied by symptoms of dizziness or syncope. It should be emphasized that the bedside testing of orthostatic blood pressure is best performed by having the patient stand quickly and taking readings immediately and again at 1 min and at 3 min, rather than using the lying-sitting-standing sequence.
Orthostatic hypotension involves the failure to maintain blood pressure in the upright posture. The maintenance of blood pressure during various levels of activity and with postural changes depends on pressure-sensitive receptors (baroreceptors) in the aortic arch and carotid sinus and mechanoreceptors in the walls of the heart. These receptors, which are the sensory nerve endings of the glossopharyngeal and vagus nerves, send afferent impulses to the vasomotor centers in the medulla, more specifically the NTS. Axons from the NTS project to the reticular formation of the ventrolateral medulla, which in turn, sends fibers to the intermediolateral cell column of the spinal cord, thereby controlling vasomotor tone in skeletal muscles, skin, and the splanchnic bed. A diminution of sensory impulses from baroreceptors increases the flow of excitatory signals, which raise the blood pressure and cardiac output, thus restoring cerebral perfusion. This subject is discussed further in relation to the regulation of blood pressure in Chap. 25.
As described by Low and colleagues, postural orthostatic tachycardia syndrome (POTS) consists of intolerance of the standing position accompanied by tachycardia up to 120 beats per minute or more, but without orthostatic hypotension. Dyspnea, fatigue, and tremulousness and a complaint of “dizziness” accompany the assumption of an upright posture, and the same constellation of symptoms may be brought out by upright tilting. There is a frequent association with longer term fatigue and with exercise intolerance. The situation is comparable to orthostatic intolerance in the chronic fatigue and postviral syndromes, with which POTS shares many features. An impairment of cerebral autoregulation has been hypothesized; others consider the condition to be a limited form of dysautonomia. The component of the syndrome that simulates anxiety makes it difficult in some cases to differentiate the anticipation of symptoms from a genuine form of autonomic dysfunction.