Bedside Evaluation of the Autonomic System

While there are numerous historical clues in a variety of organ systems to suggest the presence or absence of autonomic dysfunction, bedside evaluation of the autonomic system is confined for the most part to careful study of the cardiovascular system. The most common and most prominent clinical feature of autonomic dysfunction is orthostatic hypotension. When severe, the fall in blood pressure with assumption of the upright posture is usually accompanied by dramatic symptoms of syncope or near syncope. Early in its course, however, the symptoms associated with orthostatic hypotension may be so vague as to be misleading. In these instances the postural hypotension associated with autonomic dysfunction would be overlooked unless the measurement of blood pressure in the supine and standing positions was recorded routinely in the course of the physical examination.Normally, assumption of the upright posture is accompanied by a slight decrease in systolic pressure, usually 10 mm Hg or less, and a slight increase in diastolic blood pressure, usually less than 5 mm Hg. These changes in blood pressure may be accompanied by a physiologic increase in heart rate of 10 beats per minute or less. Under certain circumstances, normal blood pressure response to standing may include a decrease in the systolic blood pressure of up to 25 mm Hg and an increase in diastolic pressure of as much as 10 mm Hg. Although there is no uniform consensus, most authors define orthostatic hypotension as a sustained decrease in systolic pressure of 30 mm Hg or more and a decrease in diastolic pressure of 15 mm Hg or more upon standing. Orthostatic hypotension is not synonymous with autonomic dysfunction, but the association of symptomatic decrease in blood pressure of the above magnitude without an accompanying increase in heart rate must suggest a disordered autonomic response.