Recent Advances in the Classification and Management of Neurocardiogenic Syncope and Related Disorders

Abstract

“Those who do not add to their learning, diminish it.”—The Talmud While a number of new and exciting developments in the ~eld of syncope have occurred in the last few years, there are several points of particular interest to the electrophysiologist. Over the last several years it has become apparent that sudden autonomically-mediated hypotension (with or without bradycardia) is an important cause of unexplained loss of consciousness, a phenomenon now known as neurocardiogenic (or vasovagal) syncope [1]. The development of head upright tilt table testing provided not only a way to induce and therefore diagnose the condition, but also allowed for detailed assessments of the pathophysiologic processes that were at work in this disorder. In the course of these investigations it also became increasingly apparent that neurocardiogenic syncope was but one of a group of related (yet different) disturbances in autonomic nervous system function associated with orthostatic intolerance [2]. More and more it is the electrophysiologist who is called upon to diagnose and treat these disorders, thus a working understanding of them is essential. While the system of classi~cation of autonomic nervous system disorders associated with orthostatic intolerance that has evolved is to some extent arbitrary (as is any such system) it is none the less an attempt to understand these disorders in an objective manner [3]. These disturbances are divided into four large groups: (1) Re_ex Syncope, (2) Postural Orthostatic Tachycardia Syndrome (or POTS), (3) Pure Autonomic Failure, (4) Multiple Systems Atrophy (Fig. 1). The re_ex syncopes are distinguished by a sudden rapid drop in blood pressure that is frequently accompanied (although not always) by a fall in heart rate. In between syncopal episodes patients have few other complaints [1]. These paroxysmal autonomic disturbances appear to re_ect a “hypersensitive” autonomic system that over responds to stimuli that are usually within the range of normal tolerance. A common mechanism between these disorders is that rapid activation of a bed of mechanoreceptors (in the heart, bladder, rectum, carotid body, etc.) results in a sudden surge in the amount of afferent normal activity to the brainstem [5]. This sudden increase in neural traf~c seems to mimic the conditions that are normally seen only during hypertension. This provokes an apparent “paradoxic” sympathetic withdrawal that leads to peripheral vasodilation and hypotension. This group of disorders includes not only neurocardiogenic syncope but also carotid sinus hypersensitivity, micturition syncope, and defecation syncope [6]. More detailed discussions of these disorders can be found elsewhere [1,7]. The Postural Orthostatic Tachycardia Syndrome (or POTS) is an increasing recognized cause of postural tachycardia, palpitations, lightheadedness, near syncope, exercise intolerance, cognitive impairment and lastly, chronic fatigue [8]. The hallmark of this disorder is an excessive increase in heart rate on standing or during tilt, that exceeds a 30 beat/minute increase during the ~rst ten minutes or a maximum heart rate of greater than 120 during the same time period [9]. These appears to be two distinct subgroups of this disorder. The most common is referred to as the “Partial Dysautonomic” form. This represents a failure of the peripheral vasculature to adequately vasoconstrict during upright posture. This is then compensated for by a large increase in heart rate and inotropy [10]. So opposed to “inappropriate” sinus tachycardia, the increase in heart rate is both appropriate and compensating in nature. This is an energy inef~cient form of compensation where the individual is “running in place” whenever he is sitting or standing. Standing for more than ~ve to ten minutes results in a bluish mottled appearance of the lower extremities. Many of these patients may be misdiagnosed as chronic fatigue syndrome. There also appears to be a central hyperadrenergic form of POTS, where there is excessive sympathetic activation in some neural distributions almost all the time. This appears to occur above the level of the barore_ex, and excessive sympathetic activity is not adequately dampened by the barore_ex. Although quite similar to the partial dysautonomic form of POTS, the hyperadrenergics have a high incidence of migraine-like headache and cold, sweaty extremities, and respond to different treatment modalities. The interested reader can ~nd further information elsewhere [10].