A review of the classification, diagnosis, and management of autonomic dysfunction syndromes associated with orthostatic intolerance

Abstract

Syncope, the transient loss of consciousness and postural tone, is one of the oldest of recorded medical problems. Indeed, Hippocrates, the father of medicine, recorded the first description of syncope, and it is from the Greek that we derive the medical term for fainting (syncoptein -to cut short). Both a sign and a syndrome, syncope may result from a wide variety of causes. Over the last decade, considerable attention has been focused on one particular cause of syncope, the phenomenon previously called vasovagal syncope. Research into the nature of this disorder has demonstrated that it is only one aspect of a much broader group of disturbances of the autonomic nervous system that may lead to hypotension, orthostatic intolerance, and ultimately syncope. Indeed, recent discoveries have caused us to reevaluate our entire classification of autonomic disorders, and to develop a new system that better reflects our current knowledge. Because it is the cardiologist and the clinical cardiac electrophysiologist who are now frequently called upon to recognize and treat these disorders, this review is designed to acquaint the reader with these conditions, their diagnosis, and management. The autonomic nervous systemBecause these nervous disorders all result from a disturbance in normal autonomic function, it would seem appropriate to briefly review some aspects of the structure and operation of the nervous system.The human nervous system has two basic components: the central nervous system, made up of the brain and the spinal cord 1 , and the peripheral nervous system, which is comprises groups of neurons called ganglia, and of peripheral nerves that lie outside the brain and spinal cord. Although anatomically separate, the two systems are functionally interconnected. The peripheral nervous system is further divided into somatic and autonomic divisions. The somatic division is principally concerned with sensory information about the environment outside the body as well as muscle and limb position. The autonomic division (usually called the autonomic nervous system or ANS) is the motor system for the viscera, the smooth muscles of the body (especially those of the vasculature), and the exocrine glands. It is composed of three distinct parts: the sympathetic, parasympathetic, and enteric nervous systems. The sympathetic nervous system helps control the reaction of the body to stress, and the parasympathetic system works to conserve the body's resources and to restore equilibrium to the resting state. The enteric system controls the function of the gut. The organ systems governed by the ANS are, for the most part, independent of volitional control (although they sometimes can be affected by volitional or emotional inputs) and include the cardiorespiratory organs, the gastrointestinal and genitourinary tracts. The autonomic system is vital to the maintenance of internal homeostasis and achieves this by mechanisms that regulate blood pressure, fluid and electrolyte balance, and body temperature.Although representative of one ...

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