Continuum, Autonomic Disorders, December 2007, Volume 13, Issue 6

The autonomic nervous system is the critical component of a central network involved in homeostasis and adaptation. It regulates arterial blood pressure and regional blood flow in response to metabolic demands in underlying tissues, thermoregulation, motility and secretion of the gastrointestinal and respiratory tracts, micturition, and sexual function. The autonomic system consists of three subdivisions: the sympathetic, parasympathetic, and enteric nervous systems.The sympathetic and parasympathetic systems each have a central preganglionic neuron in the brain stem or spinal cord and a peripheral neuron in the autonomic ganglia. The enteric nervous system consists of neurons located in ganglia within the walls of the gut. A major component of the autonomic control systems consists of visceral afferent pathways. These pathways convey signals from the periphery that trigger visceral reflexes, transmit visceral pain, and regulate visceral function via antidromic release of neurochemical signals. The central control of autonomic function depends on a neuronal network distributed throughout the neuraxis. These neurons receive numerous afferent inputs and integrate this information according to the type of stimulus and current behavioral state. After these converging inputs have been evaluated, a specific pattern of autonomic outflow is relayed to the periphery. The central autonomic network consists of neurons in the insular and anterior cingulate cortex, amygdala, hypothalamus, periaqueductal gray, parabrachial nucleus, nucleus of the solitary tract, ventrolateral reticular formation, and medullary raphe.(C) 2007 American Academy of Neurology

Orthostatic intolerance with orthostatic hypotension and syncope are disabling features of patients with disorders of autonomic cardiovascular control. The hallmark of both central and peripheral autonomic disorders is the failure of the sympathetic postganglionic neurons to release norepinephrine appropriately upon standing. Impaired norepinephrine release is permanent in patients with autonomic failure, and upon standing, blood pressure always falls. On the other hand, in patients with neurally mediated syncopal syndromes (also known as vasovagal, vasodepressor, or reflex syncope) impaired norepinephrine release occurs episodically, typically in response to a trigger. Between syncopal episodes, patients with neurally mediated syncope usually have normal blood pressure and orthostatic tolerance. Orthostatic intolerance without a fall in blood pressure, but with a pronounced increase in heart rate, occurs in the postural tachycardia syndrome, a puzzling disorder with several possible causes characterized by excessive sympathetic activation in response to physiologic stimuli. The distinction between these disorders is important because their prognosis and management is different. Autonomic failure can be severely disabling, while neurally mediated syncopal syndromes and the postural tachycardia syndrome are always benign. Patient education is key to managing these disorders. Several simple measures should be implemented to improve orthostatic tolerance prior to pharmacologic intervention.(C) 2007 American Academy of Neurology

Autonomic failure is a frequent feature of two types of neurodegenerative disorders-multiple system atrophy and the Lewy body syndromes, which include Parkinson's disease, pure autonomic failure, and dementia with Lewy bodies. These disorders are known collectively as synucleinopathies because accumulations of the protein [alpha]-synuclein are found intracellularly in the brains of affected patients. Other neurodegenerative disorders, including the amyloidopathies or tauopathies (eg, Alzheimer's disease, progressive supranuclear palsy, frontotemporal dementia, sporadic and inherited ataxias, and prion diseases), only rarely entail clinically significant autonomic failure.(C) 2007 American Academy of Neurology

The highly distributed nature of the neural control of the bladder and genitalia means that disorders of these functions are very likely to occur with many different neurologic diseases, although the range of symptoms is limited. In this chapter the clinical presentations and recommended management of urogenital symptoms in three conditions are described: multiple sclerosis, the commonest cause of neurogenic urogenital dysfunction; multiple system atrophy, a much less common condition, but one in which urogenital dysfunction occurs early; and a disorder of voiding, which affects neurologically healthy women, now referred to as "Fowler's syndrome."(C) 2007 American Academy of Neurology

In addition to the autonomic motor system, well-defined visceral sensory systems are associated with both the parasympathetic and sympathetic limbs of the autonomic system. Parasympathetic afferents enter the brain mainly through the cranial nerves and control many autonomic reflexes as well as provide visceral sensory input to the forebrain, which may be important in taste as well as behavioral and emotional response. Disorders of the peripheral parasympathetic afferent system produce loss of taste sensation, glossopharyngeal neuralgia, and the syndrome of inappropriate secretion of antidiuretic hormone. Sympathetic afferents convey pain and other tissue injury signals. These signals control spinal autonomic reflexes and converge with the parasympathetic system in the medulla to influence many brain stem autonomic reflexes as well. Visceral pain is relayed along with parasympathetic modalities to the insular cortex, where it may influence emotional and behavioral state. Current studies are investigating treatment of visceral pain and other internal body states, such as craving for nicotine, by disrupting signaling in the insular cortex.(C) 2007 American Academy of Neurology