The autonomic nervous system (ANS) mediates neuroendocrine, visceral, and behavioral responses to environmental and inner challenges in order to ensure body homeostasis. The insular cortex, together with the medial prefrontal cortex and the extended amygdala, exerts high-order autonomic control on multiple parallel interconnections of the ANS subcortical integrative network constituted from hypothalamus, periaqueductal grey matter, parabrachial pontine complex, and ventro-lateral medullary nuclei (Benarroch 1993). Main output of the central ANS runs through pre-ganglionic sympathetic and parasympathetic cholinergic neurons, the former located in the intermediolateral columns of the thoraco-lumbar spinal cord, the latter in the brainstem and sacral spinal metameria. Parasympathetic cholinergic post-ganglionic fibers arise from ganglia which are generally located close to or within the target organ. Regional activation of parasympathetic fibers induces myosis, promotes salivation, intestinal peristalsis, vesical detrusor contraction, penis (clitoral) engorgement, and decreased heart rate. Noradrenergic sympathetic post-ganglionic neurons originate from the paravertebral ganglia and mediate midriasis, blood vessel constriction, and inotropic and cronotropic cardiac effects, among others. Further, a subgroup of cholinergic sympathetic fibers runs stimulation to cutaneous sweat glands (Goldstein 2006).
Disorders of the ANS are described as primary, if displaying a neurodegenerative origin, or secondary, if developing after brain injury of any cause or metabolic diseases like diabetes. The first report of degenerative dysautonomia dates back to 1925 when Bradbury and Egglestone described a case of orthostatic hypotension (OH) with impotence and anhidrosis of idiopathic origin (Bradbury et al. 1925). In the 1960s Shy and Drager further reported the case of two men who developed multi-domain autonomic failure along with cerebellar, parkinsonian, and pyramidal signs (Shy et al. 1960). Since then, dysautonomia (see Table 24.1) has been recognized as a major non-motor feature of multiple system atrophy (MSA), Parkinson’s disease (PD), as well as other movement disorders with prominent diagnostic, therapeutic, and prognostic implications.