Links between Autonomic Dysfunction and Metabolic Syndrome

The autonomic nervous system (ANS) plays a key role in the control of a number of vital functions including cardiovascular, endocrine/neurovascular, gastrointestinal, genitourinary, pupil and thermoregulatory functions. Its abnormalities have been associated with early mortality, sudden death, silent myocardial infarction, gastrointestinal diseases. The manifestation of the ANS dysfunction in several human diseases is underestimated. Evidences exist on the important role of ANS dysfunctions in different clinically relevant conditions, including diabetes mellitus, chronic functional constipation, scleroderma, thalassemia major. Beside the classical evaluation in patients with diabetes mellitus, little is known about the effects of metabolic factors on ANS dysfunction. The metabolic syndrome (MetS) includes a cluster of frequent abnormalities (impaired fasting glycaemia, dyslipidemia, arterial hypertension and increased visceral adiposity) predisposing to the atherosclerotic changes and increased cardiovascular mortality. Early signs of autonomic dysfunction are often found in subjects with MetS even in the absence of diabetes. Epidemiological studies demonstrated that diabetics display a cardiovascular risk which is twice that of sexand age-matched non-diabetic population. Manifestations of such a high cardiovascular risk of subjects with DM are the frequent silent myocardial infarctions (MI)s of diabetics which are often due to impaired cardiovascular autonomic function. Only recently major attention has been given to the interactions between impaired glucose tolerance (IGT) and cardiovascular autonomic dysfunctions. When increased waist circumference (one of the features of the MetS) and IGT are both present, cardiovascular autonomic dysfunction also occurs. Some adipokines (e.g. adiponectin) seem to play a role in cardiovascular risk and autonomic dysfunction. This review will therefore focus on some subtle aspects linking ANS dysfunction and MetS.