[Laboratory evaluation of autonomic function].

The sympathetic and parasympathetic systems both contain efferent and afferent neurons. The efferent pathway consists of preganglionic neurons with myelinated axons the cell bodies of which are within the central nervous system and postganglionic neurons with unmyelinated axons the cell bodies of which are peripheral. The preganglionic sympathetic fibers arise in the intermediolateral column of the thoracic and upper lumber cord segments, and reach the sympathetic chain whence postganglionic fibers are distributed to the whole body, where they have particular influence on the control of blood pressure, heart rate, swating and pupillary responses. The parasympathetic outflow is grouped into both brainstem nuclear complex and spinal sacral segments. The activities of the sympathetic and parasympathetic systems are partially reflex in nature, which, however, are controlled by numerous descending influences integrated in the hypothalamus and reticular formation. Orthostatic hypotension is the most dramatic circulatory expression of autonomic failure. As one stands, the pull of gravity pools blood in distensible veins below the heart. Diminished venous return reduces cardiac output and unless autonomic compensatory mechanism-baroreflex function performs, subsequent blood pressure fall occurs. This hemodynamic responses to upright posture can be evaluated by the head-up tilt test. More than a 30-mmHg fall of the systolic blood pressure is thought to be an abnormal response; orthostatic hypotension. The next test to study the orthostatic hypotension is to evaluate the site of the baroreflex arc lesions. Plasma noradrenaline (NA) and vasopressin (AVP) responses to the head-up tilt test are available to detect the site of the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)