The Investigation of the Cardiovascular and Sudomotor Autonomic Nervous System-A Review

Abstract

The autonomic nervous system as operating system of the human organism permeats all organ systems with its pathways permeating that it is involved with virtually all diseases. Anatomically a central part, an afferent part and sympathetic and parasympathetic efferent system can be distinguished. Among the different functional subsystems of the autonomic nervous system, the cardiovascular autonomic nervous system is most frequently examined with easily recordable cardiovascular biosignals as heart rate and blood pressure. Although less widely established, sudomotor tests pose a useful supplement to cardiovascular autonomic assessment as impaired neurogenic sweating belongs to the earliest clinical signs of various autonomic neuropathies as well as neurodegenerative disorders and significantly reduces quality of life. Clinically at first, the autonomic nervous system is assessed with a detailed history of clinical autonomic function and a general clinical examination. As a lof of confounding factors can influence autonomic testing, subjects should be adequately prepared in a standardized way. Autonomic testing is usually performed in that way that the response of the autonomic nervous system to a well-defined challenge is recorded. As no single cardiovascular autonomic test is sufficiently reliable, it is recommended to use a combination of different approaches, an autonomic test battery including test to measure parasympathetic and sympathetic cardiovascular function (deep breathing test, Valsalva maneuver, tilt, or pressor test). More specialized tests include carotid sinus massage, assessment of baroreceptor reflex function, pharmacological tests or cardiac, and regional hemodynamic measurements. Techniques to measure functional integrity of sudomotor nerves include the quantitative sudomotor axon reflex sweat test, analysis of the sympathetic skin response as well as the thermoregulatory sweat test. In addition to these rather established techniques more recent developments have been introduced to reduce technical demands and interindividual variability such as the quantitative direct and indirect axon reflex testing or sudoscan. However, diagnostic accuracy of these tests remains to be determined. We reviewed the current literature on currently available autonomic cardiovascular and sudomotor tests with a focus on their physiological and technical mechanisms as well as their diagnostic value in the scientific and clinical setting.

Keywords: autonomic nervous system; axon reflex; heart rate variability; laboratory evaluation of cardiovascular function; orthostatic tests; sudomotor function; sympathetic; valsalva maneuver.

Figure 1

Direct and axon reflex mediated sweating in sudomotor nerve fibers. While direct sweating occurs in the akin area of iontophoretic application of acetylcholine, indirect sweating is evoked by an axon reflex in adjacent sweat glands. The axon reflex response can be assessed to study functional integrity of the sympathetic C fiber which mediates the reflex.

Figure 2

Patient with left hemispheric acute ischemic stroke showing abolished sympathetic skin response on the right hand and normal responsitivity on the left hand. The graphs show skin conductance levels (micro Sievert) over time after sympathetic stimulation.