The evaluation of small fiber function - autonomic and quantitative sensory testing

Disorders of the autonomic and small nerve fibers comprise a wide spectrum of disease states that cross a multitude of clinical specialties. The evaluation of these disorders allows the investigator to determine the presence or absence of abnormalities in the lightly myelinated and unmyelinated nerve fibers with minimal discomfort to the patient. The severity of dysfunction also can be determined to guide therapy and aid in prognostication for individual patients. The study of autonomic function is based on the understanding that it is a tightly regulated control system. Most tests are designed to disturb physiologic equilibrium to determine the autonomic response in a controlled setting. The autonomic assessments discussed in this article include evaluation of cardiovascular sympathetic and parasympathetic nervous system function and the tests of sudomotor function, as these are the most likely to be performed by neurologists [1,2]. Quantitative sensory testing, focusing on its use in the assessment of the small sensory nerve fibers, also is discussed [3,4]. Autonomic and quantitative sensory testing evaluates an aspect of the nervous system that cannot be investigated through standard neurophysiologic testing. Nerve conduction studies provide information on large myelinated nerve fibers in the peripheral nervous system. Autonomic and quantitative sensory testing provides information on unmyelinated and lightly myelinated nerve fibers in the autonomic and peripheral nervous systems [4].