Neurophysiological studies of thin myelinated (A delta) and unmyelinated (C) fibers: application to peripheral neuropathies

Dysfunction of small fibers may appear in isolation or associated with larg e fiber lesions. In some acute neuropathies, such as pandysautonomia, small -fiber impairment is relatively pure but it may also appear in disorders wi th prominent somatic damage, such as Guillain-Barre syndrome, in which auto nomic failure worsens the prognosis. At the present time, chronic idiopathi c distal small-fiber neuropathy is diagnosed more frequently, and in some p revalent disorders, such as diabetic or amyloidotic polyneuropathies, small -fiber dysfunction is very noticeable. In pure autonomic failure, a periphe ral autonomic failure exists, distinguishing it from multiple-system atroph y. Complex regional pain syndrome is a severe condition in which small fibe rs are responsible for disabling signs and symptoms, and only instrumental recordings lead to the proper treatment. Standard neurophysiological techni ques evaluate large myelinated fibers exclusively. Small-fiber polyneuropat hy has been considered as a type of somatic neuropathy, but thin myelinated and unmyelinated fibers are responsible not only for temperature and pain perception but also autonomic function. For instance, full autonomic evalua tion is needed in some clinical situations such as autonomic failure in the elderly or orthostatic intolerance syndrome. To evaluate small-fiber impai rment we need a battery of sensitive, reproducible, specific and noninvasiv e tests covering somatic and autonomic systems. In this review, we describe and analyze a number of neurophysiological techniques used to diagnose and characterize small-fiber dysfunction in humans. These include cardiovascul ar monitoring, sudomotor testing, pupillary responses and quantitative sens ory tests, and also to some extent thermography and laser evoked potentials . The use of such techniques has proven useful not only for diagnosis, but also to guide adequate therapy and optimize follow-up. (C) 2000 Editions sc ientifiques et medicales Elsevier SAS.