Microneurographic recording from unmyelinated nerve fibers in neurological disorders: an update.
Microneurography is a unique neurophysiological technique allowing direct recording of unmyelinated postganglionic sympathetic or afferent nociceptive fibers by tungsten needles inserted into a peripheral nerve fascicle. In recent years, microneurography has been used to ascertain autonomic impairments in central neurological disorders such as sleep disorders, Parkinson's disease, amyotrophic lateral sclerosis, or vasovagal syncope. Abnormal resting muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA) or the abnormal sympathetic response to arousal have been described in these disorders, thereby clarifying important pathophysiological aspects of the underlying impairment. In addition, microneurography was also recently used to demonstrate absent or decreased sympathetic outflow in diseases affecting the peripheral nervous system such as Ross syndrome, pure autonomic failure, and small-fiber neuropathy. Microneurography has also been used to study nociceptor outflow in pain disorders affecting the peripheral nervous system such as small-fiber neuropathy, diabetic neuropathy, erythromelalgia, complex regional pain syndrome, and fibromyalgia. In these disorders, microneurography mainly documented mechano-insensitive C-nociceptor hyperexcitability that might account for the ongoing pain.