Microneurographic evaluation of sympathetic activity in small fiber neuropathy

ObjectiveSmall fiber neuropathy (SFN) may involve somatic and autonomic fibers. Assessment of somatic epidermal nerve fiber density (ENFs) is considered the gold standard test in the diagnosis of SFN. By contrast, autonomic involvement in SFN is more difficult to ascertain. Here we investigate peripheral sympathetic outflow by microneurography in patients with selective small nerve fiber involvement of different origin with and without autonomic symptoms to ascertain the ability of microneurography and the corresponding skin organ effector responses (sympathetic skin activity-SSR and skin vasomotor reflex-SVR) to disclose autonomic involvement.MethodsWe studied 59 patients with SFN because of reduced leg ENFs and normal conduction studies. Thirty patients reported only burning paresthesia (somatic SFN) whereas 29 patients complained of additional autonomic dysfunctions (autonomic SFN). They underwent microneurography from peroneal nerve with the recording of muscle sympathetic nerve activity (MSNA), skin sympathetic nerve activity (SSNA) and the corresponding SSR and SVR in the same innervation field. Thirty age and sex-matched healthy subjects served as controls.ResultsPatients with autonomic SFN mainly complained of loss of sweating. They showed a significant absence of indirect (SSR and SVR) and direct (MSNA and SSNA) sympathetic tests compared to somatic SFN patients and controls. SSNA, SSR and SVR were more often absent than MSNA. In addition, SSR and SVR were absent in all patients with no recordable SSNA but no significant relationship was found with MSNA recording.ConclusionsSSR and SVR, simple indirect tests of sympathetic activity, could help to disclose autonomic involvement in SFN with a good sensitivity mainly in patients with sweating dysfunctions although they expressed autonomic failure in only one sympathetic branch.SignificanceMicroneurographic evaluation of sympathetic activity, technically more difficult than indirect tests, was a useful functional tool contributing to the diagnosis and extension of autonomic involvement in SFN. Our data showed that the skin sympathetic branch is more often involved than the muscle sympathetic branch in SFN.

► SSR and SVR could help to disclose autonomic involvement in small fiber neuropathy. ► First study reporting microneurography, SSR and SVR in small fiber neuropathy. ► Microneurography showed different involvements of the sympathetic branches in SFN.