We included consecutive patients with probable or definite acute-onset SFN.

Diagnosis of SFN was based on the NEURODIAB criteria. Acute onset was considered when the maximum intensity and extension of both symptoms and signs were reached within 28 days. We performed the following investigations: clinical examination, neurophysiological assessment encompassing a nerve conduction study to rule out large fiber neuropathy, laserevoked potentials (LEPs), warm detection thresholds (WDT), and electrochemical skin conductance (ESC), epidermal nerve fiber density (ENF), and patient serum reactivity against mouse sciatic nerve teased fibers, mouse dorsal root ganglion (DRG) sections, and cultured DRG. The serum reactivity of healthy subjects (n=10) and diseased controls (n=12) was also analyzed. Data on baseline characteristics, biological investigations, and disease course were collected.

Results: Twenty patients presenting AOSFN were identified (60% women; median age: 44.2 years [IQR: 35.7-56.2]). SFN was definite in 18 patients (90%) and probable in two patients.

A precipitating event was present in 16 patients (80%). The median duration of the progression phase was 14 days [5-28]. Pain was present in 17 patients (85%). Twelve patients (60%) reported autonomic involvement. The clinical pattern was predominantly non-lengthdependent (85%). Diagnosis was confirmed by abnormal LEPs (60%), ENF (55%), WDT (39%), or ESC (31%). CSF analysis was normal in 5/5 patients. Anti-fibroblast growth factor 3 antibodies were positive in 4/18 patients (22%) and anti-contactin-associated protein-2 antibodies in one patient. In vitro studies showed IgG immunoreactivity against nerve tissue in 14 patients (70%), but not in healthy subjects or diseased controls. Patient serum antibodies bound to unmyelinated fibers, Schwann cells, juxtaparanodes, paranodes, or DRG. The patients' condition improved after a short course of oral corticosteroids (3/3). Thirteen patients (65%) showed partial or complete recovery. Others displayed relapses or a chronic course.

Discussion: AOSFN primarily presents as an acute, non-length-dependent, symmetric painful neuropathy with a variable disease course. An immune-mediated origin has been suggested, based on in vitro immunohistochemical studies.