[Small Fiber Neuropathy with Inadequate Response to Medical Therapy: Diagnosis of The Etiology of Small Fiber Neuropathy and Treatment Option].

Small-fiber neuropathy (SFN) has few significant laboratory findings and is difficult to diagnose. In 70% of the cases, the cause of SFN is unknown. Among the cases with known etiology, 50% are associated with diabetes, and the causes are autoimmune, amyloidosis, or multifactorial. In recent years, a specific autoantibody-positive group has been identified and has attracted attention because immunotherapy was successful in the autoantibody-positive SFN groups. In the cases reporting to our department, abnormalities could not be detected by various tests, including nerve conduction studies, and the response to symptomatic treatment was poor. An abnormality was identified in the current perception threshold test result, and a positive blood anti-plexin D1 antibody was detected via enzyme-linked immunosorbent assay. Therefore, autoimmune SFN was diagnosed, and plasma exchange therapy was remarkably effective. Subsequently, we aim to introduce general treatments for SFN and COVID-19-related SFN.