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Objectives: Small-fiber polyneuropathy (SFPN) has various underlying causes, including associations with systemic autoimmune conditions. We have proposed a new cause; small-fiber-targeting autoimmune diseases akin to Guillain-Barre and chronic inflammatory demyelinating polyneuropathy (CIDP). There are no treatment studies  for this  autoimmune SFPN  but intravenous immunoglobulin (IVIg), first-line for Guillain-Barre and CIDP, is prescribed off-label for SFPN despite very high cost. This project aimed to conduct the first  evaluation of IVIg effectiveness for SFPN. Methods: With IRB approval, we extracted all available paper and electronic medical records of qualifying patients. Inclusion required having objectively confirmed SFPN, autoimmune attribution and other potential causes excluded. IVIg needed to have been dosed at 1 g/kg/4 weeks for 3 months. We chose two primary outcomes  changes in composite autonomic function testing (AFT) reports of SFPN and in ratings of pain severity  to capture objective as well as patient-prioritized outcomes. Results: Among all 55 eligible patients, SFPN had been confirmed by 3/3 nerve biopsies, 62% of skin biopsies, and 89% of composite AFT. Evidence of autoimmunity included 27% of patients having systemic autoimmune disorders, 20% having prior organ-specific autoimmune illnesses and 80% having 1/5 abnormal blood-test markers associated with autoimmunity. A total of 73% had apparent small-fiber-restricted autoimmunity. IVIg treatment duration averaged 28 -25 months. The proportion of AFTs interpreted as indicating SFPN dropped from 89% at baseline to 55% (p  0.001). Sweat production normalized (p = 0.039) and the other four domains all trended toward improvement. Among patients with pre-treatment pain 3/10, severity averaging 6.3 - 1.7 dropped to 5.2 - 2.1 (p = 0.007). Overall, 74% of patients rated themselves improved and their neurologists labeled 77% as IVIg responders; 16% entered remissions that were sustained after IVIg withdrawal. All adverse events were expected; most were typical infusion reactions. The two moderate complications (3.6%) were vein thromboses not requiring discontinuation. The one severe event (1.8%), hemolytic anemia, remitted after IVIg discontinuation. Conclusion: These results provide Class IV, real-world, proof-of-concept evidence suggesting that IVIg is safe and effective for rigorously selected SFPN patients with apparent autoimmune causality. They provide rationale for prospective trials, inform trial design and indirectly support the discovery of small-fiber-targeting autoimmune/inflammatory illnesses. Keywords: autoimmune diseases, dysautonomia, immunotherapy, intravenous immunoglobulin, neuropathic pain, peripheral nervous system diseases /p>

The polyneuropathies involve widespread damage to the body and peripheral nerves. Small-fiber polyneuropathy (SFPN), also known as small-fiber neuropathy, refers to those polyneuropathies that preferentially affect peripheral neurons with the thinnest axons, including the unmyelinated C-fibers, thinly myelinated A-δ somatosensory axons and the sympathetic and parasympathetic neurons. In the past, these were dichotomized as somatic versus autonomic, but immunohistochemical studies blurred the distinction, revealing non-sensory functions of somatosensory axons including innervation and control of sweating, small blood vessels and bone.1,2 Careful evaluation showed that most patients with somatosensory complaints such as neuropathic pain, itch or sensory loss also have autonomic involvement,3 hence the current tem small-fiber polyneuropathy. Applying the only population-based estimate, 52.95/100,0004 yields an estimated 2017 global prevalence approaching four million. This is an underestimate, since it required neurologists confirmation, whereas most patients remain undiagnosed. Given recent reports that SFPN underlies 40% of the fibromyalgia syndrome,5,6 there could there could conceivably be more than 100 million cases worldwide.

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Screening and diagnosis: Small Fiber Neuropathy can be tested by a skin biopsy.

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