Background/Purpose: , Small Fiber Neuropathy (SFN) is a condition prevalent in patients with rheumatic diseases, particularly Fibromyalgia. The presence of neuropathic pain with normal nerve conduction studies raises the possibility of a neuropathy confined purely to the small unmyelinated nerve fibers: a small fiber neuropathy. Our clinical observations suggest small fiber neuropathy is an under-recognized but common manifestation of fibromyalgia that has important diagnostic significance. Testing for SFN assists providers in formulating specific diagnosis of symptoms in patients evaluated by rheumatologists. These small fiber neuropathies can be diagnosed through a 3-mm punch biopsy and may offer insight into the pathogenesis of some cases of fibromyalgia. Objective:, The purpose of this study is to discuss SFN, the importance of its association with rheumatologic conditions, diagnosis and treatment.

Methods: ,This was retrospective open label study carried out at two centers. 19 patients diagnosed with fibromyalgia according to American College of Rheumatology criteria underwent epidermal nerve fiber density to determine if the patient has SFN. A 3-mm punch biopsy samples were obtained from the right leg; 10 cm proximal to the lateral malleolus and 10 cm distal to the greater trochanter of the lateral upper thigh. Both specimens are placed in anatomically designated sample tubes. Biopsy samples were shipped to the Corinthian Reference Lab in Benbrook, Texas (with the exception of one sample sent to Therapath Neuropathology Laboratory in New York) where nerve fibers are manually counted and patient’s biopsy specimen are interpreted. Nerve fiber density is determined through pathological review.

Results: 19 patients were in the study, 16 females (84.2%) and 3 males (15.8%) underwent skin biopsies, every patient had been diagnosed with and being treated for fibromyalgia and complaining of neuropathy with normal nerve conduction test , 6 patients have Sjogren’s Syndrome (31.6%), 2 patients have Hashimoto’s thyroiditis (10.5%), 1 patient has sarcoidosis (5.3%), 6 patients have low vitamin D (31.6 %), 5 patients have low vitamin B12 (26.3%), 3 patients have Rheumatoid Arthritis (15.8%) and 3 have lupus (15.8%). Upper and lower normal values were provided specifically for each patient to which each patients nerve fiber density was compared to. 11 patients (57.9%) presented with significantly decreased upper and lower nerve fiber density. Only 1 patient (5.3%) had normal lower nerve fiber density values and 5 patients (26.3%) had a normal upper nerve fiber density value. Two patients (10.5%) had both normal upper and lower nerve fiber density.

Conclusion: Results obtained coincide with our clinical observations that SFN serves as a component in patients with rheumatologic conditions, specifically fibromyalgia. 17 out of 19 patients with fibromyalgia and neuropathic pain were found to have a small fiber neuropathy based on reduced epidermal nerve fiber density on a standard 3-mm punch biopsy. Impaired small fiber function in patients with fibromyalgia, pointing towards a neuropathic nature of pain in fibromyalgia. These results suggest testing of possible underlying conditions including connective tissue disease, glucose metabolism, vitamin D deficiency, B12 deficiency, sarcoidosis, and others is warranted. Further large prospective study is needed including treating underlying conditions and or other modalities including use of immune globulin. 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/small-fiber-neuropathy-in-rheumatology-clinics/