A Proposed Taxonomy of Isolated Small Fiber Neuropathy

The term small fiber neuropathy (SFN) is used to define a group of heterogeneous disorders that affect pe-ripheral nerves and cause structural injury of myelinated Aδ-fibers and unmyelinated C-fibers.1 In the somatosen-sory nervous system, these fibers control information about temperature, pain, and itch, and in the autonomic nervous system, they mediate thermoregulatory, cardiovascular, and gastrointestinal autonomic functions.2 Patients can have a peripheral neuropathy with predominant small fiber symp-toms such as pain, burning, and numbness, but on exam or electrophysiology one finds evidence for damage to medi-um and large fibers such as decreased reflexes or abnormal nerve conduction studies. These patients are considered to have mixed fiber neuropathies.3,4 There is a second popula-tion of patients who have small fiber neuropathic symptoms but no evidence for mixed fiber involvement on exam or electrophysiology. This second group of patients is diag-nosed objectively by demonstrating reduced intraepider-mal nerve fiber density on skin biopsy, and these patients should be considered to have isolated small fiber neuropa-thy (ISFN).

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