How is small fiber neuropathy different from painful distal symmetric polyneuropathy?
Small fiber neuropathy is a term reserved for the circumstance in which distal symmetric painful polyneuropathy is largely restricted to small myelinated and unmyelinated axons. It is often the presenting form of painful polyneuropathy, with involvement of large myelinated axons developing later. Clinically it presents with painful paresthesias and reduced sharp perception distally, as described previously. Because by definition large myelinated axons are normal, NCS, vibration perception, reflexes, and strength are normal in small fiber neuropathy. The clinical diagnosis of small fiber neuropathy can be supported by a demonstrated reduction in cutaneous axons via a skin biopsy. While neurophysiologic tests of small somatic fibers are not available, several tests of sudomotor function can be utilized to support the clinical diagnosis.