Painful peripheral neuropathies assessed by conventional electrophysiology and laser evoked potentials

Diffuse peripheral neuropathy (polyneuropathy) can be suspected in case of distal, symmetric pain affecting the lower limbs. Burning feet is usually considered a sign of small-fiber neuropathy, but this statement is probably wrong. In fact, patients with painful feet are heterogeneous, presenting both large and small fiber sensory neuropathies. Various neurophysiological methods can be applied to characterize painful neuropathies. Neurophysiological testing includes: 1) conventional electroneuromyographic assessment, comprising nerve conduction studies and needle electromyography; 2) H-reflex recording to assess proprioceptive fibers; 3) sympathetic skin reflex (SSR) recording to assess autonomic fibers; 4) quantitative sensory testing (QST) to mechanical or thermal stimuli; 5) somatosensory evoked potentials to electrical (SSEP) or laser (LEP) stimulation. An electrophysiological battery, comprising thermal QST, SSR and LEP recordings, can be used to assess small nerve fibers. An electrophysiological battery, comprising mechanical QST, H-reflex and SSEP recordings, can be used to assess large nerve fibers. The comparative analysis of results provided by these two batteries of investigation could help determine pain mechanisms involved in painful neuropathies. However, conventional electroneuromyographic assessment remains the first-line test in all cases. In the present text, the various neurophysiological methods for the evaluation of peripheral neuropathy are presented with particular emphasis on the value of LEP recordings in the diagnosis of small-fiber neuropathies.