Neurophysiological studies of pain pathways in peripheral and central nervous system disorders

Standard clinical neurophysiological assessment of somatosensory pathways by sensory evoked potentials (SEPs) is limited to the tactile and proprioceptive systems consisting of large fibers in the peripheral nerve, the dorsal columns of the spinal cord and the medial lemniscus in the brainstem. This limitation means that about half of the lesions in the somatosensory system will not be detectable. In recent years, many clinical studies have confirmed that laser evoked potentials (LEPs) allow the assessment of the other half of the somatosensory system. Rapid heating of the skin by infrared laser pulses specifically activates the nociceptive and thermoreceptive pathways consisting of small fibers in the peripheral nerve and the anterolateral spinothalamic tract in the spinal cord and brainstem. Owing to the large degree of convergence of the somatosensory pathways on to common thalamic nuclei, the differential use of LEP vs. SEP is less evident for thalamocortical lesions. In contrast to standard SEPs, the LEP technique can be applied to non-glabrous skin in any dermatome. This review summarizes the principles of clinical neurophysiological studies of pain pathways and the findings obtained in patients with peripheral and central nervous system disorders. These data provide a rational basis for developing clinical indications for LEP testing.