ASSESSMENT OF NOCICEPTIVE PATHWAYS WITH LASER-EVOKED POTENTIALS IN NORMAL SUBJECTS AND PATIENTS

This chapter reviews recent approaches to studying nociceptive pathways and pain perception in humans through the use of laser-evoked cerebral potentials (LEPs). In this technique an infrared laser generates brief radiant heat pulses which, within a few milliseconds, selectively activate thin afferent nerve fibers (A delta- and C-fibers) that project centrally via the spinothalamic tract of the spinal cord. A delta-fiber activation leads to LEPs consisting of a middle-latency component (N170), possibly generated near the secondary somatosensory area, and late components (N250, P390) that are maximal near the vertex. C-fiber activation leads to an ultra-late potential (P1300) near the vertex. Late LEPs are used clinically to document impairment of nociceptive pathways at the peripheral, spinal, and brain-stem levels. Ultra-late potentials appear when late potentials are diminished by experimental nerve block or disease. Ultra-late LEPs may thus give indirect evidence for selective loss of A delta-fiber afferents versus complete loss of all nociceptive afferents. When heat-pain sensitivity is pathologically increased, the amplitudes of late LEPs may also be increased.