The effect of nerve compression and capsaicin on contact heat evoked potentials (CHEPs) related to Aδ and C fibers.

Background/aims Brief noxious heat stimuli activate Aδ and C fibers, and contact heat evoked potentials (CHEPs) can be recorded from the scalp. Under standard conditions, late responses related to AS fibers can be recorded. This study examines C-fiber responses to contact heat stimuli. Methods A preferential A-fiber blockade by compression to the superficial radial nerve was applied in 22 healthy subjects. Quality and intensity of heat evoked pain (NRS, 0-10), and CHEPs were examined at baseline, during nerve compression, and during further nerve compression with topical capsaicin (5%). Results During the A-fiber blockade, 3 subjects had CHEPs with latencies below 400 ms, 8 subjects within 400-800 ms and 6 subjects later than 800 ms. Pain intensity to contact heat stimuli was reduced and fewer subjects reported the heat stimuli as stinging. Following acute capsaicin application, ultralate CHEPs with latencies >800 ms could be recorded in 13 subjects, pain intensity to the contact heat stimuli was increased (p <0.01) and more subjects reported the heat stimuli as being more warm/hot-burning. Conclusion The results indicate that following a compression to the superficial radial nerve, CHEPs compatible within complete A fibers or C fibers were recorded. Following sensitization with capsaicin, C-fiber responses were recorded in 62% of subjects.