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.
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%).
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.
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.
© 2012 Scandinavian Association for the Study of Pain