Somatosensory evoked potentials associated with thermal activation of type II A delta mechanoheat nociceptive afferents

We evaluated evoked potentials (EPs) to noxious contact heat pulses delivered to hairy skin of healthy adults. Heat pulses from an adapting temperature of 34 degrees C to a target temperature of 52 degrees C, produced two scalp positive waves. The first peaked at 44 degrees to 45 degrees C (similar to 500 ms following stimulus onset), while the second peaked similar to 300 ms following the 52 degrees C heat pulse (similar to 1 s after stimulus onset). The first positive wave was absent from an adapting temperature of 39 degrees C, suggesting loss of synchronized activation of warm and/or low threshold mechanothermal afferents. The second EP was observed following stimulation from both adapting temperatures and was associated with subjective report of first pain. Latency difference of the pain EP from arm and leg were consistent with conduction in AS nociceptive afferents (similar to 10/ms). EPs to painful contact thermal stimuli may be of value in the evaluation of small fiber peripheral neuropathies and assessment of altered pain states.