Cool-evoked potentials as a new method to examine somatosensory pathways

Perception of pricking pain and of non-painful cold stimuli plays an important role in quantitative sensory testing (QST) as parameters for A-delta-fiber function. Since QST depends on active participation of the patient, objective tests for A-delta-fiber function are also needed. Whereas laser-evoked potentials (LEP) have been validated as objective test to assess the pathways mediating pricking pain, no such test exists for the thermoreceptive pathways for cooling.

This study used evoked potentials (EPs) recorded from 7 scalp electrodes elicited by a contact-thermode stimulator (CHEPS, Medoc Ltd.) and subjective intensity ratings. Cool stimuli (by 3°C at nominally 40°/s) were directed bilaterally to the hand dorsum of 12 healthy volunteers from a baseline temperature of 35°C. Heat pain was induced by increasing stimulus temperature to 51°C by 70°C/s. 16 blocks of 20 stimuli each (ISI 8–10s) were performed alternating blocks of heat and cool stimuli. Following each heat stimulus volunteers rated subjective pain on a numeric rating scale (NRS; 0=“not painful“ and 100=“most intense pain imaginable“) subjective coolness was also rated (0=“not cool at all“ and 100=“as intense cool as imaginable“).

Mean pain ratings to 51°C were 23.1±6.3. Cooling was not perceived as painful and mean coolness ratings were 3.4±3.0. Preceded by a small negativity (N1), the evoked potential for both stimuli consisted of a vertex negativity (N2) followed by a positivity (P2). Latencies were shorter with cooling than with heat stimuli (N2 323 vs. 397ms, t-test; p<0.01; P2 457 vs. 529ms p<0.001). Peak amplitudes did not differ between heat and cool evoked potentials (cool 15.99±2.06µV; heat pain 19.17±2.27µV; p>0.05). Pain ratings for noxious heat stimuli steeply decreased initially (tau1: 15s) and levelled out thereafter (tau2: 88s). Cool ratings dropped very slowly (tau ges. >2000s).

The data indicate that both A-delta-fiber stimuli elicit similar vertex potentials. The shorter latency to cooling is probably due to peripheral temperature conduction due to the smaller size of the temperature step. Peripheral adaptation to cool stimuli is less pronounced than to noxious heat. Cool-EPs could be thus comfortably used as objective method to examine somatosensory pathways. Similarities and/or differences between the parts of the nociceptive system processing painful heat stimuli and for cold perception might be of use for clinical diagnostics.

Supported by DFG Tr236/11–3 and 13–3.