Cold-induced pain and prickle in the glabrous and hairy skin

Little is known concerning the mechanisms underlying the perception of cold pain in humans. An appreciation of these mechanisms is important to understand and possibly treat those disorders in which cold stimuli evoke unpleasant sensations. To study cold pain, I have conducted psychophysical experiments on 16 healthy subjects. A peltier-type stimulator (20 x 25 mm) was used to deliver stimuli to sites on the thenar eminence (glabrous skin) and volar forearm (hairy skin) of each arm. Each trial consisted of a 90 s, 2 degrees C stimulus that was preceded and followed by a 35 degrees C stimulus. A computer-based visual analog scale was used to collect continuous pain ratings throughout each trial. In experiment 1, nine subjects rated the overall evoked pain intensity (four trials/skin type) and the prickle component (four trials/skin type). Typically, subjects perceived the cold-evoked pain as prickly, cold/freezing and achy. The pain intensity and quality was similar for glabrous and hairy skin sites within individual subjects. Pain intensity gradually rose to a plateau by approximately 60 s into each trial. The prickle component differed amongst subjects due to its variable time course. Subjects consistently reported an intense, brief jab of prickle at both hairy and glabrous sites during the rewarming phase. In experiment 2, nine subjects rated the pain intensity during the cold stimulus before and during a compression-ischemic block of Abeta/Adelta fiber conduction. The dominant sensation evoked by the cold stimulus in the hairy and glabrous skin during the block was a sharp, hot/burning pain. The block did not consistently affect the total pain at the hairy sites. However, most subjects reported more pain during the block at the glabrous sites. These data suggest that noxious cold stimuli affect a mosaic of primary afferent input and central processing resulting in a complex pain experience which may differ in glabrous and hairy skin.