Figure 1. Primary hyperalgesia (right, y-axis); i.e., mechanical pain detection threshold to von Frey pinprick, in the area of the injury and secondary hyperalgesia (left, y-axis), i.e., areas of hyperalgesia in von Frey pinprick (175 mN) outside the injuries, after thermal injury in volunteers. Dotted line = legs with nerve block; full-drawn line = unblocked legs. Primary hyperalgesia in blocked legs (closed triangle); primary hyperalgesia in unblocked legs (closed diamond); secondary hyperalgesia in blocked legs (inverted closed triangle); secondary hyperalgesia in unblocked legs (closed square). Arrows indicate return of cold sensation. In diagrams with two arrows, the first arrow indicates return of cold sensation in the injury and the second indicates that cold sensation has returned all over the leg. One arrow indicates that the two events occurred at the same time. The number in each diagram identifies the subject. Subjects 1 and 3 were excluded because of pain during induction of burn injury (marked with asterisk), whereas subjects 5, 13, 14, and 15 were excluded because of prolonged block. Von Frey bristle number (1-10) represents a rank scale, where 1 indicates a force of about 3 mN and 9 about 175 mN, whereas 10 indicates that 175 mN did not produce pain or discomfort. The steps from 1 to 9 are logarithmic increases in force (see Methods). Values are medians of three observations. Time 0 h indicates measurements before thermal injury. Comparison of weighted average after return of cold sensation, reveals a significant difference between primary hyperalgesia in blocked versus unblocked legs (P = 0.005; Wilcoxon test, n = 14) and significant difference in secondary hyperalgesia (P = 0.01; Wilcoxon test, n = 14).