CEREBRAL PROCESSING OF ACUTE SKIN AND MUSCLE PAIN IN HUMANS

The human cerebral processing of noxious input from skin and muscle wa s compared with the use of positron emission tomography with intraveno us (H2O)-O-15 to detect changes in regional cerebral blood flow (rCBF) as an indicator of neuronal activity. During each of eight scans, 11 normal subjects rated the intensity of stimuli delivered to the nondom inant (left) forearm on a scale ranging from 0 to 100 With 70 as pain threshold. Cutaneous pain was produced with a high-energy CO2 laser st imulator. Muscle pain was elicited with high-intensity intramuscular e lectrical stimulation. The mean ratings of perceived intensity for inn ocuous and noxious stimulation were 32.6 +/- 4.5 (SE) and 78.4 +/- 1.7 for cutaneous stimulation and 15.4 +/- 4.2 and 73.5 +/- 1.4 for intra muscular stimulation. The pain intensity ratings and the differences b etween noxious and innocuous ratings were similar for cutaneous and in tramuscular stimuli (P > 0.05). After stereotactic registration, stati stical pixel-by-pixel summation (Z score) and volumes-of-interest (VOI ) analyses of subtraction images were performed. Significant increases in rCBF to both noxious cutaneous and intramuscular stimulation were found in the contralateral secondary somatosensory cortex (SII) and In ferior parietal lobule [Brodmann area (BA) 40]. Comparable levels of r CBF increase were found in the contralateral anterior insular cortex, thalamus, and ipsilateral cerebellum. Noxious cutaneous stimulation ca used significant activation in the contralateral lateral prefrontal co rtex (BA 10/ 46) and ipsilateral premotor cortex (BA 4/6), Noxious Int ramuscular stimulation evoked rCBF increases in the contralateral ante rior cingulate cortex (BA 24) and subsignificant responses in the cont ralateral primary sensorimotor cortex (MI/SI) and lenticular nucleus. These activated cerebral structures may represent those recruited earl y in nociceptive processing because both forms of stimuli were near pa in threshold. Correlation analyses showed a negative relationship betw een changes in rCBF for thalamus and MI/SI for cutaneous stimulation, and positive relationships between thalamus and anterior insula for bo th stimulus modalities. Direct statistical comparisons between innocuo us cutaneous and intramuscular stimulation with the use of Z scores an d VOI analyses showed no reliable differences between these two forms of noxious stimulation, indicating a substantial overlap in brain acti vation pattern. The comparison of noxious cutaneous and intramuscular stimulation indicated more activation in the premotor cortex, SII, and prefrontal cortex with cutaneous stimulation, but these differences d id not reach statistical significance. The similar cerebral activation patterns suggest that the perceived differences between acute skin an d muscle pain are mediated by differences in the intensity and temporo spatial pattern of neuronal activity within similar sets of forebrain structures.