Experimental brush-evoked allodynia activates posterior parietal cortex

Objective: To study the brain activation pattern of coexisting experimental ongoing pain and brush-evoked allodynia (pain evoked by innocuous brush) w ith the use of PET. Background: Neuropathic pain usually has two essential phenomena: ongoing pain and brush-evoked allodynia, which coexist and may i nfluence each other. Capsaicin induces both ongoing pain and brush-evoked a llodynia. Methods: Eight healthy right-handed volunteers participated in ei ght (H2O)-O-15 PET scans with two blocks of four randomized conditions: 1) rest, 2) brush, 3) capsaicin pain, and 4) capsaicin pain + brush (brush-evo ked allodynia). Capsaicin was injected intradermally on the nondominant for earm and the subjects rated pain intensity and unpleasantness on 100-mm vis ual analogue scales. Results: Pain intensity and unpleasantness were signif icantly higher during brush-evoked allodynia (74 +/- 4 and 67 +/- 4 mm) com pared with capsaicin pain alone (60 +/- 4 and 51 +/- 5 mm). Brush-evoked al lodynia, but not capsaicin pain alone, increased blood flow significantly i n the contralateral right sensory association cortex Brodmann area (BA) 5/7 , and in bilateral prefrontal cortex BA 9/10/47 and insula. No significant activity was seen in thalamus or primary somatosensory cortex (SI). Direct comparison between capsaicin pain and brush-evoked allodynia revealed signi ficant increase in contralateral BA 5/7 only. Conclusions: The specific act ivation of contralateral BA 5/7 indicates that this brain region is importa nt to the processing of brush-evoked allodynia. The involvement of BA 5/7 i n brush-evoked allodynia is claimed to reflect multisensory input to this r egion, its role in conscious pain perception, and its neuroplastic properti es.