A PET activation study of dynamic mechanical allodynia in patients with mononeuropathy

The objective of this study was to investigate the central processing of dy namic mechanical allodynia in patients with mononeuropathy. Regional cerebr al blood flow, as an indicator of neuronal activity, was measured with posi tron emission tomography. Paired comparisons were made between three differ ent states; rest, allodynia during brushing the painful skin area, and brus hing of the homologous contralateral area. Bilateral activations were obser ved in the primary somatosensory cortex (S1) and the secondary somatosensor y cortex (S2) during allodynia compared to rest. The S1 activation contrala teral to the site of the stimulus was more expressed during allodynia than during innocuous touch. Significant activations of the contralateral poster ior parietal cortex, the periaqueductal gray (PAG), the thalamus bilaterall y and motor areas were also observed in the allodynic state compared to bot h non-allodynic states. In the anterior cingulate cortex (ACC) there was on ly a suggested activation when the allodynic state was compared with the no n-allodynic states. In order to account for the individual variability in t he intensity of allodynia and ongoing spontaneous pain, rCBF was regressed on the individually reported pain intensity, and significant covariations w ere observed in the ACC and the right anterior insula. Significantly decrea sed regional blood flow was observed bilaterally in the medial and lateral temporal lobe as well as in the occipital and posterior cingulate cortices when the allodynic state was compared to the non-painful conditions. This f inding is consistent with previous studies suggesting attentional modulatio n and a central coping strategy for known and expected painful stimuli. Inv olvement of the medial pain system has previously been reported in patients with mononeuropathy during ongoing spontaneous pain. This study reveals a bilateral activation of the lateral pain system as well as involvement of t he medial pain system during dynamic mechanical allodynia in patients with mononeuropathy. (C) 1999 International Association for the Study of Pain. P ublished by Elsevier Science B.V.