Central representation of muscle pain and mechanical hyperesthesia in the orofacial region: a positron emission tomography study

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Abstract
Functional neuroimaging studies of the human brain have revealed a network of brain regions involved in the processing of nociceptive information. However, little is known of the cerebral processing of pain originating from muscles. The aim of this study was to investigate the cerebral activation pattern evoked by experimental jaw-muscle pain and its interference by simultaneous mechanical stimuli, which has been shown to evoke hyperesthesia. Ten healthy subjects participated in a PET study and jaw-muscle pain was induced by bolus injections of 5% hypertonic saline into the right masseter muscle. Repeated von Frey hair stimulation (0.5 Hz) of the skin above the masseter muscle was used as the mechanical stimulus. Hypertonic saline injections caused strong muscle pain spreading to adjacent areas. von Frey stimulation was rated as non-painful but produced hyperesthesia during jaw-muscle pain. Jaw-muscle pain was associated with significant increases in regional cerebral blood flow (rCBF) in the dorsal-posterior insula (bilaterally), anterior cingulate and prefrontal cortices, right posterior parietal cortex, brainstem, cavernous sinus and cerebellum. No rCBF changes occurred in primary or secondary somatosensory cortices. In contrast, von Frey stimulation produced a significant rCBF increase in the contralateral SI face representation. Mechanical hyperesthesia was associated with significant rCBF increases in the subgenual cingulate and the ventroposteromedial and dorsomedial thalamus. These results suggest that the cerebral processing of jaw-muscle pain may differ from the processing of cutaneous pain and that mechanical hyperesthesia, which often is encountered in clinical cases, has a unique representation in the brain.