Brain processing of tonic muscle pain induced by infusion of hypertonic saline

Most of the previous studies on the effects of pain on Regional Cerebral Blood Flow (rCBF) had been done with brief cutaneous or intramuscular painful stimuli. The aim of the present study was to investigate the effect on rCBF of long lasting tonic experimental muscle pain. To this end we performed PET investigations of rCBF following tonic experimental low back pain induced by continuous intramuscular infusion of hypertonic (5%) saline (HS) with computer controlled infusion pump into the right erector spinae on L₃ level in 19 healthy volunteers. Changes in rCBF were measured with the use of ¹⁵O labelled water during four conditions: Baseline (before start of infusion), Early Pain (4 min after start of infusion), Late Pain (20 min after start of infusion) and Post-Pain (>15 min after stop of infusion) conditions. Results of SPM analysis showed relative rCBF increase in the right insula and bilateral decrease in the temporo-parieto-occipital cortex during initial phase of painful stimulation (Early Pain) followed by activation of the medial prefrontal region and bilateral inhibition of insula, anterior cingulate and dorso-lateral prefrontal cortex mainly in ipsilateral hemisphere during Late Pain conditions. The results show that longer lasting tonic experimental muscle pain elicited by i.m infusion of HS results in decreases rather than increases in rCBF. Possible explanations for differences found in rCBF during tonic hypertonic saline-induced experimental muscle pain as compared with previous findings are discussed.