Functional MRI measurement of brain activity during rectal distension in normal volunteers and patients with irritable bowel syndrome

Abstract

Background: Perception of somatic pain is thought to be mediated in part by the anterior cingulate cortex (AC), the thalamus (Thai) and the sensory (insular) cortex (IC). The centers responsible for visceral pain are not completely understood. There may be differences in brain activation induced by pain between patients with irritable bowel syndrome (IBS) and controls. Functional MRI (fMRI) can measure brain activity during sensory stimulation. Aim: to determine activity of the AC, Thai and (IC) by fMRI in controls and IBS during painful and non-painful rectal distention. Methods: Healthy controls (n=l 1) and patients with IBS (n=3) had painful (40-55 mm Hg) and non-painful (25-60 mm Hg) rectal balloon distention by a barostat device for 40 sec x4, with 40 sec baseline intervals (6 mm Hg). Echo-planar 1.5 Tesla fMRI measured activity in 6 axial slices. The 3 ROI were selected blindly by a neuroradiologist. We measured change in signal intensity in each ROI during the last 30 sec of each stimulus and the last 30 sec of the prior baseline. Results: Eight of 9 controls and all 3 IBS activated the AC during painful rectal distention, mean signal increase of 1.1%-+ 0.1. Eight of 9 controls and all 3 IBS activated the IC during pain, with a 1.6 % ± 0.4 increase. Thai activation with pain occurred in 5 of 9 controls and all 3 IBS. The average increase in thalamic signal was 1.6 % ± 0.3 in IBS vs 0.9 % -+ 0.4 in controls (p=0.3). During distentions producing sensations of stool, AC activation was seen in 3 of 4 controls and 2 of 3 IBS, with a mean signal increase of 1.1% -+ 0.3. Non-painful stimuli activated the IC in 3 of 4 controls and 2 of 3 IBS, mean increase 0.9 % ± 0.1. Thalamic activation with nonpainful stimulation was seen in 2 of 4 controls and 2 of 3 IBS, mean increase 0.8 % ± 0.2. Activity in the AC was similar during pain and stool sensations, but IC activity was 66 % higher during pain (p=0.3) and Thai activity was 100% higher during pain (p=0.07). A typical stimulus/activity curve for the IC is displayed for a control subject during painful rectal stimulation at 50 mm Hg, Note the characteristic rapid rise of brain activity with rectal stimulation. There often is a carryover in activity after the stimulus.