Esophageal acid stimulation alters insular cortex functional connectivity in gastroesophageal reflux disease

Abstract

Background: The insula plays a significant role in the interoceptive processing of visceral stimuli. We have previously shown that gastroesophageal reflux disease (GERD) patients have increased insular cortex activity during esophageal stimulation, suggesting a sensitized esophago-cortical neuraxis. However, information regarding the functional connectivity (FC) of the insula during visceral stimulation is lacking. The primary aim of this study was to investigate the FC of insular subregions during esophageal acid stimulation.

Methods: Functional imaging data were obtained from 12 GERD patients and 14 healthy subjects during four steady state conditions: (i) presence of transnasal esophageal catheter (pre-infusion); (ii) neutral solution; (iii) acid infusion; (iv) presence of transnasal esophageal catheter following infusions (post-infusion). The insula was parcellated into six regions of interest. FC maps between each insular ROI and interoceptive regions were created. Differences in FC between GERD patients and healthy subjects were determined across the 4 study conditions.

Key results: All GERD patients experienced heartburn during and after esophageal acidification. Significant differences between GERD patients and healthy subjects were seen in: (i) insula-thalamic FC (neutral solution infusion, acid infusion, post-infusion); (ii) insula-amygdala FC (acid infusion, post-infusion); (iii) insula-hippocampus and insula-cingulate FC (post-infusion).

Conclusions & inferences: Esophageal stimulation in GERD patients revealed significant insular cortex FC differences with regions involved in viscerosensation and interoception. The results of our study provide further evidence that the insula, located at the transition of afferent physiologic information to human feelings, is essential for both visceral homeostasis and the experience of heartburn in GERD patients.

Keywords: heartburn; interoception; viscerosensation.

Figure 1. Determining functional connectivity (FC) values from blood oxygen level dependent (BOLD) time series

(A) Representative BOLD time-series from a healthy subject and a GERD patient between a seed region (right dorsal middle insula) and the target interoceptive region (left medial thalamus). (B) The scatter plot and calculated cross correlation coefficient of the corresponding BOLD time-series from panel A.

Figure 2. Insula-thalamic functional connectivity differences between GERD patients and healthy subjects during neutral solution and acid infusion and after acid stimulation

The insula seed region of interest is depicted as a red circle (right dorsal middle insula) and significantly different clusters (corrected p < 0.05) between healthy subjects and the GERD patients are shown as a yellow-orange overlay on an axial standard brain image (Panel A). Standard coordinates in Talairach-Tournoux space are also noted below each brain image. The color-scale to the right of figure corresponds to uncorrected voxel-wise p-value of unpaired t-test between healthy controls and GERD patients. Mean normalized functional connectivity scores ± SEM are shown for both healthy subjects and GERD patients for all study conditions as a bar graph (Panel B).

Figure 3. Insula-thalamic functional connectivity differences between GERD patients and healthy subjects during and after esophageal acid stimulation

The insula seed region of interest is depicted as a red circle and significantly different clusters (corrected p < 0.05) between healthy subjects and the GERD patients are shown as a yellow-orange overlay on an axial standard brain image. Standard coordinates in Talairach-Tournoux space are also noted below each brain image. The color-scale below the brain images corresponds to uncorrected voxel-wise p-value of unpaired t-test between healthy controls and GERD patients. Mean normalized functional connectivity scores ± SEM are shown for both healthy subjects and GERD patients for all study conditions as a bar graph. (A) Right ventral posterior insula and left medial thalamus. (B) Left ventral posterior insula and left medial thalamus.

Figure 4. Differences in insula-amygdala functional connectivity between GERD patients and healthy subjects

The insula seed region of interest is depicted as a red circle and significantly different clusters (corrected p < 0.05) between healthy subjects and the GERD patients are shown as a yellow-orange overlay on an axial standard brain image. Standard coordinates in Talairach-Tournoux space are also noted below each brain image. The color-scale to the right of figure corresponds to uncorrected voxel-wise p-value of unpaired t-test between healthy controls and GERD patients. Mean normalized functional connectivity scores ± SEM are shown for both healthy subjects and GERD patients for all study conditions as a bar graph. (A) Right ventral posterior insula and right amygdala. (B) Right dorsal posterior insula and right amygdala.

Figure 5. Insula-hippocampus functional connectivity differences between GERD patients and healthy subjects following esophageal acid stimulation

The insula seed region of interest is depicted as a red circle and significantly different clusters (corrected p < 0.05) between healthy subjects and the GERD patients are shown as a yellow-orange overlay on an axial standard brain image. Standard coordinates in Talairach-Tournoux space are also noted below each brain image. The color-scale to the right of figure corresponds to uncorrected voxel-wise p-value of unpaired t-test between healthy controls and GERD patients. Mean normalized functional connectivity scores ± SEM are shown for both healthy subjects and GERD patients for all study conditions as a bar graph. (A) Left ventral anterior insula and left hippocampus. (B) Right dorsal anterior insula and right hippocampus. (C) Right dorsal middle insula and right hippocampus.

Figure 6. Differences in insular-cingulate functional connectivity between GERD patients and healthy subjects following esophageal acid stimulation

The insula seed region of interest (right ventral posterior insula) is depicted as a red circle and significantly different clusters (corrected p < 0.05) between healthy subjects and the GERD patients are shown as a yellow-orange overlay on an axial standard brain image. Standard coordinates in Talairach-Tournoux space are also noted below each brain image. The color-scale to the right of figure corresponds to uncorrected voxel-wise p-value of unpaired t-test between healthy controls and GERD patients. Mean normalized functional connectivity scores ± SEM are shown for both healthy subjects and GERD patients for all study conditions as a bar graph.

Figure 7

Schematic representation of bilateral insular functional connectivity differences with interoceptive brain regions between GERD patients and healthy subjects during (1) acid infusion only (red line), (2) postinfusion only (green line), (3) acid and postinfusion (purple line), and, (4) neutral solution, acid, and postinfusion (blue line). Right and left parcellated insular regions are abbreviated : DA = dorsal anterior; VA = ventral anterior; DM = dorsal middle; VM = ventral middle; DP = dorsal posterior; VP = ventral posterior.