BACKGROUND & AIMS: In patients with functional gut disorders, abdominal distension has been associated with descent of the diaphragm and protrusion of the anterior abdominal wall. We investigated mechanisms of abdominal distension in these patients. METHODS: We performed a prospective study of 45 patients (42 women, 24-71 years old) with functional intestinal disorders (27 with irritable bowel syndrome with constipation, 15 with functional bloating, and 3 with irritable bowel syndrome with alternating bowel habits) and discrete episodes of visible abdominal distension. Subjects were assessed by abdominothoracic computed tomography (n = 39) and electromyography (EMG) of the abdominothoracic wall (n = 32) during basal conditions (without abdominal distension) and during episodes of severe abdominal distension. Fifteen patients received a median of 2 sessions (range, 1-3 sessions) of EMG-guided, respiratory-targeted biofeedback treatment; 11 received 1 control session before treatment. RESULTS: Episodes of abdominal distension were associated with diaphragm contraction (19% +/- 3% increase in EMG score and 12 +/- 2 mm descent; P < .001 vs basal values) and intercostal contraction (14% +/- 3% increase in EMG scores and 6 +/- 1 mm increase in thoracic antero-posterior diameter; P < .001 vs basal values). They were also associated with increases in lung volume (501 +/- 93 mL; P < .001 vs basal value) and anterior abdominal wall protrusion (32 +/- 3 mm increase in girth; P < .001 vs basal). Biofeedback treatment, but not control sessions, reduced the activity of the intercostal muscles (by 19% +/- 2%) and the diaphragm (by 18% +/- 4%), activated the internal oblique muscles (by 52% +/- 13%), and reduced girth (by 25 +/- 3 mm) (P <= .009 vs pretreatment for all). CONCLUSIONS: In patients with functional gut disorders, abdominal distension is a behavioral response that involves activity of the abdominothoracic wall. This distension can be reduced with EMG-guided, respiratory-targeted biofeedback therapy.

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