Magnetic resonance imaging of rectal volume in patients with irritable bowel syndrome

Extreme stool forms (1 and 2, or 6 and 7 of the Bristol stool scale) are frequent in patients with irritable bowel syndrome and are associated with colonic transit, but it is not known whether these alterations influence rectal reservoir function.

To investigate rectal reservoir function by assessing magnetic resonance imaging reconstructions of rectal volume in healthy subjects and patients with irritable bowel syndrome, and to establish whether it varies depending on the bowel habits of the patients.

Twelve healthy subjects and 20 patients with irritable bowel syndrome (with constipation, diarrhoea, mixed or undefined bowel habit according to the Rome III criteria) underwent pelvic magnetic resonance imaging in the absence of rectal sensations. T2 sagittal images were used to calculate rectal volume by multiplying inter-slice thickness by rectal area, and summing the inter-slice volumes.

Stool form was significantly different in the irritable bowel syndrome patients with diarrhoea or constipation, and bowel movements were more frequent in the irritable bowel syndrome patients than in the healthy subjects. Rectal volume was significantly smaller in the irritable bowel syndrome patients (27 ± 13 mL vs. 44 ± 21 mL; P=0.04), with no significant differences between the bowel habit sub-types.

The reduced rectal volume in irritable bowel syndrome patients suggests that rectal tone is increased and that, at least in the absence of rectal sensations, the rectum is not over-distended by stools.