Biomarkers to distinguish functional constipation from irritable bowel syndrome with constipation

Treatments for functional constipation (FC) and irritable bowel syndrome with constipation (IBS-C) differ, but symptom criteria do not reliably distinguish between them; some regard FC and IBS-C as parts of a single constipation spectrum. Our goal was to review studies comparing FC and IBS-C to identify possible biomarkers that separate them. A systematic review identified 15 studies that compared physiologic tests in FC vs IBS-C. Pain thresholds were lower in IBS-C than FC for 3/5 studies and not different in 2/5. Colonic motility was decreased more in FC than IBS-C for 3/3 studies, and whole gut transit was delayed more in FC than IBS-C in 3/8 studies and not different in 5/8. Pelvic floor dyssynergia was unrelated to diagnosis. Sympathetic arousal, measured in only one study, was greater in IBS-C than FC. The most reliable separation of FC from IBS-C was shown by a novel new magnetic resonance imaging technique described in this issue of the journal. These authors showed that drinking one liter of polyethylene glycol laxative significantly increased water content in the small intestine, volume of contents in the ascending colon, and time to first evacuation in FC vs IBS-C; and resulted in less colon motility and delayed whole gut transit in FC compared to IBS-C. Although replication is needed, this well-tolerated, non-invasive test promises to become a new standard for differential diagnosis of FC vs IBS-C. These data suggest that FC and IBS-C are different disorders rather than points on a constipation spectrum.