Irritable bowel syndrome (IBS) is a heterogeneous functional disease with a high prevalence and significant impact on quality of life. Traditionally understood as a pure disorder of brain-gut interaction, it is increasingly clear that IBS encompasses diverse pathologies, some of which involve objective alterations of intestinal structure, function, and the microbiome.
IBS is subclassified as diarrhea, constipation, or mixed type based on the most prominent stool form. We review the diagnosis and management of the diarrheal type through a pathophysiologic lens, with attention to recent developments that can inform a mechanistically based targeted approach to treatment.
Irritable bowel syndrome (IBS) remains a clinical diagnosis, and its treatment is still mostly empiric and focused on relieving symptoms. That said, our understanding of its mechanisms is progressing, and treatments are increasingly targeted to the etiology in the individual patient.
Key points
- IBS is classified as IBS-diarrhea when at least 25% of bowel movements on symptomatic days are type 6 or 7 on the Bristol Stool Scale.
- New research suggests that IBS has diverse pathologies that include intestinal inflammation, postinfectious sequelae that increase intestinal permeability, food sensitivities, microbiome alterations, and bile acid malabsorption.
- Therapies are increasingly being targeted at one or more of these pathologies, leading to the availability of new treatments such as probiotics, bile acid sequestrants, and the low-FODMAP (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) diet.
- First-line therapies still include antidiarrheals, regular exercise, psychological therapy, and the traditional IBS diet.
Functional disease
IBS is a functional disease characterized by chronic intermittent abdominal pain and altered bowel habits. Patients may also experience postprandial or stress-related abdominal bloating and sensation of incomplete emptying. Comorbid dyspepsia, mood disorder, chronic migraines, interstitial cystitis, and fibromyalgia are common.
The estimated national prevalence is 10% to 12%, although some estimates are as high as 21%. There is a well-documented 3:1 female predominance. This disorder accounts for 25% to 50% of all gastroenterology referrals nationwide, and its healthcare burden exceeds $20 billion annually.
Irritable bowel syndrome has 3 subtypes:
- IBS-diarrhea (IBS-D) is diagnosed when at least 25% of bowel movements on symptomatic days are type 6 (mushy consistency without clear edges) or type 7 (completely liquid without solid substance) on the Bristol Stool Scale
- IBS-constipation (IBS-C) is diagnosed when 25% of bowel movements are type 1 (hard and lumpy) or type 2 (sausage-like and lumpy).
- IBS-mixed (IBS-M) is diagnosed when both criteria are fulfilled.
About one-third of patients fall into each subtype. This review focuses on the diagnosis and management of IBS-D.