What defines Irritable Bowel Syndrome (IBS)?

Definition of Irritable Bowel Syndrome (IBS)

Irritable Bowel Syndrome is defined as a functional gastrointestinal disorder characterized by recurrent abdominal pain or discomfort occurring at least 3 days per month in the last 3 months, associated with two or more of: improvement with defecation, onset associated with change in stool frequency, or onset associated with change in stool form. 1

Diagnostic Criteria

The Rome III criteria are the current standard for diagnosing IBS:

• Recurrent abdominal pain or discomfort at least 3 days per month in the past 3 months
• Symptom onset at least 6 months before diagnosis
• Pain/discomfort must be associated with at least two of the following:
  • Improvement with defecation
  • Onset associated with a change in stool frequency
  • Onset associated with a change in stool form (appearance) 2, 1

Supporting Symptoms

While not required for diagnosis, these symptoms support an IBS diagnosis:

• Abnormal stool frequency (>3 bowel movements/day or <3 bowel movements/week)
• Abnormal stool form (lumpy/hard or loose/watery)
• Abnormal stool passage (straining, urgency, feeling of incomplete evacuation)
• Passage of mucus
• Bloating or feeling of abdominal distention 2

IBS Subtypes

IBS is classified into subtypes based on predominant stool patterns:

• IBS with constipation (IBS-C): Hard stools >25% of the time and loose stools <25% of the time
• IBS with diarrhea (IBS-D): Loose stools >25% of the time and hard stools <25% of the time
• IBS-mixed (IBS-M): Both hard and soft stools >25% of the time
• IBS-unclassified (IBS-U): Neither loose nor hard stools >25% of the time 2, 1

Patients whose bowel habits change from one subtype to another over time are termed "alternators" 2.

Key Diagnostic Features

Helpful Diagnostic Features

• Symptoms present for more than 6 months
• Frequent consultations for non-gastrointestinal symptoms
• Previous medically unexplained symptoms
• Patient reports that stress aggravates symptoms 2
• Female gender (female:male ratio approximately 2:1) 2
• Age typically peaks in third and fourth decades 2

Alarm Features Requiring Further Investigation

• Age >50 years at symptom onset
• Short history of symptoms
• Documented weight loss
• Nocturnal symptoms
• Family history of colon cancer
• Anemia
• Rectal bleeding
• Recent antibiotic use 2, 1

Pathophysiology

IBS has a multifactorial pathophysiology involving:

1. Altered gut reactivity (motility, secretion) in response to various stimuli
2. Visceral hypersensitivity with enhanced perception and pain
3. Dysregulation of the brain-gut axis 2

These mechanisms can be influenced by:

• Psychological stress and psychosocial factors
• Post-infectious inflammation
• Altered gut microbiota
• Genetic factors 2, 1

Common Pitfalls in Diagnosis

• Overinvestigation once a functional diagnosis is established
• Repetitive testing when unnecessary
• Failure to recognize the pain component in IBS-C
• Misclassifying functional abdominal pain (continuous pain not related to defecation) as IBS 1
• Overlooking the fact that IBS symptoms often fluctuate over time, with flares lasting 2-4 days followed by periods of remission 2

Distinguishing Features

IBS is distinguished from other functional bowel disorders by the presence of pain or discomfort associated with defecation or changes in bowel habits. Functional diarrhea, functional constipation, and functional abdominal pain are separate diagnostic categories 2.

The diagnosis of IBS should be made based on positive symptom criteria rather than as a diagnosis of exclusion, though limited investigations may be warranted to rule out organic disease, particularly in the presence of alarm features 2, 1.