What is the significance of the urge to pass stools?

Understanding the Urge to Pass Stools

The urge to defecate is a normal physiological sensation triggered by rectal distension that activates parasympathetic reflexes via the rectal sphincter, signaling the need to evacuate stool. 1

Physiological Mechanism

The urge to pass stools represents a critical sensory feedback mechanism in normal bowel function:

• Rectal distension from stool accumulation triggers mechanoreceptors in the rectal wall, initiating parasympathetic reflexes that create the conscious sensation of needing to defecate 1
• This sensation normally occurs when the rectum fills to a threshold volume (approximately 160 ml in healthy individuals) 2
• The urge sensation activates the recto-colic reflex, which coordinates left colonic contractions to continuously feed stool into the rectum during defecation until the colon empties 2

Clinical Significance

Loss of Normal Urge Sensation

Loss or diminished perception of the urge to defecate is a pathophysiologic hallmark of constipation, particularly in defecatory disorders and slow transit constipation 1:

• Patients who lose the natural call to stool typically develop abdominal discomfort only after several days without bowel movements 3
• This reduced rectal sensation represents hyposensitivity to slow rectal distension, even though these same patients may paradoxically show hypersensitivity to rapid phasic distension 3
• Elderly patients are five times more prone to constipation partly due to diminished perception of rectal distension and reduced urge to defecate 1, 4

Abnormal Urge Perception

Approximately 50% of women with chronic constipation experience abnormal urge perception, which manifests differently than normal urge 5:

• The urge is more often experienced as abdominal sensation (69.3%) rather than rectal sensation 5
• The sensory referral area is 81% larger compared to those with normal urge perception 5
• Abnormal urge is associated with more severe constipation (Cleveland Clinic score 19 vs 15), higher rates of irritable bowel syndrome (45.7% vs 22.9%), and functional evacuation disorders 5

Altered Urge in IBS

In irritable bowel syndrome, the temporal and intensity characteristics of rectal sensation are abnormal 6:

• IBS patients show prolonged persistence of urge sensations after rectal distension terminates 6
• There is a higher incidence of unpleasantness ratings exceeding pain intensity during distension 6
• These abnormalities reflect either altered peripheral neuromuscular processing or central nervous system processing of rectal input 6

Diagnostic Implications

When evaluating patients with constipation, specifically inquire about the presence, quality, and timing of the urge to defecate 1:

• Prolonged excessive straining with soft stools or inability to pass enema fluid strongly indicates defecatory disorders with reduced rectal sensation 1
• Loss of urge combined with other symptoms should prompt anorectal testing (manometry, balloon expulsion test) and colonic transit studies 1
• Need for perineal/vaginal pressure or digital evacuation is an even stronger indicator of defecatory disorders requiring pelvic floor retraining/biofeedback therapy 1

Common Pitfalls

Avoid assuming all constipation is the same—the presence or absence of normal urge sensation fundamentally distinguishes different constipation subtypes that require different management approaches 1:

• Patients with lost urge (hyposensitivity) may have slow transit constipation or defecatory disorders and may benefit from prokinetic agents or biofeedback 1
• Patients with constant urge (sensation of incomplete evacuation) typically have defecatory disorders requiring pelvic floor retraining rather than laxatives 1, 3
• Opioids suppress the urge to defecate by inhibiting intestinal motility and increasing colonic fluid absorption, requiring peripherally acting μ-opioid receptor antagonists rather than standard laxatives 1, 4