What is functional constipation?

What is Functional Constipation?

Functional constipation is a disorder of gut-brain interaction characterized by difficult or infrequent bowel movements without an identifiable organic cause, distinguished from irritable bowel syndrome with constipation (IBS-C) primarily by the absence of abdominal pain as the predominant symptom. 1, 2

Core Diagnostic Features

Functional constipation requires at least two of the following symptoms present for at least 12 weeks (not necessarily consecutive) in the previous 12 months: 3

• Straining during more than 25% of bowel movements
• Lumpy or hard stools (Bristol types 1-2) in more than 25% of defecations
• Sensation of incomplete evacuation in more than 25% of defecations
• Sensation of anorectal obstruction or blockage in more than 25% of defecations
• Manual maneuvers required to facilitate evacuation (digital evacuation, pelvic floor support) in more than 25% of defecations
• Fewer than 3 spontaneous bowel movements per week 4, 5

Critically, insufficient criteria for IBS must be present—meaning abdominal pain should not be the predominant or bothersome symptom. 1, 3 This is the key distinction: patients with painless bowel dysfunction are labeled as having functional constipation, while those with pain as a prominent feature have IBS-C, though both likely share underlying pathophysiology. 1

Pathophysiologic Subtypes

Functional constipation encompasses three mechanistic categories that are not mutually exclusive: 2, 6

Normal Transit Constipation (NTC)

• Normal anorectal function with normal colonic transit time (20-72 hours) 2, 3
• Often associated with perceived difficulty passing stools despite normal physiology
• May overlap with IBS features (bloating, abdominal discomfort not meeting IBS pain criteria) 3

Slow Transit Constipation (STC)

• Normal anorectal function but prolonged colonic transit (>72 hours) 2, 3
• Reduced colonic propulsive activity or increased uncoordinated distal colonic motor activity 2
• Histologic findings show marked reduction in colonic intrinsic nerves and interstitial cells of Cajal 2
• Abnormal colonic sensation may be present 2

Defecatory Disorders (Dyssynergic Defecation)

• Impaired rectal evacuation due to inadequate rectal propulsive forces and/or increased resistance 2, 7
• High anal resting pressure and/or incomplete relaxation or paradoxical contraction of pelvic floor and external anal sphincters during attempted defecation 2, 7
• Diminished perception of rectal distension with loss of normal urge to defecate 7
• May coexist with structural abnormalities like rectocele or intussusception 7

Clinical clue: Prolonged excessive straining even with soft stools or inability to pass enema fluid strongly indicates a defecatory disorder rather than transit problems. 3, 7

Distinguishing from Related Conditions

Versus IBS-C

The primary distinction is symptom predominance: 1, 3

• Functional constipation: Constipation symptoms predominate; pain is absent or not bothersome
• IBS-C: Recurrent abdominal pain (at least 1 day per week on average) related to defecation, associated with change in stool frequency or form 1

In IBS, abdominal pain is unrelated to the need to defecate, whereas in functional constipation, discomfort is typically related to straining or incomplete evacuation. 1

Versus Secondary Constipation

Functional constipation is diagnosed only after excluding secondary causes: 2, 3

• Medications: Opioids (causing opioid-induced constipation, a distinct Rome IV entity), anticholinergics, calcium channel blockers 2, 3
• Metabolic disorders: Hypercalcemia, hypokalemia, hypothyroidism, diabetes mellitus, uremia 2, 3
• Neurologic conditions: Parkinson's disease, spinal cord lesions 2, 3
• Structural disease: Colorectal cancer, strictures, anal fissure, proctitis 2, 3
• Radiation damage: Causing strictures or generalized dysmotility 2

Clinical Impact and Quality of Life

Despite being "functional" (no structural pathology), this condition significantly impairs quality of life: 1

• Restricts social activities, work, travel, and sexual function 1
• Average work days lost: 14.8 per year versus 8.7 in asymptomatic populations 1
• Can lead to complications: hemorrhoids, anal fissures, fecal impaction, urinary retention 1
• Quality of life reduction, rather than individual symptoms, most determines how patients rate severity. 1

Diagnostic Approach

In the absence of alarm features (rectal bleeding, anemia, unintentional weight loss, age >50 with new-onset symptoms, family history of colorectal cancer), only a complete blood count is necessary initially. 3

Metabolic testing (glucose, calcium, thyroid function) should not be performed routinely unless other clinical features warrant them. 3 Colonoscopy is not indicated unless alarm features are present or age-appropriate cancer screening is due. 3

The diagnosis is clinical and positive—made by recognizing the symptom pattern, not by excluding everything else through extensive testing. 1, 3