What is the difference between functional and non-functional constipation?

Difference Between Functional and Non-Functional Constipation

Functional constipation is a primary disorder of colonic or anorectal function with no identifiable organic cause, while non-functional (secondary) constipation results from an underlying medical condition, structural abnormality, metabolic disturbance, neurologic disorder, or medication effect. 1

Key Distinguishing Features

Functional Constipation

Functional constipation represents intrinsic problems of colonic or anorectal function without organic disease. 1 This diagnosis requires:

• At least two of the following symptoms for at least 12 weeks in the previous 12 months: straining during bowel movements, lumpy or hard stool, sensation of incomplete evacuation, sensation of anorectal blockage, manual evacuation procedures, or fewer than 3 bowel movements per week 2, 3

• Absence of alarm features such as blood in stools, anemia, unintentional weight loss, or family history of colon cancer 4

• Normal laboratory testing including complete blood count, with no indication of metabolic, endocrine, or structural abnormalities 4

Functional Constipation Subtypes

The functional category itself divides into three distinct pathophysiologic patterns 4:

• Normal transit constipation (NTC): Normal anorectal function and normal colonic transit time, often associated with irritable bowel syndrome features 3, 4

• Slow transit constipation (STC): Normal anorectal function but delayed colonic transit with reduced propulsive activity 3, 4

• Defecatory disorders: Impaired rectal evacuation from inadequate propulsive forces, high anal resting pressure, or paradoxical pelvic floor contraction 3, 4

Non-Functional (Secondary) Constipation

Non-functional constipation is caused by identifiable organic, metabolic, neurologic, or pharmacologic factors. 1 Common causes include:

• Structural/organic diseases: Colon cancer, strictures, rectocele, intussusception 3, 5

• Metabolic disturbances: Hypothyroidism, hypercalcemia, diabetes mellitus 4, 1

• Neurologic disorders: Parkinson's disease, multiple sclerosis, spinal cord injury 1

• Medications: Opioids (which cause opioid-induced constipation, a specific entity defined by Rome IV criteria), anticholinergics, calcium channel blockers 2, 5

Diagnostic Approach to Differentiate

Initial Evaluation

• Detailed symptom characterization to identify predominant patterns: infrequency alone suggests NTC or STC, while prolonged straining with soft stools indicates defecatory disorders 4

• Complete blood count only in the absence of alarm symptoms—metabolic panels are not routinely recommended unless clinical features warrant them 4

• Colonoscopy should NOT be performed unless alarm features are present 4

When to Suspect Non-Functional Causes

Critical red flags that indicate non-functional constipation requiring further investigation: 4

• Blood in stools or positive fecal occult blood
• Unintentional weight loss
• Anemia on complete blood count
• New onset constipation in patients over 50 years
• Family history of colorectal cancer

Specialized Testing Sequence for Functional Constipation

If functional constipation is confirmed but treatment fails, proceed systematically 4:

1. Anorectal testing first (manometry, balloon expulsion test) to identify defecatory disorders 4

2. Colonic transit study only if anorectal tests are normal or symptoms persist despite treatment of defecatory disorder 4

Clinical Pitfalls

• Do not assume infrequent bowel movements alone define constipation—patients with daily bowel movements may still have functional constipation if they experience straining, hard stools, or incomplete evacuation 3

• Opioid-induced constipation is a distinct entity defined by Rome IV criteria as "constipation triggered or worsened by opioid analgesics" and requires specific management approaches different from other functional constipation 2, 3

• Elderly patients have five times higher risk of constipation with age-related factors including reduced rectal sensation, polypharmacy, and reduced mobility—these represent secondary contributors overlaying potential functional disorders 5

• Excessive metabolic testing is not indicated—glucose, calcium, and thyroid-stimulating hormone should only be ordered when other clinical features suggest these disorders 4