What causes constipation and inability to pass stools?

Causes of Constipation and Inability to Pass Stools

Constipation is primarily caused by disordered colonic and/or pelvic floor/anorectal function, which can be categorized into defecatory disorders, normal transit constipation (NTC), and slow transit constipation (STC), with some patients having combination disorders. 1

Primary Causes

Defecatory Disorders

• Characterized by impaired rectal evacuation due to inadequate rectal propulsive forces and/or increased resistance to evacuation 1
• May result from high anal resting pressure ("anismus") and/or incomplete relaxation or paradoxical contraction of the pelvic floor and external anal sphincters ("dyssynergia") during defecation 1
• Structural disturbances such as rectocele and intussusception may coexist with defecatory disorders 1
• Reduced rectal sensation can contribute to defecatory disorders 1

Colonic Transit Disorders

• Normal transit constipation (NTC): Normal anorectal function with normal colonic transit 1
• Slow transit constipation (STC): Normal anorectal function with slow colonic transit 1
• Some patients with STC have reduced colonic propulsive activity or increased uncoordinated motor activity in the distal colon 1
• Resected colonic specimens from patients with STC reveal marked reduction in colonic intrinsic nerves and interstitial cells of Cajal 1
• Abnormal colonic sensation (reduced or increased) has been described in chronic constipation 1

Combination Disorders

• Some patients have overlap disorders (e.g., STC with defecatory disorders) 1
• May be associated with features of irritable bowel syndrome 1

Secondary Causes

Medications

• Opioid analgesics cause constipation with no development of tolerance over time 2
• Opioids inhibit intestinal motility by activating enteric μ-receptors, resulting in increased tonic non-propulsive contractions, increased colonic fluid absorption, and stool desiccation 1
• Other medications associated with constipation include:
  • Anticholinergics (antidepressants, antispasmodics, phenothiazines) 1
  • Calcium channel blockers 1
  • Antihistamines (OR = 1.8, PAR = 9.2%) 3
  • Diuretics (OR = 1.7, PAR = 5.6%) 3
  • Antidepressants (OR = 1.9, PAR = 8.2%) 3
  • Antispasmodics (OR = 3.3, PAR = 11.6%) 3
  • Anticonvulsants (OR = 2.8, PAR = 2.5%) 3
  • Aluminum antacids (OR = 1.7, PAR = 3.0%) 3

Medical Conditions

• Diseases of the colon (stricture, cancer, anal fissure, proctitis) 1
• Metabolic disturbances:
  • Hypercalcemia 2
  • Hypokalemia 2
  • Hypothyroidism 1
  • Diabetes mellitus 1
  • Uremia 2
• Neurologic disorders:
  • Parkinsonism 1
  • Spinal cord lesions 1
• Radiation damage causing strictures and/or generalized secondary dysmotility 1
• Abdominal or pelvic masses 2

Structural Abnormalities

• Bowel obstruction due to adhesions from previous surgeries 1
• Small bowel volvulus from band adhesion 1
• Intussusception 1
• Multiple laparotomies resulting in secondary dysmotility, especially if the bowel becomes encased in fibrous tissue (sclerosing peritonitis) 1
• Upper gut surgery (vagotomy, Whipple's resection, gastroenterostomy, bariatric procedures, bowel anastomosis) 1

Lifestyle and Dietary Factors

• Advanced age (elderly individuals are five times more prone to constipation than younger people) 2
• Inadequate fluid intake 2
• Low-fiber diet 2
• Lack of privacy when toileting 2
• Reduced physical activity 1

Clinical Presentation

Constipation manifests with various symptoms:

• Infrequent bowel movements (fewer than 3 spontaneous bowel movements per week) 1
• Straining during more than 25% of defecations 1
• Lumpy or hard stools more than 25% of defecations 1
• Sensation of incomplete evacuation more than 25% of defecations 1
• Sensation of anorectal obstruction/blockage more than 25% of defecations 1
• Need for manual maneuvers to facilitate more than 25% of defecations (digital evacuation, support of the pelvic floor) 1

Diagnostic Clues

Key findings that suggest specific causes:

• Prolonged and excessive straining before elimination suggests defecatory disorders 1
• Difficulty passing soft stools or enema fluid indicates pronounced evacuatory defects 1
• Need for perineal or vaginal pressure to allow stool passage or direct digital evacuation strongly suggests defecatory disorders 1
• Distinct transition point between dilated and normal sized bowel on imaging suggests organic obstruction 1
• Visible small bowel peristalsis, worse pain after prokinetic drugs, or giant jejunal contractions on manometry suggest organic obstruction 1

Management Considerations

Treatment should be tailored to the underlying cause:

• For defecatory disorders, biofeedback therapy is often effective 4
• For opioid-induced constipation, peripherally acting μ-opioid receptor antagonists (PAMORAs) like methylnaltrexone can provide relief while preserving pain management 1
• For slow-transit constipation, colectomy may be necessary in specific refractory cases 4
• For constipation in Parkinson's disease, fermented milk containing probiotics and prebiotic fiber can be beneficial 1

Understanding the specific cause of constipation is crucial for effective management and improving quality of life for affected patients.