Most common

• Inadequate fiber or fluid intake
• Poor bowel habits

Systemic disease

• Endocrine: hypothyroidism, hyperparathyroidism, diabetes mellitus
• Metabolic: hypercalcemia, hypokalemia, uremia, porphyria
• Neurologic: Parkinson's disease, multiple sclerosis, sacral nerve damage (pelvic surgery, tumor), paraplegia, autonomic neuropathy
• Rheumatologic: scleroderma, amyloidosis
• Medications: narcotics, diuretics, calcium channel blockers, anticholinergics, psychotropics, calcium, iron, NSAIDs, clonidine, sucralfate, cholestyramine
• Infectious: typhoid fever

Structural abnormalities

• Anorectal: rectal prolapse, rectocele, rectal intussusception, anorectal stricture, anal fissure, solitary rectal ulcer syndrome
• Pelvic floor dysfunction, e.g., hysterectomy
• Obstructing colonic mass, e.g., cancer
• Colonic stricture: radiation, ischemia, diverticulosis
• Hirschsprung's disease
• Chagas' disease
• Idiopathic megarectum

Slow colonic transit

• Idiopathic: isolated to colon
• Psychogenic
• Eating disorders
• Chronic intestinal pseudo-obstruction

Other

• Irritable bowel syndrome