Constipation

GENERAL PRINCIPLES

Definition

Constipation consists of infrequent and incomplete bowel movements, which can be associated with straining and passage of pellet-like stools.

Etiology

• Recent changes in bowel habits may suggest an organic cause, whereas long-standing constipation is more likely to be functional.

• Medications (e.g., calcium channel blockers, opiates, anticholinergics, iron supplements, barium sulfate) and systemic diseases (e.g., diabetes mellitus, hypothyroidism, systemic sclerosis, myotonic dystrophy) may contribute.

• Female gender, older age, lack of exercise, low caloric intake, low-fiber diet, and disorders that cause pain on defecation (e.g., anal fissures, thrombosed external hemorrhoids, pelvic floor dyssynergia) are other risk factors.⁴⁵

DIAGNOSIS

• Colonoscopy and barium studies help rule out structural disease and are particularly important in individuals gt;45-50 years without prior colorectal cancer screening or with alarm features such as anemia, blood in the stool, or new-onset symptoms.⁴⁵

• Colonic transit studies, anorectal manometry, and defecography are reserved for resistant cases without a structural explanation after initial workup.

TREATMENT

• Regular exercise and adequate fluid intake are nonspecific measures.

• Increased dietary fiber intake (20-30 g/d) is useful. Fecal impaction should be resolved before fiber supplementation is initiated.

• Laxatives

î Emollient laxatives such as docusate sodium, 50-200 mg PO daily, and docusate calcium, 240 mg PO daily, allow water and fat to penetrate the fecal mass. Mineral oil (15-45 mL PO q6-8h) can be given orally or by enema.

î Stimulant laxatives such as castor oil, 15 mL PO, stimulate intestinal secretion and increase intestinal motility. Anthraquinones (cascara, 5 mL PO daily; senna, one tablet PO daily to qid) stimulate the colon by increasing fluid and water accumulation in the proximal colon.

î Osmotic laxatives include nonabsorbable salts or carbohydrates that cause water retention in the lumen of the colon. Magnesium salts include milk of magnesia (15-30 mL q8-12h) and magnesium citrate (200 mL PO) and need to be avoided in renal failure. Lactulose (15-30 mL PO bid-qid) can cause bloating as a side effect.

î Lubiprostone (8-24 #956;g PO bid), a selective intestinal chloride channel activator, moves fluid into the bowel lumen and stimulates peristalsis.^45,46

° Linaclotide (145-290 #956;g PO qday) and plecanatide (3 mg PO qday) are guanylate cyclase C receptor agonists and also move fluid into the intestinal lumen as their mechanism of action.^47,48

° Prucalopride is a selective serotonin receptor agonist and a prokinetic agent that is approved for chronic constipation.⁴⁶

• Enemas: Sodium biphosphate (Fleet) enemas can be used for mild to moderate constipation and for bowel cleansing before sigmoidoscopy; these should be avoided in renal failure. Tap water enemas (1 L) are also useful. Oil-based enemas (mineral oil, cottonseed colace) as well as Hypaque enemas can be used in refractory constipation.

• Polyethylene glycol in powder form (Miralax 17 g PO daily to bid) can be used regularly or intermittently for the treatment of constipation.

• Subcutaneous or oral methylnaltrexone, oral alvimopan, oral naloxegol, and oral naldemedine are peripherally acting #956;-opioid receptor antagonists (PAMORAs) that provide rapid relief of opioid- induced constipation.⁴⁹

• Bowel-cleansing agents: Patients should be placed on a clear liquid diet the previous day and kept nothing by mouth (NPO) for 6 hours or overnight prior to colonoscopy. Patients may experience mild abdominal discomfort, nausea, and vomiting with the bowel preparation.

î An iso-osmotic polyethylene glycol solution (PEG, GoLYTELY, or NuLYTELY 1 gallon, administered at a rate of 8 oz every 10 minutes) is commonly used as a bowel-cleansing agent before colonoscopy. Lower volume preparations, such as PEG (2 L or 0.5 gallon) with ascorbic acid or other laxatives, are alternatives.⁵⁰

î Sodium phosphate (Fleet phosphosoda, 20-45 mL with 10-24 oz liquid, taken the day before and morning of the procedure), a hyperosmotic solution, draws fluid into the gut lumen and produces bowel movements in 0.5-6.0 hours. It is also available in pill form (Visicol or OsmoPrep, 32-40 tablets, taken at the rate of 3-4 tablets every 15 min with 8 oz fluid). Adverse reactions include severe dehydration, hyperphosphatemia, hypocalcemia, hypokalemia, hypernatremia, and acidosis. A dreaded rare complication is acute phosphate nephropathy, where calcium phosphate deposits cause irreversible dysfunction of renal tubules resulting in renal failure. Consequently, sodium

phosphate is only used in limited instances.

î Split preparations: Proximity of bowel preparation to procedure time improves effectiveness of cleansing and visualization during the procedure. Splitting bowel preparation into two doses, with one dose administered the evening prior and the second dose administered the morning of the procedure, can improve bowel cleansing.⁵¹

î Two-day bowel preparation is sometimes indicated in elderly or debilitated individuals when conventional bowel preparation is contraindicated, not tolerated, or ineffective. This consists of magnesium citrate (120-300 mL PO) administered on two consecutive days while the patient remains on a clear liquid diet; bisacodyl (30 mg PO or 10-mg suppository) is administered on both days.

î Tap water enemas (1-L volume) can cleanse the distal colon when colonoscopy is indicated in patients with proximal bowel obstruction.

• Other options: Biofeedback therapy and sacral nerve stimulation can be effective for idiopathic constipation resistant to medical treatment.⁵²