Making the most of currently available bowel preparations for colonoscopy

OTHER BOWEL PREPARATION AGENTS AND ADJUNCTS

Magnesium citrate

Like sodium phosphate, magnesium citrate is a hyperosmotic agent that promotes bowel cleansing by increasing intraluminal fluid volume. Since magnesium is eliminated solely by the kidney, it should be used with extreme caution in patients with renal insufficiency or renal failure.

Adding magnesium citrate as an adjunct to polyethylene glycol has been shown to reduce the amount of polyethylene glycol solution required (2 L) for the same result.¹⁷

For patients who cannot tolerate polyethylene glycol, a reasonable alternative is magnesium citrate (1 bottle, around 300 mL) the evening before the procedure plus either bisacodyl tablets at the same time as the magnesium citrate or rectal pulsed irrigation immediately before the procedure.⁷

Saline laxatives that include sodium picosulfate and magnesium citrate in combination are available primarily in the United Kingdom for bowel preparation for colonoscopy. Sodium picosulfate acts locally in the colon as a stimulant laxative and by increasing the force of laxatives, whereas magnesium citrate acts as an osmotic laxative by retaining fluids in the colon to clear the colon and rectum of fecal contents. The combination has been found to have similar efficacy and tolerability as sodium phosphate but is not currently available in the United States.²⁶

Enemas

Enemas are sufficient for flexible sigmoidoscopy, but when used alone they do not clean out the proximal colon enough for adequate visualization during colonoscopy. They are best used as adjuncts to other bowel preparation agents when patients present with poor distal colon preparation for colonoscopy.^7,27 Enemas are also useful in washing out the distal segment of bowel in patients with a proximal stoma. The common types of enemas used are tap water, sodium biphosphate (Fleet), and mineral oil.

Tap water enemas distend the rectum and mimic the natural distention by the stool to allow the rectum to empty itself. Tap water (1 L) has fewer adverse effects than sodium biphosphate or mineral oil but is less effective.

Sodium biphosphate (Fleet) enemas draw fluid into the bowel by osmotic action, prompting contraction. One or two bottles are commonly used for bowel cleansing before sigmoidoscopy. However, as with oral sodium phosphate, sodium biphosphate enemas should be avoided in the elderly and in those with renal failure because of the risk of hyperphosphatemia and subsequent hypocalcemia.

In a head-to-head comparison,²⁸ sodium biphosphate enema was found to provide significantly better bowel preparation than the sodium picosulfate-magnesium citrate combination (currently not available in the United States) for flexible sigmoidoscopy, being judged adequate or better in 93% of procedures as opposed to 74%.²⁸

Oil-based enemas such as cottonseed oil plus docusate (Colace) and diatrizoate sodium (Hypaque) are powerful lubricant laxatives that work by slowing the absorption of water from the bowel, so that the stool is softer. However, they have a number of adverse effects, such as severe allergic reactions (including angioedema and anaphylaxis), muscle cramps, and sporadic seepage that can soil the patient's undergarments for up to 24 hours. Also, their safety in children less than 2 years of age and in pregnant and breastfeeding mothers is not established.

Oil-based enemas are usually reserved for short-term use in refractory constipation, especially to soften feces that has become hardened within the rectum (as in fecal impaction).²⁷

Adjuncts

Diet. Dietary modifications alone, such as a clear liquid diet, are inadequate for colonoscopy, but they may be beneficial as adjuncts to other cleansing methods by decreasing the formation of solid residue. Clear liquids also help maintain adequate hydration during bowel preparation and are recommended with all bowel preparation regimens.

Hyperosmolar or stimulant laxatives. Bisacodyl (two to four tablets of 5 mg each), magnesium citrate (one bottle, about 300 mL), and low-dose senna (36 mg, about four 8.6-mg Sennakot tablets) have been used as adjuncts to low-volume polyethylene glycol solution, achieving results similar to those with full-volume polyethylene glycol. Depending on the type of study to be done, these agents are taken within 2 to 6 hours of starting the polyethylene glycol solution.

In contrast, the routine use of nonabsorbable carbohydrates such as mannitol and lactulose is not favored for bowel preparation, since the hydrogen gas produced by bacterial fermentation of the nonabsorbed carbohydrates increases the risk of explosion during electrosurgical procedures.²⁹

Antiemetic agents. Metoclopramide (5–10 mg), a dopamine antagonist gastroprokinetic that sensitizes tissues to the action of acetylcholine, is commonly used to prevent nausea or vomiting associated with bowel preparation agents.^7,30

Antifoaming agent. Simethicone (three tablets of 80 mg each, total dose 240 mg), an anti-flatulent, anti-gas agent, is prescribed by many gastroenterologists in an attempt to reduce bubbles during colonoscopy and improve visibility. It works by reducing the surface tension of air bubbles and causing small bubbles to coalesce into larger ones that pass more easily with belching or flatulence.

Nasogastric or orogastric tubes have been used to instill colonic preparations, especially for inpatients unable to drink polyethylene glycol solutions or for patients who are unresponsive or mechanically ventilated. This method can also be useful for rapid bowel cleansing (within 2 to 3 hours) for patients with lower gastrointestinal bleeding. However, routine use of a nasogastric tube solely for bowel preparation is discouraged as it can lead to severe complications, such as aspiration and trauma during insertion.⁷