Making the most of currently available bowel preparations for colonoscopy
ABSTRACTAdequate bowel preparation is essential before colonoscopy. Choosing an agent can be confusing, since many are available. The authors review the available regimens, offer an algorithm for choosing an appropriate regimen, and provide bowel preparation instructions for patients.
KEY POINTS
- Polyethylene glycol solutions are fast, effective, and preferred for cleansing the colon.
- Use of split dosing, a low-volume solution, or both can increase patient acceptability without compromising efficacy.
- Sodium phosphate can be prescribed for patients who cannot tolerate polyethylene glycol solutions, provided they are not at risk of electrolyte or fluid imbalances.
- Enemas, bisacodyl, magnesium citrate, and metoclopramide (Reglan) can be useful as adjuncts to polyethylene glycol but by themselves are inadequate for cleansing the entire colon.
- Educating patients about bowel preparation instructions, including correct dosing and adequate hydration, helps reduce the risk of adverse events and serious adverse events.
Increasing patient adherence to polyethylene glycol solutions
One way to increase tolerability and patient adherence is to split the dose so that the patient takes half the laxative prescription (polyethylene glycol or otherwise) the night before colonoscopy and the other half in the morning, usually about 4 to 5 hours before the scheduled time of the procedure.18,19
Split dosing not only improves patient acceptability, but also cleans the colon better.4 Traditional dosing, ie, drinking the entire volume of solution the night before, leaves a long interval between the end of the preparation process and the start of the procedure. Thick intestinal secretions empty out of the small intestine during that interval and obscure the cecum and ascending colon. With split dosing, the second dose is completed a few hours before the procedure, cleaning out the remaining intestinal secretions and obviating this problem.
Other measures that can make polyethylene glycol solutions more tolerable are:
- Chilling the solution
- Adding lemon slices or sugar-free flavor enhancers (such as Crystal Light) or lemon juice
- Giving the solution by nasogastric tube (at a rate of 1.2–1.8 L per hour) in patients with swallowing dysfunction or altered mental status
- Adding metoclopramide (Reglan) 5 to 10 mg orally to prevent or treat nausea
- Adding magnesium citrate (1 bottle, about 300 mL) in patients without renal insufficiency, or bisacodyl (two to four tablets of 5 mg each), so that the volume can be less15,16
- Stopping further ingestion of solution once the stool is watery and clear on the morning of the procedure (for patients who can clearly understand and follow bowel preparation instructions).17
SODIUM PHOSPHATE SOLUTIONS
Sodium phosphate is an osmotic laxative that draws water into the bowel lumen to promote colonic cleansing. Retention of water in the lumen of the colon stimulates peristalsis and bowel movements.
Advantages and disadvantages of sodium phosphate solutions
Sodium phosphate is widely used worldwide and has been found to be a very acceptable and effective bowel cleansing agent. A recent systematic review of 25 studies18 found that sodium phosphate was superior to polyethylene glycol in 14 studies, that there was no significant difference in 10 studies, and that only one study found polyethylene glycol to be better tolerated than sodium phosphate.18 Similarly, a meta-analysis19 found sodium phosphate to be more effective than polyethylene glycol in bowel cleansing (odds ratio 0.75; P = .0004); more easily completed by patients (odds ratio 0.16; P < .00001); and comparable in terms of adverse events (odds ratio 0.98; P = .81).19 However, most of the clinical trials excluded patients who had renal failure, ascites, or serious heart disease—the groups most at risk of significant adverse effects from sodium phosphate use. The main reasons sodium phosphate was better tolerated were better flavor and smaller volume (1.5–2 L compared with 4 L for polyethylene glycol).20–22
The main disadvantage of sodium phosphate is its potential to cause large fluid and electrolyte shifts. Its use has been associated with a variety of electrolyte abnormalities, including hyperphosphatemia, hypocalcemia, hypokalemia, increased plasma osmolality, hyponatremia, and, conversely, hypernatremia.7,8,23 Asymptomatic hyperphosphatemia alone can be seen in as many as 40% of healthy patients completing sodium phosphate preparations. It may be significant in patients with renal failure and can lead to acute phosphate nephropathy.
Rare adverse events such as nephrocalcinosis with acute renal failure also have been reported, especially in patients taking angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers.23
The significant volume contraction and consequent dehydration seen in some patients using sodium phosphate may be decreased by encouraging patients to drink fluids liberally, especially before the day of the procedure and after the procedure.7
Recently, renal failure due to hyperphosphatemia (acute phosphate nephropathy) has been reported even in patients with normal kidney function.24 Because of the risk of inappropriate use or overdose associated with over-the counter sodium phosphate, the FDA recommended on December 11, 2008, that sodium phosphate products be available only by prescription when they are used for bowel cleansing.25 The C.B. Fleet Company voluntarily recalled its oral sodium phosphate products sold over the counter (Fleet Phospho-Soda and Fleet EZ-PREP). In addition, the FDA required a black box warning on the prescription oral sodium phosphate products Visicol and OsmoPrep, alerting consumers to the risk of acute phosphate nephropathy.25 According to the FDA, health professionals should use caution when prescribing Visicol or OsmoPrep for patients who may be at higher risk of kidney injury, such as:
- Patients over 55 years of age
- Patients who are dehydrated or who have kidney disease, acute colitis, or delayed bowel emptying
- Patients taking certain drugs that affect kidney function, such as diuretics, ACE inhibitors, angiotensin receptor blockers, and nonsteroidal anti-inflammatory drugs.16
Commercially available sodium phosphate products
Sodium phosphate products can still be prescribed, but they are no longer available over the counter in the United States. Patients should be screened to make sure they can safely take these products, and the doses should not exceed the maximum recommended.
Figure 2 shows a simplified algorithm for selecting the optimal bowel preparation agent for an individual patient.
