Bowel Preparation for Colonoscopy

Magnesium Citrate

Magnesium citrate is a commonly used agent that has an osmotic effect (as described earlier) and additionally stimulates the release of cholecystokinin, which results in intraluminal accumulation of fluid and electrolytes thought to promote colonic transit. To date, there have been four randomized controlled trials evaluating the use of magnesium citrate for colonoscopy preparation, which included two trials that combined it with either PEG-ELS or sodium phosphate (NaP) solution. Park et al (2010) evaluated the efficacy and tolerance of split-dose magnesium citrate and low-volume PEG for morning colonoscopy. A total of 232 patients were randomized to receive 4-L PEG (group 1; day prior to procedure; n = 79), split-dose PEG (group 2; 2-L PEG day before the procedure followed by another 2-L PEG day of the procedure; n = 80), or magnesium citrate and PEG (group 3; 250 mL of magnesium citrate day before procedure followed by 2-L PEG day of the procedure; n = 73). They found that satisfactory bowel preparations were more frequently reported in group 3 when compared with group 1, and similar to that of group 2. In addition, they found that patients in group 3 were more willing to repeat the same preparation, if necessary, than those in group 1 (93% vs. 38%, p < 0.001) or group 2 (93% vs. 62%, p < 0.001), which led the authors to conclude that split-dose magnesium citrate low-volume PEG regimen was more efficient than the convention 4-L PEG regimen, and equally efficient but preferable to the split-dose regimen for morning colonoscopy. Magnesium citrate does result in transient elevations in serum magnesium levels; however, this has not been linked to any adverse outcomes in healthy persons. It does undergo renal excretion, however, and thus should be avoided in patients with chronic kidney disease because of possible magnesium toxicity (resulting in bradycardia, hypotension, nausea, drowsiness, and even death).

Sodium Phosphate and Oral Sodium Sulfate

NaP (Osmoprep, Salix pharmaceuticals, Raleigh, NC) is a hyperosmotic solution that has recently lost favor due to the rare occurrence of calcium and phostphate nephropathy. Patient characteristics associated with a higher predilection toward phosphate nephropathy include compromised renal function, female gender, inadequate hydration during bowel preparation, hypertension treated with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, older age, or concomitant use of certain medications (diuretics or nonsteroidal antiinflammatory drugs). Additionally, NaP can cause significant fluid shifts and electrolyte derangements, particularly in elderly patients or those with impaired gut motility, renal or hepatic impairment, or congestive heart failure. Thus, the FDA has recently issued a black box warning for the tablet form of NaP. It is not currently recommended for use in bowel preparation.

Oral sodium sulfate (OSS) (Suprep or Suclear, Braintree Laboratories, Braintree, MA) is a hyperosmotic prep that is not associated with significant fluid shifts and electrolyte derangements, owning to the fact that sulfate is a poorly absorbed anion. Di Palma et al (2009) evaluated the efficacy of OSS as a bowel preparation for colonoscopy in adult patients. This multicenter, single-blind, randomized, non-inferiority study compared the efficacy of OSS with PEG-ELS. The study, totaling 364 patients, randomized each patient to split-dose administration, in which the first portion was taken the evening before the colonoscopy and the second the morning of. They found that there was no difference between OSS and PEG-ELS, with successful preparations seen in 97.2% and 95.6%, respectively. Patients who received OSS did report slightly more gastrointestinal events (i.e., cramping, bloating, nausea and vomiting) than those that received PEG-ELS ( p = 0.009). The investigators concluded that OSS is an effective alternative to PEG-ELS with a comparable safety profile. Another multicenter, single-blind, randomized, non-inferiority study by Rex et al (2010) randomized 136 adult patients undergoing outpatient colonoscopy to 4-L SF-PEG-ELS (given the night prior to colonoscopy) or OSS given in equally divided doses the evening prior to and morning of colonoscopy. They found that a successful bowel preparation (98.4% vs. 89.6%; p = 0.04), in particular preparations rated as excellent (71.4% vs. 34.3%; p < 0.001), was achieved more frequently with OSS than SF-PEG-ELS. Given similar side effect profiles, the authors concluded that OSS was a safe alternative as a low-volume preparation for colonoscopy. A limitation of the study is the fact that SF-PEG-ELS was entirely administered the evening prior and not as a split dose.