Practical Gastrointestinal Endoscopy

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CHAPTER 6
addition of prokinetic agents or aperients improves results; the
consensus is that it doesn’t.
Mannitol
Mannitol (and similarly sorbitol or lactulose) is a disaccharide
sugar for which the body has no absorptive enzymes. It is available
as a very cheap white powder, looking and tasting similar
to glucose, or as ready-made intravenous solutions (more expensive
but easily available), which can be drunk. In solution mannitol
presents an isosmotic fluid load at 5% (2–3·L) or a hypertonic
purge at 10% (1·L) with a corresponding loss of electrolyte and
body fluid during the resulting diarrhea, although this is only
of concern in the elderly and normally can be rapidly reversed
by drinking. The solution’s sweetness can be nauseous to those
without a sweet tooth, although this is much reduced by chilling
and adding lemon juice or other flavorings. Children, in
particular, tend to vomit it back. Mannitol solution alone (1·L of
10% mannitol drunk iced over 30·minutes, followed by 1·L of tapwater)
is a useful way of achieving rapid bowel preparation (in
2–3·hours) for those requiring urgent colonoscopy or, as 2·L of 5%
solution, avoids active aperients in patients with active colitis.
There is a potential explosion hazard after mannitol, because
colonic bacteria possess the necessary enzymes to metabolize
mannitol and similar carbohydrates to form explosive concentrations
of hydrogen. If carbohydrates have been used in preparation,
electrosurgery is hazardous unless CO 2
insufflation has
been used, or all colonic gas is conscientiously exchanged several
times by aspiration and reinsufflation of room air.
Magnesium salts
Magnesium citrate and other magnesium salts are very poorly
absorbed, acting as an ‘osmotic purge’ and known, since Roman
times, as Vichy and other similar ‘spa’ waters, for their gently
cathartic properties. Picolax®, a proprietary combination,
produces both magnesium citrate (from magnesium oxide and
citric acid) and bisacodyl (from bacterial action on sodium picosulfate),
tastes acceptable and works well in most patients. Providing
enough fluid is drunk, no enema is needed. Magnesium
citrate alone (1·L of 10% solution, or 100·g magnesium oxide and
citric acid as powder) seems to be as effective and is more readily
available. When made up from powder substantial frothing and
release of heat occurs, so a large jug should be used and the solution
cooled with ice cubes when fully dissolved and clear. Some
people find the result pleasantly lemon-tasting, others find it
very bitter—which can be countered by adding sugar, sweetener
and/or any flavoring or cordial, then drinking a beaker-full at a
time, followed by other preferred clear fluids. Painless diarrhea