Practical Gastrointestinal Endoscopy

COLONOSCOPY AND FLEXIBLE SIGMOIDOSCOPY 97
dose must be taken on rising at 6–7·am, with coffee, tea or other
fluids to follow; whereas for late morning or afternoon exams the
whole second magnesium citrate dose (even both doses) can be
taken on the morning of examination. Since magnesium simply
overloads the intestinal absorptive mechanism and produces a
gentle ‘tidal wave’ without particular urgency—and rarely any
cramps or urgency—this essential final dose often appears to
the patient to have little effect, but guarantees a clean cecum for
the endoscopist.
Bowel preparation in special circumstances
Children accept pleasant-tasting oral preparations such as senna
syrup or magnesium citrate very well. Drinking large volumes
is less well accepted, and mannitol may cause nausea or vomiting.
The childhood colon normally evacuates easily except,
paradoxically, in colitis patients who prove perversely difficult
to prepare properly. Small babies may be almost completely prepared
with oral fluids plus a saline enema. Phosphate enemas
are contraindicated in babies because of the possibility of hyperphosphatemia.
Colitis patients require special care, during and after preparation.
Relapses of inflammatory bowel disease are said occasionally
to occur after over-vigorous bowel preparation, although
they can also be provoked by simple distension during an unprepared
barium enema, which perhaps suggests that the cause
is mechanical rather than chemical. Magnesium citrate, senna
preparations, mannitol, saline or balanced PEG-electrolyte solutions
are all generally well tolerated, and the latter is favored in
patients with diarrhea from active colitis. A simple tapwater or
saline enema will clear the distal colon sufficiently for limited
colonoscopy. Patients with severe colitis are unlikely to need
colonoscopy at all, since plain abdominal X-ray (or if necessary
an unprepared contrast study) will usually give enough information.
For severely ill patients even a barium enema is risky
and colonoscopy positively contraindicated, due to the potential
for perforation. When the indication for colonoscopy in a colitis
patient is to exclude cancer or to reach the terminal ileum to help
in differential diagnosis, full and vigorous preparation is necessary.
A patient fit enough for total colonoscopy is fit for full bowel
preparation, which is essential because inflammatory change
often makes the proximal colon difficult to prepare properly.
Constipated patients often need extra bowel preparation. This
is very difficult to achieve in patients with true megacolon or
Hirschsprung’s disease, in whom colonoscopy should be avoided
if at all possible. Constipated patients should continue any
habitually taken purgatives in addition to the colonoscopy preparation,
preferably in large doses for several days beforehand.
The principle is to achieve regular soft bowel actions during the