Recent data suggest that colonoscopy is superior to other screening procedures for the detection of colorectal cancer in people aged over 50.1 The American College of Gastroenterology has recently recommended that individuals over 50 at average risk of colon cancer should have elective colonoscopy every 10 years, and those at higher risk more frequently.2 In the United States, 239 000 inpatient colonoscopies were performed in 1995,3 and this figure rose by 20% to 286 000 by 1999.4 Yet complications of elective colonoscopy are reported to be infrequent: colon perforation or bleeding occurs in fewer than 1% of cases, and electrolyte disorders are not even mentioned. 1 5^–8

Preparation of the colon for colonoscopy involves a thorough cleansing of the large bowel by one of several different methods, in some of which large volumes of a liquid cleansing agent may be given: one method involves drinking 4 l polyethylene glycol solution; another involves taking 90 ml sodium phosphate solution. 5 9 Both methods can lead to diarrhoea with nausea, vomiting, and potential dehydration,9 often resulting in raised plasma concentrations of antidiuretic hormone.10 Thus electrolyte imbalance may occur, either from increased oral water intake with abnormal fluid retention or from increased fluid losses into the gastrointestinal tract.

Furthermore, preparation for colonoscopy causes substantial release of antidiuretic hormone,10 and gastrointestinal fluid losses may cause excessive …