Colonoscopy and sigmoidoscopy

What it is

Colonoscopy examines the whole colon and rectum, and often the last part of the ileum, with a flexible video endoscope passed through the anus. Flexible sigmoidoscopy looks only at the rectum and the left colon, usually up to the splenic flexure; it needs only an enema on the day and often no sedation. Colonoscopy is the reference test for colorectal cancer and its precursor, the adenomatous polyp, because it finds them and removes most polyps in the same sitting (polypectomy), which prevents cancer. It is also used for rectal bleeding, iron-deficiency anaemia, a change in bowel habit, chronic diarrhoea (with biopsies even if the mucosa looks normal), inflammatory bowel disease and follow-up after polyps or cancer. A good test depends on the preparation: the patient takes a low-residue diet, then clear fluids, then a bowel-cleansing solution, ideally in split doses (part the evening before and part on the morning of the test). The report states the quality of the preparation, whether the caecum was reached (photographed landmarks: the appendiceal orifice and the ileocaecal valve), the withdrawal time, and the size, number and shape of polyps removed. Those findings, with the histology, set the surveillance interval.

Indications

• Colorectal cancer screening, or a positive faecal immunochemical (FIT) or faecal occult blood test.
• Rectal bleeding, iron-deficiency anaemia, a persistent change in bowel habit, or chronic diarrhoea (biopsies).
• Inflammatory bowel disease: diagnosis, extent, activity and dysplasia surveillance.
• Surveillance after polyp removal or colorectal cancer, and in high-risk families (Lynch syndrome, strong family history).
• Therapy: polypectomy, control of bleeding, dilation or stenting of strictures, decompression of a sigmoid volvulus.

Contraindications

• Suspected perforation or peritonitis (absolute).
• Fulminant colitis or toxic megacolon: risk of perforation; a limited, careful sigmoidoscopy may still be used by specialists.
• Acute diverticulitis: wait until it has settled (usually several weeks) and then exclude cancer.
• Haemodynamic instability, recent myocardial infarction or severe cardiorespiratory disease: stabilise first.
• Recent bowel surgery or anastomosis, and pregnancy: only if the result will change management.

Preparation and what happens

1. Explain the test, polypectomy and its risks, and get consent — polyps are usually removed in the same sitting.
2. Review iron tablets, anticoagulants, antiplatelets and diabetes drugs with the guideline — iron darkens stool; others need a plan.
3. Low-residue diet before, then clear fluids — fibre and seeds remain in the colon and hide the mucosa.
4. Bowel preparation in split doses as the unit instructs, with plenty of clear fluid — a clean colon shows small, flat polyps.
5. Warn about frequent stools; extra care in older, frail or renal patients — preparation can cause dehydration and electrolyte shifts.
6. Sigmoidoscopy instead: an enema shortly before and often no sedation — only the left colon needs to be clean.
7. On the day: monitoring, sedation and analgesia as agreed, left lateral position — the colon has bends that can be painful to pass.
8. Rectal examination, then the scope is advanced to the caecum — landmarks are photographed to prove the test was complete.
9. Careful inspection on withdrawal, biopsies and polypectomy — most polyps are seen on the way out.
10. Recovery until awake; pass the air; escort home after sedation — bloating and cramps are expected for a few hours.

Colonoscopy and flexible sigmoidoscopy compared

Risks and complications

• Bloating and cramping from insufflated gas: common and brief.
• Bleeding after polypectomy, immediate or delayed (days later), more likely with large polyps and antithrombotic drugs.
• Perforation: rare in diagnostic tests, more likely with therapy, diverticular disease or colitis; pain, distension and fever are warnings.
• Post-polypectomy syndrome: localised pain and fever from a transmural burn without perforation.
• Sedation problems (hypoxaemia, hypotension) and preparation problems (dehydration, low sodium or potassium).

Next step

• Read the report: preparation quality, caecum reached, polyps (number, size, shape) and any tattoo marking a lesion.
• Chase the histology; the polyp findings and the guideline set the surveillance interval or the next referral.
• Incomplete test or poor preparation: arrange a repeat with better preparation, or CT colonography.
• Cancer found: staging and the multidisciplinary team; IBD: treatment and dysplasia surveillance plan.
• Severe or rising pain, distension, fever, or more than a little bleeding after the test: call a senior; CT looks for perforation.

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Elsewhere in DurgiPedia

• Upper and lower gastrointestinal bleeding
• Bowel elimination: constipation, enemas and faecal impaction
• Small bowel and colon motility
• Informed consent: elements and how to obtain it

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Colonoscopy (StatPearls (NCBI Bookshelf), 2025).

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