Colonoscopy

From Ganfyd

Colonoscopy is an investigation of the large bowel using a flexible, fibre-optic endoscope. Its main strength is that it allows direct visual inspection of the large bowel and also permits biopsies.

More recently, non-invasive virtual colonoscopy is possible using various radiological modalities that construct a 3D 'virtual' image of the colon. In CT colonography, multi-slice spiral computed tomography (CT) technology allows sufficiently rapid capture of data to compensate for artefact from bowel movement. 'Virtual' colonoscopy can also refer to MR colonography

Indications

• Investigation of lower GI symptoms, including:
  • Bleeding
  • Diarrhoea
  • Altered bowel habit
• Exclusion of colorectal cancer in symptomatic patients.
• Colorectal cancer surveillance
• Assessment of inflammatory bowel disease

Performance Characteristics

Sensitivity and Specificity

The sensitivity of a test depends on the disease or condition in question and also the gold standard it is being compared against. For most conditions, colonoscopy is regarded as the gold standard investigation with a theoretical sensitivity of 100%. In conditions like inflammatory bowel disease, direct inspection of the mucosa is essential and the added ability to biopsy the mucosa gives information not obtainable from non-invasive imaging.

In practice, however, the 'sensitivity' of colonoscopy is not 100%. The diagnostic yield is influenced by several factors including equipment, operator factors, the quality of bowel preparation and the conditions studied.

Using colorectal polyps as a disease, an idea of the true 'sensitivity' of colonoscopy has been estimated by using back-to-back colonoscopy,^[1][2] or by comparing it to newer modalities like CT colonography (see ^[3] and CT colonography page).

Risks and Consent

The main risks to be discussed during the process of consent are: (^[4], see also references in ^[5])

• Perforation 1:1000 (about 2:1000 if polypectomy)
• Bleeding, especially if biopsy performed.(about 2:1000 exams without biopsy).
• Other risks are rare and include^[6]:
  • Postpolypectomy syndrome (0.1:1000)
  • Infection including diverticulitis (0.23:1000) and appendicitis^[7]
  • Splenic rupture
  • Snare capture by a large polyp
  • Pneumoperitioneum
  • Pancreatitis

As the majority of colonoscopies are performed as day-cases, the BSG has produced consent forms and booklets that can be sent out to the patients in advance of the procedure. This has the advantage of speeding up the running of the endoscopy unit, but is also good practice as it avoids the less than ideal situation of signing consent forms in the procedure room, minutes before the endoscopy itself.^[8] The total risk of complications with morbidity is 0.2%. ^[9]

The NPSA requires the clinical practitioner ordering the procedure to assess the patients suitability for and to prescribe the bowel preparation so as to clarify lines of responsibility

Procedure

Role of Prophylactic Antibiotics

Antibiotics may reduce bacteraemia, but it is unclear if this reduction has any clinical significance. Some practitioners use antibiotics if there are heart valve lesions thought to predispose to bacterial endocarditis and sometimes if the patient has an orthopaedic prosthesis.

Antibiotics are suggested in high risk patients, but not in otherwise routine cases. The difficulty is agreeing on what constitutes a high risk. ^{[10] [11] [12] [13]}

See also Bacterial Endocarditis:Prophylaxis

Preparation

In order to obtain good views, the contents of the bowel are flushed out through the use of oral laxatives (bowel preparation or 'bowel prep' for short). Consent should be obtained (see risks).

Some patients prefer sedation, though it is not a strict requirement. In a small number of cases, where significant discomfort is anticipated, the procedure can be performed under general anaesthesia. However, over-sedation can be a dangerous thing as an almost unconscious patient may not respond to discomfort with the normal cues.

The patient is placed in the left lateral position. Prior to starting the procedure, a digital rectal examination is performed. This allows examination of the peri-anal region and is also useful excluding low rectal tumours that can sometimes be missed by the colonoscope.

Procedure Itself

The entire length of the large bowel is examined both on entry and withdrawal of the scope. A complete colonoscopy should reach the caecum and this can be confirmed by several methods. Intubation of the terminal ileum is the only sure indication of having reached the caecum, but other surrogate methods include transillumination of the abdominal wall, palpation of the right iliac fossa, and, identification of the tri-radiate fold or appendiceal orifice.^[14][15]

Training and Quality Issues

The introduction of faecal occult blood testing for colorectal cancer screening in the UK prompted an accreditation scheme to ensure a minimum standard of operator competence. A national audit in the UK, shortly before the introduction of the programme was disappointing.^[16]

Operator

The completion rates of colonoscopy are operator dependent.^[17] An accrediation scheme exists for any endoscopist wanting to participate in the UK colorectal cancer screening programme. These stipulate that the operator:

• Should have performed >1000 before entering the scheme.
• Perform >150 annually
• Should achieve caecal intubation in >90% of cases (unadjusted for case-mix).

Training

Joint Advisory Group on Gastrointestinal Endoscopy (JAG) set out training standards. Training should be carried out only in JAG accredited centres.

Before trainees can be 'signed off', they should be expected to:

• Perform ~100 procedures a year
• Achieve caecal intubation in >90% of cases (unadjusted for case-mix).

A minimum of 200 is thought to be necessary to acquire competence.

Audit

Important that parameters above are recorded and subject to scrutiny. The quality of endoscopy in general was the subject of a NCEPOD report in 2004.^[18]

Variations

Several enhancements and adjuncts can further improve the utility of colonoscopy.

Chromoendoscopy

The use of dyes such to allow characterisation and identification of lesions that appear equivocal or are difficult to see on standard optical endoscopy, e.g. flat adenomas or dysplastic regions in long-term inflammatory bowel disease.^[19][20][21] The appearance of the lesion in chromoendoscopy can be used to decide if it is suitable for endoscopic mucosal resection.

Dyes include indigo carmine, methylene blue, crystal violet and acetic acid.^[22] They work either on the principle of contrast or staining (due to absorption or reaction). Contrast dyes, e.g. indigo carmine and acetic acid, pool in the grooves and uneven contours of the lesion, accentuating abnormal surface texture. This can be washed away in contrast to staining dyes which actually stain the cells and can take some minutes to work.

Narrow-band Imaging

Use of smaller section of the light spectrum accomplished through the use of light filters. The narrower spectrum accentuates smaller, more superficial blood vessels. Normal mucosa appears blue-green, stool appears red and blood and vascular structures appear black. NBI can be toggled on and off during colonoscopy allowing easy switching betwee NBI and conventional views.

NBI allows assessment of pit pattern and vascular pattern intensity which permits more accurate characterisation of lesions.^[23] In can be particularly helpful in identifying subtle lesions such as flat adenomas or, in the case of patients with colitis, it allows better identification of DALMs.

Confocal Chromoscopic Endomicroscopy

Combination of dyes and confocal microscopy techniques allow real-time microscopic examination of deeper mucosal layers.

References