Flexible endoscopy: lower gastrointestinal tract

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The lower gastrointestinal (GI) tract of the dog and cat is much simpler in its anatomical structure compared with many other species, comprising a simple tube which is divided into the caecum, colon, rectum and anus. As a result of this simple structure and the accessibility of the lower bowel, it is particularly well suited to examination by flexible endoscopy, which is fortunate because disease of the colon is very commonly seen in small animal practice. Previously, radiographs, barium studies and possibly laparotomy to obtain biopsy samples were required, which was time-consuming and expensive. Endoscopy provides a simple and readily available method examining the entire mucosal surface of the lower bowel and permitting collection of biopsy samples to ensure a definitive diagnosis is obtained, which has revolutionized the diagnosis of lower bowel disorders in most patients. This chapter looks into Indications; Instrumentation; Patient preparation; Premedication and procedure; Patient and positioning; Procedure; and Pathological conditions.

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5.1 Anatomical structure of the lower GI tract. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.3 For carrying out an endoscopic examination of the large bowel in both dogs and cats, a forward-viewing endoscope should be selected, with an insertion tube length of at least 1 m and an outside diameter of less than 9 mm. There must be a wash and air facility, and a biopsy channel of at least 2 mm in diameter.
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5.4 Careful selection of biopsy forceps will ensure crush artefact is reduced to a minimum. Forceps with a central spike should not be used; those forceps with fenestrated biopsy cups should be selected.
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5.5 Method of collecting biopsy samples from the colon. The forceps should be advanced as near perpendicular to the mucosa as possible. This will ensure a good depth of sample is collected. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.6 Careful preparation of the large bowel is essential if the entire mucosal surface is to be thoroughly examined. The presence of faeces severely restricts the ability to carry out this examination.
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5.8 Thorough preparation of the large bowel for endoscopy is essential. A Higginson’s pump is a very effective method of administering an enema, although commercial enema preparations may also be used.
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5.9 To aid intubation of the transverse and ascending colon, and to ensure that any residual fluid does not interfere with the endoscopy, the patient should always be placed in left lateral recumbency. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.10 Once the endoscope has been advanced into the rectum, the lumen should be inflated with air. It should now be possible to visualize the descending colon extending in front of the endoscope. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.11 The mucosa of the colon should appear pale pink in colour and the submucosal blood vessels should be clearly visible through the thin mucosal layer.
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5.12 As the endoscope is advanced along the descending colon, eventually a ‘bend’ will be observed, which represents the flexure between the descending and transverse colon. This is a normal anatomical landmark, which will be observed on a second occasion as the endoscope reaches the flexure separating the transverse and ascending colon. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.13 The ascending colon is short and ends at the ileocaecocolic junction. The ileum appears as a raised red button-shaped structure whilst the caecum is a blind-ending sac. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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5.14 Lymphocytic–plasmacytic colitis results in thickening of the mucosa so submucosal blood vessels can no longer be seen, and in many cases lymphoid hyperplasia will be observed as raised ‘doughnut’-shaped structures.
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5.15 Occasionally, lymphocytic–plasmacytic colitis may be severe. In such cases proliferative changes may be observed suggesting the possible presence of neoplasia. It is essential to collect biopsy samples for histopathology and not to over interpret the visual changes.
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5.16 Eosinophilic colitis results in destructive changes to the mucosa. There are frequently small erosions and/or ulcers present. The mucosa is more friable and bleeds easily on manipulation.
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5.17 Histiocytic colitis may appear very similar to eosinophilic colitis. Ulceration, friability and bleeding are common. Proliferative changes may also be observed.
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5.18 Where granulomatous colitis is found, the majority of the colon will appear normal and only a small section of the colon will be affected. The lumen of the bowel may appear occluded by proliferative change and bleeding is not uncommon. This must be differentiated from neoplasia by collection and examination of biopsy samples.
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5.19 IBS is difficult to diagnose as there are no visual or pathological changes present. However, at endoscopy the bowel may be difficult to dilate and an excessive amount of mucus may be observed.
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5.20 Adenocarcinoma is an aggressive tumour, which invades the lumen of the colon. The tumour will appear irregular and proliferative in appearance, may bleed easily and may even appear very friable to touch. The remainder of the colon usually remains unaffected.
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5.21 Observation of an intusussception when carrying out large bowel endoscopy is rare. However, when present the ileum will appear as a normal pink colour filling the lumen of the colon, with no bleeding or ulceration in the majority of cases.
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5.22 Rectal adenomatous polyps can be diagnosed easily with the aid of endoscopy. They appear very similar to adenocarcinoma and must be differentiated from the latter.
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5.23 Rectal strictures often appear as an obvious narrowing of the lumen and a circumferential lip may prevent forward movement of the endoscope past the stricture.
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