The large intestine and perianal region

image of The large intestine and perianal region
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Constipation and diarrhoea are the most common signs of large intestinal disease. Dietary, infectious and parasitic diseases are the most common causes of large bowel diarrhoea in dogs. Inflammatory bowel disease caused by lymphocytic-plasmacytic colitis is diagnosed by ruling out other causes of diarrhoea and by performing a histological examination. Colonoscopy using a flexible endoscope has become a common follow-up procedure to survey radiography for imaging the large intestine. Survey radiographs are still important for recognizing situations in which endoscopy may not be feasible, such as obstipation. Strictures may also prevent passage of the endoscope, and contrast radiography may be the only means of diagnosing the extent and nature of disease. Ultrasonography has also replaced much of the use of contrast radiography and should be considered complementary to survey radiography. Artefacts produced by the the contents of the colon can prevent observation of much of the wall using ultrasonography. However, the wall thickness and layering of the colon can be assessed in the near-field of the transducer. The regional lymph nodes and neighbouring organs can also be examined, which can be important for determining the extent of some lesions. The chapter focuses on Normal radiographic anatomy; Contrast radiography; Ultrasonography; Overview of additional imaging modalities; Large bowel diseases; and Perianal diseases.

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11.1 Radiographs of the abdomen showing the normal appearance of the colon and caecum. Lateral radiograph of the gas-filled colon. The caecum is located in the right mid-ventral abdomen and appears as a gas-filled compartmentalized structure. VD view of the canine abdomen showing the ‘C’ shape of the gas-filled caecum. VD view of the normal feline abdomen. There are some normal appearing faeces in the colon. The caecum is not generally visualized in the cat. VD view of the abdomen in a dog. The course of the descending colon is tortuous and shifts from the left side to the right at the mid-abdomen. This is a normal variation in dogs and cats, especially when the colon is distended.
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11.2 Lateral view of a normal feline abdomen showing the normal appearance of faecal balls. The colon is not dilated and the faecal balls have a mixed soft tissue and gas opacity, and are not desiccated as seen with obstipation.
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11.3 VD radiographs taken before and after administration of 8 ml/kg bodyweight of air by rectal tube in a dog with Pepto-Bismol tablets in the descending colon. Radiography was instrumental in differentiating between a small and large bowel location of the foreign body. (Courtesy of R O’Brien) Lateral radiographs obtained before and after pneumocolonography. This technique was helpful for differentiating the large from the small intestine in a cat with a small intestinal obstruction. In (c) there is a severely dilated segment of intestine visible in the caudoventral abdomen. (d) The pneumocolonogram clearly demonstrated that the distended segment was not large intestine and that a mechanical small intestinal ileus was present.
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11.4 Double-contrast study of the colon in a dog in lateral and VD views. The barium coats the intestinal mucosa to show its smooth, thin-walled nature.
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11.5 Ultrasonograms of the normal colon. Transverse image. The near wall (between the white arrows) is well defined with a high-resolution transducer (17 MHz linear probe). The far wall cannot be identified because of the artefacts created by the presence of faeces and air. These appear as a diffuse hyperechoic region distal to the wall. The small intestine lying next to the colon shows the different appearance of the wall layering (between arrowheads). Longitudinal image. Note the five layers that can be observed with a high-resolution linear transducer.
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11.6 Transverse and longitudinal ultrasonograms of the empty and contracted descending colon (between arrows). The asterisks mark two vessels seen in cross section. When the colon is contracted, the wall layering appears irregular or stippled. This should not be confused with thickening.
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11.7 Ultrasonograms of the normal ileocolic junction in a dog and a cat. Transverse image of the ileocolic junction in the dog. The ileum appears as a small structure with concentric rings sitting at the entrance to the colon. The duodenum (DUOD) is seen as a longitudinal small intestinal segment lateral to the ileum. Sagittal image of the feline ileum taken at its entrance (wide arrow) to the colon (thin arrow).
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11.8 Ultrasonogram of normal anal sacs in a dog. A high-frequency linear probe was placed across the anus. The anal sacs (arrowed) appear as round to oval, thin-walled structures with content of varying echogenicity. The asterisk is placed over the anal sphincter, which appears as a poorly defined hypoechoic region.
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11.9 Lateral and VD radiographs of a 4-week-old kitten with atresia ani. The abdomen is severely distended and the entire small intestinal tract is markedly distended and filled with gas. On the VD view there is a large gas-filled structure containing a large amount of very opaque granular material, which was the descending colon.
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11.10 Lateral radiographs of obstipated cats demonstrating the bony opacity of the faecal content and distension of the colon. The increased opacity is caused by desiccation as the faecal balls cannot be evacuated.
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11.11 Lateral and VD radiographs of a dog with colonic torsion. The abdomen is distended and there is a loss of detail in the mid-abdomen. The majority of the small intestinal segments are dilated and either gas- or fluid-filled. The colon is severely dilated and is displaced.
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11.12 Radiograph and ultrasonogram of a thickened colonic wall. A pneumocolonographic study was performed and showed a rigid and thickened wall. Ultrasonography of the colon showed severe thickening with loss of wall layering. The medial iliac lymph nodes were also enlarged in this patient, but were only observed ultrasonographically. The diagnosis was confirmed as a lymphosarcoma.
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11.13 Longitudinal and transverse ultrasonograms of a diffusely thickened colonic wall in a dog. The colon wall (x–x) is 8 mm thick with a loss of layering. The arrow shows the air artefacts caused by the colon content distal to the near wall. The diagnosis was pythiosis. Other differential diagnoses should include neoplasia.
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11.14 Lateral radiograph of the caudal abdomen in a dog with anal sac adenocarcinoma. There is extension of the soft tissue mass into the retroperitoneal space and severe irregular periosteal new bone formation along the ventral lumbar vertebrae. The metal clips in the caudal abdomen are attributable to previous partial resection of the infiltrating mass. The small white arrows show the ventral displacement of the colon. The space-occupying lesion was caused by a combination of extension of the tumour and sublumbar lymphadenopathy. The black arrow shows destruction of the pubic bone. B = Bladder.
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11.15 CT scan of the same dog as in Prior to radiation therapy the image shows a large heterogenous and poorly marginated mass that fills the pelvic and retroperitoneal space with compression of the rectum (arrowed). After radiation therapy there is a reduction in the size of the diffuse and poorly marginated mass (arrowed) with less compression of the colon.
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11.16 Perianal ultrasonogram of a rectal stricture caused by a lymphosarcoma. A 3 cm mass (x–x) is located next to the rectum (dotted circle). The arrows indicate the anal sacs.
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11.17 Sublumbar lymph nodes in a dog. VD radiograph showing the location of the sublumbar lymph nodes (LN). Ultrasonogram of a normal medial iliac lymph node (arrowed). The node is small and elliptical with medium echogenicity. Ao = Aorta; ext. iliac a. = External iliac artery. Ultrasonogram of a lymph node (x–x) containing tumour metastases. The node is enlarged, rounded and has a heterogenous echotexture.
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