In addition to the clinical examination and laboratory database, survey radiographs and ultrasonography are important diagnostic methods in dogs and cats with acute abdomen. Radiography is important for quick detection of intestinal displacement and distension and radioopaque foreign bodies. Furthermore, the presence of focal or generalised loss of detail or space-occupying lesions can be determined. Certain radiographic findings indicate the need for an ultrasound examination. Ultrasonography is valuable for the diagnosis of ascites, pancreatitis, urinary tract obstruction, non-radioopaque foreign bodies and intestinal intussusceptions. Non-foreign body causes of intestinal obstruction such as intestinal neoplasm can also be diagnosed. Ultrasonography is indicated in icteric patients in order to rule out hepatic versus post-hepatic causes of obstruction.

Ileus

Ileus is a failure of intestinal contents to be transported and is recognised radiographically by the presence of dilated bowel segments. Survey abdominal radiographs should always be performed in vomiting animals suspected of having an ileus. Ultrasonography alone in such instances does not allow a global view of the abdomen, is much more time-consuming and non-gastrointestinal causes of the dog's clinical signs as well as any secondary abnormalities may be overlooked.

Detecting Ileus with Ultrasound

When abdominal radiographic findings are unclear, an obstruction cannot be ruled out or the radiographic findings do not explain the severity of the clinical signs, further diagnostic procedures are warranted. Transabdominal ultrasonography is the procedure of choice and is generally recommended prior to barium studies. Lack of peristalsis occurring together with generalised dilation of the small intestines may be detected with a functional ileus. Contractions can be observed in two-dimensional real-time imaging and approximately five contractions per minute are considered normal for the stomach and one to three per minute in the small intestine.

The finding of severe dilation of one or more segments of jejunum and the stomach together with empty, contracted bowel segments distally may indicate complete or partial obstruction. Radiolucent intestinal foreign bodies may be detected with ultrasonography, especially when they cause mechanical obstruction. These may be made of wood, plastic or rubber. These solid materials appear as a hyperechoic interface which casts an acoustic shadow from the intestinal lumen. Linear foreign bodies can sometimes be identified in plicated segments of small bowel. Bowel wall infiltrations can also be the source of obstruction and vomiting.

The most common neoplastic infiltrations in dogs and cats are adenocarcinoma, lymphoma and smooth muscle tumours. Sonographically, neoplastic infiltrates produce intestinal wall thickening often with a loss of wall layering.

Lymphoma is the most common intestinal tumour in cats but also occurs frequently in dogs. It commonly leads to either a symmetrical or asymmetrical, transmural, circumferential thickening. The wall layers are difficult to identify and the entire wall appears hypo-to anechoic. Regional lymph nodes may be enlarged. Complete intestinal obstructions often do not occur. Intestinal carcinoma often produces a solitary intestinal mass as can polyps, leiomyomas or leiomyosarcomas.

Carcinomas tend to be annular, irregular infiltrations that invade the lumen and cause obstructions. Regional lymphadenopathy is also commonly identified. Focal jejunal smooth muscle hypertrophy has been described in the cat and was found to be either associated with chronic enteritis, proximal to foreign bodies or with lymphoma. Tissue sampling for histology is always necessary for a definitive diagnosis.

Intestinal intussusceptions can usually be quickly diagnosed with ultrasound. Multilayered, concentric rings of bowel can be identified. In older animals, careful examination of the affected bowel for nodular infiltrations of the bowel wall and regional lymphadenomegaly is important since underlying neoplastic disease may be responsible for the intussusceptions.

Icterus

Clinical signs of hepatobiliary disease can be non-specific since they encompass the range of anorexia, apathy, vomiting, diarrhoea, polyuria, polydipsia and icterus. Abdominal ultrasonography is the most useful non-invasive diagnostic method for differentiating hepatic versus post-hepatic causes of icterus. In cats, hepatic causes of icterus such as lipidosis are more common than pre- or post-hepatic disease. The liver, gall bladder, biliary tract, duodenal papilla and pancreas should be examined with high-frequency transducers.

Non-Obstructive Biliary Tract Disease

Thickening of the gall bladder wall is a common sonographic finding in cholecystitis, cholangiohepatitis and chronic hepatitis. Varying amounts of gall bladder sediment may be identified but this is a non-specific finding. The thickened wall is generally hyperechoic possibly with an irregular inner surface. The gall bladder may also appear to have two echogenic double rims with a hypoechoic space between. This double-walled appearance of the gall bladder can be seen with hypoalbuminaemia, ascites, sepsis, acute inflammatory disease and neoplasia. Mucoceles are important findings in the presence of hepatobiliary disease and icterus and various sonographic patterns have been described. They have the potential to rupture and may cause bile peritonitis. In general, the gall bladder wall is thickened and contents with a complex echostructure and echogenicity are recognised. The presence of ascites sonographically can indicate the presence of rupture and bile peritonitis.

Obstructive Disease

Ultrasonography can be used to detect dilated and tortuous common and hepatic bile ducts, choleliths and biliary and peribiliary obstruction. Biliary calculi, pancreatitis, neoplasia, granulomas and abscesses are possible causes of obstruction. Liver flukes in cats are another cause in tropical and subtropical climates. If obstructed, the gall bladder enlarges prior to the intrahepatic bile ducts which may take days to weeks to dilate. Sonographically, 'too many vessels' are visualised in the liver, that do not show blood flow when examined with colour Doppler.

The pancreas and duodenal papilla can be sources of obstruction that can be evaluated sonographically. Both inflammatory and neoplastic disease can be the cause.

Pancreatitis

Radiography is insensitive for detecting pancreatitis. If it is suspected, the patient should be evaluated with ultrasound. In dogs with acute pancreatitis the pancreatic parenchyma becomes hypoechoic and enlarged. The surrounding mesentery of the pancreas is often diffusely hyperechoic and poorly circumscribed. When the mesentery is inflamed and in the presence of gas-filled bowel, the pancreatic tissue itself may be difficult to recognise sonographically. Mild to moderate accumulations of free fluid may be detected in the cranial abdomen and, together with a hyperechoic mesentery, may indicate a focal peritonitis. The duodenum may be dilated due to functional ileus. Pseudocyst formation associated with pancreatitis has been reported in cats. Also in cats, hepatic lipidosis can be commonly found with an enlarged hyperechoic liver.

The diagnosis of pancreatitis in cats is more difficult than in dogs and the clinical signs are more varied. Acute abdominal signs are not always present. In acute disease the pancreas can be enlarged and hypoechoic as in dogs. However, not all cats will have a hyperechoic mesentery and free fluid in the abdomen.

Abdominal Trauma

Ultrasonography is useful for detecting generalized vs. focal free fluid accumulations. Assessment of the blood supply to traumatized organs is also possible using Doppler ultrasound. Haematomas of the liver, kidney and spleen can be diagnosed. If uroabdomen is present, positive-contrast radiographic studies are preferred over ultrasound. An intravenous pyelogram (excretory urogram) allows both kidneys to be visualised and their arterial supply as well as their ability to excrete urine can be assessed. Both ureters can be traced and assessed for patency. Assessment of the patency of the urethra is limited with ultrasonography, and retrograde urography is the preferred method for this.

References

1.  Diana A, Pietra M, et al. Ultrasonographic and pathologic features of intestinal smooth muscle hypertrophy in four cats. Veterinary Radiology and Ultrasound 2003; 44(5): 566-569.

2.  Fahie MA, Martin RA. Extrahepatic biliary-tract obstruction--a retrospective study of 45 cases (1983-1993). Journal of the American Animal Hospital Association 1995; 31(6): 478-482.

3.  Lamb CR, Mantis P. Ultrasonographic features of intestinal intussusception in 10 dogs. Journal of Small Animal Practice 1998; 39(9): 437-441.

4.  Penninck, D; Smyers, B; et al. Diagnostic value of ultrasonography in differentiating enteritis from intestinal neoplasia in dogs. Veterinary Radiology and Ultrasound 2003; 44(5): 570-575.

5.  Rivers BJ, Walter PA, et al. Ultrasonographic features of intestinal adenocarcinoma in five cats. Veterinary Radiology and Ultrasound 1997; 38(4): 300-306.