Introduction

Abdominal radiographs are often an essential part of the diagnostic workup of dogs and cats presenting with acute abdominal signs and help guide treatment decisions such as surgical vs. medical approach. Being comfortable with the interpretation of abdominal radiographs of these patients is important, together with interpretation of history, signalment, physical examination and blood tests.

Survey radiographs are typically the first step in the diagnostic workup of dogs and cats with acute abdomen. Opposite lateral as well as a ventrodorsal abdominal radiographs are recommended particularly for evaluation of the gastrointestinal tract. For example, intestinal foreign bodies that were obscured by surrounding fluid and ingesta become outlined by gas in the opposite lateral projection. In the following, common causes of acute abdomen in dogs and cats and their imaging findings are discussed.

Small Intestinal Obstruction

The question if there is intestinal obstruction in a vomiting dog or cat is a common one in an emergency setting, and often one of the more challenging diagnoses. The typical radiographic findings in presence of a mechanical intestinal obstruction include gas dilation of the bowel segment orad to the obstruction whereas the intestines aborad to the obstruction are small or normal in diameter. Radiographically, two population of bowel loops are seen, gas dilated and normal loops. A small intestinal loop with a diameter that exceeds 1.5x the height of the vertebral body L5 is considered dilated. The normal colon can easily exceed this diameter and needs to be identified before making a call of a dilated small intestinal loop. It is typically best seen at the pelvic inlet and the descending colon can be followed cranially in the left lateral abdomen. The transverse colon crosses over to the left immediately caudal to the stomach and ends in a short, right sided ascending colon. The cecum in dogs is mostly gas filled and has a folded or C-shape whereas it is only a small triangular structure in cats. If the colon cannot be definitively identified a pneumocolonogram is an easy and inexpensive way to determine its location and appearance. Cloth foreign bodies often create a subtle striated appearance of a dilated bowel loop, whereas other foreign bodies such as toys are at times recognizable mostly by their angular or circular shape.

Linear foreign bodies are more challenging to diagnose due to the fact that they don't necessarily lead to gas distension. Instead, the intestinal loops are tightly bunched around the linear foreign body with plication of the wall. On a radiograph this is recognized as a typical gas patterns within the intestines. Triangular, teardrop shaped or paisley shaped gas patterns are seen within the plicated small intestines. Linear foreign material frequently has a portion anchored in the stomach. This leads to a distorted shape of the pyloric antrum and presence of soft tissue material in the pyloric antrum in the left lateral projection where the antrum should mostly be gas filled. The duodenum which normally extends along the right lateral body wall with a straight course to the caudal duodenal flexure is curved away and tortuous.

Gastric Dilatation Volvulus

Dog presenting with gastric dilatation volvulus (GDV) usually have a relatively typical signalment and clinical presentation, and radiographs are not always needed to confirm the diagnosis. If imaging is performed, a single right lateral view of the abdomen is the recommended first step, this is one of the few indications where a single projection is sufficient in most cases. In a typical GDV a gas and fluid distended stomach is has a bipartite appearance with a prominent fold in its cranial contour dividing the stomach into a smaller dorsal and larger ventral compartment. This appearance is sometimes called "compartmentalization". The pyloric antrum which, in a normal right lateral projection, should be fluid-filled and ventrally positioned, is dorsally displaced and gas filled. The duodenum may be dorsally displaced as well. The gastric wall should be closely examined for signs of gastric wall emphysema which indicates presence of gastric wall necrosis. Associated findings include a reduced size of the caudal vena cava and cardiac silhouette, a dilated caudal thoracic esophagus, an enlarged and malpositioned spleen and gas distended small intestinal loops representing a functional ileus.

Mesenteric or Colonic Torsion

This is a rare condition that is most commonly observed in dogs with a history of prior abdominal surgery.

Adhesions can develop over time and intestinal loops can be entrapped or displaced, sometimes strangulated with resulting ischemic bowel injury and eventually perforation. Animals with mesenteric torsion present with severe signs of abdominal pain and cardiovascular shock that tend not to be responsive to fluid therapy resuscitation. Suspected mesenteric or colonic torsion requires immediate surgical intervention. Severely gas distended bowel loops that are stacked on each other are the typical findings in these cases. Colonic torsions are more difficult to recognize as a gas filled colon is not necessarily an abnormal finding, but atypical positioning with caudal curvature of the colon may be seen.

Pancreatitis

The radiographic diagnosis of pancreatitis is challenging. Radiographs have a poor sensitivity and specificity for detection of pancreatitis. Findings are often subtle and include reduced peritoneal detail in the right cranial abdomen, widening of the gastroduodenal angle and caudal displacement of the transverse colon and small intestines. The duodenum or ascending colon may be dilated and spastic in appearance due to inflammation and functional ileus. It is not unusual to see a small amount of pleural effusion in the caudal thorax in patients with pancreatitis.

Splenic Torsion

Dogs with splenic torsion typically present with acute abdominal pain and cardiovascular compromise. The most helpful radiographic sign of splenic torsion is the disappearance of the normally triangular dorsal extremity of the spleen in the left cranial quadrant in the VD projection. Rotation of the spleen leads to severe congestion and enlargement of the spleen which can have a tubular, folded mass-like appearance in the mid abdomen. Peritoneal effusion is common. If the radiographic findings are equivocal, ultrasound with Doppler examination is an excellent method to determine if the spleen is rotated or not.

Peritoneal Effusion

The usefulness of abdominal radiographs in presence of peritoneal effusion is limited, but unless there is a palpable fluid wave the presence of fluid is not always recognized prior to imaging. In dogs with small intestinal ileus the presence of peritoneal effusion points to complication such as ischemia or perforation. Peritoneal effusion is recognized by deceased delineation of the abdominal organs and a wispy, striated appearance of the ventral abdominal fat. Hemoabdomen is a common cause of acute peritoneal effusion and causes for hemorrhage into the abdomen include trauma with hepatic or splenic fracture or vessel avulsions, bleeding neoplastic masses such as hemangiosarcoma or benign masses such as splenic hematomas. Thoracic radiographs should be performed to evaluate for metastatic neoplasia if a splenic mass is suspected.

Peritonitis

Survey radiographs are not rewarding in patients with peritonitis but are often the first step in the imaging workup. Peritonitis will lead to a diffuse loss of peritoneal detail with a characteristic mottled appearance. Carcinomatosis is the main differential diagnosis for this heterogeneous loss of peritoneal detail. Abdominal ultrasound is often used to determine presence and quality of peritoneal effusion (acellular or cellular) and to guide abdominocentesis if there is only a small amount of fluid present.

Urogenital Emergencies

Trauma, infection and obstruction of the urinary system are common causes of acute signs. Trauma can lead to rupture of the urinary bladder, urethra, ureters or kidney and imaging is used to determine the site of rupture. Radiographic signs of retroperitoneal effusion include increased opacity of the dorsal abdomen with a striated appearance caused by fluid tracking through the retroperitoneal fat. The colon and small intestines are ventrally displaced and the outline of the kidneys is not visible anymore. Positive contrast studies are best suited to display the site of urine leakage which can be difficult to determine using ultrasound for example. Excretory urography should only be used if the patient is well hydrated. Uroliths may be visible as focal mineralized structures in the retroperitoneal space. A distended uterus as with pyometra is typically visible radiographically as a tubular structure that extends between the colon and urinary bladder to the pelvic inlet and is present bilaterally along the body wall in a VD projection. In male dogs, prostatitis and prostatic abscessation can be very painful and present as acute abdomen. Radiographically an enlarged prostate leads to dorsal displacement and compression of the descending colon and rectum. Regional peritoneal effusion may be present.

References

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3.  Monnet E. Gastric dilatation-volvulus syndrome in dogs. Vet Clin North Am Small Anim Pract. 2003;33(5):987–1005.