Introduction

Survey radiography is one of the most important first diagnostic tests in patients with vomiting or a tense abdomen. Survey radiographs may allow an immediate diagnosis to be made, as is the case with an intestinal obstruction. Indications for plain radiography may include regurgitation, vomiting, acute abdomen, constipation, abdominal pain, abdominal distension or a palpable mass. In animals with chronic diarrhoea or marked abdominal effusion, abdominal radiographs are less beneficial.

Stomach

Following clinical history, physical examination and laboratory data base, survey radiographs and ultrasonography are important diagnostic tools in dogs and cats with vomiting. Radiography is a rapid method for assessment of extra-abdominal structures--the abdominal wall, diaphragm and vertebrae--and also for the detection of obstructive lesions and radioopaque foreign bodies. Regardless of inherent diagnostic limitations, lateral and ventrodorsal survey radiographs should always be performed prior to contrast studies, ultrasonography or endoscopy.

Gastric Dilatation and Volvulus

Severe gastric distension may be caused by excess quantities of gas, food and fluid in the stomach. If the stomach is dilated and food-filled but positioned normally, dilatation is probably the most likely diagnosis. Severe dilatation together with displacement of the stomach is a sign of gastric volvulus. In gastric volvulus the stomach appears compartmentalised or segmented with band-like soft tissue opacities apparent between the gas-filled segments. The radiographic localisation of the pylorus is key to the diagnosis. The pylorus is usually displaced dorsally and to the left. A right lateral radiograph is usually sufficient to make the diagnosis of gastric volvulus and the pylorus will be air-filled and located dorsally. The spleen is also often enlarged due to circulatory compromise and may also be displaced. The oesophagus and small intestines may often appear dilated and air-filled. Gastric volvulus without severe distension may also occur. Therefore, it is always important to interpret the radiographic findings in light of the clinical findings.

Gastric Causes of Chronic Vomiting

Primary gastric causes of chronic vomiting include diffuse inflammatory infiltration, neoplasia, foreign body, polyps, ulcers, pyloric hypertrophy and delayed gastric emptying. Their diagnosis can be challenging and often requires a combination of radiography, ultrasonography and endoscopy. Thickening of the stomach wall is commonly associated with chronic vomiting and is probably one of the most commonly over-interpreted findings on survey radiographs. Superimposition of fluid with the stomach wall may lead to the impression that the wall is thickened. The same is true for fluid within the small intestines. Thickening of rugal folds in chronic gastritis is difficult to appreciate on survey radiographs for the same reason. Liquid barium contrast studies or, preferably, ultrasonography, are required to confirm a radiographic suspicion of wall thickening. Negative-contrast studies can be obtained by filling the stomach with air and performing radiographs in various positions. This technique may allow detection of focal wall infiltrations. However, because both negative-and positive-contrast radiographic studies of the stomach are very time consuming and can be difficult to interpret, ultrasonography is generally the method of choice for imaging the stomach wall.

Radioopaque gastric foreign bodies such as ingested bone fragments are most frequently incidental findings. If the object is still located in the stomach 1-2 days after initial examination, its presence should be further investigated. Radiolucent foreign bodies can be identified by endoscopy, ultrasonography or contrast studies. Only a small amount of barium should be administered, or a double-contrast gastrogram should be performed since large volumes of barium can obscure both wall infiltrates and foreign material. Alternatively, a negative-contrast gastrogram can be performed.

Chronic pyloric obstruction can occur either due to narrowing of the lumen due to wall infiltration or mechanical blockage of the orifice. Survey radiographs usually show some degree of gastric distention. Barium studies may help to identify pyloric obstructive disease. However, differentiating hypertrophic pyloric stenosis from inflammatory infiltrates or neoplasia is often difficult since they all lead to narrowing of the pyloric orifice due to annular thickening, and have a similar appearance radiographically. Detecting intraluminal filling defects in the area of the pylorus is also possible. These may be due to foreign bodies, polyps or severe inflammatory infiltrates and neoplasms.

Gastric ulcers will lead to disruption of the mucosal surface and they usually cannot be diagnosed using survey radiographs alone. Their diagnosis requires either a contrast study, ultrasonography or endoscopy. It is important to remember that ulceration of the stomach wall has various causes and that the stomach wall should be thoroughly investigated, preferably with ultrasound, for evidence of underlying disease. Gastric neoplasia is generally only diagnosed on survey radiographs when it is large enough and when the proliferative tissue is projected into the air-filled lumen. Lack of air in the stomach may cause mural lesions to be overlooked. Diffuse stomach wall infiltrations are even more difficult to diagnose radiographically. Generalised wall thickening may be seen with a number of conditions including chronic hypertrophic gastritis, eosinophilic gastritis, fungal infiltrations and malignant histiocytosis. Both diffuse and localised gastric wall infiltrations may be detected with ultrasonography, which often eliminates the need for contrast studies.

Small Intestine

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 animals suspected of having 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. The radiographic appearance of ileus is dependent on its duration, location and type. Acute or very proximal obstructions may show little intestinal dilatation radiographically whereas chronic or more distally located obstructions will show more severely dilated bowel segments. The two major types of ileus are obstructive (mechanical) and functional. Obstructive ileus may be partial or complete and be due to foreign bodies, torsion, volvulus, herniation, intussusception, adhesions, granulomas and neoplasms.

Barium passage through the small intestines can be used to identify intraluminal, mural or extra-mural obstructive as well as non-obstructive or partially obstructive lesions. However, it is a lengthy procedure and depending on the technique and experience of the clinician, may be difficult to interpret. Barium is also contraindicated prior to endoscopy or ultrasonography, as it causes significant attenuation of sound and acoustic shadowing.

Partial Obstructions

Fasted (>12 hours) or anorectic animals should not have small bowel segments containing granular material resembling that of food. Granular or more opaque small bowel contents may be detected in patients with a partial obstruction. The intestines in such cases may be mildly dilated (1-1.5 times the width of the second lumbar vertebral body (L2)) proximal to the obstruction or may be of normal diameter. Because fluid passes through the narrowed lumen, the contents remaining proximal to the partial obstruction become more dense and, therefore, radiographically more opaque.

Complete Obstructions

More severe dilatation, usually with air, is seen in patients with complete obstructions. Dilatation (1.5-2 times the width of the body of L2) is seen proximal to the site of obstruction and the segments distal to it usually appear empty and contracted. Due to this, the jejunal segments appear to have highly variable diameters. This is due to the continued peristaltic activity in the distal segments. Faeces may still be present in the colon depending on the duration of the obstruction. Proximal duodenal or pyloric obstructions may show no radiographic abnormalities. Distal jejunal obstructions may cause generalised dilatation and resemble a functional ileus radiographically.

Functional Ileus

Another form of ileus that can be detected is a generalised and uniform mild intestinal dilatation due to lack of peristaltic activity. This is known as adynamic, functional or paralytic ileus. Functional ileus results in obstruction since the intestinal contents pool in the dependent areas of the gastrointestinal tract. It may affect the stomach, small and large bowel. Radiographically, the intestines may have a homogenous soft tissue opacity when they are fluid filled or a mixed pattern of air and fluid may be present. Such an adynamic intestinal pattern can be due to the administration of pharmaceutical agents such as parasympatholytics or sedatives. Other causes are peritonitis, blunt abdominal trauma, electrolyte imbalance or enteritis of various causes. Dysautonomia is a disorder of the autonomic nervous system that can also lead to generalised dilatation of the gastrointestinal tract in both dogs and cats. A complete obstruction in the distal jejunum or at the ileocaecal level may also lead to the same radiographic appearance. Decreased peristalsis may be limited to the duodenum in patients with pancreatitis.

References

1.  Mahaffey E, Barber D. The stomach. In: Thrall, DR. ed. Textbook of Veterinary Diagnostic Radiology. Philadelphia: WB Saunders, 2002; 615-638.

2.  Riedesel EA. The small bowel. In: Thrall, DR. ed. Textbook of Veterinary Diagnostic Radiology. Philadelphia: WB Saunders, 2002; 639-660.