Radiographic Diagnosis of Small Intestinal Mechanical Obstruction

Dogs and cats are frequently presented to veterinary practices for acute vomiting. An accurate diagnosis of the cause of the vomiting will determine if the animal should be treated medically or surgically. A common cause is mechanical obstruction of the small intestinal tract.^1,2

Common causes of mechanical obstruction include foreign bodies, intestinal neoplasia, and intussusception.³ Ingestion of a foreign body accounts for approximately 80% of cases of mechanical obstruction.¹ Mechanical obstruction results in increased intestinal contractility and increased mucosal secretory activity. The swallowing of air and saliva, increased intestinal secretions, and decreased absorption create a cycle of increased luminal pressure and distention, which ultimately leads to decreased perfusion of the intestinal wall, resulting in ischemia, electrolyte loss from secretions, clinical dehydration, and increased intestinal mucosal permeability.^1,3 As a result, it is imperative to quickly diagnose mechanical obstruction in order to mitigate intestinal damage.

Diagnostic modalities that aid in diagnosis of intestinal obstruction include radiography, ultrasonography, and computed tomography. Although radiography is reported to have the lowest sensitivity, it is often the only imaging modality available in a general practice setting. Abdominal radiography should be the first-line diagnostic imaging modality performed, even when other imaging modalities are available.

The Normal Small Intestine

The standard radiographic series most often used to survey the small bowel are the recumbent right lateral and ventrodorsal projections. The position of the patient will alter the location of gas in the gastrointestinal (GI) tract and influence what can be seen on the radiograph. For instance, in a right lateral view, gas will be seen in the fundus of the stomach and gastric fluid will be in the dependent side (the pylorus) (FIGURE 1A). On a left lateral view, gas will be seen in the pylorus and duodenum (FIGURE 1B).

Figure 1A. Right lateral radiograph of a canine abdomen.

Figure 1B. Left lateral radiograph of a canine abdomen, showing the differences in gas in the stomach with positional changes. On the left lateral view (B), gas is seen in the pylorus and descending duodenum (arrowheads).

Movement of intestinal gas during repositioning of the patient will sometimes result in intestinal tract pathology being seen clearly on only 1 view. Thus, a 3-view abdominal series is recommended.⁴ Although the order for obtaining images in this series was traditionally somewhat arbitrary, it has recently been shown that the best order is left lateral, followed by ventrodorsal, and then right lateral.⁵ When radiographs are obtained in this order, gas is more likely to be present in the pylorus and duodenum. Patient preparation (e.g., fasting, cleansing enema) is not required and often not indicated as it can alter the native appearance of the gas and fluid in the intestinal tract, which could jeopardize the radiographic evaluation.⁴

In healthy dogs and cats, small intestinal loops within each species, especially cats, are generally similar in diameter. For determining normal size, several methods have been proposed and vary between dogs and cats. In dogs, normal small intestinal diameter should be less than twice the width of a rib⁶ or less than 1.6 times the height of the L5 vertebral body at its narrowest point (FIGURE 2).⁷ Because the size of feline intestines is more consistent, direct measurements are often used. For cats, the maximum diameter of the small intestine is 12 mm.⁸ A ratio of small intestinal diameter (serosa to serosa) to the L2 vertebral endplate height of less than 2 is also considered normal (FIGURE 3).⁸

Figure 2. Right lateral view of a healthy dog. The normal intestine should be uniform in diameter and not significantly dilated. The small intestine should be less than 1.6 times the height of the L5 vertebrae (orange line) at its narrowest point or less than twice the width of a rib (white lines).

Figure 3. Left lateral radiograph of a healthy young adult cat. The small intestine diameter should be uniform, and opacity should be that of mostly soft tissue with minimal luminal gas. A ratio of small intestinal diameter (serosa to serosa) to the L2 vertebral endplate height of <2 is considered normal (white lines). Maximum diameter of the small intestine should not exceed 12 mm.

Intestinal wall thickness is not routinely evaluated on survey radiographs. A normal empty segment with a small amount of luminal gas will often appear thick because normal luminal fluid and the intestinal wall are of the same opacity.

The small intestine should be evenly dispersed throughout the peritoneal cavity. Because the intestine is highly mobile, it is easily displaced by enlarged organs or abdominal masses. In obese patients, it is common for the small intestine to be bunched (contained) into the midabdomen (cats) or into the ventral abdomen (dogs).

The normal luminal content of intestine can vary, depending on when the patient last ate. In a nonfasted animal, air, granular ingesta, and mineral/metallic debris are often seen. Gas is more commonly seen in dogs than in cats. Reportedly, 30% to 60% of small intestinal content can appear radiographically as gas in healthy dogs.⁹

Intestinal Obstruction

The radiographic features of intestinal obstruction depend on 3 factors: the completeness of the obstruction, the location of the obstruction, and the duration of the obstruction.⁴ The goal of the radiographic examination is to first determine if the cause of the clinical signs requires surgical or medical treatment, which is a subjective evaluation. The last step, which may provide more confidence in the radiographic interpretation, is objective evaluation.

Subjective Radiographic Evaluation of Intestinal Obstruction

The most common radiographic sign of intestinal obstruction is dilation of the small intestine, which is especially obvious when there is segmental dilation or 2 populations of bowel (i.e., 1 section is distended and the other sections are empty) (FIGURE 4). Distention is usually indicated by a greater than 50% difference in diameter between the normal and distended bowel or by intestinal segments that are greater than 3 to 4 times the width of a rib.^7,10

Figure 4. Left lateral radiograph of a young dog with mechanical obstruction caused by foreign material. There is marked segmental dilation of the small intestine (asterisks) relative to the normal/empty small intestine (X). This radiographic appearance is the most reliable sign of a mechanical obstruction. In addition, foreign material is seen in the lumen of the duodenum (arrowheads).

Other radiographic signs of obstruction include abnormal shape of the intestinal loops (e.g., hairpin bends, stacking of the bowel loops, plication); radiopaque foreign material (FIGURES 5–8); and potentially the gravel sign,^10,11 which is seen with more chronic partial obstruction (FIGURE 9). The gravel sign is the result of desiccated ingesta that is trapped just orad to the obstruction. It often has the appearance of granular fecal content or a mineral component,^4,10 which in the authors’ experience is often secondary to primary mural disease, as opposed to a foreign body, causing the obstruction.

Figure 5A. Right lateral radiograph of a young dog with a cloth-like foreign body (arrowheads) in the proximal small intestine.

FIgure 5B. Ventrodorsal radiograph of a young dog with a cloth-like foreign body (arrowheads) in the proximal small intestine. There is segmental dilation of the small intestine consistent with a mechanical obstruction. Within the dilated segment, the luminal content appears heterogeneous and striated, consistent with cloth-like foreign material (sock).

Figure 6A. Right lateral radiograph of a dog with a sock foreign body in the duodenum (arrowheads).

Figure 6B. Ventrodorsal radiograph of a dog with a sock foreign body in the duodenum (arrowheads). The foreign body is easy to overlook because only the duodenum is dilated and its consistency is similar to that of fecal material. The course of the dilated segment, however, is not consistent with the typical course of the colon, a finding that is helpful for diagnosis.

Figure 7A. Left lateral radiograph of a cat with a proximal duodenal foreign body (arrows).

Figure 7B. Ventrodorsal radiograph of a cat with a proximal duodenal foreign body (arrows). With proximal duodenal obstructions, the diameter of most of the small intestine is normal, making these cases more difficult to identify than jejunal obstruction. This case in particular shows the foreign material to have a consistency similar to that of fecal material and to be near the transverse colon.

Figure 8A. Right lateral radiograph of a dog with a corn cob foreign body (arrowheads).

Figure 8B. Left lateral radiograph of a dog with a corn cob foreign body.

Figure 8C. Ventrodorsal radiograph of a dog with a corn cob foreign body (arrowheads). Similar to cloth-like foreign material, this foreign body has a characteristic appearance: rectangular shape with a stippled opacity, easier to see on the right lateral (8A) than the left lateral (8B) view of this dog, which stresses the value of taking 3 views. Also note the marked segmental dilation of the small intestine, supporting a diagnosis of mechanical obstruction.

Figure 9A. Left lateral radiograph of a dog presented for chronic vomiting and weight loss. There is segmental dilation of the small intestine with small mineral debris present (arrows), which is consistent with a gravel sign and is often associated with chronic partial obstruction.

Figure 9B. Ventrodorsal radiograph of a dog presented for chronic vomiting and weight loss. There is segmental dilation of the small intestine with small mineral debris present (arrows), which is consistent with a gravel sign and is often associated with chronic partial obstruction. This radiographic appearance is usually associated with primary mural disease. For this patient, the diagnosis of intestinal mass causing the partial obstruction and gravel sign was made by ultrasonography.

The degree of dilation will depend on the completeness, location, and duration of the obstruction. Intestinal dilation may not occur if the obstruction is incomplete or if the patient recently vomited a large amount of the fluid. Obstructions in the proximal portion of the duodenum are also difficult to evaluate radiographically because most of the intestinal fluid is refluxed into the stomach (FIGURE 7).

The following are rules of thumb for interpreting intestinal dilation on radiographs^4,6,10:

• Focal/mild dilation: Involves 1 to 3 loops. Luminal distention is 1.5 to 2 times normal (3 to 4 times the width of a rib). Causes include regional enteritis, regional peritonitis, early or partial obstruction, or vascular compromise.
• Focal/severe dilation: Involves 1 to 3 loops. Luminal distention is greater than 2 times normal (4 to 5 times the width of a rib). Causes include mechanical obstruction (e.g., foreign body, intussusception, mass). Functional ileus and severe focal enteritis are possible but less common.
• Generalized/mild dilation: Involves all loops. Luminal distention is 1.5 to 2 times normal (3 to 4 times the width of a rib). Cause is usually a functional ileus induced by a drug, pain, or enteritis (FIGURES 10 AND 11).
  Figure 10A. Right lateral radiograph of a dog presented for acute vomiting.

  Figure 10B. Ventrodorsal radiograph of a dog presented for acute vomiting. The dog had a history of dietary indiscretion, which led to a high clinical suspicion of a foreign body obstruction. However, the radiographs do not support a mechanical obstruction. There is a diffuse mild dilation of the intestinal tract with concurrent fluid in the colon, which is most consistent with gastroenterocolitis. The patient was treated medically.

  Figure 11A. Left lateral radiograph of a cat presented for acute vomiting.

  Figure 11B. Ventrodorsal radiograph of a cat presented for acute vomiting. The patient had a recent history of eating a plastic remote control. The radiographs show mild diffuse dilation of the stomach and small intestine. No segmental dilation was noted, and the small intestine diameter is less than 12 mm. This radiographic appearance is consistent with gastroenteritis and negative for a mechanical obstruction. The patient was treated medically.
• Generalized/severe dilation: Involves all intestinal loops. Luminal distention is greater than 2 times normal (4 to 5 times the width of a rib). Causes include distal mechanical obstruction or severe functional ileus resulting from infection, inflammation, or a drug (FIGURE 12).

Figure 12A. Right lateral radiograph of a geriatric cat presented for vomiting and weight loss.

Figure 12B. Ventrodorsal radiograph of a geriatric cat presented for vomiting and weight loss. There is diffuse severe dilation of the intestinal tract, suggestive of distal mechanical obstruction or, less likely, severe functional ileus resulting from inflammation. Given the age of the patient, mural disease in the ileum was suspected. Ultrasonography confirmed ileocolic intussusception and lymphoma.

Objective Radiographic Evaluation of Intestinal Obstruction

Objective radiographic evaluation involves measuring small intestinal diameter. For dogs, because they vary in size, interpreting measurements involves determining the ratio of the small intestinal diameter to the height of the L5 vertebral body. This ratio should not exceed 1.6 in healthy dogs. The greater the number, the more reliable the diagnosis of intestinal obstruction. A ratio of 1.7 has been reported as 66% sensitive and specific.^4,7,11 A ratio of 1.95 represents a 77% to 80% probability of indicating obstruction, and a ratio greater than 2.07 represents a 90% probability of indicating obstruction (FIGURE 13). For cats, the diameter of the small intestine is compared with the height of the L2 vertebral endplate. A ratio greater than 2 indicates GI disease.⁸ If the ratio is greater than 2.5, an intestinal obstruction is likely.⁸

Figure 13A. Right lateral radiograph of a young dog presented for acute vomiting and anorexia. There is segmental dilation of the small intestine. The intestinal loops measure greater than 2 times the height of the L5 vertebral body (white lines), indicating a 90% probability of mechanical obstruction.

Figure 13B. Left lateral radiographs of a young dog presented for acute vomiting and anorexia. A flower-shaped foreign body is clearly delineated (arrow).

The usefulness of these measurements in dogs has been questioned. It has been shown that using the small intestinal diameter:L5 ratio is not associated with increased accuracy of a mechanical obstruction diagnosis.¹¹ That is not to say that the measurement techniques are useless, but the subjective evaluation is the most important part of the radiographic interpretation. In addition, lack of intestinal dilation does not rule out intestinal obstruction, and other diagnostics may be needed when the radiographs do not match the clinical findings.

Linear Foreign Body

Linear foreign bodies are discussed separately because their radiographic pattern tends to differ from that of other forms of mechanical obstruction. A linear foreign body trapped in the intestine will have at least 1 part of the object fixed in an orad location, usually the pylorus in dogs and the base of the tongue in cats,^4,12 and the rest passes into the small intestine. The peristaltic activity of the small intestine causes the small intestine to “climb” the foreign body, resulting in plication and bunching of the small intestine. Many affected animals do not show the marked small intestinal dilation that is seen with other forms of small intestinal obstruction. Rather, the gas becomes trapped in pockets formed by the pleated and plicated small intestine, which can appear as tapered, short tubular or crescent-/comma-shaped gas bubbles, referred to as a geometric shape (FIGURES 14–17).^4,10,12,13 Dogs and cats differ with regard to the radiographic appearance of the intestine associated with a linear foreign body. Dogs are usually older and the gas pattern is less irregular; they are more prone to secondary intestinal intussusception. Dogs tend to have more complications associated with linear foreign bodies than do cats due to the increased likelihood of bowel trauma and laceration.^12,14

Figure 14A. Right lateral radiograph of a dog with a linear foreign body. The small intestine is mildly dilated, and the sharp curves of the small intestine result in crescent-shaped gas pockets resulting from plication of the small intestine. The abdominal detail is normal, suggesting the absence of peritonitis at this time. Notice the fluid-filled stomach (arrowheads) and segmental dilation of the small intestine with abnormal curves creating cresent-shaped gas pockes (asterisks).

Figure 14B. Ventrodorsal radiograph of a dog with a linear foreign body. The small intestine is mildly dilated, and the sharp curves of the small intestine result in crescent-shaped gas pockets resulting from plication of the small intestine. The abdominal detail is normal, suggesting the absence of peritonitis at this time. Notice the fluid-filled stomach (arrowheads) and segmental dilation of the small intestine with abnormal curves creating crescent-shaped gas pockets (asterisks).

Figure 15A. Left lateral radiographs of the cranial abdomen of a 1-year-old Great Dane presented for vomiting. Soft tissue–attenuating material is visible in the pylorus of the stomach on the cranial view (arrowheads), as is marked segmental dilation of the small intestine with concurrent plication on the (B) caudal view (asterisks). This radiographic appearance is consistent with foreign material in the pylorus with a linear component in the small intestine. Unlike the case in FIGURE 14, there is marked segmental dilation of the small intestine, which is not always seen with linear foreign bodies.

Figure 15B. Left lateral radiographs of the caudal abdomen of a 1-year-old Great Dane presented for vomiting. Soft tissue–attenuating material is visible in the pylorus of the stomach on the (A) cranial view (arrowheads), as is marked segmental dilation of the small intestine with concurrent plication on the caudal view (asterisks). This radiographic appearance is consistent with foreign material in the pylorus with a linear component in the small intestine. Unlike the case in FIGURE 14, there is marked segmental dilation of the small intestine, which is not always seen with linear foreign bodies.

Figure 16. Left lateral radiograph of a young cat presented for vomiting and anorexia. There is severe plication of the small intestine, causing it to be bunched within the midabdomen. The contours of the intestinal loops are irregular, consistent with plication (arrowheads). This finding is common for cats with a linear foreign body. The normal abdominal detail suggests absence of peritonitis from intestinal compromise.

Figure 17A. Right lateral radiograph of a cat with a linear foreign body. The small intestine is bunched within the midabdomen (arrowheads). The small intestinal segments are not overly dilated, but the eccentric luminal gas and plication are consistent with a linear foreign body (asterisk). This radiographic appearance is less obvious than usual and could be mistaken for enteritis. If the radiographic diagnosis is in question, abdominal ultrasonography is suggested.

Figure 17B. Ventrodorsal radiograph of a cat with a linear foreign body.

Follow-Up Radiographs

When initial abdominal radiographs are inconclusive for diagnosis of mechanical obstruction, repeating abdominal radiographs at intervals of 7 to 24 hours has been proposed.⁴ The theory is that this delay will allow time for intestinal loops to dilate, further increasing confidence of the mechanical obstruction diagnosis. However, a recent retrospective study showed this theory to be invalid.¹⁵ The study found that follow-up abdominal radiographs taken between 7 and 28 hours did not provide a more accurate diagnosis of obstruction versus nonobstruction. However, when the results for mechanical obstruction are equivocal, it has been the authors’ experience that follow-up images taken after 12 to 18 hours are often useful (FIGURE 18). The authors find that follow-up radiographs often help rule out the need for surgery because the pattern usually improves for patients without an obstruction but not for those with an obstruction (FIGURE 19).

Figure 18A. Initial right lateral radiograph of a dog with a sock foreign body. On the initial images (A and B), the foreign body is seen in the small intestine (arrowheads), and there is segmental dilation consistent with mechanical obstruction. The clients elected medical management.

Figure 18B. Initial left lateral radiograph of a dog with a sock foreign body. On the initial images (A and B), the foreign body is seen in the small intestine (arrowheads), and there is segmental dilation consistent with mechanical obstruction. The clients elected medical management.

Figure 18C. Follow-up right lateral radiographs of a dog with a sock foreign body. The follow-up radiograph obtained 12 hours later showed the foreign body to be located in the colon (arrowheads). The patient did not need surgical intervention.

Figure 19A. Initial right lateral radiograph. There is a dilated segment of the intestine in the caudal abdomen that contains heterogeneous soft tissue opaque luminal content (arrowheads). The remainder of the intestinal tract was minimally dilated. At that time, the patient was stable. The material was suspected to be foreign material in the small intestine, but because the patient was stable, conservative treatment was elected.

Figure 19B. Follow-up right lateral radiograph obtained 12 hours later. The material (arrowheads) had not moved. The stomach and small intestine orad to this are much more dilated, which indicated a mechanical obstruction and the need for surgical intervention.

Summary

Radiographs are a valid and useful first-line diagnostic test to screen for mechanical obstruction in dogs and cats with signs of GI disease. The images should be evaluated with the clinical and physical examination findings in mind. The clinician should rely on subjective evaluation, looking for segmental dilation of the intestinal loops, as that tends to be the most reliable sign of intestinal obstruction. Other radiographic evidence of intestinal obstruction includes stacking of the bowel loops, sharp hairpin turns of the intestine, or abnormal luminal contents. These additional findings can further bolster confidence when diagnosing intestinal obstruction. Nonremarkable radiographs do not rule out the presence of a mechanical obstruction but do make it less likely. When the radiographs and clinical examination findings are in disagreement, additional diagnostics are needed and might include serial radiography, abdominal ultrasonography, or, if available, computed tomography.