Case 18 - Test Your Skill in Distinguishing Bowel Obstruction From Ileus

Volume 3, Case 18
Corinne C. Chan-Nishina, MD
Patrice M.L. Tim-Sing, MD
Kapiolani Medical Center For Women And Children
University of Hawaiʻi John A. Burns School of Medicine

Abdominal radiographs can be difficult to analyze. A mechanical obstruction is often difficult to differentiate from an adynamic ileus. The goal of this case discussion is to help one to have a better understanding of a mechanical obstruction versus an adynamic (paralytic) ileus, and be able to make a distinction between these two conditions. Sixteen abdominal radiographs will be displayed to test your skill in distinguishing a bowel obstruction from an ileus.

It is important to first look at those components that are common to all films, such as the stomach, rectum, and the hepatic and splenic flexures of the colon. These areas are relatively fixed. Then, one should look at the solid abdominal viscera, such as the liver, spleen, kidneys, psoas muscles and bladder. Finally, an examination of the lungs (lower portions), diaphragms, bony structures and pelvis are important.

A mechanical obstruction is an impedance to the passage of air or intestinal contents secondary to a mechanical hindrance. Examples of this include incarcerated inguinal hernia, bowel adhesions, intussusception, volvulus, intestinal atresias, intraluminal masses (tumors, bezoars, large stool masses), and extrinsic bowel compression (Ladd's bands, annular pancreas, etc.). In a paralytic (adynamic) ileus, there is a temporary impedance to the passage of air or contents secondary to uncoordinated peristalsis or hypoperistalsis.

Adynamic ileus frequently occurs with major abdominal, retroperitoneal and spinal surgery. It also occurs frequently with inflammatory processes such as sepsis, pneumonia, gastroenteritis, appendicitis, peritonitis, pancreatitis and urinary tract infection. One may have bowel disturbances and a resultant ileus with hypokalemia, electrolyte disturbance, dehydration, vasculitis, renal disease, neurogenic shock, sepsis, drugs, hypothyroidism and idiopathic intestinal pseudoobstruction. Although the most common cause of an ileus is gastroenteritis, an ileus is not necessarily a benign condition.

There are different criteria that one must look at when trying to distinguish an ileus from an obstruction on an abdominal radiograph. These include, the fixed anatomy, gas distribution, degree of bowel distention, air fluid levels, and arrangement of the bowel loops. It should be noted that none of these criteria are necessarily definitive in always distinguishing an ileus from an obstruction.

Gas distribution: A gasless abdomen is usually abnormal. Only rarely is the abdomen truly gasless. However, radiographs with an extreme paucity of gas (i.e., almost gasless) should be treated with the same degree of suspicion as a gasless abdominal radiograph. Although a gasless abdomen is highly suggestive of a high obstruction, this can also be seen with excessive vomiting, and/or diarrhea. This picture can also occur in the early stages of appendicitis, as well as in Addisonian crisis (adrenal crisis). Occasionally, this occurs in patients with marked cerebral depression such that their swallowing is impaired.

In a mechanical obstruction, there is preferentially more air proximal to the obstruction than distal to it. Thus, in an obstruction, there is either too much gas in the small bowel (and not much gas in the large bowel), or too much gas in the large bowel (and not much gas in the small bowel). In an adynamic ileus, there usually is no preferential collection of air. There is too much air or not much air in both the small and large bowel. This pattern of distribution is not necessarily definitive.

When there is too much air in the small bowel, this may be a small bowel obstruction which has been present long enough to have allowed the colon gas to clear. When there is too much air in the colon, this may be a large bowel obstruction (e.g.., sigmoid volvulus) with a competent ileocecal valve. If, however, there is too much air in both parts of the bowel, you may have a paralytic ileus, or a large bowel obstruction with an incompetent ileocecal valve, or a small bowel obstruction which is early or intermittent.

Another important point is that sometimes in a mechanical obstruction, there is very little air present and the intestinal loops are filled with fluid. In these cases, the loops may appear as opaque sausage-like structures in the abdomen or the bowel may be isodense with the rest of the abdomen showing a paucity of gas. On the upright view, the air may get trapped in the valvulae conniventes (small bowel plicae circulares [circular folds]) giving a "string of pearls" gas pattern appearance.

Bowel dilatation: Bowel dilatation is another important criteria that needs to be considered. In a mechanical obstruction one usually sees dilatation proximal to the site of obstruction. In a bowel obstruction, the bowel dilatation appearance in children is different from that generally seen in adults. In infants and children, an obstruction characteristically shows dilated bowel with SMOOTH bowel walls. The degree of dilatation is not necessarily excessive, but the smoothness of the bowel wall is most notable. This smoothness is due to the loss of plicae (circular folds) and haustration of the bowel due to gaseous distention. In an obstruction where the bowel is dilated, the bowel resembles "hoses" or "sausages" where the bowel walls are smooth (the normal bowel wall irregularity is lost).

Determining the level of the obstruction is often difficult. It is often difficult to radiographically distinguish small from large bowel in the infant. In older children you may see cross striations which represent the valvulae conniventes when the small bowel is distended. These resemble the haustra of the large bowel, however, they are more numerous and more narrowly spaced. Haustra appear as indentations which do not cross the lumen like these do, and the indentations of haustra do not necessarily line up with the opposite side. In paralytic ileus, the bowel loops all dilate in proportion to each other. The colon usually remains larger than the small intestine.

It is worth mentioning here that one can see short segments of bowel dilatation adjacent to areas of inflammation ("sentinel" loops). These are areas of short segment paralytic ileus and when found in the right upper quadrant, can represent cholecystitis, pyelonephritis, hepatitis or traumatic disease. In the left upper quadrant these are seen with pancreatitis, pyelonephritis, or splenic injury. In the right lower quadrant, it is seen with appendicitis, Meckel's diverticulitis, or regional enteritis. These loops are rare in the left lower quadrant, but can be seen with salpingitis or cystitis in females.

Air-Fluid levels: In mechanical obstruction, air-fluid levels can be seen on the upright view. One can see short air-fluid levels in both limbs of what look like hairpin loops of intestine. The heights of the fluid levels are usually different in any two limbs of one loop (resembles candy canes). In a paralytic ileus, there may be few to numerous sluggish air-fluid levels scattered throughout the abdomen. An obstruction characteristically shows many dilated air-fluid levels, while an ileus characteristically shows fewer air-fluid levels that are not dilated.

Arrangement of Bowel Loops: One could also look at how orderly the intestinal loops are arranged. In a mechanical obstruction the dilated loops are often stacked one under the other in a "step ladder" appearance (in a more orderly fashion) on the SUPINE view (not the upright view). With an ileus, the dilated loops tend to be less orderly, scattered throughout the abdomen from top to bottom and side to side. Perhaps another way at describing this "orderliness", is that an obstruction resembles a bag of sausages (a more orderly arrangement), while an ileus resembles a bag of popcorn (a less orderly arrangement). The sausages of a bowel obstruction are due to dilated bowel while the popcorn of an ileus is due to a generalized distribution of bowel gas and better preservation of the bowel plicae and haustra.

In summary, one should evaluate abdominal films in a stepwise fashion.

1. Look at the fixed anatomy. Do not forget the lungs.

2. Gas Distribution.

• Obstruction: Too much air in the small bowel (and not much gas in the large bowel) or too much air in the large bowel (and not much gas in the small bowel). Poor gas distribution or gasless.
• Ileus: Good gas distribution over most of the abdomen. Too much air in both large and small bowel.
• Warning: This could also appear in large bowel obstruction with an incompetent ileocecal valve, or in an early or intermittent small bowel obstruction.

3. Bowel Dilatation.

• Obstruction: Smooth bowel walls (resembles sausages or a hose). Preferential dilatation of the bowel proximal to the obstruction.
• Ileus: Dilatation of the bowel in proportion to each other, so that the colon remains larger than the small intestine. Look for sentinel loops.

4. Air-fluid Levels.

• Obstruction: Many dilated air-fluid levels in both limbs of a given loop, at different heights (candy canes).
• Ileus: Fewer and/or smaller (less dilated) air-fluid levels scattered throughout the abdomen.

5. Arrangement of loops (supine view only).

• Obstruction: Dilated loops arranged in "stepladder" fashion. Orderly. A bag of sausages.
• Ileus: Disorderly loops scattered throughout the abdomen. A bag of popcorn.

Remember, presentations are variable, and not always clear cut. Often, it is difficult to distinguish the two, especially when there is a mixed paralytic and mechanical obstruction. A high index of suspicion should remain when the clinical and radiographic information is unclear. Conditions such as intussusception, volvulus, and appendicitis are surgical emergencies that require a timely diagnosis and intervention. These conditions may not have definitive findings on plain radiographs. Other diagnostic studies or surgical intervention may be necessary if these conditions are still suspected after the completion of plain film radiographs.

Now test your skill in distinguishing obstruction from ileus in this series of 16 pediatric abdominal radiographs. All of these patients are vomiting with varying degrees of abdominal pain. No histories are given here except for the patient's age and sex. In reality, the radiographic findings should be interpreted in conjunction with the patient's clinical findings. Two views are shown in each case. The view on the left is a supine view. The view on the right is an upright view unless otherwise specified.

Case A: 18-month old male.

View Case A.

Interpretation of Case A

• Gas Distribution: There are pockets of gas scattered in several areas of the abdomen. There is gas in the small bowel, colon, and rectum.
• Bowel Dilatation: No excessively dilated bowel. The bowel walls are not smooth. Haustra and plicae are preserved.
• Air-Fluid Levels: None.
• Arrangement of Loops: Large loops are not present.
• Impression: Within normal limits.

Case B: 7-day old female.

View Case B.

Interpretation of Case B

• Gas Distribution: There are pockets of gas scattered in several areas of the abdomen. There is gas in the small bowel, colon, and rectum.
• Bowel Dilatation: There is mild dilation of the bowel, mostly in the colon. The dilated segment of bowel in the left upper quadrant shows relatively smooth bowel walls. However, most of the bowel does not show this. In other words, the haustra and plicae of most of the bowel are well preserved.
• Air-Fluid Levels: None.
• Arrangement of Loops: The loops are not arranged in an orderly pattern.
• Impression: Ileus.

Case C: 17-day old male.

View Case C.

Interpretation of Case C

• Gas Distribution: There is gas over most of the abdomen. There are loops of bowel mostly in the central abdomen. The dilated loops are mostly small bowel.
• Bowel Dilatation: The bowel walls are smooth indicating that the bowel is dilated.
• Air-Fluid Levels: There are multiple short air fluid levels on the upright film (hair pin loops).
• Arrangement of Loops: Orderly, although not truly in a stepladder fashion. The arrangement here resembles a bag of sausages more so that a bag of popcorn.
• Impression: Small bowel obstruction. In this age, the mostly likely cause is an incarcerated inguinal hernia. This is confirmed clinically.

Case D: 1-month old female.

View Case D.

Interpretation of Case D

• Gas Distribution: There is a lot of gas in the small and large bowel distributed throughout the abdomen.
• Bowel Dilatation: The degree of bowel dilation here is proportional throughout. In other words, the large bowel is slightly dilated, as is the small bowel.
• Air-Fluid Levels: None.
• Arrangement of Loops: Disorderly arrangement of dilated bowel. This resembles a bag of popcorn rather than a bag of sausages.
• Impression: Ileus. The differential is extensive, including gastroenteritis, urinary tract infection, etc. However, an ileus is still compatible with several surgical conditions such as appendicitis.

Case E: 3-1/2 year old male.

View Case E.

Interpretation of Case E

• Gas Distribution: Increased gaseous distribution in both small and large bowel, with more colonic involvement. Gas is also present in the rectum.
• Bowel Dilatation: Note the smooth bowel walls resulting in the "sausage-like" appearance of some of the loops. There are several areas of extreme dilation. The stomach is also very dilated.
• Air-Fluid Levels: Multiple loops of bowel with air fluid levels. The typical "candy cane" appearance is not very dramatic.
• Arrangement of Loops: The loops are stacked in a somewhat orderly fashion. However, this is not definite. The "arrangement" should be best determined on the supine flat view and not the upright view. Although this arrangement resembles a bag of sausages more so than a bag of popcorn, this is not as clear-cut as in other cases.
• Impression: The gas distribution throughout the bowel suggests that this is not an obstruction. However, the reason for the extreme bowel dilatation is uncertain. This is still suspicious for an obstruction. Note the frothy density over the left flank area (supine view). This probably represents fecal matter. Though a fecal obstruction is possible, a BE or an UGI series would be helpful to evaluate other causes of obstruction such as malrotation or Hirshsprung's disease. A contrast enema and an UGI series were performed on this patient. Both were normal. His symptoms and bowel dilation gradually resolved after several enemas and bowel movements.

Case F: 7-month old male.

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Interpretation of Case F

• Gas Distribution: Relatively gasless in both large and small bowel. This is a poor gas distribution.
• Bowel Dilatation: In some of the few bowel loops that are seen, the bowel walls appear smooth.
• Air-Fluid Levels: There are no obvious air-fluid levels. However, in the upright view, the central abdomen shows the presence of two bowel loops resembling arches that are air-fluid levels which do not have the typical candy cane appearance. The candy cane appearance of air-fluid levels is usually not seen in infants.
• Arrangement of Loops: It is difficult to comment on the arrangement given the minimal gas pattern.
• Impression: Probable obstruction based mainly on the paucity of gas and its distribution. Since these radiographs are highly suspicious, the next recommended exam should be an ultrasound and/or a BE to evaluate the possibility of intussusception or appendicitis. An intussusception is often the cause of a bowel obstruction associated with a paucity of gas on plain radiographs A BE performed in this patient demonstrated an intussusception.

Case G: Newborn male.

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Interpretation of Case G

In this case, only a supine view is shown on the left. The image on the right is a contrast enema study.

• Gas Distribution: There is poor gas distribution with only 3 dilated loops of bowel, triple bubbles, probably representing high (i.e., proximal) small bowel loops. There is some gas in the left lower quadrant. This cannot be the colon since there is no gas in any other intervening bowel segments evident.
• Bowel Dilatation: As noted above, dilation is present in the loops seen. There is no colon gas evident.
• Air-Fluid Levels: An upright or lateral decubitus view is not shown here.
• Arrangement of Loops: Too few to comment.
• Impression: This is a proximal small bowel obstruction. The contrast enema on the right shows a microcolon indicating the absence of bowel contents passing to the colon during gestation. In a proximal small bowel obstruction, a microcolon is usually not present. The presence of a microcolon suggests that the distal small bowel is also atretic. This patient was ultimately diagnosed with a long segment small bowel atresia. Note that the contrast enema study also shows the cecum in the wrong position. It should be in the right lower quadrant, but it appears to be more medial than its expected positions. Malpositioning of the cecum is highly indicative of a malrotation.

Case H: 3-day old female.

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Interpretation of Case H

• Gas Distribution: Generalized presence of gas throughout all quadrants.
• Bowel Dilatation: The degree of bowel dilatation is proportional. The right lower quadrant may demonstrate some smooth bowel walls, but this is probably just the descending colon. Some of the haustra in these segments are still preserved. For the remainder of the bowel, the haustra and plicae are well preserved.
• Air-Fluid Levels: None.
• Arrangement of Loops: Disorderly arrangement resembling a bag of popcorn.
• Impression: Ileus.

Case I: 2-1/2 year old female.

View Case I.

Interpretation of Case I

• Gas Distribution: Well distributed throughout all quadrants.
• Bowel Dilatation: There are two dilated regions seen on the supine view in both lower quadrants. However, the bowel walls do not appear smooth. The typical sausage or hose appearance of dilated small bowel is not present. The haustra and plicae are still fairly well preserved.
• Air-Fluid Levels: The upright view shows many small air fluid levels. The typical hairpin or candy cane appearance is not present indicating that these air fluid levels are small and not present in large loops.
• Arrangement of Loops: Disorderly loops resembling a bag of popcorn more so than a bag of sausages (supine view).
• Impression: Moderate ileus versus partial obstruction. An ileus is more likely.

Case J: 3-year old female.

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Interpretation of Case J

• Gas Distribution: There is gas distributed throughout the abdomen. Most of the gas present is in the colon.
• Bowel Dilatation: There is moderate dilation of the colonic regions. There is a dilated loop of small bowel on the left (supine view) which overlaps the colon. The haustra and plicae are preserved. No sausages or hoses are seen (i.e., no smooth bowel walls are present).
• Air-Fluid Levels: None.
• Arrangement of Loops: Disorderly arrangement resembling a bag of popcorn more so than a bag of sausages.
• Impression: Ileus.

Case K: 9-day old male.

View Case K.

Interpretation of Case K

• Gas Distribution: Poor distribution. Although gas is present throughout most of the abdomen, its distribution appears to be limited to just a few bowel segments.
• Bowel Dilatation: Marked bowel distention though difficult to determine small versus large bowel. The bowel walls are smooth.
• Air-Fluid Levels: Multiple air-fluid levels mostly on the left. Hair pins and candy canes are not present.
• Arrangement of Loops: Not very helpful in this case. The arrangement is best evaluated on the supine view which is not obviously orderly or disorderly. In other words, it is not easy to say whether this arrangement resembles a bag of sausages or a bag of popcorn.
• Impression: Obstruction based mainly on the gas distribution and the degree of bowel dilatation. This is not a normal abdominal series for a 9-day old. A contrast enema demonstrated a transition zone consistent with Hirschsprung's disease.

Case L: 12-month old female.

View Case L.

Interpretation of Case L

• Gas Distribution: Small areas of gas are present throughout the entire abdomen. Many of the areas are foamy suggesting the presence of excessive amounts of stool.
• Bowel Dilatation: Most of the bowel is not dilated. There is a modest paucity of gas. There are two dilated loops in the RLQ on the supine view (RLQ sentinel loops).
• Air-Fluid Levels: None.
• Arrangement of Loops: Disorderly. Despite the paucity of gas, the supine view resembles a bag of popcorn more so than a bag of sausages.
• Impression: Ileus. RLQ sentinel loops raise the possibility of appendicitis.

Case M: 7-month old female.

View Case M.

Interpretation of Case M

• Gas Distribution: There is a definite paucity of gas which is poorly distributed.
• Bowel Dilatation: Nothing obvious.
• Air-Fluid Levels: None.
• Arrangement of Loops: Not a useful sign here because of the paucity of gas.
• Other comments: There is a "target sign" in the right upper quadrant. The target sign is discussed in detail in Case 2 of Volume 1. The target is faintly visible as a doughnut shape (with the doughnut center still present) in the right upper quadrant below the liver (supine view). This is subtle. You may have to turn down the room lights and adjust the contrast and brightness on your monitor. This sign indicates the presence of an intussusception. This radiograph also demonstrates the "absent liver edge" sign (liver edge not well defined in any view), which is also a sign of intussusception (though less specific than the target sign). If you have difficulty identifying the target and liver edge findings in this radiograph, review Case 2 of Volume 1 for other examples that are easier to identify.
• Impression: Suggestive of an obstruction based mainly on the paucity of gas. The target sign indicates the presence of an intussusception. A barium enema confirmed an intussusception.

Case N: 22-month old.

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Interpretation of Case N

• Gas Distribution: Good distribution except for one portion in the LUQ. Although the upright view appears to be somewhat gasless with most of the gas seen localized to the upper abdomen only, the supine view shows a better distribution of gas.
• Bowel Dilatation: There are no dilated regions. The haustra and plicae are well preserved.
• Air-Fluid Levels: None.
• Arrangement of Loops: Disorderly.
• Other Comments: The supine view demonstrates "thumb printing" suggesting bowel wall edema such as that seen in colitis. This is best seen in the LUQ region (or left middle region) where the colon shows thumb-shaped indentations into its lumen.
• Impression: Ileus, colitis.

Case O: 11-month old male.

View Case O.

Interpretation of Case O

• Gas Distribution: Poorly distributed. Gas is concentrated in the left upper quadrants in both the supine and upright views.
• Bowel Dilatation: There are two dilated bowel segments seen on the supine view. The bowel walls are smooth and resemble sausages.
• Air-Fluid Levels: None.
• Arrangement of Loops: Orderly. Note the two dilated bowel segments on the supine view are stacked on top of each other resembling a step ladder. Also, this view clearly resembles a bag of sausages (only two big ones), rather than a bag of popcorn.
• Impression: Obstruction. A barium enema in this case demonstrated intussusception.

Case P: 6-1/2 year old male.

View Case P.

Interpretation of Case P

• Gas Distribution: Well distributed except for a paucity of gas in the left lower quadrant.
• Bowel Dilatation: The haustra and plicae are well preserved. No smooth bowel walls are visible. The caliber of the bowel is proportional to the normal bowel size.
• Air-Fluid Levels: None.
• Arrangement of Loops: Disorderly. Does not resemble a bag of sausages. Nor does it truly resemble a bag of popcorn. However, there is no order to the arrangement.
• Impression: Ileus. There is a possible appendicolith in the right lower quadrant (spherical density). This is highly suggestive of acute appendicitis. This again stresses the point, that an ileus is not necessarily benign.

References

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2. Swischuk LE. The Alimentary Tract. In: Radiology of the Newborn and Young Infant, second edition. Baltimore, Williams & Wilkins, 1980, pp. 487-490.
3. Kirks DR. The Gastrointestinal Tract. In: Practical Pediatric and Diagnostic Radiology of Infants and Children. Boston, Little, Brown and Company, 1984, pp. 551-553.
4. Parker BR. The Abdomen and Gastrointestinal Tract. In: Silverman FN, Kuhn JP. Caffey's Pediatric X-Ray Diagnosis, Ninth edition. St. Louis, Mosby, 1993, pp. 1059-1089.
5. Squire LF, Novelline RA. The Abdominal Plain Film: Distended Stomach, Small Bowel, Colon, Free Fluid and Free Air. In: Fundamentals of Radiology, 4th edition. Cambridge, MA, Harvard University Press, 1988, pp. 194-205.