Ileus

The term ileus appears to have undergone a subtle change in meaning/usage. The term Ileus originally was used to denote stasis of bowel contents. The term had two common uses

• Adynamic Ileus ( syn paralytic ileus, non-obstructive ileus)

This referred to non-functioning bowel for reasons other than obstruction

• Obstructive Ileus

This referred to stasis of bowel contents for reasons related to obstruction

Adynamic Ileus ----------> was shortened to ----------> Ileus
Obstructive Ileus----------> was shortened to ----------> obstruction

• Motility Disorder

This term refers to abnormal motility of bowel contents without specifying whether it is obstructive or non-obstructive in nature.

The hallmark of small bowel obstruction is the presence of gaseous loops of small bowel which are distended over 30mm. The absorptive capacity of the small bowel is so great that even extreme amounts of air swallowing will not distend normal small bowel.



Correlation with patient history and clinical signs can assist in arriving at a more specific diagnosis. The difficulty in differentiating obstruction from ileus has led some radiologists to use the blanket term "motility disorder" when describing dilated loops of bowel.

The bowel could reasonably be said to be a very sensitive organ. It has a propensity to stop functioning with little provocation. Amongst the possible causes are infection (anywhere), abdominal inflammation, chemical/pharmacological causes, post-operation peritonitis, electolyte imbalance, and trauma. 
Abdominal surgery commonly results in generalised adynamic ileus in which the bowel is temporarily non-functioning. This typically manifests on around day 4 post-op.

ADYNAMIC / PARALYTIC ILEUS

The appearance of generalised adynamic ileus on plain film is quite characteristic. The large and small bowel are extensively airfilled but not dilated. This is often described as the large and small bowel "looking the same".

LOCALISED ILEUS / SENTINEL LOOP SIGN

The cause of the inflammation is unknown but would be typical of ulcerative colitis or Crohn's disease. This appearance is known as "thumbprinting".

Just inferior to the diseased segment of colon are a few prominent air-filled loops of jejunum. It is possible that this is localised ileus of the jejunum associated with the diseased colon. This is known as "sentinel sign".


SMALL BOWEL OBSTRUCTION

Vomiting may release some of the proximal bowel contents and reduce the amount of proximal dilation. The bowel hyperperistalses. Bowel distal to the point of obstruction (i.e. colon and sometimes distal small bowel) empties over time. Strangulation of the bowel may result from vascular compromise of the affected loops and is a cause of increased mortality.

Causes: Overwhelmingly, the most common cause of a mechanical small bowel obstruction are adhesions related to prior surgery /appendectomy, colorectal surgery and gynecologic surgery/ (60%). Hernias Most often femoral or inguinal.   Intussusception. Volvulus. Tumor, either primary or metastatic. Wall lesions such as leiomyomas or strictures. Crohn’s disease.Foreign bodies. Gallstones.

  Imaging findings:

o       Conventional radiography is the study of first choice

§         Loops proximal to the point of obstruction will become dilated and fluid-filled

·         Usually greater than 2.5-3 cm in size

§         Differential height of air-fluid levels in the same loop of small bowel no longer considered reliable sign of mechanical SBO

§         Absence of, or disproportionately smaller amount of, gas in the colon, especially the rectosigmoid

§         Loops of small bowel may arrange themselves in a step-ladder configuration from the left upper to the right lower quadrant in a distal SBO

§         Mostly fluid-filled loops of bowel may demonstrate a string-of-beads sign caused by the small  amount of visible air in those loops

Closed-loop obstructions

o       Most (75%) are caused by adhesions

o       In a closed-loop obstruction, the twisted loop itself remains dilated with gas and fluid thus producing a dilated, U-shaped loop of small bowel

§         Does not change in position or size over time

·         Coffee bean sign or pseudotumor may be seen

o       Closed-loop obstructions are not usually diagnosable by conventional radiography and require CT

§         CT findings may include a U- or C-shaped loop of small bowel

§         A spoke-like configuration of the mesentery demonstrating stretched vessels converging on the site of the twist may be seen

·         The appearance of the tightly twisted mesentery has been called the whirl sign

§         The beak sign may be seen as a fusiform tapering at the site of the obstruction

·         Treatment of small bowel obstruction

o       Many patients are treated conservatively with small bowel decompression and intravenous fluids

o       Surgical intervention may be necessary if there are signs and symptoms of strangulation, peritonitis or lack of response to conservative treatment

Differentiating SBO from Paralytic Ileus
	SBO	Ileus
Etiology	Patient with prior surgery weeks to years prior	Recent (hours) post-operative patient
Pain	Colicky	Not a prominent feature
Abdominal distension	Frequently prominent	Sometimes not apparent
Bowel sounds	Usually increased	Usually absent
Small bowel dilatation	Present	Present
Large bowel dilatation	Absent	Present

Large bowel obstruction

The most common causes of large bowel obstruction are colo-rectal carcinoma and diverticular strictures.  Less common causes are hernias or volvulus (twisting of the bowel on its mesentery). Adhesions do not commonly cause large bowel obstruction.

Radiological appearances of large bowel obstruction differ from those of small bowel obstruction, however, with large bowel obstruction there is often co-existing small bowel dilatation proximally.

Dilatation of the caecum >9cm, and >6cm for the rest of the colon is considered abnormal.

Volvulus

Twisting of the bowel, or volvulus, is a specific cause of bowel obstruction which can have characteristic appearances on an abdominal X-ray.

The two commonest types of bowel twisting are sigmoid volvulus and caecal volvulus.

Sigmoid volvulus

Unlike the majority of the large bowel, the sigmoid colon has its own mesentery. It is therefore more prone to twisting at the root of its mesentery, which is located in the left iliac fossa. The result is the formation of an enclosed bowel loop which becomes dilated. If untreated this can lead either to perforation due to excessive dilatation, or to ischaemia due to compromise of the blood supply.

Sigmoid volvulus - coffee bean sign 

Caecal volvulus

The caecum is most frequently a retroperitoneal structure, and therefore not susceptible to twisting. However, in up to 20% of individuals there is congenital incomplete peritoneal covering of the caecum with formation of a 'mobile' caecum on a mesentery, such that it no longer lies in the right iliac fossa. This is a normal variant but is associated with increased incidence of folding or twisting of the caecum (caecal volvulus), which may be complicated by obstruction, vascular compromise, or perforation. 

Bowel wall inflammation

Occasionally, abdominal X-rays show signs of inflammation in patients with inflammatory bowel disease. Abnormalities may relate to either acute or chronic stages of disease.

• Abdominal X-rays sometimes demonstrate signs of bowel inflammation such as mucosal thickening 'thumb-printing' or a featureless colon 'lead pipe' colon.

Mucosal thickening - 'thumbprinting' - ACUTE EXACERBATION

Lead pipe colon - CHRONIC INFLAMMATION (ulcerative colitis)

 

Toxic megacolon

Toxic megacolon is a potentially life-threatening condition characterized by dilatation of the large bowel without obstruction, in the context of acute bowel inflammation. This may be due to inflammatory bowel disease, especially ulcerative colitis, or other causes of colitis such as infection.

  Gasless abdomen

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.

DISCUSSION

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 apaucity 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.

SUMMARY

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.

SOURCES:
http://www.wikiradiography.com/page/The+Abdominal+Plain+Film-+Ileus
http://www.learningradiology.com/archives06/COW%20216-SBO/sbocorrect.htm
http://radiologymasterclass.co.uk/tutorials/abdo/abdomen_x-ray_abnormalities/pathology_large_bowel_obstruction.html 
http://www.hawaii.edu/medicine/pediatrics/pemxray/v3c18.html