Although not immediately obvious, this patient’s radiographs
have findings suggestive of mechanical small bowel obstruction (SBO).
Detection of SBO is the primary role for abdominal radiography in
patients presenting to the ED with abdominal pain. However, radiographs
are diagnostic of obstruction in only 50% of cases; in
30% of cases, they are suggestive, but not diagnostic;
and in 20%, they are negative (Mucha 1987). Interpretation
of bowel gas patterns can be difficult, and there is considerable
interobserver variability, even among radiologists (Suh et al. 1995,
Markus et al. 1989).
of Small Bowel Obstruction
There is a wide range of clinicial presentations of mechanical
SBO. The classical clinical presentation
includes intermittent, crampy, periumbilical abdominal pain accompanied
by vomiting, and nonpassage of feces or flatus. On examination,
the patient’s abdomen is distended and tympanitic, and
the bowel sounds are typically high-pitched and hyperactive. The
abdomen may be mildly tender, but there should be no rigidity or
rebound tenderness, unless the obstructed bowel is ischemic. Most
patients have had prior abdominal surgery causing postoperative
In patients with classical clinical presentations, the diagnosis
is usually obvious on clinical examination. Radiographic studies
serve mainly to confirm the clinical impression.
Many patients with SBO have atypical
clinical presentations. Pain may be constant, mild, and even
localized. Abdominal distention and vomiting may be minimal, and
passage of feces and flatus may continue until bowel distal to the
obstruction has evacuated its contents which can take 1–3
days or more. Muted clinical presentations are common in elderly
and debilitated patients.
SBO should be considered as a cause of abdominal pain in all
patients with prior abdominal surgery. However, obstruction can
have etiologies other than postoperative adhesions (Table 1).
Table 1 Causes of SBO |Favorite Table|Download (.pdf)
Table 1 Causes of SBO
|Crohn disease:||up to 7%|
|Bowel wall lesions (causing intussusception)|
|Intraluminal mass: foreign body, gallstone ileus|
|Extrinsic inflammatory lesions (appendiceal)|
Patients with SBO
Initial management of patients
with SBO includes nasogastric suction to decompress the stomach,
prevent further intestinal distention, and reduce the risk of aspiration.
Intravenous fluids are administered to restore intravascular volume
When bowel ischemia is suspected, emergency surgery is indicated. Some
surgeons will operate when radiography shows complete/high-grade
obstruction because of the increased risk for ischemia.
The rationale for the traditional aggressive surgical approach
(“never let the sun rise or set on a bowel obstruction”)
is that clinical parameters are unreliable in excluding ischemia,
and delay in operative intervention results in increased morbidity
and mortality. However, current selective surgical management allows
many patients with SBO to be managed without surgery. Nonetheless,
morbidity and mortality are still substantially increased if there
is delay in operating on patients with ischemic bowel obstruction
(Fevang et al. 2003).
When there are no clinical signs of bowel ischemia, and no high
risk factors such as an incarcerated hernia or closed-loop obstruction,
most surgeons will attempt a trial of nonoperative
(conservative) care. Surgery is performed if signs of ischemia
develop or if the obstruction does not resolve over a period of
time—usually 24–48 hours. If successful, a conservative
approach avoids the short-term morbidity associated with laparotomy
and the potential creation of additional adhesions (Cox et al. 1993,
Serror et al. 1993, Fevang et al. 2002, Shih et al. 2003).
and Closed-Loop Obstruction
Bowel wall ischemia is the most
dangerous complication of bowel obstruction, and is the principal
indication for emergency surgery. There are two
mechanisms: (1) marked bowel distention that impedes bowel
wall perfusion and (2) mesenteric vascular compression that occurs
with an incarcerated hernia or small bowel volvulus. Volvulus occurs when
a loop of bowel twists around its mesentery. Small bowel volvulus is also called a “closed-loop obstruction” because
a single loop of bowel is occluded at both ends (Figure 2).
Closed-loop SBO and volvulus.
(A) Adhesive band causes obstruction.
(B) Small bowel volvulus closed-loop.
[From Balthazar et al. Closed-loop and strangulating
intestinal obstruction: CT signs. Radiology 1992;185:769,
The spectrum of bowel wall ischemia
ranges from reversible ischemia that resolves when the obstruction
is relieved, to irreversible ischemia and infarction that requires
resection of the involved bowel segment. The incidence of
ischemia in patients with SBO ranges from 8% to 38% (Miller
et al. 2000, Shih et al. 2003, Fevag 2002).
Clinical signs of ischemia include
pain that is constant (rather than
intermittent), marked abdominal tenderness, rigidity or rebound
tenderness, fever or leukocytosis, tachycardia, and signs of sepsis
(Table 2). Small bowel volvulus typically
causes an abrupt onset of abdominal pain.
Table 2 Ischemic Bowel Obstruction |Favorite Table|Download (.pdf)
Table 2 Ischemic Bowel Obstruction
|Pain is constant, not intermittent|
|Marked abdominal tenderness, rigidity, rebound tenderness|
|Fever or leukocytosis|
|Signs of sepsis|
|Small bowel markedly distended (on abdominal radiography)|
|Incarcerated hernia |
|Closed-loop obstruction (on CT)|
|CT signs of bowel wall ischemia |
The absence of these clinical signs, however, does not reliably
exclude ischemia. Therefore, patients being managed nonsurgically
must be observed closely. Patients at higher risk of ischemia are
those with advanced high-grade obstruction (marked bowel distention
on the abdominal radiograph), an incarcerated hernia, or a closed-loop
Computed tomography (CT) can detect
signs of bowel wall ischemia or closed-loop obstruction (Donecker
1998). However, CT has only moderate sensitivity (70–96%)
and specificity (60–95%) for bowel ischemia (Sheedy et
al. 2006, Zalcman et al. 2000, Frager et al. 1996). Therefore,
although helpful, CT should not be used alone to diagnose ischemic
bowel obstruction; the decision to operate or to continue nonoperative
care is based on the entire clinical scenario.
Principles of SBO
With SBO, both gas and fluid accumulate within bowel proximal
to the obstruction, and bowel distal to the obstruction empties
its contents. Distal small bowel becomes collapsed soon after the onset
of obstruction, whereas the colon does not empty for 1–2
days or longer. The principal diagnostic
imaging finding of SBO is, therefore,
distention of small bowel proximal to the obstruction and collapse
or a relative paucity of gas and fluid in bowel distal to the obstruction.
Bowel distention also occurs in adynamic
ileus; however, with adynamic ileus, there is no disparity
in distension between proximal and distal bowel. Adynamic ileus
is caused by diminished gut motility due to a wide variety of abdominal and
systemic disorders (see “Introduction to Abdominal Radiology,” Table
6). With mild or localized ileus, only a portion of bowel
is distended, whereas with severe adynamic ileus, the entire large
and small bowel is distended. Diffuse bowel dilation also occurs
with distal large bowel obstruction.
Abdominal radiography readily demonstrates
bowel gas and is therefore useful for the diagnosis of bowel obstruction, when
there is a considerable quantity of air within bowel proximal to
the obstruction. Collapsed distal small bowel cannot be seen on
abdominal radiography and the colon is therefore the portion of
distal bowel that is observed to ascertain whether distal bowel
is collapsed. However, the colon is often not totally devoid of
gas and feces in patients with SBO because it can take two days
or longer for large bowel to evacuate its contents.
When there is less disparity in caliber between small and large
bowel, SBO has an appearance similar to adynamic ileus. In addition,
when the obstructed small bowel is entirely fluid-filled, it is not
visible on abdominal radiography. Abdominal radiography should therefore
not be used to exclude the diagnosis of SBO.
Computed tomography can diagnose
SBO when conventional radiography is not conclusive (50% of
patients). On CT, collapsed distal small bowel can be directly visualized.
In addition, CT can visualize dilated loops of bowel that are completely
fluid-filled and are therefore not visible on conventional radiography.
The sensitivity of CT for SBO is as high as 95% (Megibow
et al. 1991).
CT can also determine the cause of obstruction (adhesions, tumors,
hernias, etc.) and detect signs of bowel ischemia and closed-loop
Strategy in Patients with Suspected SBO
Abdominal radiography is the traditional
diagnostic test in patients with suspected SBO. However, abdominal
radiography is diagnostic in only 50% of patients with
SBO. Abdominal radiography is most likely to be diagnostic in patients
with abdominal distention due to gas-filled bowel.
CT is indicated when abdominal
radiography is nondiagnostic. In many hospitals, CT is also used whenever
SBO is being managed nonoperatively to detect signs of bowel ischemia
or a closed-loop obstruction that might not be evident clinically.
CT is also indicated when the cause of obstruction is uncertain
such as in patients who have not had prior abdominal surgery or
in patients who have had both an intra-abdominal malignancy and
prior abdominal surgery.
In EDs where CT is readily available, CT can be used as the initial
imaging test. CT is more often diagnostic and can provide information
about other disorders responsible for the patient’s abdominal
Abdominal radiography still has a role in the diagnosis of SBO,
although its use should be limited to patients in whom the clinical
presentation is classic—crampy abdominal pain and vomiting with
gaseous distention of the abdomen, particularly in a patient who
has had prior abdominal surgery. Abdominal radiography can be performed
more quickly than CT and, when the diagnosis of SBO is confirmed,
an earlier surgical consultation can be made. If CT will be ordered
irrespective of the abdominal radiograph results or the clinical
presentation is not classic for SBO, it is reasonable to omit abdominal
radiography and proceed directly to CT, because abdominal radiography is
unlikely to be diagnostic.
One precaution in the use of CT is that patients with SBO often
cannot tolerate oral contrast due to vomiting, which could lead
to substantial delays in obtaining CT. In these cases, oral contrast should
be omitted because it is not essential to making the diagnosis of
SBO on CT.
Signs of Mechanical SBO
On the supine abdominal radiographs, the principal diagnostic
finding is dilated gas-filled, small bowel proximal to the obstruction
and a relative paucity of air and feces in large bowel (Table 3). The
presence of gas in the distal bowel (colon or rectum) does not negate
the diagnosis of obstruction; it means simply that the distal bowel
has not yet evacuated all of its contents.
Table 3 Radiographic Signs of SBO |Favorite Table|Download (.pdf)
Table 3 Radiographic Signs of SBO
|Distended small bowel (>3 cm) (“stack-of-coins”)|
|Relative paucity of air and feces in colon|
|Differential air/fluid levels (“step-ladder”)|
|Broad air/fluid levels (“tortoise-shell”)|
Distended small bowel has a diameter greater than 3 cm. The valvulae
conniventes make distinctive thin transverse bands extending completely
across the bowel lumen (Figures 3 and 4). Numerous valvulae conniventes
in distended small bowel can have a “stack-of-coins” appearance
(Figure 5A). Valvulae conniventes are not present in the distal
ileum, which, when distended, has a smooth tubular appearance.
Complete small bowel obstruction.
(A) There is marked distention of
small bowel and nearly complete absence of air in distal bowel (a small
amount of air remains in the rectum).
(B) The upright radiograph shows
characteristic broad air/fluid levels (“tortoise-shell” sign)
(arrows) and a “string of-pearls” (arrowheads).
The patient had adhesions due to a prior laparotomy for a stab
wound to the abdomen. Because of the marked bowel distention, surgery
was performed shortly after admission for lysis of adhesions.
“Partial,” although definite, SBO
and differential air/fluid levels.
(A) On the supine radiograph, there
are dilated loops of small bowel (SB) and a relative paucity of air
in the colon. Air in the rectum (R) does not negate the diagnosis
of bowel obstruction, but indicates only that the distal colon has
not yet expelled all of its contents. This radiographic pattern may
be referred to as a “partial” small bowel obstruction
even though it is advanced and is “definite” for
the diagnosis of SBO.
(B) The upright radiograph shows differential (dynamic) air/ fluid
levels— air/fluid levels that are at different
heights in a single loop of bowel (arrows).
Differential air/fluid levels are suggestive of mechanical
obstruction. Simple (nondifferential) air/fluid levels
are seen in both mechanical obstruction and adynamic ileus (see “Introduction
to Abdominal Radiography,” Figure 3B in Abdominal Radiography).
This patient developed SBO two weeks after a laparotomy performed
for lysis of adhesions due to a prior stab wound to the abdomen.
This early postoperative bowel obstruction was successfully managed
using nasogastric decompression.
“Stack-of-coins” and “string
of pearls” signs.
(A) On the supine radiograph, a
single distended loop of dilated small bowel has numerous valvulae conniventes
extending across the bowel lumen, giving it a “stack-of-coins” appearance.
(B) The upright radiograph illustrates
the genesis of the “string-of-pearls sign”. There
is a row of small air pockets representing gas that had become trapped
under valvulae conniventes when the patient assumed an upright position
(arrows). In addition, there are two
broad air/fluid levels, which are also characteristic of
mechanical small bowel obstruction (arrowheads).
This patient presented with periumbilical abdominal pain and
vomiting and had not had prior abdominal surgery. After abdominal
radiography, she was reexamined and a small incarcerated umbilical
hernia was found (C). The hernia was
reduced in the ED and the patient was hospitalized for urgent, rather
than emergent, hernia repair.
The upright radiograph can be helpful in confirming the diagnosis
of SBO. Occasionally, SBO is more evident on the upright than the
Fluid and gas within the bowel lumen create distinctive air/fluid levels. The upright
radiograph is especially helpful when the obstructed bowel is mostly
fluid-filled and signs of SBO are less evident on the supine view.
Air/fluid levels occur in both obstruction and adynamic
ileus. However, the air/fluid levels associated with SBO
have several distinguishing features. With SBO, the air/fluid
levels at opposite ends of an inverted U-shaped loop of bowel are
often at different heights. These are known as differential
(dynamic) air/fluid levels (Figure 4B). This effect
is presumably due to the action of peristalsis. Air/fluid
levels are more specific for SBO when there is greater height differential
(>15 mm) (Harlow et al. 1993, Lappas et al. 2001). Multiple differential
air/fluid levels have a “step-ladder” appearance.
Broad air/fluid levels (“tortoise-shell” sign) are also characteristic of
mechanical obstruction (Figure 3B). With adynamic
ileus; air/fluid levels tend to be less prominent
and, when present, are usually small and isolated.
One type of air/fluid level that is highly suggestive
of obstruction is the string-of-pearls sign.
Small pockets of air are trapped under the valvulae conniventes
when the patient assumes an upright position. This appears as a
row of small air bubbles on the radiograph (Figures 3B, 5B, 6, and
7). The string of pearls sign is especially helpful when the small
bowel is nearly entirely fluid-filled, which makes bowel distention
impossible to detect on the supine radiograph.
“String-of-pearls” sign depicted on
In another patient with SBO, the abdominal radiographs were negative
because the bowel was nearly entirely filled with fluid. On CT,
obstructed small bowel is markedly dilated and fluid-filled. There
are several small pockets of air trapped under valvulae conniventes
producing a CT “string-of-pearls” sign (arrowheads).
(A) This patient’s supine
radiograph showed a single loop of dilated small bowel in the mid-abdomen
(large arrow) and air in the colon
(C). Although this dilated small bowel loop is abnormal, SBO cannot
be diagnosed on the supine view because there is no clear disparity
in distention between proximal and distal bowel. The single dilated
loop of small bowel could be interpreted as a localized or mild
Most of the obstructed small bowel was not visible on the radiograph
because it wasfilled with fluid. Gas in large bowel was not relatively diminished compared
to small bowel because the obstruction developed rapidly and the colon
did not have time to empty its contents.
Incidentally, two small phleboliths are seen in the pelvis (small arrows).
upright radiograph is suggestive of obstruction. Most of the bowel
is fluid-filled, so there are few air/fluid levels. However,
there is a string-of-pearlssign, which is indicative of mechanical
small bowel obstruction (arrow).
When the patient is too ill to stand upright, a left lateral
decubitus view should be obtained to detect air/fluid levels
(a cross-table AP view of the abdomen with the patient lying left-side
down) (see Figure 9B). This view is also used to detect
free intraperitoneal air (see Patient 3, Figure 8 in Pneumoperitoneum).
Small bowel obstruction with negative radiographs.
An 84-year-old woman presented with abdominal pain. Abdominal
radiographs were “negative.” A CT of the abdomen
revealed distal SBO.
CT slices through the lower pelvis demonstrated that the cause
of obstruction was an incarcerated femoral hernia (E). The large
incarcerated femoral hernia was not noted initially due to an incomplete
A nearly gasless abdomen can be seen in patients with high-grade
obstruction when the obstructed bowel is entirely fluid-filled.
(A) Supine radiograph is “negative,” i.e.,
no signs of obstruction. The abdomen is nearly entirely gasless.
(B) Left lateral decubitus abdominal
radiograph (left-side down) was obtained because the patient was
too weak to stand upright.
There is a single dilated loop of bowel with an isolated air/fluid
(C and D)
CT slices through the lower abdomen at the level of the iliac crests.
There is markedly dilated small bowel (SB). The obstructed small
bowel is nearly entirely fluid-filled, which accounts for the negative
(gasless) abdominal radiograph. Oral contrast is present in the
obstructed small bowel. There is collapsed small bowel (Coll SB)
in the right lower quadrant. The colon is also collapsed (Colon).
slice of the lower pelvis reveals a large femoral hernia (arrow), which was the cause of the
femoral hernia follows the course of the femoral artery and vein
into the anterior thigh inferior to the inguinal ligament.
In traditional radiographic nomenclature, when there is a total
absence of gas in large bowel and marked proximal bowel distention,
obstruction is termed “complete” (Figure
3). When there is any gas in the colon or rectum, the obstruction
is termed “partial” or “early” (Figure
This terminology can be problematic because it does not distinguish
between a “partial” obstruction, in which there
is a great disparity in caliber between proximal small bowel and
colon, i.e., the radiographs are definitely diagnostic of SBO (Figure
4), and a “partial” obstruction in which there
is little disparity in bowel distention, i.e., the radiographs are
merely “suggestive” of or “equivocal” for
SBO (Figure 8). In addition, the terminology is potentially confusing
because a radiographically “partial obstruction” can
be physiologically complete (i.e., no bowel contents can pass the
point of obstruction) or advanced (if proximal bowel is markedly
distended), when the distal bowel has simply not emptied all of
its contents, i.e., an “early” SBO. The term “partial” or “early” SBO
could create an erroneous impression that the obstruction is mild
or the diagnosis is questionable.
Small bowel obstruction with suggestive but not diagnostic
A 45-year-old man presented with abdominal pain, vomiting, and
abdominal distention. Two years earlier, he had undergone laparotomy
following a stab wound to the abdomen.
The initial radiographs were suggestive but not diagnostic of
obstruction. Adynamic ileus can have a similar radiographic appearance.
An abdominal CT obtained several hours later revealed a high-grade
mechanical SBO. The patient was treated with laparotomy for lysis
A. Supine radiograph. There is gaseous distention of small bowel
and considerable gas and feces in large bowel. Small bowel is centrally
located. One distended loop of ileum has a “bent finger” appearance
(*). The colon (C) is peripheral and contains feces .
B. Upright radiograph. A few nondifferential air/fluid
levels are present. One broad air/fluid level is seen in
the left lower quadrant—the “tortoise-shell” sign
(C) Small bowel is markedly distended
and mostly fluid-filled (SB). The small bowel wall is normal — thin
and uniformly enhancing with visible valvulae conniventes. Large
bowel is not distended but contains air and feces (Colon), which is why the abdominal radiographs
were not diagnostic of obstruction.
The obstruction could be termed “partial” even
though it is a “high-grade” obstruction with markedly
distended small bowel proximal to the obstruction. Oral contrast
was administered, but it did not reach the obstructed small bowel.
Enteric contrast is not necessary to visualize distended bowel containing
fluid or air.
(D) The distended bowel tapers to the transition point (Tr).
There is no lesion at the transition point, implying that postoperative
adhesions are the cause of obstruction.
(E) Distal small bowel
is collapsed (Coll SB).
Rather than categorize the radiographic findings of SBO as “partial
(early)” or “complete,” an interpretation
can be based on diagnostic certainty (Harlow et al. 1993). “Definite SBO” or “probable SBO” is reported
when there is marked distention in proximal small bowel and little
or no gas or feces in the colon (Figures 3, 4, and 5). “Possible SBO” is reported
when there is more gas in the colon and less distention in proximal
small bowel such that SBO is questionable and further testing with
CT is needed (Figure 8). Finally, radiographs can be negative for
signs of SBO even though obstruction is present (Figure 9).
of Abdominal Radiography for SBO
There are several reasons that abdominal radiographs may be nondiagnostic
in patients with small bowel obstruction (Table 4). These are:
(1) partial or early obstruction with less disparity in distention
between small and large bowel makes differentiation from adynamic
ileus difficult (Figure 8); (2) obstructed bowel is filled with
fluid and therefore not visible on the radiographs (Figure 9);
and (3) proximal small bowel obstruction such that only a short
segment of bowel becomes dilated. When obstructed bowel is entirely
filled with fluid, the abdomen may appear relatively gasless, i.e., radiographs are normal
or negative for signs of SBO. Therefore, negative, nonspecific,
and mild ileus patterns do not exclude bowel obstruction.
Table 4 Reasons for Nondiagnostic Radiographs in
Patients with SBO |Favorite Table|Download (.pdf)
Table 4 Reasons for Nondiagnostic Radiographs in
Patients with SBO
|Partial or early SBO|
The specificity of abdominal radiography is
limited, particularly when there is a partial” or “early” obstruction—because
adynamic ileus has a similar radiographic picture (Figure 10). In
addition, acute mesenteric ischemia can produce a “pseudo
SBO” pattern in which a segment of small bowel is ischemic
and therefore dilated and there is a disparity in distention between
small bowel (the ischemic segment) and large bowel.
Radiographs suggestive of obstruction but not due to
A 25-year-old woman presented with lower abdominal pain and nausea.
The abdominal radiographs were “suggestive” of
partial SBO. The supine radiograph shows moderately dilated small
bowel and a relative paucity of air in the colon. The upright radiograph shows
several nondifferential air/fluid levels.
An abdominal CT revealed markedly thickened bowel wall of the
distal small bowel, including the terminal ileum, consistent with
Crohn disease. There was no bowel obstruction; bowel dilation was
due to adynamic ileus.
Radiographs that are “suggestive” of obstruction
(especially partial SBO) should not be considered diagnostic of
obstruction. Clinical correlation and further diagnostic testing,
i.e., CT, is necessary. (See Introduction to Abdominal Radiology,
Figure 3—radiographs in a patient with appendicitis
showing “early SBO versus localized ileus.”)
Finally, undue equivocation in the radiologist’s
report can also be a source of misdiagnosis. When gas is present
in large bowel, the obstruction may be termed “partial” or “early.” However,
this does not adequately differentiate high-grade partial obstruction
that has relatively definitive radiographic findings from partial
obstruction in which the radiographs are equivocal. Even when there
is great disparity in caliber between proximal and distal bowel,
the radiograph might still be reported as “possible partial
or early obstruction versus ileus,” even though the diagnosis
of obstruction is likely and the obstruction is more advanced. Because
radiologist’s reports of bowel gas patterns often are equivocal,
the clinician should review the radiographic images themselves rather
than rely solely on the radiologist’s report (Figure 11).
Delayed diagnosis of bowel obstruction due to an unduly
equivocal radiologist’s report.
A. Supine view. B. Upright view. C. CT showed mechanical bowel obstruction with markedly dilated
A 42-year-old woman presented to the ED with upper abdominal
pain and vomiting. She had a uterine myomectomy four months earlier.
The radiographic findings were correctly noted in the radiologist’s report, although
the “impression” understates their significance.
With gas in the colon, obstruction is traditionally called “partial” or “early.” These
terms do not, however, distinguish between “definite” obstruction
(present in this patient, despite the gas in the colon) and “possible” obstruction, which
cannot be differentiated from adynamic ileus. This ambiguity contributed
to the delay in diagnosis.
After the radiologist’s interpretation was reviewed,
an ultrasound of the right upper quadrant was ordered because the
patient had tenderness in that location. The sonogram revealed gallstones without
definite signs of cholecystitis. On the advice of a surgical consultant,
a HIDA (hepatobiliary) scan was obtained to determine whether the
gallstones were responsible for the patient’s abdominal
pain. The scan was normal.
After that, the patient had an abdominal
CT scan which revealed SBO (C).
Twenty hours after her arrival, the patient was taken to surgery.
Bowel obstruction with mural ischemia was found and treated with
lysis of adhesions. Bowel resection was not necessary.
Review of the initial radiographs (A and B)
reveals that they were, in fact, diagnostic of obstruction. Small
bowel was markedly dilated and only a small amount of air and feces
were in the cecum (C in Figure 11A). There were several differential air/fluid levels
on the upright radiograph (arrows in
Because written reports of bowel gas patterns are often vague,
it is important for the clinician to examine the radiographs, rather
than rely solely on the radiologist’s report.
Had cholecystitis been a plausible concern, CT would have been
a better test than HIDA scan because CT has good sensitivity for
cholecystitis (>90%), can detect other abdominal disorders, and
can be performed more quickly. However, had the radiologist’s
report been less equivocal, the diagnosis of SBO would have been
After the abdominal radiographs were reviewed, the patient was
carefully reexamined and a small left inguinal hernia was found.
The hernia could not be reduced in the ED. An incarcerated (irreducible)
hernia requires emergency surgery when it is causing bowel obstruction
or ischemia (strangulation).
The patient was taken to the operating room for reduction and
repair of the incarcerated hernia.
In all patients with abdominal pain, a careful search should
be made for hernias, even if the patient is not complaining of localized
pain or swelling.
CT scanning is a major advance in diagnostic imaging for bowel
obstruction. In addition to being more sensitive and specific than
radiography, CT can determine the cause of obstruction in 75–90% of
cases, and can detect signs of bowel ischemia and closed-loop obstruction
(Table 5) (Megebow 1991, Balthazar 1994, Maglinte 1997, Macari 2001).
Table 5 Role of CT in SBO Diagnosis |Favorite Table|Download (.pdf)
Table 5 Role of CT in SBO Diagnosis
|Confirm obstruction—sensitivity up to 95%|
|Cause of obstruction—adhesions, hernia, tumor|
|Bowel ischemia—sensitivity 70–96% |
The CT diagnosis of SBO is based
upon seeing: (1) dilated small bowel proximal to the obstruction,
(2) collapsed small bowel distal to
the obstruction, and (3) a transition zone (Table 6, Figures 8 and 9). A transition zone without a lesion or mass implies that
the obstruction is due to an adhesion (Figure 8). With an adhesion,
the obstructed small bowel may taper at the transition zone and
have a “bird’s beak” appearance. Large
bowel is usually not totally collapsed and devoid of gas
and feces (Figure 8).
Table 6 CT Signs of SBO |Favorite Table|Download (.pdf)
Table 6 CT Signs of SBO
|Dilated proximal small bowel (> 2.5 cm diameter)|
|Collapsed distal small bowel|
|Transition zone |
|Obstructing lesion (adhesions presumed if no lesion seen)|
|Small bowel feces sign|
The principal advantages of CT over
abdominal radiography are (1) that the diagnosis of SBO does not
depend on collapse of the colon (collapsed distal small bowel is
directly visualized), and (2) CT can detect dilated fluid-filled
small bowel that is not visible on radiography (Figure 9).
A secondary sign of SBO is the small
bowel feces sign (Figure 12). Enteric contents within obstructed
bowel undergo digestion and develop feces-like air stippling. This
sign is suggestive, although not entirely specific, for obstruction.
In addition, it can help locate the transition zone because it usually
occurs in the distal portion of obstructed small bowel (Mayo-Smith
et al. 1995).
Small bowel feces sign.
Feculent enteric contents in obstructed small bowel (arrow).
and descending colon.
Intravenous contrast causing bowel
wall enhancement is needed to detect bowel wall thickening, which
is a sign of ischemia, as well as other bowel wall and solid organ
lesions. Oral contrast is not essential
to diagnose SBO because distended bowel containing fluid and air
can be visualized without intraluminal contrast (Figure 8) (Macari
and Megibow 2001). Oral contrast is often not tolerated by patients
who are vomiting due to SBO, and the oral contrast may not reach
the lumen of the obstructed bowel. However, when the obstruction
is less severe and the patient is able to tolerate oral contrast,
enteric contrast provides a more comprehensive CT examination and better
evaluation of other abdominal conditions that could be responsible
for the patient’s pain. Some authors recommend administration
of oral contrast whenever possible because passage of contrast into
the colon is a sign that obstruction is not complete and that nonoperative
care is more likely to be successful (Frager 2002).
CT is indicated in patients with
suspected SBO whenever abdominal radiography is nondiagnostic (50% of cases).
CT can confirm the diagnosis of SBO more reliably than abdominal
radiography and, when obstruction is not found, it can often detect
other disorders responsible for the patient’s symptoms
(Figure 10). CT is also useful when nonoperative management of SBO
is being considered because it can detect signs of ischemia and
closed-loop obstruction that mandate surgery (Donecker 1998).
CT can frequently identify the lesion
causing obstruction. This is particularly important when a
lesion other than an adhesion is found, which may require specific
treatment, such as a tumor, abscess, or hernia (Figures 9 and 13).
CT is therefore especially useful in patients with SBO who have
not had prior abdominal surgery because etiologies other than adhesions
are likely. Even when urgent surgery is planned, many surgeons
obtain a preoperative CT if it does not delay operative intervention
in order to delineate intra-abdominal pathology prior to laparotomy.
Ileoileal intussusception causing SBO.
A pedunculated ileal polyp was the lead point for intussusception
causing SBO. A segment of ileum, the intussusceptum (asterisk), along with mesenteric fat
and vessels were pulled into an outer segment of bowel, the intussuscipiens
The reported sensitivity of CT for
SBO varies among published studies, although differences in patient
selection account for much of this variation. CT is most useful
in patients with acute onset of symptoms, i.e., those most likely
to be seen in the ED. Sensitivity and specificity in such patients
is as high as 90–100% (Megibow et al. 1991, Fukuya
et al. 1992, Frager et al. 1994, 1995, Gazelle et al. 1994). Other investigators
found considerably lower sensitivity (64%) and specificity
(80%) (Maglinte et al. 1993 and 1996). However, all of
their patients had been referred for enteroclysis and
are therefore not representative of patients with SBO seen in the
ED. Sensitivity was especially low (50%) in patients with
low-grade partial obstruction (as defined by enteroclysis). (CT sensitivity
was 83% in patients with high-grade obstruction). Despite
its limitations, this study does demonstrate that CT can miss the
diagnosis of low-grade partial SBO.
CT’s accuracy has likely been improved with the advent
of helical CT and particularly, multidetector
CT (Caoili and Paulson 2000, Furukawa et al. 2001, Boudiaf
et al. 2001, Khurana et al. 2002, Taourel et al. 2002). Reformatted
images in the coronal or sagittal planes further improve CT’s capabilities
by, for example, helping to identify a transition zone that does
not lie in the axial plane (Jaffe et al. 2006).
Macari and Megibow (2001) proposed an approach for examining
CT scans in patients with suspected SBO. First, the colon is followed
on sequential images from the rectum and sigmoid colon proximally
to the cecum looking for lesions that could be causing obstruction.
Identification of the entire colon also allows the reader to confidently
distinguish small bowel from colon on the CT images. Next, the terminal
ileum is identified near the ileocecal valve. If the terminal ileum
is dilated, SBO is excluded. If the terminal ileum is collapsed,
the remaining small bowel is examined to identify dilated and collapsed
segments of bowel and a transition zone.
CT Signs of
Ischemic Obstruction and Closed-Loop Obstruction
CT signs of bowel ischemia include
bowel wall thickening, bowel wall edema (target sign), congestion
of mesenteric fat due to edema and hemorrhage (increased attenuation),
and hemorrhagic ascites (Table 7, Figure 14). Absence of bowel wall
enhancement and intramural gas are signs of infarction.
Table 7 CT Signs of Ischemic and Closed-Loop Obstruction |Favorite Table|Download (.pdf)
Table 7 CT Signs of Ischemic and Closed-Loop Obstruction
|Bowel wall thickening, “target sign,” increased enhancement |
|Mesenteric congestion—edema, hemorrhage|
|Infarcted bowel—poorly enhancing, paper-thin bowel
wall; intramural gas|
|Closed-loop obstruction |
|Dilated loops of bowel in a cluster or radial distribution|
|Mesenteric vessels converging to a central point—“Spoke-wheel” sign|
|C-shaped or U-shaped loop of dilated bowel|
|Two adjacent collapsed, triangular-shaped loops of bowel|
|Whirl sign—twisted mesenteric vessels at point
Ischemic SBO—Target sign.
Adhesive SBO with bowel wall thickening. At laparotomy, the bowel
was ischemic but salvageable following relief of obstruction. Other
disorders that can cause a target sign include infectious enteritis,
inflammatory bowel disease and acute mesenteric ischemia.
Detecting ischemia is one of CT’s most important roles
in patients with SBO because ischemic bowel mandates surgical treatment
and clinical signs of ischemia are not always present (Table 2) (Donecker
1998). However, CT has limited sensitivity for detecting bowel ischemia,
ranging from 70% to 96% (the latter using helical
CT) (Balthazar et al. 1997, Zalcman et al. 2000). Therefore, despite
a negative CT, when there is clinical suspicion of ischemia, the
patient should undergo laparotomy.
A closed-loop obstruction is at
high risk of ischemia due to the potential for mesenteric vascular
occlusion at the neck of the obstruction (Figure 15) (Balthazar
et al. 1992, Batlhazar 1994). CT signs of
closed-loop obstruction include: (1) a cluster of dilated fluid-filled
loops of bowel arranged in a radial distribution; (2) mesenteric
vessels converging to a central point, (“spoke wheel sign”); (3) a
C-shaped loop of dilated bowel; (4) two adjacent collapsed, triangular-shaped
loops of bowel; and (5) the “whirl sign”—twisted
mesenteric vessels at point of volvulus (Table 7, Figures 15, 16, 17, and 18). Because of the increased risk of ischemia, surgery is indicated
even when there are no CT signs of bowel wall ischemia.
CT signs of closed-loop obstruction.
Upper slice shows two or more adjacent dilated loops of bowel.
At the level of twisting, compressed triangular-shaped bowel is
seen. A “whirl” sign may be present at this level.
When the closed-loop is in the same plane as the CT slice, a
C-shaped loop of bowel is seen (Figure 16).
[From: Balthazar, et al: Closed-loop and strangulating
intestinal obstruction: CT signs. Radiology 1992;185:770.]
A C-shaped loop of obstructed bowel lies in the plane of the
(Non-contrast CT was done in a patient with abrupt onset lower
abdominal pain for suspected renal colic).
CT depiction of closed-loop obstruction.
A cluster of distended loops of bowel arranged in a radial distribution.
The mesentery radiates from the central point of the volvulus. This
forms the “spoke-wheel” sign.
[From: Balthazar EJ: CT of small bowel obstruction. AJR 1994;162: 258 with permission.]
Closed-loop small bowel obstruction with bowel ischemia
diagnosed by CT.
A 64-year-old woman presented with abrupt onset of mid-abdominal
pain. Initially, her abdomen was soft and mildly tender, and her
pain seemed out of proportion to the abdominal tenderness. The abdominal
radiographs and blood tests were “negative” (WBC
count 8,400 cells/mm3)
(A) Abdominal radiography showed
a considerable amount of air in the transverse colon and practically
no gas in small bowel. This finding would seem to go against SBO
because it is opposite the expected pattern of small bowel dilation
and large bowel collapse.
After two hours of observation in the emergency department, the
patient’s abdomen was distended and increasingly tender.
CT of the abdomen revealed a closed-loop small bowel obstruction.
A cluster of dilated fluid-filled small bowel loops was arranged
in a radial distribution, the “spoke
wheel sign.” The congested edematous mesentery radiated
from the point of volvulus. The bowel wall was thickened, indicative
(C) A second CT slice showed ascites
(A) surrounding the liver (L), also a sign of bowel ischemia. The
ascites extended along the portal vein (*).
A radiopaque nasogastric tube was in the dilated fluid and contrast-filled
At surgery, an internal hernia was
found and was reduced. Blood flow to the bowel returned, and there
was no need for bowel resection. Bloody ascites was present.
An abdominal radiographic finding associated
with closed-loop obstruction is the “pseudo-tumor
sign.” The fluid-filled obstructed bowel appears like
a soft-tissue mass displacing adjacent abdominal organs (Balthazar
et al. 1992). This is rarely seen.
The gasless mid-abdomen on this radiograph (A) may represent
a pseudo-tumor sign (fluid-filled loops of dilated bowel), although
this finding would be impossible to identify without knowing the
CT results. Gas in the colon is often seen in closed-loop obstruction
because the onset of obstruction is rapid and gut motility is diminished
due to the severity of pain.
An internal hernia (herniation
of bowel through a defect in the mesentery) has a similar CT appearance
and risk of ischemia as a closed-loop obstruction. Defects in the
mesentery are either congenital (paraduodenal is most common) or
postsurgical in origin (Takeyama et al. 2005, Blanchar 2001).