Small bowel obstruction (SBO) accounts for 80% of all mechanical intestinal obstruction, the remaining 20% results from a large bowel obstruction. It has a mortality rate of ~5%.
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Clinical presentation
The classical presentation is cramping abdominal pain and abdominal distension with nausea and vomiting. Radiographic findings can be evident 6-12 hours before the onset of clinical symptoms 9.
Pathology
Etiology
Causes can be divided into congenital and acquired. Acquired causes may be extrinsic causing compression, intrinsic, or luminal.
In developed countries, adhesions are by far the most common cause, accounting for ~75% of obstructions while in developing countries incarcerated hernias are much more common accounting for 80% of obstructions 3.
Congenital
ileal atresia or stenosis
Extrinsic causes
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the main cause in developed countries (75% of cases)
almost all are related to post-operative adhesions with a small percentage secondary to peritonitis
diagnosis of exclusion as adhesive bands are not seen on CT
an abrupt change in caliber without mass lesion, inflammation or bowel wall thickening at the transition point
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10% of cases in developed countries
external hernia related to abdominal or pelvic wall defect (congenital weakness or previous surgery)
internal hernia with protrusion of viscera through peritoneum or mesentery into another abdominal compartment
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a rare cause of SBO
endometrial implants are typically on the anti-mesenteric edge of the bowel
solid enhancing nodule contiguous with or penetrating the thickened bowel wall
may infiltrate the submucosa with a hypoattenuating layer between the muscularis and mucosa
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masses
extrinsic neoplasm
hematoma
Intrinsic bowel wall causes
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inflammation, e.g. Crohn, tuberculosis, eosinophilic gastroenteritis
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small bowel obstruction in Crohn disease may relate to:
acute flare with luminal narrowing secondary to transmural inflammation
cicatricial stenosis in long-standing disease
adhesions or incisional hernias from previous surgery
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tumor (rare)
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primary small bowel neoplasms are rare (<2% all GI malignancy) and usually advanced at the time of SBO.
asymmetric and irregular mural thickening at the transition point
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small bowel involvement by metastatic disease is more common
peritoneal carcinomatosis with an extrinsic serosal disease in association with the transition point
cecal malignancy involving ileocecal valve
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produces adhesive and fibrotic changes in the mesentery with luminal narrowing and dysmotility
may cause an obstruction in the late phase (>1 year after therapy)
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occlusion or stenosis of the mesenteric arterial or vascular supply
produces small bowel wall thickening and obstruction
pneumatosis and portal venous gas if advanced
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intramural hematoma
trauma, iatrogenic, anticoagulant therapy, Henoch-Schonlein purpura
produces luminal narrowing
better seen on non-enhanced CT with homogenous, regular and spontaneously hyper-attenuating wall
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rare in adults (<5% of SBO)
a lead point may relate to neoplasm, adhesion or foreign body
bowel-within-bowel with or without mesenteric fat and mesenteric vessels
leading mass should be carefully interpreted and differentiated from the soft-tissue pseudotumor that represents the intussusception itself
Intraluminal causes
swallowed, e.g. foreign body, bezoar
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a rare complication of recurrent cholecystitis
biliary-intestinal fistula with impaction of a gallstone in the small bowel
meconium ileus (or meconium ileus equivalent, distal intestinal obstruction syndrome)
migration of a gastric balloon
Radiographic features
Abdominal radiograph
Abdominal radiographs are only 50-60% sensitive for small bowel obstruction 3. In most cases, the abdominal radiograph will have the following features:
dilated loops of small bowel proximal to the obstruction (see 3-6-9 rule)
predominantly central dilated loops
three instances of dilatation > 2.5 - 3 cm ref required
valvulae conniventes are visible
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gas-fluid levels if the study is erect, especially suspicious if 8
>2.5 cm in width
in the same loop of the bowel but at different heights (> 2 cm difference in height)
However, obstruction (which may be high-grade mechanical obstruction) may also present with the following features:
gasless abdomen: gas within the small bowel is a function of vomiting, NG tube placement and level of obstruction
string-of-beads sign: small pockets of gas within a fluid-filled small bowel
Ultrasound
Bedside tests help to diagnose small bowel obstruction, findings suggestive of small bowel obstruction 7:
dilated bowel loop (diameter > 3 cm)
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ineffective peristalsis
results in "to-and-fro" or "whirling" appearance of intraluminal contents
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prominence of the valvulae conniventes
present in dilated jejunal loops
The extent of obstruction is typically implied rather than sought directly based on the involvement of the ascending/descending colon, the morphology of the small bowel loops (high mucosal folds pattern present in the jejunum, absent in the ileum), and involvement of the stomach.
Findings suggestive of bowel ischemia/infarction (will need urgent surgical evaluation):
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extraluminal free fluid
the "pointy" triangular appearance of interloop free fluid is sometimes referred to as the tanga sign
loss of peristalsis
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bowel wall thickening >3 mm
with effacement of mural architecture
mural gas
CT
CT is more sensitive than radiographs and will demonstrate the cause in ~80% of cases 3. Features on CT may include:
dilated small bowel loops >2.5 cm up from outer wall to outer wall
normal caliber or collapsed loops distally
Closed-loop obstructions are diagnosed when a bowel loop of variable length is occluded at two adjacent points along its course. May be partial or complete with characteristic features:
radial distribution of several dilated, fluid-filled bowel loops
stretching of prominent mesenteric vessels converging towards the point of torsion
U-shaped or C-shaped configuration
beak sign at the site of fusiform tapering
whirl sign reflecting rotation of bowel loops around a fixed point
Strangulation is defined as closed-loop obstruction associated with intestinal ischemia. Mainly seen when the diagnosis is delayed (up to 10% of small bowel obstructions) and associated with high mortality. Features are non-specific and include:
thickened and increased attenuation of the bowel wall
halo or target sign
localized fluid or hemorrhage in the mesentery
Positive oral contrast is not usually necessary for the diagnosis of small bowel obstruction 4:
usually, becomes dilute in the setting of SBO and does not usually reach the transition point before the scan occurs
may obscure the evaluation of the small bowel wall, limiting evaluation of bowel ischemia
In cases of adhesional small bowel obstruction with no complications such as ischemia or perforation, a water-soluble contrast challenge may be administered. This has both a diagnostic and therapeutic effect 5.
Differential diagnosis
sentinel loop: primarily to be considered for small bowel dilatation on abdominal radiographs
