Small Bowel Obstruction
Review of small bowel obstruction for radiology board review. Download the free study guide by clicking here. Prepare to succeed!
Show Notes/Study Guide:
What are classic plain film findings of small bowel obstruction?
On radiographs, look for distended loops of small bowel with air fluid levels. Note that small bowel loops can be identified as they have valvulae conniventes, sometimes termed plicae circulares, which are thin mucosal folds that span the entire bowel loop, whereas the colon has haustral folds that are thicker and do not span the entire width of the colon. Small bowel loops are typically also more central in the abdomen than colonic loops which tend to be more peripheral.
What are basic characteristics of a mild versus a more severe small bowel obstruction?
Classic imaging findings of small bowel obstruction typically include bowel dilatation, and a transition point from distended to collapsed bowel distal to the transition point. An obstructing lesion at the transition point may or may not be seen. Mild small bowel obstructions tend to be partial or incomplete, meaning bowel contents can pass beyond the site of obstruction. Vascularity will not be compromised in a mild small bowel obstruction. More severe small bowel obstructions tend to be high-grade with complete or near-complete obstruction of bowel contents. These have a higher risk of vascular compromise with associated bowel ischemia and possible bowel infarction and/or perforation. As such, one must look for evidence of pneumoperitoneum to indicate perforation, and signs of strangulation and/or bowel ischemia including pneumatosis intestinalis as signs of complication with severe bowel obstructions.
Clinically, a mild bowel obstruction may have transient abdominal pain, decreased bowel movements and emesis. More severe small bowel obstructions may have a sudden, severe onset of pain and in cases of vascular compromise a rapid onset of shock. Emesis may not always be present with ischemic bowel due to acute vascular compromise as pain may precede bowel dilatation and backup of enteric contents which leads to vomiting. Vomiting initially is mostly of gastric contents and progresses to bile and feculent material with more severe, unresolved small bowel obstruction.
What is the so-called 3-6-9 rule of the bowel?
This is a generalization of normal bowel diameter in centimeters as follows:
-Small bowel: <3 cm
-Large bowel: <6 cm (Some add appendix < 6 mm (not cm) here as well)
-Cecum: <9 cm.
Note that some advocate for slightly different measurements, such as 2.5 cm for small bowel to increase sensitivity for detection of small bowel obstruction, but the 3-6-9 rule is a general rule of thumb mnemonic that is easy to remember. Also note that this rule is not specific for small bowel obstruction but rather bowel distention which can result from obstruction or ileus.
What are imaging findings that can help distinguish between small bowel obstruction and ileus?
Ileus classically shows gaseous distention of the small and large bowel diffusely but can also be more focal involving a so-called sentinel loop. If present, a sentinel loop is near a focal inflammatory process such as an inflamed gallbladder or pancreas, causing short segment adynamic ileus of the adjacent small bowel. Small bowel obstruction classically shows a transition point between dilated small bowel loops proximally and collapsed small bowel loops distally and should not have distention of the colon.
What is the most common cause of a small bowel obstruction?
Adhesions, most related to prior abdominal or pelvic surgery. Note that adhesions can be occult on imaging, so in many cases adhesions as the etiology of small bowel obstruction is made by excluding other causes. Secondary signs of adhesions include tethering of bowel non-dependently to the peritoneum, as well as abrupt angulation of an obstructed bowel loop.
What are some classic causes of a closed loop small bowel obstruction?
Like small bowel obstructions in general, adhesions are the most common cause of a closed loop small bowel obstruction. Other causes include bowel hernias which can be internal and often challenging to diagnose, external hernias such as an inguinal or ventral hernia, as well as a small bowel volvulus.
What are imaging features of a closed-loop small bowel obstruction?
Closed loop small bowel obstructions occur when the same bowel loop is obstructed twice at a nearby or often even the same point. A closed-loop bowel obstruction classically presents as a dilated small bowel loop with narrowing and obstruction at two nearby or adjacent points with a transition point proximally and another transition point distally. Dilated mesenteric vessels pointing towards the site of closed loop obstruction transition point(s) is sometimes present. Because the bowel loop is obstructed both proximally and distally, it has no route of decompression if complete and this can be a precarious situation. The clinical significance of a closed loop obstruction is a higher rate of vascular compromise with associated bowel ischemia and possible bowel infarction due to strangulation of vasculature as well as compromised perfusion due to distention causing venous compromise and congestion due to increased intraluminal pressures combined with necrosis and subsequent bowel perforation if not resolved. CT findings supporting possible ischemia include adjacent vascular engorgement, ascites, and abnormally decreased enhancement of the affected bowel. Pneumatosis intestinalis is a sign of potentially infarcted bowel. The most classic imaging finding confirming potential closed loop obstruction is to identify two areas of occlusion among a bowel loop. Secondary signs can include asymmetric mesenteric edema about the obstructed bowel loop, stretched or angulated mesenteric vessels, C- or U-shaped dilated bowel loop(s), and a whirl sign from rotation of bowel loops and mesenteric vessels.
True or false? Roux-en-Y gastric bypass can be a cause of closed loop bowel obstruction.
True. Roux-en-Y gastric bypass creates a rift in the mesentery which can result in bowel obstruction, including closed looped obstructions. Additionally, an afferent limb obstruction can occur after Roux-en-Y bypass wherein the pancreaticobiliary limb is obstructed which is closed-loop because this is a blind ending loop with the remnant stomach at the other end which does not have outflow. Clinically this would present as a bowel obstruction with no improvement following nasogastric tube placement and attempted decompression as the obstructed portion is the pancreaticobiliary limb and not the alimentary limb.
What are potential signs of small bowel ischemia?
Pneumatosis intestinalis where air is seen in the bowel wall as well as portal venous gas in which intrahepatic gas is seen near the periphery of the liver are likely the most classic imaging findings. Additionally, hemorrhage manifesting as increased attenuation of the thickened bowel wall and/or intraluminal bowel contents or hemorrhage within the mesentery may be noted. To prevent bowel ischemia and subsequent infarction early surgical intervention should be pursued. Of course, remember serum lactate elevation as a potential laboratory measure of ischemia.
Gas seen in the bowel wall in the neonate is classic for which entity?
Necrotizing enterocolitis. Risk is elevated in premature neonates and with congenital heart disease or other neonatal insults.
What are some potential causes of so-called benign pneumatosis?
Benign pneumatosis can be caused by pulmonary disease such as COPD, scleroderma, lupus, and other types of collagen vascular disease, various causes of intestinal inflammation, recent endoscopy procedures, GI tube placement, or bowel surgery, as well as certain medications include chemotherapy and steroids. Benign pneumatosis classically creates air-containing circumferential rings in the bowel wall and would lack additional findings suspicious for ischemia to include an absence of bowel wall thickening, mesenteric or intraperitoneal fluid or stranding, or evidence of vascular occlusion including abnormal, often decreased, bowel wall enhancement, and no associated bowel or mesenteric hemorrhage. Clinical features are key to help differentiate benign/incidental pneumatosis, often clinically asymptomatic, from pneumatosis concerning for bowel ischemia in which severe patient compromise and distress may be present.
What is the small bowel feces sign?
The small bowel feces sign is when one identifies the appearance of feces with air bubbles in the small bowel, whereas feces is typically only seen in the large bowel, potentially the result of delayed small intestinal transit. The small bowel feces sign, when seen with other findings of small bowel obstruction like distended bowel proximally, and collapsed bowel distally, supports the diagnosis of small bowel obstruction. However, while this finding increases the likelihood of small bowel obstruction when other imaging findings of obstruction are also seen, the small bowel feces sign can also be seen in settings outside of small bowel obstruction. Note that the small bowel feces sign most commonly forms in low-grade obstructions rather than acute high-grade obstruction, is most common in the distal small bowel, and is frequently seen just proximal to the site of obstruction. Therefore, when you see this sign carefully look for a nearby transition point.
What are classic imaging findings of gallstone ileus?
Gallstone ileus is a commonly tested concept, so make sure you are prepared to recognize this on a board exam. Gallstone ileus occurs when chronic cholecystitis creates a fistula from the chronic inflammation between the gallbladder and small bowel, allowing one or more gallstones to enter the small bowel. The gallstone(s) can subsequently become impacted at the ileocecal valve causing small bowel obstruction (not ileus, the name is a misnomer). Note that gallstone ileus is rare but is more common in the elderly.
The tricky thing with gallstone ileus is they typically don’t make it clear that the patient has had chronic cholecystitis, and you are expected to consider this possibility when seeing a calcified structure causing small bowel obstruction at the level of the ileocecal valve. They can test this on a radiograph or CT showing the Rigler triad of small bowel obstruction, right lower quadrant calcification, and biliary tree gas. However, most gallstones do not calcify so one must also be on the lookout for a non-calcified gallstone about the ileocecal valve on CT when biliary gas and bowel obstruction coexist. Especially if oral contrast is given, the fistula between gallbladder and adjacent bowel may be identifiable.
Bouveret syndrome is a variant of gallstone ileus in which the gallstone erodes into and obstructs the duodenal bulb. Biliary gas, duodenal obstruction, and an obstructing stone are classically seen.
What are other potential causes of small bowel obstruction to consider?
Besides adhesions, which is the most common cause, and gallstone ileus which is an infrequent cause of small bowel obstruction, other causes include external and internal hernias with resultant obstruction, benign or malignant obstructing masses, small bowel volvulus, Crohn’s disease with terminal ileitis causing fibrotic strictures, acute inflammatory processes, and small bowel intussusception. Note that external hernias causing bowel obstruction can include inguinal, umbilical, incisional, and Spigelian hernias. Internal hernias can be congenital or surgically created openings within the mesentery. Small bowel intussusception is more common in the pediatric age group as a cause of obstruction but can also occur in adults, including with a Meckel’s diverticulum or after surgery such as gastric bypass.
What are classic causes of small bowel volvulus?
Small bowel volvulus is more common in the pediatric age group, and mid gut volvulus is classic within the first month of life secondary to malrotation wherein no rotation or incomplete rotation of the bowel around the superior mesenteric artery axis occurs during embryogenesis which may lead to acute volvulus of the midgut. Small bowel volvulus in adults can also be secondary to malignancy, pregnancy, adhesions within the peritoneum, or a small bowel diverticulum.
What are classic imaging findings of small bowel volvulus?
On radiographs, look for findings of proximal small bowel obstruction with dilated small bowel loops and air fluid levels within the proximal bowel. On CT, a so-called whirl sign suggests volvulus of the bowel with swirling of mesenteric vessels, mesenteric edema, and swirling or twisting of obstructed small bowel loops. However, the whirl sign can also be seen without small bowel volvulus, so this is a sensitive but not a specific sign for this entity. Small bowel volvulus has a strong risk for small bowel ischemia. With midgut volvulus one can often see the whirl sign wherein the superior mesenteric vein and loops of small bowel are seen swirling around the superior mesenteric artery.
