Case 1: Introduction to Small Bowel Obstruction
The patient has distended loops of small bowel with multiple air fluid levels. The distal small bowel is decompressed and the large intestine is normal. There is a transition point in a right femoral hernia. This is a small bowel obstruction (SBO) with a clearly identified transition point.
The plain film criteria for a small bowel obstruction follows the rule of 3's: small bowel dilated to 3 cm, greater than 3 air-fluid levels, or a small bowel wall greater than 3 mm thick. However, the routine KUB is neither sensitive nor specific for obstruction and many patients in the ER will have a "non specific" bowel gas pattern requiring followup with either clinical exam, repeat KUB, or CT of the abdomen. In our ER most patients with suspected bowel pathology go on to a CT examination. Many do not start with a KUB. This is probably OK. It is easier to identify obstruction on CT and it is often possible to identify the transition point. The KUB remains useful as a quick check for free intraperitoneal air in an acutely ill patient though many attendings in our ED have begun ordering non contrast CT scans to evaluate for bowel perforation.
A patient with SBO will have dilated small bowel proximal to the obstruction and relatively decompressed small bowel distal to the obstruction. We use 3cm as the maximum diameter for normal small bowel. However, common sense must prevail. If the proximal bowel is 2.8 cm and the distal bowel is completely collapsed there is an obstruction. The dilated bowel can be completely fluid filled, or have multiple air fluid levels. The distal bowel does not need to be completely collapsed. In a complete obstruction the distal bowel will eventually collapse. However, depending on the severity of the obstruction and the timing of the scan, air and fluid can still be seen distally. For this reason, a patient with SBO may still pass flatus and can have continued bowel movements. A scan with dilated proximal bowel and incompletely collapsed distal bowel is indicative of an early or partial obstruction.
When looking at a CT to assess for SBO, I first look for two things: dilated proximal bowel and decompressed distal bowel. Look in the right lower quadrant for the decompressed bowel. This is where the distal portion of the small bowel is located. Once you see these two things you know there is an obstruction. Next look for a transition point by following the bowel backward from the cecum and forward from the duodenum. If the obstruction is distal, it is easier to work backward from the cecum. If the obstruction if proximal, it is easier to work forward from the duodenum. The transition point is a focal area where the caliber of the bowel abruptly changes. In this case, if you follow the bowel backwards from the cecum you can easily see the transition point in a right femoral hernia.
Causes of hernia include adhesions from prior surgery, hernia, intra-abdominal mass (which can be either extrinsic or intrinsic to the bowel), stricture from inflammatory bowel disease, or edema from recent bowel surgery or inflammatory bowel disease. Adhesions are the most common cause, and while the transition point may be easily seen, the adhesion itself is often not identified.
If you cannot find the exact location of the transition point that is OK. You have identified the patient has a SBO and treatment can begin. The initial treatment for SBO is nasogastric tube decompression, IV fluid hydration, and serial physical exams. However, several imaging features of SBO can push the patient to the operating room and you should be aware of what they are.
Is there small bowel wall thickening or focal areas of mesenteric stranding? Distended bowel should have a paper thin wall. Edema in a distended wall suggests ischemia. Pneumatosis in distended loops of small bowel suggests necrosis. Check to make sure there is no air in the SMV or portal vein. In this example there is stranding in the mesenteric fat within the hernia. This suggests that the hernia is strangulated and at risk of ischemia or necrosis.
Is there ascites? If the patient has cirrhosis, low albumin, CHF, or other causes for third spacing fluid then the ascites may be unrelated. However, if there is no good explanation for the ascites the patient may have ischemia secondary to the obstruction.
Is there a closed loop obstruction? In a closed loop obstruction you will see a loop of distended bowel that loops back onto itself. The loop itself will be distended. The bowel just proximal to the loop will also be dilated. The bowel distal to the loop will be decompressed. The transition point is located where the bowel crosses itself. This can be caused by an adhesion, an internal hernia, or a volvulus (twisting of the bowel). Closed loop obstructions are more likely to be treated with surgery because of the increased rate of perforation. The loop cannot drain its internal secretions and is prone to rupture.
Is there free air? This means the patient has a perforated viscus and probably needs an operation.
Some important caveats to remember.
Oral contrast does not have to progress to the transition point. It frequently will not because of fluid and air backs up in the obstructed bowel. If the proximal loops are dilated and the distal loops are collapsed, call it SBO. You do not need to re-scan to check the passage of contrast unless you are unsure if the proximal loops are really different in caliber from the distal loops.
If you think there is a caliber change in the bowel but contrast is seen distal to this area then the patient does not have a complete SBO. They may have a partial or intermittent obstruction and you should tell the surgeons this. Also, ask if the patient has had a recent scan with oral or rectal contrast.. You may be seeing barium from a prior study.
Followup scans can be misleading. If the patient is being decompressed via nasogastric tube then the proximal small bowel will look less dilated. This can make a complete obstruction look like a partial obstruction because it appears as if some of the fluid is passing past the transition point. Double check the end of the contrast column. If it is still proximal to the transition then there is no real evidence of resolution. You can also ask the surgeon to correlate the results of the scan with the amount of NGT output. A large volume of aspirate will explain the interval decrease in size of the proximal small bowel.