Large bowel obstruction: Clinical sciences

Large bowel obstruction occurs when a blockage interferes with the passage of intraluminal contents like stool and gas through the colon. When this leads to infrequent or difficulty passing stool or flatus, it’s called constipation; while total inability to pass stool and flatus is known as obstipation.

Blockages can be mechanical, which is when there is a physical obstruction, such as a tumor or volvulus; or pseudo-obstructions, also known as Ogilvie syndrome, which means that the obstruction is functional and the bowel isn't working properly, so normal peristalsis is disrupted. Because the content of the bowel is unable to pass, large bowel obstruction can lead to bowel dilatation, ischemia, perforation, and finally sepsis.

When assessing a patient with signs and symptoms suggestive of large bowel obstruction, first perform an ABCDE assessment to determine if the patient is unstable. If the patient is unstable, start acute management immediately to stabilize their airway, breathing, and circulation. This means that you might need to intubate the patient, obtain IV access, administer fluids, and monitor their vitals before continuing with your assessment.

The next step is to obtain a focused history and physical exam, as well as labs like CBC, CMP, and lactate. Now, history might reveal abdominal bloating, abdominal pain ranging from crampy to diffuse in nature, obstipation, and sometimes nausea and vomiting. The onset of symptoms usually ranges from hours to days. Physical exam typically shows abdominal distension and signs of peritonitis, such as diffuse tenderness to palpation, rebound pain, and guarding. Finally, labs may show leukocytosis, as well as lactic acidosis from bowel ischemia.

Alright, if you see these signs and symptoms, suspect large bowel obstruction. The next step is to start supportive care. Supportive care involves IV fluid resuscitation and vasopressor support, electrolyte replacement, broad-spectrum antibiotics, as well as bowel rest, and nasogastric tube placement for bowel decompression if the patient is having nausea and vomiting. Once supportive care is initiated, you can order an abdominal x-ray series.

Now, the x-rays might show large bowel dilatation and signs of volvulus, which is the twisting of the colon along its mesentery. The first being northern exposure sign which occurs when a single dilated loop of the large bowel extends above the level of the transverse colon. The second potential sign called coffee bean sign refers to a massively dilated loop of the large bowel, twisted around itself, that resembles a giant coffee bean.

Additionally, an x-ray might show some serious signs like pneumatosis, which means that there might be necrosis; cecal diameter greater than 12 centimeters, which is concerning for impending perforation, and finally pneumoperitoneum, which indicates that perforation has already occurred. If you see any of these signs, you can diagnose complicated large bowel obstruction. Lastly, immediately obtain abdominal and pelvic CT scan and surgical consultation for emergent laparotomy.

Okay, now that unstable patients are taken care of, let’s return to the ABCDE assessment and talk about stable patients. Your first step here is to obtain a focused history and physical exam, as well as labs like CBC, CMP, and lactate. Stable patients typically report bloating, abdominal pain, and obstipation, with or without nausea and vomiting.

Here is a high-yield fact! Some causes of obstruction can be suspected based on history. For example, growing tumors increasingly obstruct the large bowel. So, these patients might report a longer duration of symptoms, progressively narrower stool caliber, rectal bleeding, and unintentional weight loss. On the flip side, a physical exam often reveals abdominal distention, and sometimes abdominal tenderness to palpation. Finally, labs might demonstrate leukocytosis or lactic acidosis. At this point, you can suspect large bowel obstruction, so be sure to start supportive care.

As before, supportive care includes IV fluid resuscitation, electrolyte replacement, and broad-spectrum antibiotics, as well as bowel rest and nasogastric tube placement for bowel decompression if the patient is having nausea and vomiting. Once the supportive care is initiated, you can move on to imaging. Order a CT of the abdomen and pelvis with oral and IV contrast to diagnose the condition and possibly find the underlying cause.

Next, let’s consider possible CT findings of mechanical large bowel obstruction. Over half of the large bowel obstructions are mechanical obstructions caused by colorectal malignancies. On CT, this may appear as a segment of the colon narrowed by an annular mass, which is frequently referred to as an “apple core sign,” because of its similar appearance. Another cause of mechanical large bowel obstruction is a volvulus. In this case, CT might show signs like the whirl sign, which looks like a swirling pattern in the mesentery caused by the twisting around its axis.