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Bowel obstruction: Clinical practice



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Bowel obstruction: Clinical practice


0 / 8 complete

USMLE® Step 2 style questions USMLE

8 questions

A 50-year-old woman comes to the emergency department because of colicky abdominal pain for the past hour. She has a medical history of endometriosis and she has had an appendectomy and multiple cesarean sections. Physical examination shows a distended abdomen that is tympanitic to percussion. An abdominal radiograph shows dilated loops of bowel with air-fluid levels. Which of the following is the most likely underlying condition in this patient?     


Content Reviewers:

Rishi Desai, MD, MPH

Bowel obstruction is when the normal flow of contents moving through the intestines is interrupted.

Bowel obstruction can be defined as partial or complete.

Partial obstruction is when gas or liquid stool can pass through the point of narrowing, while complete obstruction is when no substance can pass - and this is defined as obstipation.

The causes of bowel obstruction can be either functional or mechanical causes.

Functional, or adynamic causes disrupt peristalsis, which are the waves that move through the smooth muscles of the bowel walls.

Mechanical causes are actual blockages in the large or small intestines of the bowel, and these mechanical causes are the ones that are usually treated surgically.

Among mechanical causes, one cause of small bowel obstruction in individuals who have previously undergone surgery, are postoperative adhesions.

Adhesions are fibrous bands of scar tissue that causes organs to attach to the surgical site or to other organs, causing the lumen of the bowel to get kinked or pinched tight in certain spots.

Another cause of small intestinal obstructions is hernias - which is when a portion of the bowel protrudes out of the abdominal cavity and can get trapped or tightly pinched at the point where it pokes out.

Large bowel obstructions, on the other hand, are most often due to a volvulus - which is when a loop of intestine twists upon itself, kinking off the lumen.

Sometimes the volvulus can occur around a mass like in colorectal cancer.

Other mechanical causes of both small and large bowel obstruction include inflammatory bowel disease which can cause strictures and adhesions, ingestion of a foreign body which can get lodged along the gastrointestinal tract, and intussusception - which is where a part of the intestine folds into the lumen of an adjacent section of bowel.

One unique situation is called a closed-loop obstruction, and it’s where the intestine is obstructed at two locations, creating a segment where nothing goes in, and nothing comes out.

Soon after a bowel obstruction, the air and bowel contents distal to the obstruction get passed, but after that happens, proximal to the obstruction, air and stool start to accumulate, causing the bowel to dilate, causing cramping abdominal pain.

Over time, the bowel walls become edematous, so they stop absorbing any more nutrients and water.

Individuals can develop nausea and vomiting which can cause electrolyte abnormalities and dehydration.

Finally, some individuals develop respiratory distress from upward pressure on the diaphragm from their distended abdomen.

Now, generally speaking, in small bowel obstruction, vomiting is more common and the pain tends to be periumbilical, cramping, and intermittent - with bouts that last for a few minutes at a time.

In large bowel obstruction, vomiting is less common and the pain is lower in the abdomen and the bouts of pain are less frequent but last a bit longer.

If bowel dilation becomes excessive, or if there’s bowel strangulation, the bowel wall can face extreme pressure causing the blood vessels within the walls to collapse.

When that happens perfusion to the intestine can be compromised, and it can lead to ischemia, and eventually necrosis and perforation.

When there’s progression from cramping to more focal and constant pain it can indicate localized peritoneal irritation due to a microperforation of the intestine.

Alternatively, a large intestinal perforation may cause a sudden relief of pain due to the nerves in the bowel wall no longer being stretched, but then it’s usually followed by progressive worsening of pain, as generalized peritonitis sets in.

Finally, hematochezia can be a sign of colorectal cancer, which can be a cause of bowel obstruction or bowel ischemia.

Next up, lab studies include a CBC with differential, as well as a basal metabolic panel including electrolytes, BUN, and creatinine.

Leukocytosis with a neutrophil predominance is a specific sign of an underlying inflammatory response - such as inflammatory bowel disease or diverticulitis, while anemia may point to inflammatory bowel disease or colorectal cancer.

Individuals with more severe symptoms like fever, tachycardia, hypotension, and altered mental status, should have an ABG and a serum lactate, because metabolic acidosis and an elevated lactate level suggest bowel ischemia.

Finally blood cultures should be sent if there are clinical signs of peritonitis or sepsis.

Diagnosis of bowel obstruction requires imaging studies, starting with an abdominal X ray.

In bowel obstruction, there are dilated loops of bowel with air-fluid levels, which show that both fluid and gas are collecting in the intestines. Usually the proximal bowel is dilated, and the distal bowel is not.

Specific X-ray signs can suggest the cause of bowel obstruction.

For example, an "apple core sign”, suggests constriction of the colonic lumen and it’s often due to a ring-shaped colon cancer.

Now if there’s bowel perforation, there’s free intraperitoneal air, called a pneumoperitoneum, and it requires immediate surgical intervention.

In hemodynamically stable individuals who don’t need emergency surgery, an abdominal CT scan with contrast - given either orally or through enema - is done next to identify the potential cause of the obstruction.

Typical abdominal CT findings include bowel wall thickening, submucosal edema or hemorrhage, and mesenteric edema.