USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 30-year-old woman gravida 1, para 1, comes to the office because of colicky abdominal pain for 12 hours. She states that along with the pain she has felt nauseated and vomited twice. She has never experienced these symptoms before, but her medical history includes appendicectomy 2 years ago. Her temperature is 36.5ºC (97.7ºF), pulse is 87/min, respirations are 18/min, and blood pressure is 116/82 mm Hg. Examination shows abdominal distension. The abdomen is tender to palpation, especially in the right lower quadrant. Bowel sounds are present and high-pitched. There are no signs of peritonism. Which of the following is the most likely diagnosis?
The causes of bowel obstruction can be either mechanical or functional, also called ileus.
Partial obstruction is when gas or liquid stool can pass through the point of narrowing, while complete obstruction is when nothing can pass.
Now if we zoom into a cross-section of the intestinal wall, it’s lined by four layers of tissue: First, there’s the adventitia, or serosa; which is the outermost layer that faces the abdominal or peritoneal cavity. This is the space between the abdominal wall and the abdominal organs, and it’s lined by peritoneal membranes that contains a thin film of serous fluid.
Deep beneath this layer is the submucosa, which has connective tissue as well as glands, blood and lymph vessels that supply the intestinal wall.
And finally, the innermost layer is the mucosa and it’s composed of a few of its own layers: the muscularis mucosae, which has smooth muscle, the lamina propria, which is rich with blood and lymph vessels, and the innermost layer which is the epithelial lining that faces the lumen.
Okay, so let’s go over some mechanical causes for bowel obstruction.
The most common cause in the small intestine is postoperative adhesions.
After a surgery, the scar tissue that forms during the healing process can form fibrous bands that cause 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 obstruction is hernias, and they can occur 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.
Sometimes the volvulus can occur around a mass like in colorectal cancer.
Some 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, kind of like retracting a telescope. This is the most common cause of bowel obstruction in children under the age of 2.
Now, regarding functional obstruction, causes include anything that decreases smooth muscle contractility.
The most common one is postoperative ileus, which is transient paralysis of the smooth muscles in the intestinal wall, and it’s usually caused by trauma during surgery.
Other causes include infection or inflammation, such as appendicitis or peritonitis, hypothyroidism, meaning the thyroid gland does not produce enough thyroid hormones, and electrolyte abnormalities like hypokalemia, meaning low potassium in the blood, or hypercalcemia which is high calcium in the blood, or various medications such as opioids.
So, when there's a bowel obstruction, whatever the cause, the bowel contents distal to the obstruction get passed; but after that happens, proximal to the obstruction, gas and stool start to accumulate, causing the bowel to dilate, and therefore, the overall abdominal cavity to distend.
The pressure pushes the water in these vessels into the surrounding tissue, leading to mucosal edema.
At the cellular level, this is accompanied by the production of reactive oxygen species; which can damage DNA, RNA, and proteins of the cells in the epithelial layer and lamina propria of the mucosa, leading to cell death, or mucosal infarction.
These macrophages then release inflammatory cytokines like tumor necrosis factor-alpha, which cause blood vessels to become more permeable to fluid and to more immune cells, further increasing mucosal edema, inflammation, and damage.
The overall result is the compromised ability of the mucosa to absorb food and water, which may lead to dehydration and loss of electrolytes, like sodium, potassium and chloride.
Now, as all these lumen contents continues to build up, intraluminal pressure rises even higher, making the problem even worse if not corrected.