Small bowel ischemia and infarction

Definitions & Key takeaways

Small bowel ischemia and infarction refer to a medical condition characterized by reduced blood flow and tissue death in the small intestine. It often results from something like a blood clot or a nearby tumor, a hernia, a volvulus, or intussusception, which blocks blood vessels supplying the small intestine. It may also be due to low blood pressure, or a decrease in the overall amount of blood flowing into the area.

Small bowel ischemia and infarction can complicate into an ileus in which food lingers and doesn't get pushed along, or even peritonitis and sepsis. Symptoms include severe abdominal pain( out of proportion to the physical examination), nausea, vomiting, diarrhea, and fever. Treatment typically involves addressing the underlying cause and restoring blood flow to the affected area, using medications or surgery. Treatment involves IV fluids, pain management, antibiotics, and surgery if needed.

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Small bowel refers to the small intestine and infarction is when ischemia, which is an inadequate blood supply, causes necrosis, or tissue death.
So, a small bowel infarction happens when there’s a reduced blood supply to the small intestine causing parts of the intestinal wall to necrose or die which can be life threatening.
Now, the small intestine is made of several layers. The innermost layer is the mucosal layer and it’s composed of a few of its own layers.
The first layer is the epithelial lining and it faces the lumen; next is the lamina propria, which is rich with blood and lymph vessels; and finally the muscularis mucosae, which has smooth muscle.
Deep to this mucosal layer is the submucosal layer, which has connective tissue with proteins like collagen and elastin, as well as glands, and additional blood vessels.
The submucosal layer also contains the Meissner plexus which is a part of the enteric nervous system. Below the submucosal layer is the muscularis propria which is basically two layers of smooth muscle with the myenteric plexus, another part of the enteric nervous system, sandwiched between them.
These muscles are particularly important in helping to move food through the bowel. Finally, there’s the serosal layer which is the outermost layer of the small intestines that faces the abdominal cavity.
The superior mesenteric artery is the main supplier of blood to the small intestine. Branches of the artery spread through the mesentery - called mesenteric arteries - and penetrate the serosa layer and travel to the submucosa where they branch further into arterioles.
Because the small intestine has a high demand for oxygen and nutrients to sustain digestion, it is highly susceptible to tissue injury from ischemia.
To reduce the risk of that happening, the mesenteric arteries branch and reconnect at points forming collateral circulation.
That’s protective because if blood flow is reduced in one pathway, then the tissue can still receive blood through another pathway.
Once the small intestines have gotten oxygenated blood, that blood leaves through the superior mesenteric vein. Small bowel infarction happens when there’s a significant decrease in blood flow to the small intestine.
This reduction in blood flow decreases blood pressure, and can cause an insufficient blood flow throughout the collateral circulation which initiates ischemic injury in a wide region of tissue.
At the cellular level, ischemic injury can lead to the production of reactive oxygen species which can damage DNA, RNA, and proteins in the cell, leading to cell death.
If blood flow returns to the ischemic tissue, it’s called reperfusion. Unfortunately, though, that process can cause further injury - called reperfusion injury.
In reperfusion injury, the influx of oxygen into an already damaged cell can be overwhelming and can cause even more oxidative stress, which worsens the cell damage.
As damaged cells release reactive oxygen species, it triggers an inflammatory response which attracts immune cells, like neutrophils.
The immune cells remove dead and damaged cells and release of cytokines, like Tumor necrosis factor-alpha. The cytokines cause blood vessels to become more permeable to fluid and more immune cells - resulting in bowel edema or swelling of the small intestinal wall.
Small bowel ischemia and infarction becomes more severe as the damage extends from just the mucosal layer, called a mucosal infarct, to all layers, known as a transmural infarction.
Early on bowel ischemia can make the bowels simply not work - resulting in an ileus - where food lingers and doesn’t get pushed along.
Severe damage to the small intestines can also cause a break in the epithelial lining of the small intestines, allowing bacteria in the lumen to get into the blood vessels in the wall.
Alternatively, bacteria can completely cross the small intestinal wall and get into the peritoneal space, and from there get into lymphatics or blood vessels.
Ultimately, if bacteria get into the bloodstream then it can lead to a massive inflammatory response called sepsis. In sepsis, blood vessels throughout the body can get leaky, and if enough fluid moves from the blood vessels into the interstitial space, it can lead to septic shock, which is where organs throughout the body get insufficient blood.
And this can lead to organ failure and death. Small bowel ischemia and infarction, can happen due to occlusive and nonocclusive causes.
Occlusive causes are physical blockages that prevent blood flow through the vasculature, and they usually cause transmural infarcts.
This can happen when a thrombus, a blood clot, forms in the superior mesenteric artery or vein and causes thrombosis or occlusion of the vasculature; it can also happen when a thromboembolism, which is a piece of a blood clot that has broken off, travels through the blood and becomes lodged in the superior mesenteric artery.
Another type of occlusive cause is when something like a tumor, hernias, volvulus--or a twisting of the bowel--or intussusception-- which is telescoping of the bowel, physically compresses the vasculature and occludes blood flow.
Nonocclusive causes of small bowel ischemia and infarction are related to systemic decreases in blood flow, and they usually cause mucosal infarcts.
This can happen in the setting of hypovolemia due to severe hemorrhage or dehydration, or low cardiac output conditions like after a myocardial infarction.
A classic symptom of small bowel ischemia is severe abdominal pain, even though the abdomen itself may be soft and easy to press into.
Small bowel infarction usually occurs about twelve hours later and commonly causes vomiting, and sometimes even bloody diarrhea.
Over time, the abdomen can get distended and bowel sounds can go away as the bowels stop moving. In severe cases, when there’s sepsis, a person can develop a fever, low blood pressure, and a fast heart rate and breathing rate to help compensate.
If fluid accumulates in the abdomen, it can cause signs of peritoneal inflammation like rebound tenderness and guarding.
Early diagnosis of small bowel ischemia and infarction is crucial. An abdominal CT can show bowel dilation, and bowel wall thickening from edema and inflammation, as well as intestinal pneumatosis which refers to air in the bowel wall.
CT angiography can help visualize blood flow through the small intestines. Typically, lab studies are done and these show a high white blood cell count and metabolic acidosis.
Treatment of small bowel ischemia and infarction is generally aimed at giving enough fluids, managing pain, and giving antibiotics if needed.
Definitive treatment, though, requires reestablishing blood flow through surgery or by using a thrombolytic enzyme if a clot is suspected.
Sometimes, surgical resection of infarcted tissue might be needed as well. Alright, as a quick recap … Small bowel ischemia and infarction can happen when blood vessels are occluded from something like a blood clot or a nearby tumor, a hernia, a volvulus, or intussusception.
It can also occur due to a nonocclusive event like low blood pressure throughout the body. In either situation, the wall of the small intestines can be severely damaged and inflamed.
The result can be anything from an ileus to bacterial peritonitis. Rapid diagnosis by CT scan, and treatment to reestablish blood flow, are essential to survival.