Superior mesenteric artery syndrome

Definitions & Key takeaways

Superior mesenteric artery syndrome (SMAS) is a condition caused by compression of the small intestine (jejunum) between the aorta and the superior mesenteric artery. It is a rare disorder, usually occurring in people who have lost a large amount of weight, including the initially present mesenteric fat. This causes the aortomesenteric angle to go from a normal 45-degree angle down to as little as 6 degrees. The result is small bowel obstruction which can cause early satiety and vomiting and can be corrected with weight gain, and in certain cases, surgery.

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Introduction0:00–0:25

Superior mesenteric artery syndrome is when the first part of the small intestine—which is the duodenum—gets squeezed between two large arteries: the abdominal aorta and the superior mesenteric artery.
So, when the duodenum gets squashed, food can’t easily pass by, and it leads to intestinal obstruction. Normally, blood heading toward the lower parts of the body exits the heart, swoops through the aortic arch, and then flows downward through the descending aorta, which is a large, muscular blood vessel about as wide as a thumb.

Physiology0:25–1:43

The descending aorta runs along the back of the abdominal wall in front of the spine, and that part is called the abdominal aorta.
The abdominal aorta then forks into the common iliac arteries near the fourth lumbar vertebra, or L4. Along the way, it gives rise to a number of paired arteries like the renal arteries, as well as three unpaired arteries—the celiac trunk, the superior mesenteric artery, and the inferior mesenteric artery—all of which come off of the anterior or front wall of the aorta and supply blood to the stomach and intestines.
Usually, the angle between the aorta and the superior mesenteric artery as it branches off—the aortomesenteric angle—is around 45 degrees.
It turns out that the duodenum, which is the C-shaped first section of the small intestines - passes through this little archway.
More specifically, it’s the third or transverse section of the duodenum, and as it passes through it’s cushioned by the mesenteric fat pad which is a collection of fat that protects the duodenum from getting crushed by the two arteries.

Pathology1:43–2:04

In superior mesenteric artery syndrome the mesenteric fat pad starts to thin out which reduces the aortomesenteric angle, allowing the aorta and the superior mesenteric artery to pinch down on the transverse duodenum.
This blocks food from passing from the duodenum to the jejunum, and the backup of food leads to intestinal obstruction. There are a few reasons why this might happen - and the most common one is extreme weight loss from any cause like an illness or even intentional weight loss like in anorexia nervosa.

Causes2:04–2:50

In young people, another common cause of superior mesenteric artery syndrome is surgery for scoliosis, which is a condition in which the spine is abnormally curved.
During the surgery, the spine is straightened, and the aorta and superior mesenteric artery get stretched upward, causing the aortomesenteric angle to narrow.
Rarely, there can be congenital anatomic abnormalities, like when the ligament of Treitz, which holds the duodenum in place, is too short, or when the superior mesenteric artery branches off of the aorta further down than it’s usual spot.

Signs and Symptoms2:50–3:21

Regardless of the cause, the main symptom of superior mesenteric artery syndrome is small bowel obstruction and the onset can be gradual or pretty quick depending on the underlying cause.
Depending on the severity, symptoms can range from abdominal pain and early satiety, which is where someone feels full after eating only a little bit, to vomiting and significant weight loss.
Vomit can contain bile, which is secreted into the digestive tract before the point of compression along the transverse duodenum.

Diagnosis3:21–4:01

Diagnosis starts with identifying the small bowel obstruction - which might be due to many things including superior mesenteric artery syndrome.
Typically, on exam a small bowel obstruction can cause high-pitched bowel sounds as well as a succussion splash, which is the sloshing sound of built-up gas and fluid in the distended digestive tract upstream of the obstruction.
If superior mesenteric artery syndrome is suspected as the underlying cause of the obstruction, imaging studies like abdominal X-rays or CT scans with oral contrast can be helpful, as well as an abdominal ultrasound to measure the aortomesenteric angle, which can get as narrow as 6 degrees.

Treatment4:01–4:30

Treatment of a small bowel obstruction includes inserting a nasogastric tube to decompress the stomach and early duodenum, as well as includes offering fluids and electrolytes.
The major treatment for superior mesenteric artery syndrome is weight gain to help regrow the mesenteric fat pad. Sometimes that requires a feeding tube inserted past the site of obstruction.
If the underlying problem is the ligament of Treitz, treatment could include surgery to allow the duodenum to move more freely.
All right, as a quick recap...superior mesenteric artery syndrome is when the duodenum is squeezed between the superior mesenteric artery and the aorta, usually due to loss of surrounding mesenteric fat.

Review4:30–5:50

This causes the aortomesenteric angle to go from a normal 45 degree angle down to as little as 6 degrees. The result is small bowel obstruction which can cause early satiety and vomiting, and can be corrected with weight gain, and in certain cases, surgery.