Summary of Superior mesenteric artery syndrome
Transcript for Superior mesenteric artery syndrome
Content Reviewers:Rishi Desai, MD, MPH, Charles Davis, MD, Tanner Marshall, MS, Yifan Xiao, MD, Christa Morris
Superior mesenteric artery syndrome
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.
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.
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.
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.