Chapter 406. Superior Mesenteric Artery Syndrome

Superior mesenteric artery syndrome (SMAS), also known as Wilkie syndrome, arteriomesenteric duodenal compression, chronic duodenal ileus, or cast syndrome, was first described by von Rokitansky in 1842, and is an unusual form of gastrointestinal obstruction resulting from compression of the duodenum between the abdominal aorta posteriorly and the superior mesenteric artery anteriorly (Fig. 406-1).1 In 1878, Willet documented the development of SMAS in association with orthopedic casting; however, it was not until 1927 that Wilkie formally characterized SMAS in a series of 75 patients.2,3 To date, the diagnosis, and to some extent the existence, of SMAS remains controversial as symptoms do not always correlate with radiologic findings and may not improve with surgical correction)

Epidemiology

SMAS is a rare form of intestinal obstruction with a reported incidence ranging from 0.01% to 0.3%.4,5,6 SMAS is found more commonly in females than males and typically affects individuals of slender build after acute weight loss, such as from hyperthyroidism, anorexia nervosa, or even gastroenteritis. SMAS also affects individuals following orthopedic casting, especially following scoliosis surgery or following the application of a hip-spica cast.7 Acute or chronic neurologic injury is associated with superior mesenteric artery syndrome.8 Other associated disorders include fungal abscess, gastric bezoar, pregnancy, and aneurysms of the superior mesenteric artery.

Pathophysiology

The superior mesenteric artery (SMA) usually arises from the anterior aspect of the aorta at the level of the first lumbar vertebral body and is surrounded by fat and lymphatic tissue. The artery then extends caudally at an acute angle into the mesentery. The duodenum passes between the aorta and the proximal SMA commonly at the level of the third lumbar vertebral body. The duodenum is suspended within the angle created by the aorta and SMA by the ligament of Treitz (Fig. 406-1). In the majority of the population, the angle between the superior mesenteric artery and the aorta is between 45 and 60 degrees. This angle is in part due to the presence of an anterior mesenteric fat pad, the size of which correlates with body mass index.9 In SMAS, the angle between the aorta and the SMA is reduced to as little as 6 degrees, allowing the SMA to compress the duodenum against the aorta, resulting in mechanical obstruction.

Although, to date, no genetic mutations have been associated with SMAS, Iwaoka documents the development of SMAS in identical twin males without any previous risk factors or surgeries.5 In addition, Ortiz reported a case of familial SMAS with 5 out of 8 family members developing SMAS, including a father and his 4 daughters.10 Taken together, these case reports suggest that there may be a genetic component that predisposed some individuals to the development of SMAS.

Clinical Features and Differential Diagnosis

Symptoms of SMAS may be acute or chronic. The acute presentation is characterized by symptoms similar to small bowel obstruction, with vomiting, nausea, anorexia, abdominal distention, and abdominal pain.4,8,11 However, in some individuals the onset is insidious, with symptoms including postprandial epigastric pain, early satiety, and weight loss. In severe cases, individuals may also present with acute metabolic alkalosis, dehydration, and altered mental status resulting from profuse vomiting. In those individuals with abdominal pain, nearly half describe pain as epigastric in location.8

Physical findings are commonly nonspecific but can include abdominal distension, epigastric tenderness, a succussion splash, and/or high-pitched bowel sounds.

Diagnostic Evaluation

The diagnosis of superior mesenteric artery syndrome (SMAS) is commonly made by upper gastrointestinal radiography that demonstrates a dilated stomach and duodenum with an abrupt cut-off or narrowing of the duodenum at the level of the third lumbar vertebra just proximal to the ligament of Treitz (eFig. 406.1). In addition, upper gastrointestinal radiography may demonstrate intermittent reversal of peristaltic activity of the contrast as it attempts to pass the obstruction. The diagnosis can also be made by computerized tomography (eFig. 406.2) or at laparotomy.12,13 Endoscopic ultrasound has also been documented for diagnosis of SMAS for measurement of the distance between the aorta and the anterior mesenteric artery and the presence of preduodenal adipose tissue.14

Treatment

Treatment of superior mesenteric artery syndrome (SMAS) is usually conservative. Acute management is focused on bowel decompression, maintenance of the fluid and electrolyte balance and nutritional support or rehabilitation. Removal of the body cast as soon as practical is required when SMAS is associated with orthopedic casting.15 Once the patient is stabilized, small, frequent high-caloric oral feeds may be effective. Changes in feeding position (knees to chest or right lateral decubitus positioning) may also be useful in bypassing the obstruction. If symptoms persist, a nasojejunal tube placed past the obstruction allows nutritional rehabilitation for continuous enteral feeding15,16 or parenteral nutrition may be required. In the rare instance where medical treatment fails, surgical intervention may be necessary, particularly when profound chronic dilatation and stasis of the proximal small bowel occurs.15,16 Successful surgical therapies have included duodenojejunostomy, gastrojejunostomy, or resection of the ligament of Treitz.

Despite these surgical successes, advances in both enteral and parenteral nutrition over the last 30 years have dramatically impacted the medical management of SMAS. In a 1974 review, over 70% of patients were treated surgically,17 whereas a more recent review found that less than 20% required surgery and most were treated with nasojejunal feeding.8

Reported complications of SMAS include dehydration, metabolic imbalances, and even death.18 However, the vast majority of patients have a good prognosis.8