By SRINIVAS SEELA, MD

Superior mesenteric artery syndrome (SMAS) is a digestive condition that occurs when the duodenum is compressed between two arteries (the aorta and the superior mesenteric artery). This compression causes partial or complete blockage of the duodenum. Superior mesenteric artery (SMA) syndrome is an uncommon but well recognized clinical entity.

Superior mesenteric artery syndrome was first described in 1861 by Von Rokitansky, who proposed that its cause was obstruction of the third part of the duodenum as a result of arteriomesenteric compression. Some studies report the incidence of superior mesenteric artery syndrome to be 0.1-0.3 percent. Approximately 0.013-0.78 percent of barium upper GI studies evaluating for superior mesenteric artery syndrome support the diagnosis.

Despite the fact that about 400 cases are described in the English language literature, many have doubted the existence of superior mesenteric artery syndrome as a real entity; indeed, some investigators have suggested that superior mesenteric artery syndrome is over diagnosed because it is confused with other causes of megaduodenum. Nonetheless, the entity (also called cast syndrome) is a well-known complication of scoliosis surgery, anorexia, and trauma. It often poses a diagnostic dilemma; its diagnosis is frequently one of exclusion

The superior mesenteric artery usually forms an angle of approximately 45° (range, 38-56°) with the abdominal aorta, and the third part of the duodenum crosses caudal to the origin of the superior mesenteric artery, coursing between the superior mesenteric artery and aorta. Any factor that sharply narrows the aortomesenteric angle to approximately 6-25° can cause entrapment and compression of the third part of the duodenum as it passes between the superior mesenteric artery and aorta, resulting in superior mesenteric artery syndrome.

In addition, the aortomesenteric distance in superior mesenteric artery syndrome is decreased to 2-8 mm (normal is 10-20 mm). Alternatively, other causes implicated in superior mesenteric artery syndrome include high insertion of the duodenum at the ligament of Treitz, a low origin of the superior mesenteric artery, and compression of the duodenum due to peritoneal adhesions.

Mortality/Morbidity

Delay in the diagnosis of superior mesenteric artery syndrome can result in malnutrition, dehydration, electrolyte abnormalities, gastric pneumatosis and portal venous gas, formation of an obstructing duodenal bezoar, hypovolemia secondary to massive GI hemorrhage, and even death secondary to gastric perforation.

Symptoms:

The patient often presents with chronic upper abdominal symptoms such as epigastric pain, nausea, eructation, voluminous vomiting (bilious or partially digested food), postprandial discomfort, early satiety, and sometimes, subacute small bowel obstruction. Symptoms of superior mesenteric artery (SMA) syndrome often develop from 6-12 days after scoliosis surgery.

• abdominal fullness,
• bloating after meals,
• nausea and vomiting of partially digested food, and
• Mid-abdominal "crampy" pain that may be relieved by the prone (lying on the stomach) or knee-chest position.

An asthenic habitus is noted in about 80 percent of cases. Abdominal examination may reveal a succussion splash. Peptic ulcer disease has been noted in 25-45 percent of the patients, and hyperchlorhydria has been noted in 50 percent. Patients can present with signs of subacute small bowel obstruction.

Important etiologic factors that may precipitate narrowing of the aortomesenteric angle and recurrent mechanical obstruction include the following:

• Constitutional factors
1. Thin body build
2. Exaggerated lumbar lordosis
3. Visceroptosis and abdominal wall laxity
4. Depletion of the mesenteric fat caused by rapid severe weight loss due to catabolic states such as cancer, surgery, burns, trauma, or psychiatric problems
• Severe injuries, such as head trauma, leading to prolonged bedrest
• Dietary disorders
1. Anorexia nervosa
2. Malabsorption
• Spinal disease, deformity, or trauma (use of body cast in the surgical treatment of scoliosis or vertebral fractures): Superior mesenteric artery syndrome cases after corrective spine surgery are due to the result of spinal elongation, which decreases the superior mesenteric/aortic angle. Postoperative weight loss is an important factor for development of superior mesenteric artery syndrome. Although use of Harrington rods for corrective surgery commonly used in the 1950s and 1960s was an important contributory factor for development of superior mesenteric artery syndrome, newer derotation/translation corrective techniques can also rarely be associated with this disease entity.
• Rapid linear growth without compensatory weight gain, particularly in adolescents: Adolescents with low body mass index (< 18 kg/m²) may be at higher risk for developing superior mesenteric artery syndrome after spinal fusion for scoliosis than patients with a higher body mass index.
• Anatomic anomalies (rare)
1. Abnormally high and fixed position of the ligament of Treitz with an upward displacement of the duodenum
2. Unusually low origin of the superior mesenteric artery
• Unusual causes
1. Traumatic aneurysm of the superior mesenteric artery after a stab wound
2. Abdominal aortic aneurysms and mycotic aortic aneurysms
3. Familial superior mesenteric artery syndrome
4. Recurrent superior mesenteric artery syndrome
5. Idiopathic neonatal superior mesenteric artery syndrome

Diagnostic Considerations:

The differential diagnosis includes anorexia nervosa and bulimia. In addition, superior mesenteric artery (SMA) syndrome should be differentiated from other causes of megaduodenum or duodenal ileus, including diabetes mellitus, collagen vascular conditions, and chronic idiopathic intestinal pseudoobstruction. Mechanical obstruction secondary to peptic ulcer disease or duodenal web should also be considered.

Diagnosis:

The diagnosis of superior mesenteric artery (SMA) syndrome is difficult. A high index of suspicion is required since symptoms can be nonspecific. A diagnosis of superior mesenteric artery syndrome is often a diagnosis of exclusion. If not already performed, patients should undergo judicious testing for other disorders that can cause similar symptoms. Confirmation usually requires radiographic studies, such as an upper GI series, hypotonic duodenography, and CT scanning.

Treatment

Reversing or removing the precipitating factor is usually successful in a patient with acute superior mesenteric artery (SMA) syndrome. Conservative initial treatment is recommended in all patients with superior mesenteric artery syndrome; this includes adequate nutrition, nasogastric decompression, and proper positioning of the patient after eating (i.e., left lateral decubitus, prone, knee-to-chest position, or Goldthwaite maneuver) Enteral feeding using a double lumen nasojejunal tube passed distal to the obstruction under fluoroscopic assistance is an effective adjunct in treatment of patients with rapid severe weight loss and also eliminates the need for intravenous fluids and the risks associated with total parenteral nutrition.

In some instances, both enteral and parenteral nutritional support may be needed to provide optimal calories. The patient's weight should be monitored daily. Subsequently, the patient can be started on oral liquids followed by slow and gradual introduction of small and frequent soft meals as tolerated. Finally, regular solid foods are introduced. Metoclopramide treatment may be beneficial. Review of the orthopedic literature reveals that the success rate is 100 percent with medical management, only in cases with an acute presentation of superior mesenteric artery syndrome.

Surgery:

Surgical intervention is indicated when conservative measures are ineffective, particularly in patients with a long history of progressive weight loss, pronounced duodenal dilatation with stasis, and complicating peptic ulcer disease. A trial of conservative treatment should be instituted for at least 4-6 weeks prior to surgical intervention.

Options for surgery include: Strong's procedure, Gastrojejunostomy and Duodenojejunostomy.

Srinivas Seela, MD, is board certified in both Internal Medicine and Gastroenterology. He is a member of the American Gastroenterological Association (AGA), the American Society for Gastrointestinal Endoscopy (ASGE), the American Association for the Study of Liver Diseases (AASLD), and Crohn's Colitis Foundation (CCF).

In addition to being an Assistant Professor at the University of Central Florida School of Medicine, he is also a teaching attending physician at both the Florida Hospital Internal Medicine Residency and Family Practice Residence programs. Seela is a gastroenterologist at Digestive and Liver Center of Florida.