What is Superior Mesenteric (SM) Syndrome?

What is Superior Mesenteric Artery Syndrome?

Superior mesenteric artery (SMA) syndrome is a rare anatomical condition where the third (transverse/horizontal) portion of the duodenum becomes compressed between the superior mesenteric artery anteriorly and the abdominal aorta posteriorly, resulting in duodenal obstruction. 1, 2, 3

Pathophysiology

The syndrome occurs due to narrowing of the aortomesenteric angle, which normally measures 25-60 degrees with a distance of 10-28 mm between the SMA and aorta 3. The key mechanism involves:

• Loss of the retroperitoneal fat pad that normally cushions the duodenum between these vessels 4, 3
• Acute angulation of the SMA leading to compression of the third part of the duodenum against the aorta 5
• This results in chronic, intermittent, or acute duodenal obstruction that can be complete or partial 1

Clinical Presentation

The syndrome presents with a characteristic constellation of symptoms that should raise immediate suspicion 6:

• Postprandial epigastric abdominal pain that worsens after meals 6, 1
• Nausea and vomiting (often bilious and recurrent) 1, 2, 3
• Early satiety 1, 3
• Significant weight loss due to reduced food intake 6, 2
• Fear of eating (sitophobia) 6
• Abdominal distension 2, 4

Patient Demographics

• Median age: 23 years (range 0-91 years) 3
• Female predominance with a 3:2 ratio over males 3

Diagnostic Approach

Computed tomography angiography (CTA) is the standard diagnostic tool for confirming SMA syndrome 6, 3. The diagnostic workup should include:

• CT angiography: Shows narrowed aortomesenteric angle and compression of the duodenum 6
• Upper GI series with barium: Demonstrates compression of the horizontal duodenum with characteristic vertical linear filling defect 6, 1
• Upper gastrointestinal endoscopy: Can visualize duodenal compression but may not be diagnostic alone 1
• Ultrasonography: Has advantages in safety and real-time assessment of SMA mobility and duodenal passage 3
• MR angiography: Alternative imaging modality 6

Important Diagnostic Considerations

This syndrome can mimic anorexia nervosa or functional dyspepsia, making it a diagnostic challenge 3. Early diagnosis is critical because recurrent vomiting can lead to aspiration pneumonia or respiratory depression via metabolic alkalosis 3.

Management Algorithm

Initial Conservative Treatment (First-Line)

Conservative management should be attempted first in all cases with success rates of 70-80% 1, 3:

• Postural changes: Prone or left lateral decubitus positioning to relieve compression 3
• Gastroduodenal decompression: Nasogastric tube placement 3
• Nutritional rehabilitation: Parenteral nutrition or nasojejunal feeding to promote weight gain and restore retroperitoneal fat 2, 4, 3
• The goal is weight gain to increase the fat pad and relieve the obstruction 2

Surgical Treatment (When Conservative Fails)

If conservative therapy fails, surgical intervention is indicated with success rates of 80-100% 3:

• Laparoscopic duodenojejunostomy (Roux-en-Y configuration) is the most frequently used procedure with high success rates 1, 3
• The procedure involves dividing the ligament of Treitz and performing a side-to-side duodenojejunostomy 2
• Gastrojejunostomy is an alternative surgical option 1
• Mean postoperative hospitalization is approximately 7 days with generally uneventful recovery 1

Clinical Pitfalls

• Do not dismiss as functional dyspepsia or eating disorder: The symptoms overlap significantly with anorexia nervosa, but SMA syndrome has an anatomical cause requiring specific treatment 3
• Maintain high index of suspicion: In patients with unexplained postprandial pain and progressive weight loss, especially young females 6
• Do not delay imaging: Repeat gastroscopies alone may miss the diagnosis; CT imaging is essential 2
• Recognize that the main problem is the narrowed aortomesenteric angle, which typically requires surgical correction when conservative measures fail 1