What is Superior Mesenteric Artery (SMA) syndrome?

Superior Mesenteric Artery (SMA) Syndrome

Superior Mesenteric Artery syndrome is a rare condition characterized by duodenal obstruction caused by compression of the third portion of the duodenum between the superior mesenteric artery anteriorly and the aorta posteriorly, leading to significant morbidity if left untreated. 1

Pathophysiology

SMA syndrome occurs when the angle between the SMA and the aorta narrows to ≤22° (normal: 38-65°) and the aortomesenteric distance decreases to ≤8 mm (normal: 10-28 mm), causing external compression of the third portion of the duodenum 2. This anatomical abnormality leads to partial or complete duodenal obstruction.

Epidemiology

• Predominantly affects females with a ratio of 3:2
• Median age of 23 years (range 0-91 years)
• Rare condition, though exact prevalence is not well established 1

Risk Factors

• Significant weight loss
• Recent surgery (particularly spinal surgery with casting or correction)
• Prolonged bed rest
• Neurological injury
• Conditions that reduce retroperitoneal fat (malabsorption, anorexia nervosa, malignancy) 3

Clinical Presentation

The clinical presentation includes:

• Postprandial abdominal pain (59%)
• Vomiting, often bilious (50%)
• Nausea (40%)
• Early satiety (32%)
• Anorexia (18%)
• Weight loss
• Symptoms may worsen when lying supine and improve in left lateral decubitus or prone position 1, 3

Diagnostic Approach

1. CT Angiography: Standard diagnostic tool

   • Demonstrates narrowed aortomesenteric angle (≤22°)
   • Reduced aortomesenteric distance (≤8 mm)
   • Shows duodenal dilation proximal to the compression

2. Upper GI Series with Barium:

   • Shows dilation of the first and second portions of the duodenum
   • Abrupt vertical or oblique compression of the third portion
   • Delayed gastroduodenal emptying 2

3. Ultrasound:

   • Advantages include safety and real-time assessment
   • Can evaluate SMA mobility and duodenal passage 1

4. Upper Endoscopy:

   • To exclude other causes of obstruction
   • May show pulsatile extrinsic compression 4

Management

Conservative Treatment (First-Line)

Conservative management is successful in 70-80% of cases and includes:

• Postural changes (left lateral decubitus or prone position)
• Gastroduodenal decompression via nasogastric tube
• Nutritional support:
  • Small, frequent meals
  • Enteral feeding beyond the point of obstruction
  • Parenteral nutrition if enteral feeding is not tolerated 1

Surgical Treatment

Surgical intervention is indicated when conservative management fails and is successful in 80-100% of cases 1. Options include:

1. Duodenojejunostomy: Most commonly performed procedure

   • Can be performed laparoscopically
   • Creates a bypass around the compressed segment

2. Strong's procedure: Division of the ligament of Treitz to mobilize the duodenum

3. Gastrojejunostomy: Alternative if duodenojejunostomy is not feasible 5, 2

Prognosis and Outcomes

• With appropriate treatment, significant symptom improvement occurs in most patients
• Following surgery, studies show significant reduction in symptom scores and increase in BMI 2
• Complications if untreated include:
  • Electrolyte abnormalities
  • Malnutrition
  • Aspiration pneumonia
  • Metabolic alkalosis from persistent vomiting 1

Important Considerations

• SMA syndrome can be mistaken for anorexia nervosa, functional dyspepsia, or other gastrointestinal disorders
• The condition can create a vicious cycle where symptoms lead to decreased intake and weight loss, which further worsens the anatomical compression
• Early diagnosis and intervention are crucial to prevent complications and improve quality of life 4