What Causes SMA Syndrome
SMA syndrome is caused by compression of the third portion of the duodenum between the superior mesenteric artery and the abdominal aorta, most commonly triggered by significant weight loss that depletes the mesenteric fat pad, reducing the aortomesenteric angle to less than 25 degrees and the aortomesenteric distance to less than 8 mm. 1, 2
Primary Mechanism
The fundamental pathophysiology involves anatomic compression where the duodenum becomes trapped between two vascular structures 3:
- Loss of the mesenteric fat cushion is the critical factor that allows the SMA to compress the duodenum against the aorta 4
- The normal aortomesenteric angle (45-60 degrees) narrows to less than 25 degrees 2
- The normal aortomesenteric distance (10-28 mm) decreases to less than 8 mm 2
Clinical Conditions That Precipitate SMA Syndrome
Weight Loss-Related Causes
Rapid or significant weight loss from any etiology is the most common precipitant 1, 5:
- Eating disorders, particularly anorexia nervosa, create a vicious cycle where food aversion leads to further weight loss and worsening obstruction 2, 5
- Malignancy and chronic illness causing cachexia 2
- Substance abuse, particularly methamphetamine use for weight loss 2
- HIV/AIDS-related wasting 2
Surgical Causes
Bariatric surgery can precipitate SMA syndrome through rapid postoperative weight loss 2:
- Scoliosis corrective surgery is a unique risk factor where surgical lengthening of the vertebral column mechanically reduces the aortomesenteric distance 4
- Underweight status or low BMI prior to spinal surgery significantly increases risk 4
Patient Demographics at Highest Risk
The American College of Surgeons identifies specific high-risk populations 1:
- Young females with low BMI (mean BMI in case series: 15.73) 6
- History of gastrointestinal surgery 1
- Prolonged bed rest or immobilization (historically called "cast syndrome") 3
Important Clinical Distinction
SMA syndrome must be distinguished from acute mesenteric ischemia, which involves vascular occlusion rather than mechanical compression 1:
- Acute mesenteric ischemia presents with sudden severe pain out of proportion to exam findings and requires urgent CTA to evaluate for arterial occlusion 7
- SMA syndrome presents with postprandial symptoms that develop more gradually 1, 8
The Perpetuating Cycle
A critical pitfall is that SMA syndrome creates a self-perpetuating cycle 8, 5:
- Postprandial pain and vomiting lead to fear of eating (sitophobia) 1
- Reduced food intake causes further weight loss 1
- Additional weight loss further depletes the mesenteric fat pad 4
- This worsens the aortomesenteric compression, intensifying symptoms 5
Key Caveat
The median age of presentation is 23 years with female predominance (3:2 ratio), but the condition can occur across the entire age spectrum (0-91 years) 3. While the provided evidence focuses on acute mesenteric ischemia from embolic/thrombotic causes 7, these represent entirely different pathophysiologic processes from SMA syndrome and should not be confused despite both involving the superior mesenteric artery.