Superior Mesenteric Artery Syndrome (SMAS)
Adil Abbasi, MD, FACP
Learning Objectives
Introduction
Superior Mesenteric Artery Syndrome (SMAS), also known as Wilkie’s syndrome or cast syndrome, is a rare cause of upper gastrointestinal obstruction resulting from compression of the third portion of the duodenum between the superior mesenteric artery (SMA) and the abdominal aorta (Welsch et al., 2007).
Under normal anatomical conditions, the SMA originates from the anterior surface of the abdominal aorta at approximately the L1 vertebral level, forming an angle known as the aortomesenteric angle. Normally this angle ranges between 38–65 degrees, and the distance between the SMA and aorta is approximately 10–28 mm. These anatomical relationships allow the third part of the duodenum to pass safely between these two vessels without compression (Neri et al., 2005).
In SMAS, the aortomesenteric angle narrows significantly, often to 6–25 degrees, and the distance between the vessels decreases to 2–8 mm, leading to mechanical compression of the duodenum. This results in partial or complete obstruction, producing symptoms such as early satiety, postprandial abdominal pain, nausea, and vomiting (Shiu et al., 2020).
The syndrome is uncommon, with estimated prevalence between 0.013% and 0.3%, though the true incidence may be underestimated due to frequent misdiagnosis (Welsch et al., 2007). SMAS most commonly occurs in individuals who experience rapid or significant weight loss, because the mesenteric fat pad that normally maintains the aortomesenteric angle becomes depleted.
Although historically described in patients with severe weight loss from chronic illness, the syndrome is also observed in patients with eating disorders, trauma, spinal surgery, burns, malignancy, or prolonged immobilization (Biank & Werlin, 2006).
Because symptoms can mimic other gastrointestinal disorders, SMAS often requires high clinical suspicion and confirmatory imaging.
Etiology and Pathophysiology
Etiology
The fundamental cause of SMAS is narrowing of the angle between the SMA and aorta, usually due to loss of the mesenteric fat cushion.
Common etiologic factors include:
1. Rapid Weight Loss
2. Post-surgical Causes
3. Congenital or Anatomical Factors
4. Other Causes
(Welsch et al., 2007; Neri et al., 2005)
Pathophysiology
The third portion of the duodenum (D3) passes between the abdominal aorta posteriorly and the superior mesenteric artery anteriorly.
Under normal conditions:
When weight loss or anatomical changes reduce the fat cushion, the SMA angle narrows.
This results in:
This creates a vicious cycle:
Weight loss → decreased mesenteric fat → duodenal compression → vomiting and reduced intake → further weight loss.
The obstruction can also cause:
Symptoms and Signs
Clinical presentation varies depending on the severity of obstruction.
Common Symptoms
Pain often worsens after meals because gastric distension increases pressure on the compressed duodenum.
Many patients report relief of symptoms in certain positions, particularly:
These positions increase the aortomesenteric angle, temporarily relieving compression.
Physical Findings
Physical examination may reveal:
Severe cases may develop:
Diagnostic Workup
Diagnosis requires clinical suspicion and confirmatory imaging.
1. Computed Tomography (CT scan)
CT angiography is currently the most useful diagnostic test.
Typical findings include:
(Shiu et al., 2020)
2. Upper GI Contrast Study (Barium Study)
Characteristic findings:
3. Endoscopy (EGD)
Endoscopy helps:
Typical findings:
4. Ultrasound
Doppler ultrasound can measure:
However, it is operator dependent and less commonly used.
Diagnostic Criteria
Common radiologic criteria include:
Parameter | Normal | SMAS |
Aortomesenteric angle | 38–65° | <22° |
Aortomesenteric distance | 10–28 mm | <8 mm |
(Neri et al., 2005)
Management
Management depends on severity and chronicity.
Medical (Conservative) Management
Initial treatment is usually non-surgical, especially in early disease.
1. Nutritional Rehabilitation
Goal: restore the mesenteric fat pad
Methods include:
Weight gain often restores the aortomesenteric angle.
2. Postural Therapy
Patients may obtain relief with:
These positions reduce duodenal compression.
3. Gastric Decompression
Nasogastric tube may be used to relieve:
4. Prokinetic Agents
Medications may help improve gastric emptying:
However, evidence remains limited.
Surgical Management
Surgery is considered when:
1. Duodenojejunostomy (Most Common)
This procedure creates a bypass between the duodenum and jejunum.
Advantages:
Currently performed laparoscopically in most cases.
2. Strong’s Procedure
This involves division of the ligament of Treitz, allowing the duodenum to fall inferiorly and relieve compression.
Success rates are lower than duodenojejunostomy.
3. Gastrojejunostomy
This bypasses the obstruction but does not relieve duodenal compression, so it is used less frequently.
Summary
References
Biank, V., & Werlin, S. (2006). Superior mesenteric artery syndrome in children: A 20-year experience. Journal of Pediatric Gastroenterology and Nutrition, 42(5), 522–525.
Neri, S., Signorelli, S., Mondati, E., et al. (2005). Ultrasound imaging in diagnosis of superior mesenteric artery syndrome. Journal of Internal Medicine, 257(4), 346–351.
Shiu, J. R., Chao, H. C., Luo, C. C., et al. (2020). Clinical and nutritional outcomes in superior mesenteric artery syndrome. World Journal of Gastroenterology, 26(24), 3419–3430.
Welsch, T., Büchler, M. W., & Kienle, P. (2007). Recalling superior mesenteric artery syndrome. Digestive Surgery, 24(3), 149–156.