Superior Mesenteric Artery (SMA) Syndrome
SMA syndrome is a rare condition where the third portion of the duodenum becomes compressed between the superior mesenteric artery and the aorta, causing proximal intestinal obstruction due to loss of the intervening mesenteric fat pad 1, 2.
What Causes SMA Syndrome
The syndrome results from anatomical narrowing characterized by:
- Aortomesenteric angle <22 degrees (normally 25-60 degrees)
- Aortomesenteric distance <10 mm (normally 10-28 mm)
This critical narrowing occurs when the protective mesenteric fat pad is lost, allowing the SMA to compress the duodenum against the aorta 1, 2.
Key Risk Factors and Precipitants
Weight loss is the dominant trigger - the syndrome typically affects:
- Young, thin females (female:male ratio 3:2)
- Median age 23 years (range 0-91 years)
- Patients with rapid, significant weight loss from any cause
- Low body mass index patients 2
Associated conditions include:
- Anorexia nervosa or restrictive eating disorders 3
- Post-surgical states (spinal surgery, bariatric procedures)
- Severe burns or trauma
- Malignancy with cachexia
- Connective tissue disorders (hypermobile Ehlers-Danlos syndrome, scleroderma) 3, 4
Signs and Symptoms
Classic Presentation (Chronic/Subacute)
The hallmark symptom triad consists of:
- Postprandial abdominal pain (pain after eating)
- Nausea and vomiting (often bilious, providing temporary relief)
- Progressive weight loss (which paradoxically worsens the condition)
Additional common symptoms:
Important Clinical Pitfall
The presentation can mimic anorexia nervosa or functional dyspepsia, leading to delayed diagnosis and significant psychological distress 2. Maintain high clinical suspicion in patients with unexplained upper GI symptoms and weight loss, even without typical chronic prodromal symptoms.
Acute Presentation (Less Common but Critical)
While classically chronic, SMA syndrome can present acutely with:
- Sudden onset severe vomiting
- Acute epigastric pain
- Signs of proximal small bowel obstruction
- No prior chronic symptoms 6, 7
Life-threatening complications from acute presentation:
- Aspiration pneumonia from recurrent vomiting
- Respiratory depression via metabolic alkalosis
- Gastric ischemia from massive distension
- Pancreatic ischemia
- Renal ischemia 6, 2
Physical Examination Findings
Physical exam is often nonspecific and unremarkable, which contributes to diagnostic delay 5. You may find:
- Epigastric tenderness
- Succussion splash (from gastric distension)
- Tympany on percussion
- Weight loss/cachexia
- Postural symptom relief (left lateral decubitus or knee-chest position may temporarily relieve obstruction)
Laboratory Findings
Labs are typically unremarkable or show nonspecific abnormalities 5:
- Hypokalemia (from vomiting)
- Metabolic alkalosis
- Possible leukocytosis (if complications present)
Diagnostic Approach
Computed tomography angiography (CTA) is the standard diagnostic tool, demonstrating:
- Reduced aortomesenteric angle (<22°)
- Decreased aortomesenteric distance (<10 mm)
- Duodenal compression at the third portion 1, 2
Upper GI contrast studies show:
- Gastric and proximal duodenal dilation
- Abrupt contrast cutoff at the third portion of the duodenum
- Delayed gastric emptying
Ultrasonography offers advantages in real-time assessment of SMA mobility and duodenal passage, with benefits of safety and repeatability 2.
Upper endoscopy (EGD) can confirm duodenal compression and exclude other pathology 5.
Critical Diagnostic Caveat
Whether SMA syndrome represents a true clinical entity versus a radiological finding remains contentious 3. Some clinicians dispute its existence and believe it is overdiagnosed. The anatomical findings may represent a consequence rather than cause of malnutrition. Therefore, carefully weigh the risks of any surgical intervention, as benefits are not always predictable or clear, particularly given that conservative management succeeds in 70-80% of cases 3, 2.