SMA syndrome
Summary
Superior mesenteric artery (SMA) syndrome is a rare cause of upper intestinal obstruction caused by compression of the third (transverse) portion of the duodenum between the SMA and the aorta due to loss of the intervening mesenteric fat pad. It typically occurs in patients with rapid or severe weight loss (e.g., anorexia nervosa, prolonged bed rest, severe burns, spinal surgery with body casting) and presents with postprandial epigastric pain, nausea, vomiting (often bilious), and early satiety.
Detail
Pathophysiology: The SMA normally branches off the aorta at approximately a 45-degree angle, with mesenteric fat and lymphatic tissue cushioning the duodenum as it passes between the SMA and aorta. Severe weight loss (from any cause—anorexia nervosa, malignancy, burns, trauma, prolonged immobilization/body casts, bariatric surgery) depletes this fat pad, narrowing the aortomesenteric angle (normally 25-60°) and distance (normally 10-28 mm) to critical levels (<25° and <8 mm respectively), causing extrinsic compression and obstruction of the third/transverse duodenum.
Clinical presentation: Chronic, intermittent, or acute postprandial epigastric pain, early satiety, nausea, bilious vomiting, and weight loss—creating a vicious cycle since weight loss worsens the compression, which worsens symptoms, which causes further weight loss and food avoidance. Symptoms are classically relieved by positioning that increases the aortomesenteric angle (left lateral decubitus, prone, or knee-chest position) and worsened by supine positioning.
Diagnosis: Upper GI series or CT/MR angiography showing dilated proximal duodenum and stomach with abrupt vertical/linear extrinsic compression of the third duodenal portion, along with a narrowed aortomesenteric angle and distance. Barium studies may show a 'cutoff' sign.
Management: Conservative treatment is first-line—nutritional rehabilitation (enteral or parenteral nutrition), small frequent meals, and postural changes to restore the fat pad and widen the aortomesenteric angle. Nasojejunal feeding tubes placed distal to the obstruction may be needed. Surgical options (duodenojejunostomy, Strong's operation to lyse the ligament of Treitz) are reserved for refractory cases after failed conservative management.
Key associations for boards: Rapid weight loss, anorexia nervosa, spinal surgery with body cast (Milwaukee brace), association with low body mass index, differentiate from other causes of duodenal obstruction (annular pancreas, duodenal atresia in neonates, malrotation with Ladd's bands).
Sources
- First Aid for the USMLE Step 1
- UpToDate: Superior mesenteric artery syndrome
- Sabiston Textbook of Surgery
- Robbins Basic Pathology
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