Abdominal pain in a 20-year-old woman

CASE CONTINUED: SHE UNDERGOES CT

We send our patient for computed tomography (CT) of the abdomen with contrast (Figure 1). The stomach and duodenum are distended, and the duodenum is compressed under the superior mesenteric artery (SMA). Upper GI endoscopy shows a normal esophagus, normal gastric antrum, and normal duodenal bulb. The second and third portions of the duodenum are narrowed with prominent pulsations.

2. Which of the following is the most likely diagnosis at this point?

• SMA syndrome
• Chronic mesenteric ischemia involving the SMA
• Megaduodenum due to a connective tissue disorder

SMA syndrome is the most likely diagnosis. Despite its name, this syndrome is not a vascular condition. It is an uncommon cause of proximal intestinal obstruction in which the duodenum is compressed between the SMA and the aorta. First described in 1861, it has also been known as cast syndrome, Wilkie syndrome, and arteriomesenteric duodenal obstruction.⁵

The SMA usually arises from the anterior aspect of the aorta at the level of the L1 vertebral body. It is surrounded by fatty and lymphatic tissues that protect the duodenum from compression. In most patients, the angle between the SMA and the aorta is about 25 to 60 degrees, due in part to the mesenteric fat pad, and the angle correlates with the body mass index.⁶ In SMA syndrome, loss of the mesenteric fat pad reduces the angle to as little as 6 degrees, allowing the SMA to compress the duodenum against the aorta (Figure 2).

To date, more than 400 cases of this syndrome have been reported, twice as many in women as in men. Most patients are between 20 and 40 years of age at the time of diagnosis. Common presenting symptoms include postprandial abdominal pain, nausea, vomiting, and weight loss, which may further reduce the angle between the SMA and the aorta. Diarrhea is not generally associated with this syndrome, and in our patient’s case the diarrhea was thought to be unrelated to the SMA syndrome, since it subsided spontaneously.

Conditions and events that cause, contribute to, or worsen SMA syndrome include:

• Rapid weight loss (as in cancer or burns) or lean body habitus
• Prolonged bed rest
• Use of a body cast
• Malabsorption
• Spinal disease, deformity, or trauma
• Scoliosis surgery
• Rapid linear growth without compensatory weight gain
• Abnormally high and fixed position of the ligament of Treitz
• Abdominal surgery
• Cardiac cachexia
• Unusually low origin of the SMA.⁷

More common causes of mechanical smallbowel obstruction are adhesions, hernias, and tumors.⁸ Hyperactive, high-pitched peristalsis with rushes coinciding with cramps is typical. Abdominal cramps are centered around the umbilicus or in the epigastrium and are associated with vomiting; obstipation develops in patients with complete obstruction. Patients with partial obstruction may develop diarrhea. Paralytic ileus secondary to hypokalemia is an important consideration in partial obstruction. However, abdominal radiography and CT did not confirm an obstruction, and her symptoms persisted despite correction of the potassium level.

Chronic mesenteric ischemia can be caused by vasculitis, nonocclusive conditions that cause prolonged vasoconstriction (eg, cocaine ingestion), or reduced cardiac output.⁹ Symptoms are due to the gradual reduction in blood flow to the intestine that occurs during eating. Our patient’s toxicology report did not suggest cocaine abuse, and her history and the workup thus far do not suggest heart failure. A workup for vasculitis was negative.

Megaduodenum, SMA-like syndrome. In rare cases, dilation of the duodenum at the level of the SMA may be part of a generalized duodenal dilation caused by something other than obstruction due to mechanical compression. There are conditions, as described below, that cause an SMA-like syndrome.

A compression defect of the duodenum at the site where the SMA crossed the duodenum was found in a series of 11 cases of systemic sclerosis.¹⁰ These patients had definite dilation of the duodenum, but it was a result of atrophy of the muscle layers and replacement by collagenous tissue, changes that result in diminished peristalsis, loss of muscle tone, and dilation. The duodenum yields to pressure in its third portion under the SMA.

Several pathologic conditions, particularly connective tissue disorders, may predispose to the development of a megaduodenum that may result in an imprint on the duodenum at the level of the SMA. The most noteworthy of these conditions is scleroderma. Other conditions that can cause reduced duodenal peristalsis include diabetes, pancreatitis, dermatomyositis, lupus erythematosus, myxedema, and amyloidosis.¹¹

It is important to distinguish SMA syndrome from SMA-like syndromes for several reasons.¹² SMA-like syndromes result in loss of normal peristalsis. Further, the conditions have different outcomes, even though they are managed similarly initially, ie, with rehydration and parenteral nutrition. Surgery is to be avoided if possible in conditions that affect widespread areas of the intestine, such as scleroderma or diabetic neuropathy.