Superior mesenteric artery syndrome (also known as SMA syndrome) is a rare condition first described in 1861. It occurs when the third (transverse) portion of the duodenum gets entrapped under the superior mesenteric artery. Anatomically, the duodenum passes across the abdominal aorta just below the origin of the SMA. In some cases, structural abnormalities cause the artery, which exits the aorta at a downward angle of about 45°, to experience downward traction, which pinches the duodenum between the artery and the body of the aorta. In the English-language medical literature, there are over 400 cases of SMA syndrome reported. Some researchers and physicians still doubt the syndrome's validity however.

Presentation and Clinical features
SMA syndrome is quite rare. Due to the general nature of the symptoms, it is difficult to estimate the frequency of cases in the general population. It does not seem to be more common in any particular race, but seems to strike women about twice as often as men. The syndrome usually occurs in older children and teenagers. About 3/4 of all cases occur between ages 10 and 30.

The presentation of SMA syndrome can be chronic, intermittent, or acute, and can result in either complete or partial duodenal obstruction. Patients often present with chronic symptoms related to the upper abdomen. These can include:

These symptoms are often relieved by lying down on the left side, prone (face down), or with the knees to the chest. They can be aggravated in the supine (face up) position.

Pathophysiology and etiology
Anything that causes narrowing of the angle between the aorta and SMA can cause entrapment and compression of portion of the duodenum that passes under the SMA. Differing physical makeup or prior surgical procedures may also play roles in the development of SMA syndrome. Some factors that may result in this condition include:

Other rare but recognized causes include trauma to or aneurysm of the SMA, and familial predisposition toward SMA syndrome.

Diagnosis
Physical findings in SMA syndrome are often vague or generalized, and the diagnosis of this condition often comes by exclusion. About eight out of ten patients are thin and sickly (asthenic habitus). They often suffer from peptic ulcer disease (25-45%), or hypersecretion of stomach acid (hyperchlorhydria). The presentation can mimic that of subacute small bowel obstruction. Examination can reveal tenderness or pain, fullness, or a succussion splash. Pressure from below the navel upward and toward the back (Hayes maneuver) can lift the SMA, releasing the obstruction, and cause relief of both subjective and objective signs. Repositioning during the examination may also afford symptomatic relief.

The definitive diagnosis of SMA syndrome is difficult. Xray and other imaging studies that may be helpful include:

Differential diagnoses for SMA syndrome include eating disorders such as anorexia nervosa and bulimia and other metabolic conditions including diabetes and collagen vascular disease. There are other causes of obstruction and pseud-obstruction that should also be considered.

Treatment
Initially, conservative treatment including intravenous and oral fluids, nutrition, decompression, and proper positioning after food intake is recommended. Enteral feeding by nasogastric tube can be helpful as part of the treatment of rapid or severe weight loss. Metoclopramide (brand name Reglan), a pro-motility agent, may be beneficial. Medical management of SMA syndrome works best in cases which present acutely.

Surgical intervention is indicated when medical management fails. It is especially appropriate when a prolonged history of progressive weight loss, intestinal obstruction, or peptic ulcers is present. Duodenojejunostomy is most most often performed, and has about a 90% success. Laparoscopy is being used investigationally.

Natural course
Delay in the diagnosis of subacute or chronic SMA syndrome can result in malnutrition, dehydration, electrolyte disorders, and death. The acute form if untreated can result in dehydration and even intestinal perforation.

Prognosis
With proper conservative treatment in acute cases, and appropriate medical or surgical management in more chronic cases, the success rate in treating SMA syndrome is high. It is imperative to discover the underlying cause of the condition and treat that as well.

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