Superior Mesenteric Artery (SMA) Syndrome

Superior mesenteric artery (SMA) syndrome is an uncommon but well recognized clinical entity characterized by compression of the third, or transverse, portion of the duodenum between the aorta and the superior mesenteric artery. This results in chronic, intermittent, or acute complete or partial duodenal obstruction.[1] Superior mesenteric artery syndrome was first described in 1861 by Von Rokitansky, who proposed that its cause was obstruction of the third part of the duodenum as a result of arteriomesenteric compression. Some studies report the incidence of superior mesenteric artery syndrome to be 0.1-0.3%.[2] Approximately 0.013-0.78% of barium upper GI studies evaluating for superior mesenteric artery syndrome support the diagnosis.[3, 4]

Despite the fact that about 400 cases are described in the English language literature, many have doubted the existence of SMA syndrome as a real entity; indeed, some investigators have suggested that superior mesenteric artery syndrome is overdiagnosed because it is confused with other causes of megaduodenum. Nonetheless, the entity (also called cast syndrome) is a well-known complication of scoliosis surgery, anorexia, and trauma. It often poses a diagnostic dilemma; its diagnosis is frequently one of exclusion.

The superior mesenteric artery usually forms an angle of approximately 45° (range, 38-56°) with the abdominal aorta, and the third part of the duodenum crosses caudal to the origin of the superior mesenteric artery, coursing between the superior mesenteric artery and aorta. Any factor that sharply narrows the aortomesenteric angle to approximately 6-25° can cause entrapment and compression of the third part of the duodenum as it passes between the superior mesenteric artery and aorta, resulting in SMA syndrome.

In addition, the aortomesenteric distance in superior mesenteric artery syndrome is decreased to 2-8 mm (normal is 10-20 mm). Alternatively, other causes implicated in superior mesenteric artery syndrome include high insertion of the duodenum at the ligament of Treitz, a low origin of the superior mesenteric artery, and compression of the duodenum due to peritoneal adhesions.[5, 6]

Frequency of SMA Syndrome

United States

The precise incidence of this entity is unknown. In a review of the literature, approximately 0.013-0.78% of the findings from upper GI tract barium studies support a diagnosis of superior mesenteric artery syndrome.

SMA Syndrome Mortality/Morbidity

Delay in the diagnosis of SMA syndrome can result in malnutrition, dehydration, electrolyte abnormalities, gastric pneumatosis and portal venous gas, formation of an obstructing duodenal bezoar, hypovolemia secondary to massive GI hemorrhage, and even death secondary to gastric perforation.[7, 8, 9, 10]

Race

No racial differences have been identified.

Sex

More females are affected by SMA syndrome. In one large series of 75 patients with superior mesenteric artery syndrome, two thirds of the cases involved women, with an average age of 41 years; one third of cases involved men, with an average age of 38 years.

Age

SMA syndrome usually occurs in older children and adolescents. In one report, 75% of the cases occurred in patients aged 10-30 years.

The patient often presents with chronic upper abdominal symptoms such as abdominal pain, nausea, eructation, voluminous vomiting (bilious or partially digested food), postprandial discomfort, early satiety, and sometimes, subacute small bowel obstruction. Symptoms of superior mesenteric artery (SMA) syndrome often develop from 6-12 days after scoliosis surgery.

The symptoms are typically relieved when the patient is in the left lateral decubitus, prone, or knee-to-chest position, and they are often aggravated when the patient is in the supine position. These maneuvers are thought to reduce the small bowel mesenteric tension at the aortomesenteric angle.[11]

An asthenic habitus is noted in about 80% of cases. Abdominal examination may reveal a succussion splash. Peptic ulcer disease has been noted in 25-45% of the patients, and hyperchlorhydria has been noted in 50%. Patients can present with signs of subacute small bowel obstruction.

Important etiologic factors that may precipitate narrowing of the aortomesenteric angle and recurrent mechanical obstruction include the following:

• Constitutional factors

  • Thin body build

  • Exaggerated lumbar lordosis

  • Visceroptosis and abdominal wall laxity

  • Depletion of the mesenteric fat caused by rapid severe weight loss due to catabolic states such as cancer, surgery, burns, trauma, or psychiatric problems

• Severe injuries, such as head trauma, leading to prolonged bedrest

• Dietary disorders

  • Anorexia nervosa

  • Malabsorption

• Spinal disease, deformity, or trauma (use of body cast in the surgical treatment of scoliosis or vertebral fractures): Superior mesenteric artery syndrome cases after corrective spine surgery are due to the result of spinal elongation, which decreases the superior mesenteric/aortic angle. Postoperative weight loss is an important factor for development of superior mesenteric artery syndrome. Although use of Harrington rods for corrective surgery commonly used in the 1950s and 1960s was an important contributory factor for development of superior mesenteric artery syndrome, newer derotation/translation corrective techniques can also rarely be associated with this disease entity.[12]

• Rapid linear growth without compensatory weight gain, particularly in adolescents: Adolescents with low body mass index (< 18 kg/m2) may be at higher risk for developing superior mesenteric artery syndrome after spinal fusion for scoliosis than patients with a higher body mass index.[13]

• Anatomic anomalies (rare)

  • Abnormally high and fixed position of the ligament of Treitz with an upward displacement of the duodenum

  • Unusually low origin of the superior mesenteric artery

• Unusual causes

  • Traumatic aneurysm of the superior mesenteric artery after a stab wound

  • Abdominal aortic aneurysms and mycotic aortic aneurysms[14]

  • Familial superior mesenteric artery syndrome

  • Recurrent superior mesenteric artery syndrome

  • Idiopathic neonatal superior mesenteric artery syndrome[15, 16]

The diagnosis of superior mesenteric artery (SMA) syndrome is difficult. Confirmation usually requires radiographic studies, such as an upper GI series, hypotonic duodenography, and CT scanning.

Fluoroscopic findings suggestive of superior mesenteric artery syndrome include dilation of the first and second portions of the duodenum with an abrupt narrowing at the third portion (see the image below), delayed gastroduodenal emptying, and antiperistaltic waves proximal to the obstruction. Additionally, the obstruction of the duodenum may be relieved by a change in position, especially left lateral decubitus position.[17]

Prompt gastric emptying of residual contrast is present within the stomach into the second portion of the duodenum. Additional contrast was hand injected through the transpyloric feeding tube to further distend the second portion of the duodenum. The second portion of the duodenum is dilated with very slow transit across the spine into the jejunum. This occurred despite difference in position of the patient. The high-grade partial obstruction at the distal second portion of the duodenum is due to superior mesenteric artery (SMA) syndrome.

A Hayes maneuver (ie, pressure applied below the umbilicus in cephalad and dorsal direction), which elevates the root of small-bowel mesentery, may also relieve the obstruction. In equivocal cases, hypotonic duodenography may depict the site of obstruction and dilation of the proximal duodenum, with antiperistaltic waves within the dilated portion.

CT scanning is useful in the diagnosis of superior mesenteric artery syndrome and can provide diagnostic information, including aorta-superior mesenteric artery distances and duodenal distension. Also, it can be used to assess intra-abdominal and retroperitoneal fat. CT criteria for the diagnosis of superior mesenteric artery syndrome include an aortomesenteric angle of less than 22 degrees and an aortomesenteric distance of less than 8-10 mm. In children, an angle of less than 20° has been correlated with superior mesenteric artery syndrome.[2] CT can also identify problems that may require intervention, like compression of the left renal vein that results in renal vein thrombosis, pneumatosis or portal venous gas, or an abdominal aortic aneurysm.[17]

Upper GI endoscopy may be necessary to exclude mechanical causes of duodenal obstruction. However, the diagnosis of superior mesenteric artery syndrome may be missed with this study.

Abdominal ultrasonography may be helpful in measuring the angle of the superior mesenteric artery and the aortomesenteric distance. When combined with endoscopy, this may offer an alternative way to diagnose superior mesenteric artery syndrome in children to avoid other tests with a risk of radiation exposure.[18]

Manometry may be used to differentiate between the possibility of a myopathic form of chronic intestinal pseudo-obstruction syndrome (hollow visceral myopathy) by demonstrating low-amplitude waves throughout the duodenum, and frequently the stomach, versus an irregular or absent phase III and no postprandial motility changes in a neuropathic form of chronic intestinal pseudoobstruction syndrome. The pattern of increased amplitude of propagated contractions and retrograde contractions should hint or suggest the mechanical obstruction in superior mesenteric artery syndrome.

Reversing or removing the precipitating factor is usually successful in a patient with acute superior mesenteric artery (SMA) syndrome. Conservative initial treatment is recommended in all patients with superior mesenteric artery syndrome; this includes adequate nutrition, nasogastric decompression, and proper positioning of the patient after eating (ie, left lateral decubitus, prone, knee-to-chest position, or Goldthwaite maneuver).[19] Enteral feeding using a double lumen nasojejunal tube passed distal to the obstruction under fluoroscopic assistance is an effective adjunct in treatment of patients with rapid severe weight loss and also eliminates the need for intravenous fluids and the risks associated with total parenteral nutrition.

In some instances, both enteral and parenteral nutritional support may be needed to provide optimal calories. The patient's weight should be monitored daily. Subsequently, the patient can be started on oral liquids followed by slow and gradual introduction of small and frequent soft meals as tolerated. Finally, regular solid foods are introduced. Metoclopramide treatment may be beneficial. Review of the orthopedic literature reveals that the success rate is 100% with medical management only in cases with an acute presentation of SMA syndrome.

Surgical intervention is indicated when conservative measures are ineffective, particularly in patients with a long history of progressive weight loss, pronounced duodenal dilatation with stasis, and complicating peptic ulcer disease. A trial of conservative treatment should be instituted for at least 4-6 weeks prior to surgical intervention.

Options for surgery include a duodenojejunostomy or gastrojejunostomy to bypass the obstruction or a duodenal derotation procedure (otherwise known as the Strong procedure) to alter the aortomesenteric angle and place the third and fourth portions of the duodenum to the right of the superior mesenteric artery.[12] This is called derotation because the final position of the midgut is in direction opposition to the normal embryonic rotation of the midgut. This may be best suited for pediatric patients in whom the superior mesenteric artery syndrome may be related to congenital anatomic conditions that predispose to their symptomatology.[12]

Surgical duodenal derotation for superior mesenteric artery syndrome usually requires a laparotomy, during which the duodenum is mobilized after division of the ligament of Treitz. Once the duodenojejunal junction has been fully mobilized, the jejunum is passed behind the superior mesenteric artery and is positioned to the right of the superior mesenteric artery so it does not lie in the acute angle between the aorta and the superior mesenteric artery.[12, 20] This procedure can also be done laparoscopically and can be converted to a gastrojejunostomy or duodenojejunostomy if it fails to improve the patient's symptoms.

Another surgical approach to treating superior mesenteric artery syndrome is a duodenojejunostomy, in which the compressed portion of the duodenum is released and an anastomosis is created between the duodenum and jejunum anterior to the superior mesenteric artery. This is the classic approach to superior mesenteric artery syndrome. Advantages include the ease of procedure. Complications include risk of bleeding, leakage or stricture at the anastomotic site, and a creation of a nonphysiologic bilious circulation loop of unknown consequence.[12]

Successful laparoscopic duodenojejunal bypass has been described. The operation consists of a loop of jejunum anastomosed to the dilated duodenal segment, which is seen below the transverse mesocolon. Although experience is limited to case reports and small studies, laparoscopic approaches are feasible and provide a less invasive surgical option.[21, 22, 23] A gastrojejunostomy may be another surgical option but is usually reserved for patients who have contraindication to a duodenojejunostomy, such as duodenal ulcer disease or if both the stomach and duodenum are severely dilated.[12] Care in pursuing surgical correction should be taken in patients with significant premorbid conditions and malnutrition, such as end-stage renal disease, because these have a high surgical mortality when performed for superior mesenteric artery syndrome.[24]

A retrospective study evaluated 12 patients with superior mesenteric artery syndrome who were treated with laparoscopic enteric bypass. The study concluded that laparoscopic duodenojejunostomy is safe and effective and should be considered the optimal treatment for patients presenting with duodenal obstruction from superior mesenteric artery syndrome. The study also concluded that advances in minimally invasive surgery have demonstrated the safety and low morbidity of laparoscopically created enteric anastomoses. The authors add that the shorter hospital stay, low morbidity, and that the high success of laparoscopic enteric bypass make this approach favorable to traditional open techniques.[25, 26]  A retrospective chart review by Chang et al on patients who underwent minimally invasive duodenojejunostomy found that at follow-up only 6 of 18 patients reported symptomatic improvement or resolution even though 14 of 18 patients reported initial symptom improvement.[27]

The following consultations may be indicated:

• Pediatric gastroenterologist

• Nutritionist[28]

• Pediatric surgeon

Medical therapy usually begins with the initiation of intravenous fluids and, once no significant emesis, with the frequent administration of small amounts of liquids. In some cases, nasojejunal or nasogastric tube feedings with a standard liquid diet may be indicated. If the patient is completely obstructed or unable to tolerate liquids, total parenteral nutrition is indicated.

Drug therapy currently is not a component of the standard of care for this syndrome. Metoclopramide may be used to provide a prokinetic effect. Caution should be taken if patient has signs of complete obstruction.

Class Summary

These stimulate motility of the GI tract. GI smooth muscle is regulated by autonomic innervation, local reflexes, and hormones. Peristalsis is induced and causes the gut contents to move, encouraging digestion.

Metoclopramide (Reglan, Clopra)

Dopamine antagonist that stimulates acetylcholine release in the myenteric plexus. Acts centrally on chemoreceptor triggers in the floor of the fourth ventricle, causing important antiemetic activity.

Nasojejunal or nasogastric tube feedings with a standard liquid diet may be indicated in some patients with superior mesenteric artery (SMA) syndrome. These feedings may be continued at home until the obstruction resolves spontaneously.

Complications include the following:

• Electrolyte imbalance (eg, hypokalemia, hypochloremia, metabolic alkalosis)

• Dehydration

• Malnutrition

• Oliguria

• Hypotension

• Peptic ulcer disease

• Aspiration pneumonia

The outcome is excellent if the entity is diagnosed in a timely fashion and if the patient receives appropriate therapy. In the past, deaths due to progressive dehydration, hypokalemia, and oliguria have been reported; most of these occurred in patients in whom the diagnosis was delayed or missed.