gillianmackay@sky.com
SMJ 2009 54(3): 58
Abstract
Superior
mesenteric artery syndrome is characterised by vascular compression of the third
part of the duodenum, resulting in obstructive symptoms. This case has been
selected for its unusual presentation with pneumomediastinum and for the
strikingly abnormal cross-sectional findings. We present the case of a
young male of asthenic build, who was admitted to hospital with a history
mimicking Boerhaave’s syndrome (spontaneous oesophageal perforation).
Radiological features included pneumomediastinum, but also an astonishing degree
of gastric and duodenal dilatation. Extraluminal retroperitoneal air was also
noted, further complicating the patient’s presentation. We explore the
relevant imaging and surgical findings, and relate these to an initially
puzzling diagnostic dilemma.
Introduction
First described by Van Rokitansky in 1842 (and subsequently popularised to Wilkie syndrome), superior mesenteric artery syndrome is characterised by narrowing of the angle between the superior mesenteric artery and aorta (in a normal subject, usually around 45-65°). The artery compresses the third part of the duodenum, leading to obstruction. Active peristalsis is present proximal to the abrupt point of transition.
This feature helps to exclude a neurological cause of megaduodenum (for example, in cases of autonomic neuropathy) in which the normal peristaltic motion of the gut is generally reduced or absent.
A thin build is present in around 80% of patients, who are most commonly young females between 10 and 39 years of age 1.
Conditions
predisposing to high catabolic states are described as underlying aetiologies,
and reported cases included wasting due to HIV 2 or anorexia nervosa 3,
and after spinal surgery where prolonged immobilisation is required.
Most commonly occurring in adolescence or late childhood, this condition is also described following rapid growth without compensatory weight gain.
Our patient was not known to possess any of the above predisposing factors, making his presentation all the more unusual.
Case Presentation
A healthy 15 year old male presented as a surgical emergency with a 36 hour history of repetitive forceful vomiting and abdominal pain, which began acutely after eating a meal. There were no associated lower gastrointestinal symptoms. He had no significant past medical history, and was a keen athlete, playing football with a local junior league team.
Clinical
examination revealed a teenager of thin build. Although haemodynamically stable,
he was clinically dehydrated, with rebound guarding on abdominal palpation.
An initial plain film of the chest surprisingly revealed extensive surgical emphysema within the neck and chest wall, with evidence of pneumomediastinum 4,5. A left pneumothorax was also noted (Figure 1).
Figure 1 Diffuse subcutaneous air in tissues of neck and chest wall, with air outlining the superior mediastinum and aortic arch.
These radiographic findings initially supported the clinical suspicion of Boerhaave’s syndrome. Water-soluble contrast swallow was performed, showing no evidence of an oesophageal tear. However, moderate dilatation of the oesophagus was noted during the procedure. At intervals, a fluid level was noted within the distal oesophagus, with the impression of a “to and fro” 6 motion (Figure 2a, 2b, 2c) suggesting a possible distal obstruction.
Figure 2a, 2b and 2c Consecutive images from contrast swallow. Figure IIb shows a small reduction in the level of contrast, before rising again.
Multi-slice CT of the chest, abdomen and pelvis (64 slice MDCT, slice thickness 0.5mm, with oral and intravenous contrast) demonstrated shallow bilateral apical pneumothoraces, and extensive pneumomediastinum (Figure 3). The dilated oesophagus was noted to enter a massively distended stomach (some 35cm in maximum craniocaudal dimension) with gross dilatation of the first, second and third parts of the duodenum. Very little intra-abdominal fat was noted.
Figure 3a CT on lung window settings. Air is seen in the pleural space bilaterally with extensive subcutaneous emphysema. The distended, fluid-filled oesophagus is noted (white arrow). Figure 3b Air tracks around the mediastinal structures.
Oral contrast tapered to an abrupt cut-off at the third part of duodenum. Multiple unilateral locules of free retroperitoneal air were noted (Figure 4), thought most likely to have tracked downwards from the mediastinum.
Figure 4 Axial CT images of the transition point of the duodenum (straight white arrow). Several pockets of right-sided retroperitoneal air are marked (curved white arrow).
The patient’s clinical symptoms briefly worsened, prompting further surgical investigation. Upper GI endoscopy was performed under general anaesthesia, which confirmed a normal oesophageal mucosa with no evidence of perforation. After aspiration of over 4 litres of fluid from the stomach and desufflation of the upper GI tract, no abnormality of either the gastric or duodenal mucosa was seen. However, given the presence of extraluminal air in the retroperitoneum, and the ongoing abdominal pain, it was felt particularly important to exclude a duodenal perforation. A laparotomy was therefore performed.
During surgery, the free retroperitoneal air was discovered after Kocherising (mobilising) the duodenum, but reassuringly there was no evidence of perforation of the GI tract. The stomach and duodenum had been decompressed endoscopically, and, despite the high volume of aspirate, there was no evidence of an obstructing lesion.
Overall, the upper GI tract appeared unremarkable. The abdomen was closed without further intervention.
Post-operatively, there were exceptionally high nasogastric aspirates (7 litres/ day), and episodic vomiting occurred following clamping of the nasogastric tube. The post-operative situation prompted a review of the CT scan.
Coronal oblique reformatting of the axial CT data showed the superior mesenteric artery crossing the third part of duodenum at its exact point of transition (Figure 5a). Sagittal reformatting displayed the narrow angle between the aorta and SMA (Figure 5b). These images were the most helpful in determining the cause of the patient’s obstruction.
Figure 5a The superior mesenteric artery can be seen to cross at exactly the level of obstruction (white arrow). Massive distension of the stomach is obvious. Figure 5b The abnormal aortomesenteric angle is arrowed.
Nutrition was maintained with parenteral feeding, initially via a peripheral line and latterly via a dedicated, tunnelled, central feeding line. After a 2 week period of parenteral nutrition, he settled spontaneously, and was able to eat normally. He was discharged shortly after, and at recent clinic review was found to be in excellent health. He had returned to football training and was eating a normal diet. At present, the need for bypass surgery (duodenojejunostomy is considered in those patients refractory to conservative management 7) is not necessary.
Discussion
In the first instance, this case is unusual owing to the presenting feature of pneumomediastinum. This initially led to a presumptive diagnosis of Boerhaave’s syndrome. However, the suspicious radiological features contrasted the patient’s overall clinical condition. Although tender on abdominal palpation, he had no signs of sepsis or haemodynamic compromise. Should an oesophageal perforation have occurred during the 36 hours prior to presentation, a more precipitous clinical decline would usually be expected. In addition, the plain film of the chest revealed no evidence of a pleural effusion, which may have further supported this initial suspicion.
Acute gastric and duodenal dilatation with spontaneous pneumomediastinum was also considered as a possible diagnosis following the negative contrast swallow. The possible causes of such massive gastric distension in a young patient include eating disorders, medication, electrolyte abnormalities, connective tissue diseases, autonomic neuropathy, psychogenic polyphagia and gastric volvulus. No such underlying conditions were apparent, either from the history or clinical examination. Neither was there any evidence of small bowel malrotation to support the development of volvulus. Furthermore, spontaneous pneumomediastinum is rare in children, with asthma being the most common cause 8.
The degree of gastric dilatation is a further point of particular interest in this case. As little as 3 litres of fluid can distend a normal stomach to the point of ischaemia or eventual rupture, and in this case, a significantly higher volume of gastric aspirate was obtained. However, there was no evidence of gastric wall pneumatosis or portal venous gas to give cause for such concern, both of which have previously been described with such gross distension 9,10 ,11.
The transition point in the third part of the duodenum confirmed a proximal small bowel obstruction, and suggested the underlying aetiology. Oral contrast examination of the upper GI tract (for example, barium meal or hypotonic duodenography 4) has long been felt the gold standard investigation for possible superior mesenteric artery syndrome. The classical findings have been described as dilatation of first and second parts of duodenum with or without gastric dilatation; abrupt, partial or complete, vertical obstruction of barium/flow at site where superior mesenteric artery crosses the third part of duodenum (“duodenal clamp”); and antiperistaltic waves proximal to obstruction, producing a “to and fro” movement 6 . However, we have shown that thin slice multi-detector CT can beautifully depict, not only the exact point of obstruction, but the course of the vessel over the duodenum and the narrowed aortomesenteric angle. Previous series have mentioned that cross-sectional imaging may be helpful as an adjunct to other modalities 12 but, as a further distinguishing aspect to this case, our images are able to demonstrate the CT diagnosis with accuracy. Multi-slice CT (and the ability to reformat the acquired volume of data) is now widely used, and is offered routinely as an out-of-hours examination. Suspected cases of superior mesenteric artery syndrome can therefore be imaged promptly, optimising the management of such patients.
The pneumomediastinum is most likely the result of a rise in the intra-alveolar pressures during vomiting 7. Air dissects along peribronchial vascular sheaths, ending with free air in the mediastinum (Macklin effect). The mediastinum also communicates with vascular sheaths in the neck, along which the air may track. Less commonly, as in this case, air may also track into the retroperitoneum. Two routes of communication with the retroperitoneum are known, via a tissue plane through the sternocostal attachment to the diaphragm, as well as periaortic and perioesophageal fascial planes. Rising intra-alveolar pressure has, in this case, also given rise to the small pneumothoraces, as air can enter the pleural space via distended and damaged alveoli.
Conclusion
Superior mesenteric artery syndrome is a rare but recognised cause of duodenal obstruction. It should be considered if the obstruction presents in a young adult, even in the absence of any predisposing factors. Its presentation may mimic other pathologies of the GI tract, and if suspected, can be investigated by oral and IV contrast-enhanced multi-slice CT.
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