Superior mesenteric artery (SMA) syndrome is an uncommon gastro-vascular condition that is well recognized clinically. It is characterized by the compression of the traverse or the third, portion of the duodenum between the superior mesenteric artery and the aorta. The compression can lead to a partial or complete obstruction of the duodenum. SMA syndrome was first reported by Von Rokitansky who, in 1861, described it as a causal agent for the blockage of the third part of the duodenum as a result of the compression. Studies have indicated the incidence to be low (0.1-0.3% (Salem et al., 2017)). English literature indicates that there have been 400 reported cases of SMA syndrome. Despite this fact, many people have not accepted the existence of this condition. It has remained a controversial entity in medicine as it has been confused with various other conditions. However, it is now widely acknowledged.
Pathophysiology
The abdominal aorta normally forms an angle of about 45 degrees (ranging between 38 and 56 degrees) with the superior mesenteric artery. The traverse part of the duodenum always crosses caudal to the origin of the mesenteric artery hence coursing between the two vessels. When there is any factor that makes the aortomesenteric angle to narrow to a range of 6 to 25 degrees, a sharp entrapment of the third part of the duodenum occurs. This causes it to compress as it passes the between the aorta and the mesenteric artery leading to superior mesenteric artery syndrome. The reduction in the angle is highly attributed to the reduction or lack of the visceral fat and retroperitoneal. Peritoneal adhesion can cause the compression of the duodenum as well (So, 2016). The distance between the two vessels also reduces to 2-8 millimeters instead of the normal range of between 10 and 20 millimeters. However, the reduction in the angle can also be attributed to a lower BMI especially in children. Thus, this is not enough to act as a basis for the diagnosis of SMA syndrome. Another cause of the condition can be due to the high insertion of the duodenum at the ligament of Treitz.
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Clinical Description
The duodenum is normally cushioned by the lymphatic tissue and retroperitoneal fat hence preventing its compression by the superior mesenteric artery. Thus, the SMA syndrome is caused by any situation that leads to an insubstantial cushion and reduction in the mesenteric angle. The presentation of this syndrome can take place in two forms: acute or induced, and chronic or congenital (So, 2016). The latter form makes the patients have a lengthy or in some cases a lifelong history of abdominal conditions with sporadic exacerbations which depend on the extent of the compression on the duodenum.
The risk factors of SMA syndrome are inclusive of the anatomic conditions such as intestinal malrotation around the axis formed by the SMA, a lower origin of the vessel, an abnormally high insertion of the duodenum at the Treitz ligament, or very thin (aesthenic) body build. The conditions that predispose an individual to these anatomical problems involve a history of neurological injury, catabolic states which are experienced in cancer and burns, cachexia, peritoneal adhesions, abdominal wall laxity, and poor motility of the digestive tract. Dietary disorders also act as factors which may aggravate the predisposition as they can lead to the depletion of the fat that surrounds the mesenteric artery. The factors include loss of appetite, starvation, weight loss and anorexia, and malabsorption.
The individuals who have a rapid linear growth that lacks a compensatory weight gain acts as a catalyst to the SMA syndrome (Salem et al., 2017). This is particularly in adolescents who have a lower BMI and have had a spinal fusion for scoliosis. Spinal trauma, injury, or deformity can also aggravate the condition, especially when there has been the use of body cast after surgical performances on vertebral fractures. Corrective vertebral surgery can lead to the elongation of the spine which may, in turn, reduce the angle between the aorta and the mesenteric artery. There are other unusual causes of SMA syndrome that include visceroptosis, exaggerated lumbar lordosis, and idiopathic neonatal SMA syndrome.
The acute form of the syndrome can result rapidly after a traumatic occurrence that makes the SMS to hyper-extend across the duodenum, and causing obstruction. It can also be brought by a sudden loss of weight as a result of any cause. The predisposing factors include ileoanal pouch surgery, left nephrectomy, scoliosis surgery, and a prolonged supine bed rest (So, 2016).
As observed from the causes, it is imperative to note that the SMA syndrome can mimic some eating disorders. However, distinguishing the two is extremely important as misdiagnosis can lead to complicated situations.
Signs and Symptoms
A patient of SMA syndrome exhibits a variety of symptoms which develop from 6 to 12 days after scoliosis surgery. They include extreme postprandial abdominal pain which comes as a result of both duodenal compression and the peristalsis that acts to reverse the effects. The victim may feel an early satiety (gets satisfied faster) after a meal. Also, nausea and voluminous vomiting of bilious or partially digested food are experienced (Salem et al., 2017). The patient may have eructation, abdominal distention, heartburn, external hypersensitivity of the abdominal area, and reflux. In children, there is difficulty in feeding and poor weight gain. In some instances, spontaneous loss of weight can result accompanied by severe malnutrition. In turn, this heightens the compression of the third section of the duodenum which leads to worsening of the underlying condition, hence bringing the cycle of worsening symptoms.
The symptoms can be aggravated when the individual assumes a supine (face up) position and can be relieved when they are in the left lateral decubitus or knee-to-chest position (So, 2016). The symptoms can also be eased when the patient is in the face down pose. The pressure exerted below the umbilicus in dorsal and cephalad direction (also known as Hayes maneuver) can help in elevating the superior mesenteric artery hence easing the constriction slightly. The measures are meant to reduce the tension in the small bowel at the angle between the two vessels.
Diagnosis
The diagnosis of superior mesenteric artery syndrome is problematic as its beginning is insidious. This makes the syndrome to be considered after the patient has undergone thorough checks after the report on abdominal discomfort. The gastrointestinal tract should undergo an extensive evaluation including upper endoscopy. Also, the evaluation should be done for the inflammatory intestinal, ulcerative, and malabsorptive conditions whose diagnostic frequency is high. The diagnosis is done through imaging which may begin by x-ray examination which indicates dilation in the duodenum and the abrupt compression near the overlying SMA, followed by a delayed transit time in the gastrointestinal region from four to six hours. The analysis includes the use of computed tomography (CT) scan to examine the abdomen and the pelvis with an IV and oral contrast (Salem et al., 2017). Alternatively, upper gastrointestinal series (UGI) can be carried out and hypotonic duodenography done for equivocal cases. Other examination processes that can be used include vascular imaging studies such as contrast angiography and ultrasound which may indicate the increased velocity of flowing blood through the SMA or a reduced angle.
There is always controversy surrounding the diagnosis of the SMA syndrome as the symptoms do not relate concurrently with the imaging findings. Another reason is that the symptoms may not improve even after surgical intervention. But, the rationale for the persistence of the gastrointestinal symptoms even after the condition is surgically corrected in some instances has been attributed to the remaining eminence of reversed peristalsis as compared to direct peristalsis. The syndrome is more attributed to the females between the age of 10 and 30 (Salem et al., 2017). Therefore, it is commonplace for medical practitioners to misinterpret the emaciation as a personal choice rather than a clinical condition resulting from SMA syndrome. The symptoms in the early stages of the disease are subtle and the patients are likely to ignore them until there is a substantial damage to the health of the person. They may be unaware of their illness until the pain becomes prolonged.
The adaptation of a single imaging process is done when there is an unclear diagnosis from the upper gastrointestinal series examination. It can also be done when there is a suspicion of the underlying etiology of the condition. The CT scan is therefore adopted as it can measure to precision the angle between the aorta and the SMA. It can also measure the distance in the sagittal plane. The mode of carrying this out is to use a contrast-enhanced CT scan.
Treatment
The treatment of SMA syndrome can take place in both conservative and surgical procedures. The former includes gastric decompression, nutritional support, and correction of fluid electrolytes imbalance. The conservative treatment is aimed at restoring the retroperitoneal fat and weight gain. The nutrition can be administered through parenteral or post-pyloric tube feeding (So, 2016).
When the conservative methods fail, surgical procedures can be performed to bypass the obstruction. The procedure of choice here is the duodenojejunostomy which has shown a 90 percent success rate. Strong’s procedure is also another intervention which involves the division of the ligament of Treitz (Salem et al., 2017).
Conclusion
In conclusion, SMA syndrome is a rare occurrence whose diagnosis is challenging as it has some similarity to nutrition disorders. It happens due to the narrowing of aortomesenteric angle to a range of 6 to 25 degrees, which causes a sharp entrapment of the third part of the duodenum. The diagnosis of superior mesenteric artery syndrome is problematic as its beginning is insidious but there are imaging options which can be used after other examinations. Adaptation of the imaging method depends on the ability to measure the precise angle between the aorta and the mesenteric artery.
References
Salem, A., Al Ozaibi, L., Nassif, S. M. M., Osman, R. A. G. S., Al Abed, N. M., & Badri, F. M. (2017). Superior mesenteric artery syndrome: A diagnosis to be kept in mind (Case report and literature review). International journal of surgery case reports, 34, 84-86.
So, C. Y., Chan, K. Y., Au, H. Y., Chan, M. L., & Lai, T. (2016). Superior mesenteric artery (SMA) syndrome: an unusual cause of intestinal obstruction in palliative care. Annals of palliative medicine, 6(1), 91-93.
