Malrotation of the bowel indicates developmentally abnormal positioning of the bowel. Malrotation of the bowel covers a wide spectrum of abnormalities in the process of large and small bowel rotation during development. Included in this spectrum are abnormalities in fixation of the bowel to the retroperitoneum. Because of the abnormalities in rotation and also in fixation of the bowel, all of these patients are at an increased risk for volvulus. The majority of the patients with malrotation will present early in life and frequently in the first year of life. However not all patients present early and may in fact be plagued with chronic intermittent volvulus which can be difficult to diagnose. Intermittent volvulus was a suspicion in the patient presented here.

The abnormal position of the SMA and SMV in the setting of malrotation was described in 1983 by Nicholas and Li in three patients who on CT examination had the SMV to the left of the SMA instead of the normal right ventral position.[1] In 1992, Weinberger et al reported their attempt to sonographically assess the relative position of the SMA and SMV in 337 children.[2] The visualization was successful in 74% and of these children, 5 had the SMV to the right of the SMA. All 5 of these patients had malrotation. Several other authors have reported similar findings demonstrating the superior mesenteric vein to lie anterior to or to the left of the superior mesenteric artery rather than to the right on transverse ultrasound images of the upper abdomen.[3,4,5] On the other hand, in 1982, Lieberman and Haaga reported the case of a patient with malrotation who had a normally positioned superior mesenteric artery and vein on CT scan.[5] Inversion of the normal vascular orientation may be caused by other entities besides malrotation. Recently, Papadopoulou reported abnormal positioning of the SMA and SMV caused by ileocolic intussusception.[6]

On barium examinations, the key finding in malrotation of the small bowel is positioning of the duodenum and proximal small bowel to the right of the patient's spine. In malrotation, the third and fourth portions of the duodenum will not cross to the left side of the spine. The small bowel may all lie to the right of the spine. Again however, there is a spectrum of findings on barium exams and at least seven different duodenal positions in the setting of malrotation have been described.[7]

In conclusion, since CT scans and ultrasound exams are often first line studies in evaluating patients with unexplained abdominal pain, familiarity with the normal orientation of the SMA and SMV is helpful. Although there will be false-positives and false-negatives, the orientation of the SMA and SMV may be an indicator for underlying pathology such as malrotation. Detection of an abnormal relationship of the SMA and SMV should lead to further evaluation typically with an upper GI series to assess for possible malrotation as occurred in this patient's work-up.

References:
1. Nichols DM, Li DK. Superior mesenteric vein rotations:A CT sign of
midgut malrotation. AJR 1983;141:707-708.

2. Weinberger E, Winters WD, Lindell RM, Rosenbaum DM, Krauter D.
Sonographic diagnosis of intestinal malrotation of infants:Importance of the
relative positions of the superior mesenteric vein and artery. AJR October 1992;159:825-828.

3. Gaines PA, Saunders AJS, Drake D. Midgut malrotation diagnosed by ultrasound.
Clinical Radiol 1987;38:51-53.

4. Loyer E. Eggli K. Sonographic evaluation of superior mesenteric vascular relationships
in malrotation. Pediatr Radiol 1989;19:175-178.

5. Lieberman JM, Haaga JR. Duodenal malrotation. J Comput Assist Tomogr 1982;6:1019-1020.

6. Papadopoulou F, Efremidis SC, Raptopoulou AG, et al. Distal ileocolic intussusception:
Another cause of inversion of superior messenteric vessels in infants. AJR November 1996;167:1234.

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