▶ Transverse ultrasound image (left) shows that the superior mesenteric vein (SMV) (arrow) is located to the left of the superior mesenteric artery (SMA) (arrowhead). An urgent upper GI series showed malrotation.
▶ CT image of an older patient shows the same finding of the SMV (arrow) located to the left of the SMA (arrowhead).
The imaging differential diagnosis for emesis in a young infant depends on the character of the emesis. Bilious emesis suggests an obstruction distal to the ampulla of Vater. The diagnosis that must be excluded is midgut volvulus.
▶ Malrotation is a congenital anomaly of the development of the midgut. During weeks 5 to 6 of gestation, the midgut herniates into the umbilical cord and normally rotates 270 degrees counterclockwise before returning to the abdomen with the jejunum in the left upper quadrant and the cecum in the right lower quadrant. These loops of bowel form the ends of the root of the mesentery and they are normally widely separated, so that the bowel is stable in fixation. Abnormal rotation of the midgut causes malfixation of the bowel such that the ends of the root of the mesentery are close together. This allows the bowel to pivot around this narrow point of fixation, potentially causing midgut volvulus.
▶ Normally the SMV is on the right side of the SMA as it joins the splenic vein to form the portal vein. The finding of inversion of the SMA and SMV relationship has a high association with malrotation, but evaluation of the SMA/SMV relationship is not a good screening study for malrotation, since approximately one third of patients with surgically proven malrotation have a normal SMA/SMV relationship.
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