DUODENAL ATRESIA, STENOSIS, WEB, AND ANNULAR PANCREAS

Duodenal atresia, stenosis, web, and annular pancreas are all part of a spectrum of similar abnormalities. All cause either complete or partial duodenal obstruction and usually present at birth or within the first few days of life. Often, components of more than one diagnosis are present. For example, many cases of duodenal atresia have a component of annular pancreas, and annular pancreas almost never occurs without a component of intrinsic duodenal stenosis.

The duodenum is the most common site of intestinal atresia. Duodenal atresia and stenosis almost always occur in the region of the ampulla of Vater. Approximately 30% of cases of duodenal atresia are associated with Down syndrome. Other associations include other intestinal atresias, biliary abnormalities, congenital heart disease, and associations with the complex known as VATER (vertebral defects, imperforate anus, tracheoesophageal fistula, and radial and renal dysplasia). Radiographs of neonates with duodenal atresia typically demonstrate a dilatated stomach and a dilatated proximal duodenum with no gas distal to the proximal duodenum. The two dilatated structures are referred to as a “double-bubble” sign (Fig. 5-7). In the appropriate clinical setting, a double bubble is diagnostic of duodenal atresia, and additional imaging by an upper gastrointestinal series (UGI) is unnecessary. The question that often arises is how do we know that this is not an acute obstruction caused by a midgut volvulus, which is, a surgical emergency? The answer is that dilatation of the duodenal bulb is seen only with chronic causes of obstruction. There is not enough time for the bulb to become dilated in acute obstruction, such as with midgut volvulus. If it is not clear, a UGI can be performed to document the cause of obstruction. With duodenal stenosis, the double bubble is seen in association with the presence of distal bowel gas.

FIGURE 5-7. Duodenal atresia in a newborn infant. Radiograph shows air-filled, dilatated stomach (S) and dilatated duodenal bulb (D), giving the appearance of a double bubble. There is no distal bowel gas.

Duodenal web is another cause of congenital duodenal obstruction. Typically, a web consists of an obstructing membrane; a pin-sized hole in its center is the only lumen. The web may stretch downstream, forming a wind-sock configuration seen on UGI (Fig. 5-8). Because the obstruction is not complete, these patients may present later in life than those with atresia. The presence or absence of a component of annular pancreas is not something that can be determined on UGI in the setting of congenital duodenal obstruction.

FIGURE 5-8. Duodenal web in an infant with vomiting. Lateral view from a UGI shows obstruction of the duodenum with a rounded, windsocklike appearance. The duodenal bulb (D) is dilatated. There was eventually delayed passage of some contrast into more distal bowel. S, stomach.

MALROTATION AND MIDGUT VOLVULUS

Midgut volvulus is one of the few true emergencies in pediatric gastrointestinal imaging. A delay in the diagnosis of midgut volvulus can result in ischemic necrosis of large portions of the bowel and possibly in death. An understanding of midgut embryogenesis is often emphasized, but an understanding of the end result is more important. With normal embryonic rotation, both the duodenojejunal and ileocolic portions of the bowel rotate 270 degrees about the axis of the superior mesenteric artery. The result is that the duodenojejunal junction (DJJ) is positioned in the left upper quadrant and the cecum is positioned in the right lower quadrant. This results in a long, fixed base that keeps the small bowel mesentery from twisting. If the duodenojejunal and ileocecal junctions are not in their normal positions, the base of the small bowel mesentery may be short and predispose the small bowel to twisting, resulting in a midgut volvulus.

For clarification, note the following definitions:

The diagnosis of malrotation is made on UGI by determining that the DJJ is abnormally positioned. The duodenojejunal junction, the point at which the proximal bowel turns inferiorly on a frontal view, is considered normal when it meets the following two criteria: (1) it is to the left of the spine and (2) it is at the same level as or superior to the duodenal bulb. It is important to evaluate the position of the DJJ during the first pass of contrast through the duodenum and jejunum (see subsequent material; Figs. 5-9A-C, 5-10A-C).

FIGURE 5-9. Normal position of the duodenojejunal junction (DJJ, ligament of Treitz) on standard views from a UGI. A, Lateral view shows antrum (A), duodenal bulb (B), and duodenal sweep (D). Note that the duodenum normally extends posteriorly and inferiorly during the retroperitoneal course. B, Frontal view obtained during first pass shows the DJJ (arrow) identified by the point in the bowel that angles inferiorly. The DJJ is normally to the left of the spine and at the same level (superiorly to inferiorly) as the level of the duodenal bulb. C, Oblique view of patient with left side down again shows the level of the DDJ (arrow) at the level of duodenal bulb and proximal jejunum in left upper quadrant.

FIGURE 5-10. Abnormal position of the DJJ. A, Marked abnormal position of the DJJ. A UGI performed via nasogastric tube shows the DJJ (arrow) to be lower than the duodenal bulb and not to the left of the spine. Proximal jejunum is on right of spine. Findings are consistent with malrotation. B, Abnormal position of the DJJ associated with dilatation of the proximal duodenum. UGI shows that the DJJ (arrow) is not to the left of the spine and is lower than the duodenal bulb. The proximal jejunum is on right of spine. Findings are consistent with malrotation. Dilatation of the proximal duodenum (D) is suspicious for obstruction due to volvulus or Ladd bands. C, DJJ that does not quite meet normal criteria. The DJJ (arrow) is not quite to the left of the spine and not quite at same level as the bulb. In such cases, most radiologists will follow the contrast through the small bowel. If the cecum is in the right lower quadrant, the small bowel mesentery is most likely long enough not to be at risk for volvulus.

In many cases of malrotation the findings are grossly obvious; the duodenum courses rightward, rather than leftward and never crosses the spine (Fig. 5-11; and see Fig. 5-10). However, when performing UGIs in children, there are many cases that do not quite meet the criteria for normal but are very close. It is probably inappropriate to send all of these cases to the operating room. As you become more experienced in imaging, some things become more and more clear. Others become less clear, and you realize that many of the rules you were taught should really be thought of more as guidelines. Although germane to the fabric of pediatric radiology, the diagnosis and exclusion of malrotation is not straightforward or easy. In cases with borderline-positioned DJJs (not quite as high as the bulb, not quite to the left of the spine), most pediatric radiologists follow the contrast through the small bowel (see Fig. 5-10). If the jejunum is in the left upper quadrant and the ileum and cecum are in the right lower quadrant, the patient is probably not at risk for midgut volvulus. Also, the DJJ is a mobile structure in children and can “factitiously” be moved into an abnormal position by a space-occupying lesion, such as a mass or distended bowel loops. In addition, the presence of a nasojejunal tube may alter the apparent position of the DJJ.

FIGURE 5-11. Malrotation. The duodenum courses rightward and never crosses the spine. The opacified proximal small bowel is in the right upper quadrant. The unopacified colon is in the left abdomen.

In patients who are malrotated, midgut volvulus may happen at any age; however, more than 90% are present during first 3 months of life. Midgut volvulus can be seen on UGI as a corkscrew appearance of the duodenum and proximal jejunum or as duodenal obstruction (Fig. 5-12A-C). The presence of bilious vomiting and findings of malrotation on UGI, with or without findings of midgut volvulus, is considered a surgical emergency.

FIGURE 5-12. Midgut volvulus. A, Lateral view from UGI shows corkscrew appearance of the duodenum and distention of the proximal duodenum. Findings are consistent with volvulus. B, Frontal view from UGI in another patient shows distention of proximal duodenum (D) and beaklike area of narrowing. Note that the DJJ is not in the normal position and the proximal small bowel is in the right lower quadrant. Findings are consistent with malrotation and volvulus. C, Surgical image from a different patient shows multiple loops of necrotic small bowel secondary to infarction from the volvulus.

Malrotation and midgut volvulus may also be encountered on cross-sectional imaging studies, such as computed tomography (CT) or ultrasound, when these studies are ordered to evaluate abdominal pain or vomiting. This is particularly true in older children in whom malrotation is often not initially suspected as the cause of acute abdominal symptoms. On cross-sectional imaging, the bowel may be seen to form a swirling pattern around the superior mesenteric vessels (Fig. 5-13A, B). In addition, the superior mesenteric vein, which is normally to the right of the superior mesenteric artery, is more often to the left of the superior mesenteric artery in malrotated patients (see Fig. 5-13). However, this is neither sensitive nor specific. The superior mesenteric artery is smaller and rounder than the superior mesenteric vein and is surrounded by fat, giving the artery an echogenic wall, as compared to the vein.

FIGURE 5-13. Malrotation presenting with intermittent abdominal pain secondary to midgut volvulus in an obese teenager. A, CT image of the upper abdomen shows inversion of the normal relationship between the superior mesenteric artery (arrow) and the vein (arrowhead), with the vein situated to the left of the artery. Note swirling vascular structures surrounding the superior mesenteric artery. B, A more caudal image shows characteristic swirling pattern of bowel loops (arrows) twisting about the axis of the mesenteric vessels, consistent with volvulus.

HIRSCHSPRUNG DISEASE

Hirschprung disease is related to the absence of the ganglion cells that innervate the colon. The denervated colon spasms and causes a functional obstruction. Therefore, the affected portions of colon are small in caliber, and the more proximal, normally innervated colon is dilatated secondary to the obstruction. The rectum and a variable amount of more proximal colon are affected in a contiguous fashion; there are no skip lesions. Most patients with Hirschprung disease (90%) present in the neonatal period with failure to pass meconium (Fig. 5-19; and see Fig. 5-14). However, patients can present later in life with problems related to constipation. Hirschprung disease is much more common in boys (4:1) and is associated with Down syndrome in 5% of cases.

FIGURE 5-19. Hirschprung disease in an infant. Frontal radiograph demonstrates the diameter of the rectum (arrows) to be smaller than the diameter of the sigmoid colon, an abnormal rectosigmoid ratio. Note the sawtooth appearance of the abnormal contracted segment.

When an enema is being performed to evaluate for possible Hirshprung disease, it is essential to obtain early filling views, collimated to include the rectum and sigmoid colon, in both the lateral and then the frontal position. Findings of Hirschprung disease include a transition zone from an abnormally small rectum and distal colon to a dilatated proximal colon (see Figs. 5-14, 5-19). In a normal patient, the rectum has the largest luminal diameter of the left-sided colon. When the rectum alone is involved by Hirschprung disease, the sigmoid colon is larger than the rectum (see Figs. 5-14, 5-19). This is referred to as an abnormal rectosigmoid ratio. Another, but less common, finding is fasciculations or saw-toothed irregularity of the denervated segment (see Fig. 5-19). If the entire colon is involved by Hirschprung disease (very rare), the entire colon may appear small in caliber and may mimic a microcolon. Patients with Hirschprung disease may present with associated colitis. Therefore, in patients who are suspected to have Hirschprung disease and are ill, contrast enemas should be avoided.

Definitive diagnosis is obtained by rectal biopsy, and patients are treated by surgical resection of the denervated segment. The transition zone depicted on enema does not always accurately predict where the transition from absent to present ganglion cells occurs histiologically.

MECONIUM PLUG SYNDROME

Meconium plug syndrome, also referred to as functional immaturity of the colon or small left colon syndrome, is a common cause of distal neonatal obstruction. It is the most commonly encountered diagnosis in neonates who fail to pass meconium. It is thought to be related to functional immaturity of the ganglion cells. As in Hirschprung disease, the distal colon does not have normal motility, which causes functional obstruction. Unlike Hirschprung disease, it is a temporary phenomenon and resolves. Although most neonates with meconium plug syndrome are otherwise normal and have no abnormal associations, increased incidence occurs in patients who are infants of diabetic mothers or of mothers who have received magnesium sulfate for eclampsia. In neonates with meconium plug syndrome, there is always concern about underlying Hirschprung disease so at many centers, all neonates who have findings of meconium plug syndrome undergo rectal biopsy. In contrast to meconium ileus, there is no significant relationship between meconium plug syndrome and cystic fibrosis.

Infants with meconium plug syndrome present with failure to pass meconium. On contrast enema, multiple filling defects (meconium plugs) are seen within the colon (Fig. 5-20). The right and transverse colons maybe more dilatated than the left colon (small left colon syndrome; see Fig. 5-20), although these findings are variable. Microcolon does not occur. The rectum tends to be normal in luminal diameter, as compared to the rectums in infants with Hirschprung disease. The enema is often therapeutic; plugs of meconium are commonly passed during or shortly after the enema, and symptoms of obstruction often resolve within hours after the enema.

FIGURE 5-20. Meconium plug syndrome (small left colon syndrome) in a newborn infant born of a diabetic mother. Contrast enema demonstrates multiple filling defects (arrows) within the colon, consistent with meconium plugs. The left colon is small in caliber. Rectal biopsy, performed to exclude Hirshprung disease, demonstrated normal ganglion cells.