Normal Development

The duodenum derives from the distal portion of the foregut. During the fourth to eighth weeks of gestation, rapid growth of the epithelial lining of the foregut results in obliteration of the lumen. This is the solid core stage. Recanalization occurs by the process of vacuolization, beginning during the 8th to 10th weeks. Failure of vacuoliza tion can lead to congenital obstruction, in the form of either atresia or stenosis. Congenital atresias in the jejunum, ileum, and colon do not apparently occur by this mechanism, but rather result from intrauterine bowel ischemic events.

Because the pancreas and the extrahepatic biliary ductal system develop at the same time as the duodenum, combined anomalies sometimes occur. The dorsal portion of the embryonic pancreas arises from the wall of the duodenum and the ventral portion arises between the duodenum and the hepatic bud. During the process of bowel rotation, the ventral pancreas migrates along the right side of the duodenum to merge with the dorsal pancreatic bud. Incomplete rotation of the ventral pancreas can partially or completely surround the duodenum; this annular pancreas anomaly is often associated with atresia or stenosis of the duodenum. Other biliary and pancreatic anomalies that occasionally occur in association with duodenal atresia and stenosis include biliary atresia, choledochal cyst, pancreatic lipomatosis, dual pancreatic duct, and anomalous bile duct insertion.

Normal fetal development of the gut includes rotation of the intra-abdominal components. Early in embryogenesis, the primitive gut develops in an extracoelomic location. As the gut returns to the coelomic cavity, the bowel undergoes rotation. Before 6 weeks gestational age, the duodenum and cecum rotate 90° counterclockwise, as viewed from the front. Between 6 and 8 weeks gestation, the midgut (i.e., the portion of bowel extending from the duodenal–jejunal junction to the midportion of the transverse colon) develops in an extracoelomic location. During this period, the duodenum undergoes an additional 90° counterclockwise rotation, and the cecum remains relatively stationary. At approximately 10 weeks of gestation, the midgut begins to return to the coelomic cavity, and the duodenum rotates 90° and the cecum rotates 180°. During this rotation, the cecocolic loop passes anterior and superior to the origin of the superior mesenteric artery. The cecocolic loop then continues toward the right and descends into the right lower quadrant where it becomes fixed to the retroperitoneum. As rotation is completed, the mesentery fuses from the ligament of Treitz to the right lower quadrant. The various forms of malrotation result from arrested or abnormal progression in the process of fetal intestinal rotation.

An omphalocele results from failure of appropriate return of the embryonic midgut to the coelomic cavity from the umbilical stalk. Failure of normal bowel rotation also occurs in these patients. Persistence of the bowel within the umbilical stalk results in a markedly enlarged umbilical cord that contains a variable segment of intestine. If the membrane of the umbilical cord ruptures, the fetal bowel passes into the amniotic cavity.

An internal hernia can occur due to failure of appropriate mesenteric fixation of the bowel. A right paraduodenal hernia is caused by passage of intestine into a pocket of unfused ascending colon mesentery. A left paraduodenal hernia is due to extension of intestine into a pocket of unfused ascending colon mesentery.

Duplication Cyst

An enteric duplication cyst is a developmental lesion that can occur anywhere in the digestive tract. The small intestine is the most common location. The lesion morphology is cystic or tubular. Enteric duplication cysts are invariably located along the mesenteric border of the bowel.

The peak age for the clinical presentation of an enteric duplication cyst is the first year of life. Potential clinical manifestations of the abnormality include vomiting, abdominal distension, a palpable abdominal mass, and GI hemorrhage. Rarely, there are manifestations of urinary tract involvement, such as frequency due to compression of the bladder. Bowel obstruction in patients with an enteric duplication cyst can occur due to intussusception, volvulus, or extrinsic compression. Acute abdominal symptoms can also result from perforation of a duplication cyst. Malignant tumors can arise in a duplication cyst, but this complication is rare, and most reported cases are in adults with an otherwise asymptomatic cyst. Enteric duplication cysts occasionally occur in association with other congenital anomalies.

Abdominal radiographs are usually normal in an asymptomatic patient with an enteric duplication cyst. Rarely, the lesion is large enough to cause visible displacement of adjacent loops of bowel. Symptomatic lesions are occasionally associated with radiographic signs of a mechanical bowel obstruction. If barium studies are performed to evaluate for a suspected obstruction, such as intussusception, the lesion sometimes appears as a submucosal mass projecting into the bowel lumen. The mass may be somewhat malleable on external palpation.

Sonography is often the most useful and specific imaging technique for the detection and characterization of a suspected duplication cyst. A duplication cyst consists of an inner mucosal layer and an outer smooth muscle layer. This “GI signature” is often visible with a careful sonographic examination: a hyperechoic inner rim surrounded by a ring that is isoechoic to abdominal wall muscle (the “muscular rim sign” or “double wall sign”) (Figure 36-1). The use of a high frequency transducer aids in the demonstration of this finding. The layers are often not uniform in thickness. This double-layered wall pattern is reported to occur in over 50% of patients.¹ Use of the muscular rim sign for the diagnosis of an enteric duplication cyst requires caution, as this finding is not pathognomonic.² Occasionally, up to 5 layers are visible sonographically in the wall of an enteric duplication cyst: superficial mucosa, deep mucosa, submucosa, muscularis propria, and serosa.³ The contents of enteric duplication cysts are variable, ranging from solid, to debris-filled fluid, to clear fluid (Figure 36-2). Rarely, peristalsis of a duplication cyst is visible on real-time sonography.⁴

In children with GI hemorrhage, scintigraphy with ^99mTc-pertechnetate is an excellent technique to survey the abdomen for a lesion with ectopic gastric mucosa, such as a Meckel diverticulum. Ectopic gastric mucosa is present in approximately 25% of small bowel duplication cysts; in these instances, the scintigraphic appearance can mimic that of a Meckel diverticulum. The unusual tubular variant of duplication cyst has a characteristic long looping pattern of uptake if it contains gastric mucosa (Figure 36-3). In addition to ulceration related to ectopic gastric mucosa, an enteric cyst may cause GI hemorrhage by way of pressure necrosis, intussusception, or perforation. The lesion is usually not visible on scintigraphy in these instances, unless there is rapid blood loss that causes extravasation of radiopharmaceutical into the bowel lumen.

CT demonstrates a duplication cyst as a round or tubular mass along the mesenteric border of the bowel. The cyst cavity does not opacify with ingested contrast material (Figure 36-4). The contents of the cyst determine the attenuation characteristics of the lesion on CT, that is, clear fluid, debris, or hemorrhage. The mucosal layer of the cyst wall enhances with IV contrast, but the contents do not enhance. Variations in the shape of the lesion may be observed if more than 1 series of images is obtained during the examination.⁵

On MRI, a duplication cyst appears as a fluid-containing lesion with well-defined margins (Figure 36-5). Most often, the contents are homogeneously hyperintense on T2-weighted images and hypointense on T1-weighted sequences. Debris from prior hemorrhage may produce a fluid–fluid level. The cyst wall enhances with IV contrast. MR cholangiopancreatography (MRCP) is helpful in selected patients with an upper abdominal duplication cyst, especially when associated with the duodenum, to define the anatomic relationship with the common bile duct and pancreatic duct. This examination also aids in the differentiation of a duodenal duplication cyst from a choledochal cyst.^6,7

Anomalies of the Omphalomesenteric Duct

Meckel Diverticulum

Meckel diverticulum is a congenital outpouching from the distal ileum that is due to improper closure and resorption of the omphalomesenteric duct during fetal development. It is the most common clinically significant congenital lesion of the GI tract; the general population prevalence is 2% to 3%. Meckel diverticulum occurs with approximately equal frequency in males and females, but is more often symptomatic in males. There is a weak association between the presence of a Meckel diverticulum and the development of Crohn disease.^8–13

Meckel diverticulum is an anomaly of the omphalomesenteric duct. The omphalomesenteric (vitelline) duct is the embryonic communication between the yolk sac and the midgut. The midgut elongates and herniates into the umbilical cord during the sixth week of embryogenesis. Within the umbilical cord, the midgut rotates 90° counterclockwise around the axis of the superior mesenteric artery. At the same time, the midgut elongates to form the jejunum and ileum and the lumen of the omphalomesenteric duct closes. The midgut returns to the abdominal cavity by the 10th week of embryogenesis and the omphalomesenteric duct becomes a thin fibrous band that eventually disintegrates and is absorbed.

Failure of complete atrophy of the omphalomesenteric duct leads to a spectrum of anomalies (Table 36-1). Meckel diverticulum accounts for greater than 90% of omphalomesenteric duct anomalies. The pathogenesis of Meckel diverticulum involves appropriate fibrous obliteration of the umbilical end of the omphalomesenteric duct and persistent patency with continued growth of the ileal end of the duct. The resultant diverticulum contains all layers of the intestinal wall and, therefore, represents a true diverticulum. It is located on the antimesenteric border of the ileum. In teens and adults, Meckel diverticulum is located approximately 80 to 100 cm from the ileocecal valve. The vascular supply is via omphalomesenteric vessels from the mesentery of the small intestine. If the umbilical end of the duct is incompletely absorbed, the Meckel diverticulum connects to the umbilicus by a fibrous band.

Meckel diverticulum is asymptomatic in most individuals. The most common complications are peptic ulceration (GI hemorrhage), small bowel obstruction, and inflammation (i.e., diverticulitis). Small bowel obstruction in these patients can occur at any age; potential mechanisms for obstruction include intussusception, volvulus, torsion, extension of the lesion into a hernia (termed a Littre hernia), or diverticular inversion. Neoplasia within a Meckel diverticulum is a rare complication. Approximately 60% of patients who suffer clinical manifestations of Meckel diverticulum do so prior to 10 years of age. Those diverticula that cause GI hemorrhage nearly always contain ectopic gastric mucosa.

In children, the most common clinical manifestation of a Meckel diverticulum is GI hemorrhage. This is due to mucosal ulceration related to acid production by ectopic gastric mucosa within the diverticulum. The actual ulcer may be located in the diverticulum or in the adjacent ileum. In some children with a bleeding ulcer related to a Meckel diverticulum, there is passage of large amounts of blood per rectum. Classically, this results in a “currant jelly” appearance of the stool. In other patients, there is chronic occult bleeding that is detected clinically by guaiac-positive stools or laboratory evidence of anemia.

Acute Meckel diverticulitis is a rare complication of this lesion in children. The pathogenesis involves acquired obstruction at the mouth of the diverticulum, with subsequent infection and distention of the isolated lumen. The obstruction can be caused by inflammation, fibrosis, an enterolith, a foreign body, or parasitic infection. The clinical features of Meckel diverticulitis are sometimes indistinguishable from those of appendicitis. Patients may suffer abdominal pain, fever, and leukocytosis. Sup puration of a Meckel diverticulum can lead to perforation and peritonitis.

A Meckel diverticulum is a true diverticulum, with all layers of the intestinal wall. The lesion is lined entirely or predominantly by small intestine mucosa. Heterotopic tissue is present in about half of Meckel diverticula. Heterotopic gastric mucosa is the most common form, occurring in 25% to 50% of Meckel diverticula. Other tissue types that can occur in these lesions include pancreatic, duodenal, colonic, and biliary mucosa. Heterotopic pancreatic tissue occasionally serves as a lead point for an intussusception.

In the child with GI hemorrhage and a suspected Meckel diverticulum, ^99mTc-pertechnetate scintigraphy is the diagnostic procedure of choice. As described above, nearly all patients with GI hemorrhage due to a Meckel diverticulum have ectopic gastric mucosa within the lesion. The mucous glands in the ectopic gastric mucosa accumulate and secrete the pertechnetate ion. These cells secrete fluid that is similar to an ultrafiltrate of plasma with added mucus.

Pertechnetate scintigraphy for Meckel diverticulum is performed with sequential images of the abdomen and pelvis following the IV injection of radiopharmaceutical. Images should be obtained for at least 30 minutes. Oblique, lateral, or single-photon emission computed tomography (SPECT) images help differentiate ectopic activity from normal gastric and urinary activity. The most common scintigraphic appearance of a Meckel diverticulum is that of a small focus of radiopharmaceutical accumulation within the mid or lower portion of the abdomen (Figure 36-6). The ectopic focus appears simultaneously with, or slightly after, normal uptake in the stomach. The intensity of uptake increases with time, also commensurate with normal gastric activity (Figure 36-7). Typically, the location of the ectopic activity does not change substantially on sequential images.^14–17

Pharmacological agents that enhance the sensitivity of pertechnetate scintigraphy for the diagnosis of Meckel diverticulum include cimetidine, glucagon, and pentagastrin. Cimetidine facilitates the accumulation of pertechnetate within gastric mucous cells and inhibits the secretion into the GI lumen, thereby “trapping” the radiopharmaceutical. Diminished intestinal peristalsis in response to glucagon administration lessons dilutional effects at the site of the ectopic gastric mucosal secretion of radiopharmaceutical. Pentagastrin enhances gastric mucosal secretion directly, but is generally reserved for rare instances in which standard scintigraphy is negative or indeterminate and there is a high clinical suspicion.

The overall accuracy of pertechnetate scintigraphy for the diagnosis of a Meckel diverticulum that contains ectopic gastric mucosa is greater than 95%; the sensitivity is at least 85% and the specificity is approximately 95%. There are several important considerations for the optimal performance of this examination. The bowel should be free of barium from prior studies, as this substance attenuates the emitted radiation and can result in a false-negative result. Bowel preparation or endoscopy can sometimes cause mucosal hyperemia that leads to increased tracer accumulation and a potentially false-positive study. Potassium perchlorate should not be utilized prior to pertechnetate scintigraphy, as this substance inhibits the accumulation of tracer within the gastric mucosa. The bladder should be as empty as possible during the study, to prevent obscuration of a pelvic or lower abdominal Meckel diverticulum.

Intrinsic factors can also lead to false-negative and false-positive pertechnetate scintigraphic studies for Meckel diverticulum. Factors that can potentially interfere with detection of a Meckel diverticulum include a small quantity of gastric mucosa, interruption of blood supply to the mucosa, emptying of the contents of the cyst into the lumen of the bowel, and dilution of radiopharmaceutical within the diverticulum by fluid entering the lesion. False-positive results can occur in association with ulcerative or inflammatory processes of the intestine unrelated to Meckel diverticulum (e.g., intussusception, inflammatory bowel disease), heterotopic gastric mucosa in the wall of the small intestine, and heterotopic gastric mucosa in an intestinal duplication cyst. Ectopic gastric mucosa can occur anywhere within the GI tract, although these foci are usually quite small and not associated with symptoms.

Standard radiographs of children with a Meckel diverticulum are frequently normal. If there is an acute volvulus or other form of bowel obstruction, radiographs show nonspecific proximal small bowel dilation. Occasionally, a large diverticulum is visible as a lower abdominal mass. Calculi or gas collections are sometimes present within the lesion. Enteroliths occurring in association with a Meckel diverticulum typically contain calcification peripherally and have radiolucent centers; a laminated appearance is less common. In patients with Meckel diverticulitis, there are nonspecific radiographic features of peritoneal inflammation, unless the diverticulum itself is visible.¹⁸

Barium examination of the small intestine occasionally allows visualization of a Meckel diverticulum. The lesion appears as a saccular, blind-ending diverticulum located on the antimesenteric border of the ileum. There sometimes are filling defects due to enteroliths or foreign bodies. Heterotopic tissue or inflammation may result in mucosal irregularity or nodules. A Meckel diverticulum is most often located in the right lower quadrant or pelvis; periumbilical or midabdominal locations can occur due to adhesions or congenital fibrous bands.^19,20

CT is occasionally useful for the evaluation of patients with Meckel diverticulum and bowel obstruction or diverticulitis. As with standard radiographs, the CT findings often fail to provide a specific diagnosis. Occasionally, the diverticulum is visible as a blind-ending tubular structure that communicates with the antimesenteric border of the small intestine (Figure 36-8). The diverticulum may contain fluid, gas, enteroliths, or fecal-like material (Figure 36-9). Thickening of the diverticular wall and edema within the adjacent mesenteric fat suggest inflammation. With inversion (invagination) of a Meckel diverticulum into the lumen of the small intestine, CT shows a central core of mesenteric fat surrounded by a collar of soft tissue attenuation that represents the wall of the diverticulum. With progression to a true intussusception, CT may demonstrate concentric rings of alternating fat and soft tissue attenuation surrounding the central core of mesenteric fat of the inverted diverticulum.^21–24

With sonography, a Meckel diverticulum appears as a round or tubular cyst that has a thick irregular hyperechoic internal wall and a hypoechoic external wall (i.e., GI signature). The inner hyperechoic layer represents the mucosal and submucosal layers. With obstruction and inflammation, the wall of the diverticulum is thick and there is reduced compressibility. Doppler studies may show hyperemia within the wall. With an inverted Meckel diverticulum serving as a lead point for intussusception, sonography shows a blind-ending segment of thick-walled bowel that projects from the apex of the intussusceptum. Mesenteric fat may produce an echogenic appearance centrally.^24–26

Superior mesenteric angiography of patients with a Meckel diverticulum demonstrates a persistent vitellointestinal artery arising from the distal aspect of the ileal artery. This appears as an elongated nonbranching artery that supplies a group of tortuous vessels. The visualization of this artery may be difficult because of overlying vessels, however. Selective catheterization of distal ileal arteries may be helpful. Other angiographic findings include a vascular blush, early venous return, and arterial irregularity.²⁷

Other Anomalies of the Omphalomesenteric Duct

Meckel diverticulum accounts for greater than 90% of omphalomesenteric duct anomalies. The omphalomesenteric duct anomalies that occur in addition to Meckel diverticulum are uncommon to rare; these include omphalomesenteric fistula, sinus, and cyst, and fibrous connection of the ileum to the umbilicus (Table 36-1).

Omphalomesenteric duct cyst is the result of abnormal persistent patency along the course of the omphalomesenteric duct despite appropriate obliteration of both ends of the embryonic duct. Secretion of mucus from cells within the intestinal mucosa that lines the ductal remnant leads to distention of the isolated segment and formation of a cyst. This lesion is frequently asymptomatic unless there is secondary infection.

Omphalomesenteric duct sinus is the counterpart to Meckel diverticulum, with fibrous obliteration of the ileal end of the duct and persistent patency of the umbilical end. An omphalomesenteric duct sinus can present with oozing of mucus from the umbilicus or it may be asymptomatic unless infection occurs.

A fibrous remnant of the omphalomesenteric duct can form a connection between the ileum and the umbilicus, with or without a cyst, diverticulum, or fistula, and can potentially precipitate the acute onset of a bowel obstruction or volvulus. This umbilicoileal cord can act as an obstructing band or as a point of fixation about which a volvulus can occur. These patients may present with clinical manifestations of acute or intermittent bowel obstruction.^9,28

Umbilicoileal fistula is the least common of the omphalomesenteric duct anomalies. This is due to residual patency of the entire duct, thereby resulting in communication between the umbilicus and the ileum. Rarely, a persistent omphalomesenteric duct communicates with the cecum or appendix. Most patients with an umbilicoileal fistula present during infancy with drainage of fluid from the small bowel through the umbilicus. Intussusception of the ileum into a patent omphalomesenteric duct can also occur.²⁹

A radiographic sinogram using water-soluble contrast material is sometimes helpful in the evaluation of a child with drainage from the umbilicus (see Chapter 39). This study shows an omphalomesenteric duct sinus as a thin blind-ending tract. With a fistula, the injected contrast material opacifies the ileum (see Figure 39-3). An omphalomesenteric duct cyst appears on CT and sonography as a fluid-filled structure immediately dorsal to the umbilicus. Most often, the contents have characteristics of clear fluid. With acquired hemorrhage or infection, the contents have higher attenuation on CT and greater echogenicity on sonography, and there sometimes are inflammatory changes in the adjacent soft tissues. A fibrous omphalomesenteric duct remnant is too small to be directly imaged. The presence of a fibrous band can sometimes be inferred by findings of a mechanical bowel obstruction in association with a Meckel diverticulum. The most characteristic contrast enema features of intestinal obstruction related to an omphalomesenteric duct remnant are beak-like obstruction of the terminal ileum or cecum and medial deviation of the cecum.³⁰

Clinical Presentations: Neonatal Small Bowel Obstruction

Bowel obstruction in the neonate is nearly always related to a congenital anomaly of the GI tract. On the basis of the clinical findings and the appearance of standard abdominal radiographs, neonatal intestinal obstructions are classified as high (proximal) or low (distal). Most radiologists consider an obstruction proximal to the mid-ileum or jejunal–ileal junction to be “high,” and those beyond this level to be “low.” The differential diagnosis of a high intestinal obstruction in a neonate includes various forms of stenosis or atresia at the gastric outlet, duodenal, or jejunal levels. Low obstructions include ileal atresia, meconium ileus, colonic dysmotility, colonic atresia, and Hirschsprung disease (Table 36-2).^31,32

In general, high intestinal obstruction in the neonate results in vomiting. With high-grade obstruction, vomiting may begin after the first feeding. The vomiting due to a gastric outlet obstruction is nonbilious; obstruction beyond this level typically causes bilious vomiting. In the appropriate clinical situation, no additional imaging is required prior to surgical therapy when standard abdominal radiographs show manifestations of a high intestinal obstruction in a neonate.

Common clinical manifestations of a low intestinal obstruction include abdominal distention, vomiting, and failure to pass meconium. Radiographs show numerous dilated loops of bowel throughout the abdomen. A water-soluble contrast study per rectum helps to characterize the lesion and localize the level of obstruction.

Congenital Duodenal Obstruction

Atresia and stenosis of the duodenum are common causes of bowel obstruction in the newborn. The lumen is completely obstructed in patients with atresia, whereas stenosis refers to incomplete obstruction. Duodenal atresia is approximately 4 times more common than duodenal stenosis. Intrinsic congenital obstruction of the duodenum occurs in 1 in 4000 to 1 in 6000 livebirths. Approximately 25% to 35% of children with duodenal atresia or stenosis have trisomy 21. Other associated anomalies are present in 20% to 30% of cases, including congenital heart disease, esophageal atresia, intestinal malrotation, renal abnormalities, imperforate anus, other regions of small bowel atresia, annular pancreas, preduodenal portal vein, and biliary atresia.

Intrinsic lesions account for about two-thirds of all congenital duodenal obstructions. The etiology of duodenal atresia and stenosis is usually related to failed recanalization following the solid cord stage of embryonic development. Some instances, especially those in the distal portion of the duodenum, may be related to an intrauterine vascular insult.

Duodenal Atresia

The estimated prevalence of duodenal atresia is 1 in 10,000 to 1 in 40,000 livebirths. As with other GI atresias, duodenal atresia can occur as complete separation of 2 blind-ending lumina, a string-like fibrous band bridging the lumina, or a diaphragm that completely blocks the lumen of otherwise intact bowel. An obstructing membrane composed of mucosa is the most common form. Duodenal atresia occurs just distal to the ampulla of Vater in 85% and is preampullary in 15%. Duodenal atresia directly at the level of the ampulla is rare; this type causes both jaundice and duodenal obstruction. Multiple atresias are present in approximately 10% to 15% of children with duodenal atresia. Annular pancreas and intestinal malrotation are the most common coexisting anomalies.³³

A history of polyhydramnios is present for about half of infants with duodenal atresia. The initial postnatal presentation is vomiting. The vomitus is usually bile stained as a result of the typical location of the atresia distal to the ampulla. Because of the high location of the obstruction, the vomiting begins shortly after birth, usually with the first or second feeding. The abdomen usually is not distended. Dehydration, electrolyte imbalance, and obstipation occur if treatment is delayed.

The diagnostic evaluation of a vomiting newborn should always begin with abdominal radiographs, including supine and either upright or decubitus views. Classically, the stomach and duodenal bulb are markedly distended, and there is absence of gas distally. In some instances, the distention of the duodenal bulb is greater than that of the stomach. The air-containing stomach and duodenum produce the “double bubble sign” (Figure 36-10). This is usually visible on both supine and upright radiographs.

The double bubble sign of duodenal atresia is often demonstrable on prenatal ultrasound, due to the distended fluid-filled stomach and proximal duodenum. Polyhydramnios is usually present as well. However, a normal fetal ultrasound in the presence of polyhydramnios does not entirely rule out the diagnosis of duodenal atresia or other intrinsic obstruction. False-positive examinations can also occur, due to transient distention of the stomach and duodenum in the absence of an underlying anatomic obstruction. Postnatally, sonography serves a complementary role to standard radiographs for documentation of duodenal bulb dilation.^34,35

The presence of a small amount of bowel gas beyond the level of the duodenum does not exclude the diagnosis of duodenal atresia. On occasion, the hepatopancreatic duct bifurcates, thereby forming a Y configuration at its insertion into the duodenum. If 1 limb of the duct inserts above the atretic region and the second inserts distal to the atresia, the ducts may allow the passage of air around the obstruction. This small communication is usually not visible on fluoroscopic contrast studies.³⁶

For most infants with duodenal atresia, the findings on standard abdominal radiographs are diagnostic; additional imaging prior to surgery is usually not necessary. In those unusual circumstances in which there is insufficient air in the stomach and duodenum for adequate visualization of these structures radiographically, air or nonionic contrast can be introduced via an enteric tube (Figure 36-11). Contrast studies of the upper GI tract generally provide little useful additional information for most infants with duodenal atresia, and carry the risk of regurgitation and aspiration. Some clinicians request a contrast examination of the colon to evaluate for possible malrotation and midgut volvulus as the cause of duodenal obstruction. However, the demonstration of a normally-located cecum does not entirely exclude the diagnosis of volvulus. In addition, malrotation occurs as a concomitant lesion in a substantial minority of infants with duodenal atresia. Despite complete obstruction of the proximal small bowel in infants with duodenal atresia, contrast enema examinations typically show the colon to be of normal caliber.

It is important to recognize that dilation of the duodenal bulb in infants with duodenal obstruction due to midgut volvulus is much less than the dilation that occurs with a congenital high-grade obstruction. In duodenal atresia and annular pancreas, there is marked distension of both the duodenal bulb and the stomach, whereas the degree of distention is less prominent in patients with obstruction due to a peritoneal band or volvulus. In addition, obstruction from the latter 2 conditions is usually more distal in the duodenum than is typical with duodenal atresia and annular pancreas. Therefore, if radiographic studies indicate the site of obstruction to be in the proximal portion of the duodenum just distal to a dilated duodenal bulb, the lesion is probably duodenal atresia or an annular pancreas (although the latter is less likely). Volvulus is unlikely in this situation, especially if there is no bowel gas distal to the duodenum.

Surgical repair of duodenal atresia is by either duodenoduodenostomy or duodenojejunostomy; the former is generally preferred in current practice. The surgeon may select a side-to-side or proximal transverse to distal longitudinal anastomosis. An antimesenteric tapering duodenoplasty can be performed if the portion of the duodenum proximal to the obstruction is markedly distended. Fluoroscopic contrast studies of the upper GI tract in patients following duodenal atresia repair typically show residual dilation of the duodenal bulb and a small-caliber distal portion of the duodenum.³⁴

Duodenal Stenosis

Congenital duodenal stenosis can occur as a segmental intrinsic wall narrowing or as a membrane (duodenal diaphragm or web). Concomitant annular pancreas is present in some patients with duodenal stenosis; this usually represents a coexistent developmental lesion rather than the direct cause of duodenal obstruction. The clinical features and the age at presentation of congenital duodenal stenosis vary between patients. The severity of the obstruction is the most important factor in determining the presentation. Those patients with high-grade obstruction usually suffer vomiting in the neonatal period. Those with lesser severity obstruction may present with vomiting, abdominal pain, or failure to thrive later in childhood. Occasionally, there is delay in the diagnosis until adulthood. As with duodenal atresia, conditions that are commonly associated with duodenal stenosis include Down syndrome, annular pancreas, and malrotation.

The imaging features of congenital duodenal stenosis vary with the severity of obstruction. If there is severe obstruction, the findings are similar to those of duodenal atresia (Figure 36-12). The double bubble pattern with a dilated stomach and duodenal bulb is often present, although the incomplete nature of the obstruction allows gas to pass into nondilated bowel distal to the obstruction (Figure 36-13). Abdominal radiographs are often normal in children with mild forms of duodenal stenosis.

A fluoroscopic contrast examination is usually indicated for evaluation of the child with suspected duodenal stenosis. This shows a variable degree of duodenal dilation proximal to the obstruction and an abrupt change in caliber at the level of the obstruction (Figure 36-14). If the bowel appears twisted in the region of the obstruction or has a beaked configuration, the possibility of midgut volvulus should be entertained. In addition, an extrinsic form of congenital duodenal obstruction can occur due to the presence of an anomalous band; this type of stenosis is more common in the distal portion of the duodenum than in the proximal aspect.

Duodenal stenosis due to a membranous diaphragm or web can be associated with radiographic manifestations of severe high intestinal obstruction, with findings similar to those of duodenal atresia. In others, contrast studies show a rounded obstruction or narrowing of the duodenum that is indistinguishable from other forms of duodenal stenosis (Figure 36-15). If sufficient contrast passes through the orifice of the membrane to allow visualization of both margins, the diaphragm appears as a thin, convex, curvilinear structure extending across the lumen of the duodenum. Most often, the attachment of the diaphragm is near the level of the ampulla of Vater. In older children and adults with a duodenal diaphragm, stretching of the lesion may result in a windsock configuration (Figure 36-16). Careful clinical and radiographic evaluation of the GI system is important in patients with a congenital duodenal diaphragm as this lesion can occur in association with malrotation, duodenal bands, or midgut volvulus (Figure 36-17).

The differential diagnosis of a focal duodenal stenosis includes adhesion, postinflammatory fibrosis, trauma, neoplasm, volvulus, and compression from an extrinsic congenital mass (e.g., duplication cyst) (Figure 36-18).

Anomalies Associated with Congenital Duodenal Obstruction

Preduodenal Portal Vein

Preduodenal portal vein can occur in association with duodenal stenosis. Most often, this anomaly is not the direct cause of obstruction. The normal portal vein develops by way of persistence of the major cephalic portion of the right vitelline vein, and regression of the left vitelline vein. If there is anomalous persistence of the left vitelline vein, the portal vein assumes a preduodenal course. This anomaly is common in patients with heterotaxy syndrome. The anomalous course of the portal vein anterior to the pancreas and duodenum can be demonstrated in these patients with sonography, CT, or MR.

Annular Pancreas

Annular pancreas refers to an anomalous band of pancreatic tissue that encircles the second portion of the duodenum. Although annular pancreas occasionally occurs as an isolated abnormality, it is usually accompanied by congenital duodenal narrowing. Some degree of extrinsic compression of the duodenum occurs with annular pancreas, but most patients with symptomatic obstruction have intrinsic duodenal narrowing. Annular pancreas can occur as a complete ring or as an incomplete band. Anomalies that are sometimes associated with annular pancreas include trisomy 21, malrotation, esophageal atresia, anorectal malformations, and congenital heart disease.

The complete ring form of annular pancreas usually is associated with overt clinical manifestations of a high small bowel obstruction in the neonatal period. With an incomplete band, symptoms of obstruction may not occur until later in life, if at all. As in other forms of congenital duodenal obstruction, infants with annular pancreas typically develop vomiting after feedings, with progressive increase in the severity of vomiting with time. The vomitus may or may not contain bile, depending on the relationship of the narrowed segment of bowel to the common bile duct. Jaundice is often present if the common bile duct enters the area of the annular pancreas.

The findings on standard radiographs of infants and neonates with annular pancreas are due to the associated duodenal obstruction. As with other forms of obstruction, the appearance varies with the severity of the narrowing. Although the appearance is usually nonspecific, a type of double bubble sign that is highly suggestive of annular pancreas can occur: absence of gas in the constricted portion of the duodenum and prominent collections of gas in the duodenum proximal and distal to the area of narrowing. Both components of the double bubble are therefore within the duodenum, unlike the classic double bubble sign of duodenal atresia. The constriction of the duodenum associated with annular pancreas may be either concentric or more prominent along the right side of the duodenal sweep.

Fluoroscopic contrast examination in the presence of annular pancreas typically shows concentric or asymmetric constriction of the midportion of the descending duodenum. In general, the findings on standard radiographs and upper GI examination are sufficient for determining the need for surgical therapy, even if a specific diagnosis of annular pancreas is not provided. When clinically useful, the anomalous configuration of the pancreatic head can be demonstrated with cross-sectional imaging studies, that is, sonography, CT, or MR (Figure 36-19).^5,37

Intestinal Malrotation

Disruption of the complex process of GI development can result in a variety of anomalies of rotation and fixation. These conditions can be complicated by midgut volvulus or bowel obstruction due to peritoneal bands or internal hernias. Anomalies of intestinal rotation and fixation comprise the fourth most common malformation of the GI system. The prevalence is up to 1 in 500 livebirths, although not all patients become symptomatic.^38–40

Intestinal malrotation (usually nonrotation) is present in all patients with omphalocele or gastroschisis, as the abnormal in utero position of the bowel interferes with normal rotation and mesenteric development. Malrotation is common in children with duodenal atresia or stenosis, congenital diaphragmatic hernia, or heterotaxy syndrome. Other conditions associated with malrotation include Hirschsprung disease, Meckel diverticulum, trisomies 13, 18, and 21, familial intestinal malrotation, Cornelia de Lange syndrome, intestinal atresia, Marfan syndrome, and prune belly syndrome.

The term malrotation indicates lack of normal developmental rotation of any part of the intestinal tract. Midgut malrotation is any deviation from the normal 270° rotation of the small intestine and colon that occurs during embryonic development. Malrotation anomalies comprise a continuum that reflects the stage in development of the midgut at which arrest of rotation occurs. Malfixation is the most dangerous consequence of malrotation; a narrow mesenteric root predisposes the patient to midgut volvulus. There are various classification schemes for intestinal malrotation.⁴¹

Nonrotation (type I malrotation) results from early failure of intestinal rotation. The initial 90° of rotation does occur, however, such that the duodenum is to the right of the superior mesenteric artery and the distal portion of the colon is to the left of the superior mesenteric artery. Lack of additional rotation causes most of the small bowel to be located in the right hemiabdomen and the colon to be located in the left. Individuals with nonrotation are usually asymptomatic and the risk of midgut volvulus is low.

Developmental failure during the final 180° counterclockwise rotation of the small bowel or the final 180° counterclockwise rotation of the colon results in incomplete rotation. The course of the bowel varies in these patients from a nonrotation pattern to a nearly normal pattern. Failure of the duodenojejunal limb to rotate posterior and to the left of the superior mesenteric artery results in the lack of a ligament of Treitz or malposition of the ligament to the right of the midline. Incomplete rotation of the cecocolic limb leads to Ladd bands that extend from the cecum across the duodenum. Deficient mesenteric fixation results in a substantial risk for midgut volvulus.

Rotational anomalies that occur during the extracoelomic phase of embryonic bowel development are classified as type II malrotations. The most common of these anomalies is nonrotation of the duodenum, with the duodenojejunal junction to the right in the midline and a normal location of the cecum (Figure 36-20).

Reversed rotation is an uncommon form of type II malrotation that is caused by an initial return of the caudal portion of the midgut to the abdomen and subsequent clockwise rotation of the duodenum rather than the normal counterclockwise rotation. This results in extension of the duodenum anterior to the superior mesenteric artery and a posterior location of the colon. The cecum is usually located in the right lower quadrant in these patients. Occasionally, the colon passes anterior to both the duodenum and the superior mesenteric artery, and the small intestine becomes trapped in the mesentery; this is a type of congenital internal hernia.

Type III malrotation, a form of incomplete rotation, is due to rotational abnormalities that occur during return of the midgut to the peritoneal cavity. Because the final 90° rotation of the duodenum occurs early in this process, the duodenal location in these patients is often normal or only mildly anomalous. The final 180° rotation of the colon is more gradual; the stage at which arrest occurs determines the final position of the cecum, which may be anywhere from the left upper quadrant to the right lower quadrant.

Duodenum inversum (duodenum reflexum) is a minor anomaly of proximal small bowel fixation. The distal portion of the duodenum ascends to the right of the spine to the level of the duodenal bulb and then crosses the spine horizontally behind the superior mesenteric artery. The duodenojejunal junction is fixed in a normal location by the ligament of Treitz. Duodenum inversum is usually clinically inconsequential; there is no predilection for volvulus. Occasional patients have symptoms of mild obstruction.

Peritoneal bands (Ladd bands) are common in patients with intestinal malrotation. These presumably represent disordered embryonic attempts at fixation of the malpositioned bowel. These peritoneal bands extend from the retroperitoneum in the right upper quadrant inferiorly to the cecum and proximal aspect of the colon. The bands frequently compress the descending or transverse portions of the duodenum, and may or may not result in clinically significant obstruction. Occasionally, peritoneal bands result in severe obstruction and thereby mimic the clinical and imaging findings of duodenal atresia. Distortion of the course of the duodenum by peritoneal bands can result in a “Z” configuration on contrast studies.

In those forms of malrotation in which there is a short mesenteric attachment, there is an elevated risk for midgut volvulus. Type III malrotation in which the duodenojejunal junction is at the midline and the cecum overlies the duodenojejunal junction results in a very short mesenteric attachment and a high likelihood of catastrophic volvulus. Volvulus can also develop as a complication of other forms of malrotation and there are no unequivocal criteria to establish a “safe” form of malrotation. However, the rarity of volvulus in patients with nonrotation is such that the risk for a surgical complication (such as an obstructive adhesion) is similar to that for volvulus without surgical therapy.

Midgut volvulus presents as a clinical catastrophe when there is substantial vascular compromise of the intestine. With severe ischemia, necrosis of the entire jejunum and ileum can occur. In other patients, there is preservation of arterial perfusion despite compromised venous and lymphatic drainage. Occasional patients have intermittent symptoms that suggest episodes of volvulus and spontaneous detorsion. Patients with malrotation can also have symptoms due to the presence of an obstructing peritoneal band or internal hernia.

Although volvulus due to malrotation can occur at any age, it is most common in the infant. Infants with incomplete rotation most often present with signs of intestinal obstruction during the first few weeks of life; 80% present during the first month. The neonate often appears normal until the third or fourth day of age, when there is sudden onset of bilious vomiting. This clinical sequence should always initiate prompt diagnostic evaluation. Other potential clinical manifestations of midgut volvulus include abdominal distention, GI hemorrhage, shock, and disseminated intravascular coagulation. A volvulus that does not cause compromised arterial perfusion may have a less dramatic clinical presentation. Venous obstruction in these patients can lead to melena, hematochezia, or, rarely, hematemesis. Lymphatic obstruction can cause protein loss into the intestine and chylous ascites.

Standard radiographs of patients with malrotation are often normal in the absence of an acute bowel obstruction. An abnormal location of the cecum is sometimes discernible. With nonrotation, the small intestine is located in the right side of the abdomen and the colon in the left. In the presence of a volvulus, there are sometimes radiographic findings of a bowel obstruction and/or bowel ischemia (Figure 36-21). However, the appearance is often deceivingly normal, particularly on radiographs obtained near the time of the acute clinical presentation. If sufficient time has passed for the development of findings of a small bowel obstruction or severe ileus, intestinal infarction is likely. In general, a relatively gasless abdomen in a patient with midgut volvulus suggests patency of the venous drainage. Radiographic evidence of bowel obstruction at the level of the duodenum is present in some patients with midgut volvulus. Because the obstruction is acute, dilation of the proximal portion of the duodenum is much less pronounced than the dilation that occurs with congenital duodenal stenosis or atresia. Ischemia of the small intestine results in nonspecific gaseous distension of the bowel, sometimes with bowel wall thickening (Figure 36-22).

Contrast studies usually allow a definitive diagnosis of intestinal malrotation. The most useful technique is the fluoroscopic upper GI examination and small bowel follow-through. A contrast study of the colon alone is insufficient to rule out malrotation or volvulus, as the cecum is in a normal location in some of these patients. When performing an upper GI tract contrast study of any child, symptomatic or not, accurate demonstration of the location of the duodenojejunal junction is essential. Fluoroscopic observation of the first bolus of contrast as it passes through the duodenum allows precise depiction of the crucial anatomy. In normal individuals, the duodenojejunal junction is to the left of the left-sided pedicles at the level of the duodenal bulb. On the lateral projection, the duodenojejunal junction is relatively posterior, due to the normal retroperitoneal course (Figure 36-23). A meandering course of the duodenum (wandering or mobile duodenum) is a normal variation. In addition, inferior displacement of the duodenojejunal junction can occur without malrotation in patients with gastric dilation, splenomegaly, scoliosis, or ipsilateral renal agenesis.

With type I and II malrotations, the duodenum is located to the right in the midline (Figure 36-24). The course of the duodenum is often normal in patients with reversed rotation, whereas the anatomy of the duodenum is variable in patients with type III malrotation. The most important fluoroscopic feature of malrotation is lack of appropriate extension of the duodenum into the left upper quadrant. The duodenojejunal junction is low and to the right of the spine or at the midline. Excessive redundancy of the duodenum is an additional sign, but is not independently diagnostic. Unlike the normal “wandering duodenum,” a pathologically redundant duodenum often has an angular or kinked appearance. The important clue to the presence of malrotation on the lateral projection of an upper GI examination is an abnormal anterior and inferior location of the duodenojejunal junction (Figure 36-25). Determination of the location of the cecum on the small bowel follow-through portion of the examination is essential when the initial images indicate an anomalous duodenal course. Approximately 80% of individuals with intestinal malrotation have an abnormal cecal position. An anomalous high location of the cecum in conjunction with a midline or right-sided duodenojejunal junction indicates a high propensity for volvulus.^42,43

In the presence of a midgut volvulus, a contrast study of the upper GI tract shows mild-to-moderate dilation of the proximal portion of the duodenum. There is often distention of the stomach as well. The contrast column typically assumes a beaked appearance at the level of the duodenal obstruction (Figure 36-26). Depending on the severity of twisting, contrast eventually opacifies the narrowed segment of the duodenum in most instances, thereby allowing visualization of the characteristic corkscrew or pigtail appearance (Figure 36-27). If sufficient contrast enters the jejunum and ileum to allow adequate visualization, manifestations of mucosal thickening may be visible.

The findings with contrast enema in patients with malrotation or volvulus are variable. Approximately 15% of normal children have a mobile or somewhat high cecum, due to incomplete mesenteric attachment; this is usually of no clinical significance. With nonrotation, the entire colon is located in the left hemiabdomen and the cecum is in the left lower quadrant. With reversed rotation, there may be narrowing of a portion of the transverse colon due to impression from the superior mesenteric artery. An ominous form of type III malrotation is suggested by a transverse colon that crosses the midline and then doubles back such that the cecum is located near the midline in the upper portion of the abdomen. In the presence of an acute volvulus, reflux of contrast into the distal ileum may have a beaked appearance at the site of obstruction.

Inversion of the superior mesenteric vein and artery is a potential sign of intestinal malrotation on sonography, CT, and MR. The documentation of normal SMA and SMV relationships implies a very low likelihood of a dangerous form of malrotation. However, the specificity of mesenteric vessel inversion for the diagnosis of malrotation is limited; estimates range from 60% to 87%. This finding can occur as a normal variation or in association with various unrelated conditions, such as an abdominal mass. With volvulus, cross-sectional imaging studies may demonstrate twisting of the involved bowel and the mesentery; this is the “whirl” or “whirlpool” sign. Contrast-enhanced CT sometimes shows mesenteric venous congestion and/or lack of normal superior mesenteric artery enhancement. Ancillary signs of midgut volvulus on cross-sectional imaging studies are duodenal dilation, gastric distension, and mural bowel thickening. CT findings of bowel ischemia include mural thickening of the small intestine, mesenteric edema, lack of bowel wall enhancement, intramural gas, portal venous gas, and pneumoperitoneum. There can be hyperperfusion of the proximal aspect of the duodenum and an abrupt cessation of enhancement in the distal portion, that is, the “perfusion cutoffsign.”^44–46

Doppler sonography is useful for demonstrating abnormalities in mesenteric vessel flow due to volvulus, particularly in infants. The twisted mesentery produces the “whirlpool sign” on sonography: the mesenteric vein wraps around the superior mesenteric artery, which is at the center of the “whirlpool.” With caudal movement of the transducer, the superior mesenteric vein rotates around the artery in a clockwise manner. The “clockwise whirlpool sign” is highly suspicious for midgut volvulus. This finding is lacking in patients with vascular occlusion, however. Occasionally, there is flow in the superior mesenteric artery only, due to compression of the mesenteric veins. Another potential finding is truncation or hyperdynamic pulsation of the superior mesenteric artery. Ancillary sonographic findings of midgut volvulus include dilation and tapering of the proximal duodenum, bowel wall thickening, and peritoneal fluid.^47–50

Jejunal or Ilial Obstruction

Jejunal–Ileal Atresia and Congenital Stenosis

Atresia of the jejunum or ileum is twice as common as duodenal atresia. The reported prevalence varies from 1 in 400 to 1 in 1500 livebirths. Atresias in the small intestine are equally distributed between the jejunum and ileum. The site of atresia is in the proximal portion of the jejunum in 30%, distal portion of the jejunum in 20%, proximal aspect of the ileum in 15%, and distal portion of the ileum in 35%. Multiple atresias occur in 5% to 20% of cases. The most popular classification system for jejunoileal atresia recognizes 4 major types. Type I has an intact mesentery and bowel wall, with atresia of the mucosa in the form of a thin-walled diaphragm. Type II has an intact mesentery, with a fibrous cord that connects the atretic segments. Type III has a mesenteric defect and no connective tissue between the atretic segments. Type IIIb is the apple-peel variant (see below). Type IV refers to multiple small bowel atresias.

Approximately 5% to 10% of infants with jejunoileal atresia have cystic fibrosis, and nearly 15% have intestinal malrotation. Other GI anomalies that are associated with small bowel atresia include malrotation, volvulus, gastroschisis, and intussusception. There are rare instances (<1%) of jejunal or ileal atresia occurring in association with trisomy 21.

The pathogenesis of jejunal or ileal atresia involves a fetal intestinal vascular occlusion. The ischemic episode can be a late embryologic event; therefore, bile and meconium may be present in the bowel distal to the site of atresia. The midportion of the second trimester is apparently the most common time. There is subsequent resorption of the ischemic portion of the bowel, leading to an atretic segment of variable length. Occasionally, the pathogenesis of the prenatal small bowel ischemia is evident at the time of surgical therapy. Potential causative lesions include volvulus, malrotation, herniation, congenital bands, and intussusception. Fetal segmental volvulus due to meconium ileus accounts for the association with cystic fibrosis. There are localized mesenteric defects in some patients. The small bowel is foreshortened in some infants with jejunal or ileal atresia.

There are 2 unusual variants of jejunoileal atresia that are recognized as distinct entities; each often has a familial incidence pattern. The first type is termed apple-peel small bowel. This is a rare, often fatal, form of intestinal atresia that includes proximal jejunal atresia near the ligament of Treitz, absence of the superior mesenteric artery beyond its origin, foreshortening of the bowel, and absence of the mesentery. Because of the mesenteric defect, this is a type III atresia. The distal small bowel assumes a spiral configuration about a narrow vascular pedicle, resulting in an appearance that resembles an apple peel. There is occlusion of the superior mesenteric artery; the vascular supply to the distal bowel is compromised, and the intestine is perfused in a retrograde manner by anastomotic arcades from the ileocolic or right colic branches of the inferior mesenteric artery. In some patients, apple-peel small bowel is transmitted as an autosomal recessive trait. These infants have low birth weight and there is usually a history of polyhydramnios; the amniotic fluid is often meconium stained. Jaundice occurs in approximately 40% of infants with apple-peel small bowel. Malrotation of the colon is common. There is a predilection for necrotizing enterocolitis.

An additional unusual type of small bowel atresia is multiple GI atresias with intraluminal calcification. This is a very rare fatal disorder that has an autosomal recessive inheritance pattern. There are multiple atresias of the intestine in these patients, located anywhere from the stomach to the rectum. The intestinal atresias are usually type I lesions. A characteristic feature is extensive calcification of the intraluminal contents between the areas of atresia; this calcification is readily demonstrable on standard radiographs.

In about one-fourth of infants with small bowel atresia, there is a history of polyhydramnios. Polyhydramnios is more common with duodenal and jejunal atresias than with ileal atresias, as some degree of absorption of swallowed amniotic fluid can occur despite the presence of a distal small bowel obstruction. All infants with intestinal atresia develop vomiting as newborns. With jejunoileal atresias, the vomiting is often delayed until after the first several feedings, after which the symptoms progressively worsen. Because the obstruction is distal to the ampulla of Vater, the vomitus is bilious. Jaundice is occasionally present. Abdominal distention in infants with small bowel atresia is usually substantial. The obstruction can be complicated by intestinal perforation; meconium peritonitis results if the perforation occurs in utero. The passage of meconium stools by the newborn infant does not exclude the diagnosis of intestinal atresia, but rather indicates that the obstruction developed late in gestation.

Congenital stenosis of the small intestine is much less common than atresia. The clinical manifestations of congenital intestinal stenosis vary with the severity of obstruction. High-grade stenosis of the small intestine usually results in vomiting and abdominal distention, thereby mimicking the clinical findings of atresia. With a lesser degree of narrowing, these symptoms are mild and may be delayed in onset. When the lumen is only slightly narrowed, vague abdominal pain may be the only patient complaint. Other patients experience abdominal distention, with or without vomiting.

The diagnostic evaluation of a vomiting neonate should always begin with radiographs of the abdomen, including supine and either upright or decubitus films. These radiographs often establish the diagnosis of an intestinal obstruction that requires surgical attention, and additional studies may not be necessary. Although precise localization of a congenital small intestine obstruction is not usually possible with standard radiographs, the number and distribution of dilated bowel loops allow estimation of the level of obstruction (Figure 36-28). In general, there are air–fluid levels above the site of obstruction and a paucity of gas distally. Often, gas is not visible in the colon or the rectum. When an obstruction involves the terminal portion of the ileum, the small intestine is grossly distended and it is sometimes impossible to distinguish colon from small bowel on standard radiographs (Figure 36-29).

If standard radiographs are equivocal for the diagnosis of a bowel obstruction or additional information concerning the level of the obstruction is required, a contrast enema can be performed. Nonionic water-soluble contrast material should be utilized and the contrast should be injected gently, as the small bowel distal to the site of atresia sometimes opens freely into the peritoneal cavity. Likewise, the bowel adjacent to the atresia can be necrotic and susceptible to perforation.

Contrast enema shows a variable degree of diminished size of the colon and the portion of the small bowel distal to the atresia (Figure 36-30). Two major factors influence the intestinal caliber in these patients: the gestational age at which the atresia occurred and the level within the small bowel at which the atresia is located. Both the volume of intestinal secretions (succus entericus) and amniotic fluid passing through the fetal colon and the duration of the intestinal obstruction play roles in determining the size of the colon. With lesions that occur early in gestation and those that involve the distal small bowel, the colon is usually small (i.e., a microcolon). If the occlusion occurs late in gestation or is high in the small bowel, the colon is usually nearly normal in size. The presence of an abnormally small colon in conjunction with manifestations of a proximal small bowel atresia raises the possibility of multiple atresias. This appearance also may occur with apple-peel atresia. An additional finding with apple-peel atresia is a corkscrew appearance of the distal ileum if it is opacified by retrograde filling during a contrast enema.

The differentiation of ileal atresia from meconium ileus is sometimes difficult. The presence of a soap bubble pattern of meconium admixed with gas in the dilated small bowel is generally diagnostic of meconium ileus. However, a similar appearance occasionally occurs with isolated ileal atresia as well as with total colonic Hirschsprung disease. The demonstration of air–fluid levels on abdominal radiographs and rapid filling of the colon and distal ileum on a contrast enema are suggestive of ileal atresia. With meconium ileus, there is usually slow filling of the colon during a contrast enema, and it is often difficult to reflux contrast into the ileum. The diagnosis of meconium ileus is established if refluxed contrast outlines the meconium pellets in the ileum; in many cases, contrast can successfully be refluxed into dilated loops of small bowel, thereby ruling out atresia.

Meconium peritonitis can occur in association with ileal atresia. The calcifications due to meconium peritonitis are fine and plaque-like. They may occur anywhere in the peritoneal cavity, but are most often on the right side. Intramural bowel wall calcification is a rare complication of intestinal atresia; this is apparently due to calcification of necrotic fetal bowel.

Radiographs of an infant with low intestinal obstruction sometimes show signs of intraperitoneal fluid. This is suggested by separation of small bowel loops and generalized haziness of the abdomen. The presence of peritoneal fluid does not necessarily indicate a diagnosis of peritonitis, as this finding can occur with an otherwise uncomplicated obstruction of the small bowel or colon. The identification of pneumoperitoneum, however, indicates perforation; the use of oral or rectal contrast is generally contraindicated in this situation.

In infants with congenital small bowel stenosis, the degree of stenosis determines the severity of the obstructive pattern on radiographs. In those patients with only minimal radiographic signs of intestinal obstruction, a contrast enema or upper GI study is necessary to demonstrate the presence of the stenosis and to determine its location and severity.

Internal Hernia

An internal hernia refers to extension of bowel through a mesenteric defect. These lesions sometimes accompany intestinal malrotation. An internal hernia becomes clinically significant if bowel incarceration occurs. This complication can occur at any age from infancy to adulthood. The most common congenital internal hernia is paraduodenal. Less common locations include the foramen of Winslow, the pericecal region, the small bowel mesentery, the mesocolon, and the omentum.

The paraduodenal hernia is the result of a congenital deficiency in mesenteric peritoneal fixation. At least three-quarters of these hernias occur on the left, with the peritoneal defect located to the left of the distal portion of the duodenum. Small intestine can herniate through this defect in the mesentery into the left portion of the transverse mesocolon. A specific type of paraduodenal hernia can occur in patients with reversed rotation of the intestine. In these patients, the duodenum is anterior to the superior mesenteric artery and the colon is anterior to the duodenum, thereby forming a mesenteric pocket that can trap the small bowel.

Although most internal hernias are congenital lesions, many are not identified until adulthood. The symptoms may be intermittent. The clinical manifestations of an internal hernia form a spectrum that ranges from chronic mild abdominal pain, cramping, distention, and vomiting to acute severe pain that is accompanied by dehydration and signs of peritonitis.

The diagnosis of an internal hernia is sometimes suggested on standard radiographs if there are dilated loops of bowel crowded into a localized area of the abdomen. Contrast studies may show delayed transit through this area. Overlying loops of bowel often obscure the actual site of obstruction. If herniation through a mesenteric defect results in complete obstruction, the radiographic findings are indistinguishable from those of any other form of intestinal obstruction. The CT features of an internal hernia include an abnormal central location of the small bowel, proximal small bowel dilation due to obstruction, clustering of small bowel loops, displacement of adjacent structures by the clustered small bowel, and stretching, displacement, crowding, or engorgement of mesenteric vessels.

In patients with a paraduodenal hernia, contrast studies typically demonstrate a mass of jejunal loops in the left upper quadrant lateral to the ascending portion of the duodenum. The herniated loops may displace the duodenojejunal junction anteriorly and medially. Mass effect from the trapped small bowel loops may also cause indentation of the posterior wall of the stomach and displacement of the distal portion of the transverse colon. The trapped loops of bowel are often dilated and there is slow transit of contrast through the affected loops. Lateral radiographs show the abnormal loops of intestine to be in a posterior location. CT shows bowel interposed between the stomach and the body of the pancreas.

Mesodiverticular Band

Mesodiverticular band is due to an anomalous remnant of the embryonic vitelline artery. The band is composed of the mesenteric portion of a patent or involuted vitelline artery remnant that extends from the caudal portion of the mesentery to the tip of a Meckel diverticulum. There is no connection with the umbilicus. This anomalous arterial supply from the mesentery to the Meckel diverticulum makes the structure subject to ischemic complications.

The most common symptomatic complication of a mesodiverticular band is herniation of a portion of the small intestine through a hiatus in the band, resulting in intestinal obstruction. These patients may present with nausea, vomiting, abdominal distention, and abdominal pain of acute onset. There is right lower quadrant abdominal tenderness, with or without a palpable mass. Although there is a broad age range at presentation, the most common onset of symptoms is during infancy or early childhood. There is a 2:1 male predominance of symptomatic mesodiverticular band.^51–53

The typical radiographic features of an obstructing mesodiverticular band are those of a distal small bowel obstruction. Occasionally, there is a closed loop obstruction, and radiographs show a fluid-filled lower abdominal mass in association with dilation of the more proximal bowel. Most often, the imaging findings are not specific to this entity; the differential diagnosis includes other forms of small intestine obstruction such as adhesions, intussusception, internal hernia, volvulus, duplication, and appendicitis. A contrast study of the intestine occasionally demonstrates the Meckel diverticulum that occurs in association with a mesodiverticular band; the band itself can sometimes be visualized due to impression on contrast-opacified bowel or by way of palpation during fluoroscopy.⁵⁴

Meconium Ileus

Meconium ileus refers to obstruction of the distal portion of the ileum by inspissated intestinal contents. Greater than 99% of children with meconium ileus have cystic fibrosis. Approximately 10% to 15% of newborns with cystic fibrosis suffer meconium ileus. Up to half of infants with meconium ileus have complications or associated lesions of the bowel, such as volvulus of meconium-filled loops of bowel, intestinal ischemia, bowel necrosis, perforation, peritonitis, or meconium pseudocyst formation.

The intestinal obstruction in patients with meconium ileus is caused by extremely thick, tenacious meconium that adheres to the bowel wall. This material results from the abnormal physical and chemical properties of the mucous secreted by the intestinal glands in infants with cystic fibrosis. The mucous contains an abnormal mucoprotein that has a high protein content. Deficient pancreatic enzyme formation also plays a role in the pathogenesis of meconium ileus. The swallowed amniotic fluid is inadequately digested, thereby facilitating the accumulation of macromolecules within the meconium. There is also hyposecretion of water and electrolytes, which contributes to the tenacity of the meconium. Water resorption within the distal portion of the ileum causes further desiccation of the meconium, and the formation of pellet-like plugs that accumulate within the terminal ileum and colon. The dilated proximal portion of the small intestine contains tenacious meconium that has a putty-like consistency; this meconium often becomes admixed with air after birth.

The predominant clinical manifestations of meconium ileus are bilious vomiting and abdominal distention. The clinical onset is typically within the first 24 hours of life. There is failure of appropriate meconium passage per rectum in the newborn, although expulsion of a small plug of inspissated mucus sometimes occurs. Approximately 95% of normal infants pass at least 1 meconium stool during the first 24 hours of life. The distal location of the obstruction results in prominent abdominal distention. Masses of inspissated meconium are sometimes palpable.

Abdominal radiographs of the newborn with meconium ileus usually show nonspecific abdominal distension. Calcifications from meconium peritonitis are occasionally visible (Figure 36-31). As swallowed air fills the bowel, radiographs demonstrate distented gas-filled loops of proximal small intestine, often with mucosal effacement. Bowel wall thickening causes separation of adjacent loops of gas-filled intestine. Typically, the colon is not visible. A characterizing feature of meconium ileus on abdominal radiographs is variation in size of the dilated loops, ranging from marked dilation to normal caliber (Figure 36-32). Another useful radiographic finding is a “soap bubble” or a granular appearance of the intestinal contents, due to the admixture of air and viscous meconium. (The soap bubble pattern also occasionally occurs in infants with ileal atresia, Hirschsprung disease, imperforate anus, or meconium plug syndrome.) Upright or decubitus radiographs of infants with meconium ileus show a paucity of air–fluid levels, despite the severity of bowel dilation; this is related to the failure of thick meconium to form layers where interfaced with air.

Sonography can be helpful for the differentiation of meconium ileus from ileal atresia, as both are common forms of distal ileal obstruction that can have similar appearances on standard radiographs. With meconium ileus, the dilated distal ileum contains echogenic material that is relatively static in appearance on sequential images. With ileal atresia, the dilated bowel contains fluid and air, and there is constant alteration in the appearance as peristalsis occurs. In some instances, sonography allows a prenatal diagnosis of meconium ileus.

A water-soluble contrast enema is a useful diagnostic and therapeutic procedure for infants with suspected meconium ileus. Reflux of hyperosmolar water-soluble contrast material into the area of the meconium impaction draws water into the lumen and lubricates the tenacious meconium. More than 1 therapeutic enema may be required to cleanse the terminal ileum and colon of impacted meconium. Successful clearance of meconium obviates the need for surgical intervention. Most pediatric radiologists use warmed ionic water-soluble contrast for this indication. Some advocate dilution to an osmolality of approximately 600 mOsm/L. A mixture of dilute contrast and N-acetylcysteine has also been advocated. Because enemas with hyperosmolar contrast cause fluid shift into the bowel, appropriate hydration and electrolyte balance should be maintained by the administration of IV fluids. There is a risk of bowel perforation in infants undergoing therapeutic contrast enema for meconium ileus, particularly when there is an associated lesion such as volvulus, atresia, or pseudocyst. Antibiotics should be administered prior to the procedure. Spontaneous perforation can also occur (Figure 36-33).

Contrast enema examination of the infant with meconium ileus typically shows a very small-caliber colon that is normal in length and is appropriately rotated. Meconium ileus causes the most severe form of microcolon of the distal intestinal obstructing lesions, including ileal atresia (Figure 36-34). However, microcolon is not specific for meconium ileus, and an attempt should be made to reflux contrast into the terminal ileum. The distal 10 to 20 cm of the ileum is usually normal in caliber (although slightly larger in size than the colon) and is filled with inspissated meconium pellets (Figure 36-35). The colon frequently contains pellets of meconium as well. Once retrograde filling of the terminal ileum is achieved, additional gentle introduction of contrast eventually may allow opacification of dilated loops of small intestine that contain putty-like meconium; this finding allows an unequivocal diagnosis of meconium ileus.