Dr. Sumner is with the Department of Radiology, Wake Forest University Baptist Medical Center, Winston-Salem, NC; Dr. Cox is with St. Paul Radiology, St. Paul, MN; Dr. Auringer is with Forsyth Radiological Associates, Winston-Salem, NC.

The most common abdominal emergency in the neonate is intestinal obstruction. The typical clinical presentation includes poor feeding and vomiting, often bilious, followed by abdominal distension and delayed passage of meconium (after 24 hours). Radiologists have a critical role in the diagnosis and management of causes of abdominal distension in neonates. If conventional radiographs of the abdomen reveal dilated bowel, recognition of certain patterns and correlation of these with patient age can facilitate subsequent imaging.

Diagnosis of surgical causes of intestinal obstruction is made easier by first determining whether the obstruction is high or low. An obstruction is considered high if it is proximal to the distal ilium; it is low if located at the level of the distal ileum or colon. Intestinal gas should reach the colon and rectum by the 16th hour of life. ¹ If conventional radiographs of the abdomen reveal one, two, or a few dilated bowel loops, the obstruction is high; if a larger number of dilated bowel loops are present, low obstruction is suspected. Infants with high obstruction may go directly to surgery, or an upper gastrointestinal (GI) series may be indicated, particularly when the infant has bilious vomiting with proximal intestinal obstruction. Although low obstruction is usually obvious on conventional radiographs, it may be impossible to determine how low it is (whether obstruction involves only the small bowel or the small bowel and the colon) because of the lack of small-bowel folds or colonic haustra in the neonate. This distinction can usually be made on the basis of contrast enema results.

Low intestinal obstruction

The differential diagnosis of low intestinal obstruction lesions consists of four conditions. Two of these, Hirschsprung's disease and meconium-plug syndrome, involve the colon; meconium ileus and ileal atresia involve the distal ileum. Other rare etiologies include anorectal malformations, duplications, intussusception, and colon atresia.

Again, contrast enema is the examination of choice to determine whether only the small bowel is dilated as a result of distal ileal obstruction, or whether dilation involves the small bowel and the colon, because of colon obstruction. Although the choice of contrast agent varies, most pediatric radiologists use a water-soluble contrast solution. A low-osmolar water-soluble contrast agent is recommended to avoid two potential problems: perforation and electrolyte imbalance. In addition, two of the conditions that can cause low obstruction--meconium plug syndrome and meconium ileus--may be treated effectively with the water-soluble contrast.

Contrast-enema technique merits a brief discussion. A small, soft catheter should be used; the exact size depends upon the size of the anus and catheter availability. If a balloon catheter is used, the balloon should not be inflated, as it might obscure abnormalities or even perforate the rectum. Inserting the catheter too high could obscure a low rectal transition zone in infants with Hirschsprung's disease. Beginning the enema in the lateral projection is recommended, because distal pathology such as a transition zone is easier to recognize. If a transition zone, small left colon, or other colonic abnormality is identified, no additional colon filling proximal to the obstruction is necessary. However, if the colon is a microcolon, an attempt to reflux contrast into the terminal ileum is recommended.

Microcolon is a key differential diagnostic finding on the contrast enema in a newborn with low bowel obstruction. ² A microcolon is an unused colon, generally <1 cm in diameter. Because its caliber depends on the amount of succus entericus reaching the colon, it will be small in caliber with distal sites of bowel atresia, usually ileal, and meconium ileus. Rare conditions associated with microcolon are total aganglionosis and microcolon of prematurity. More proximal bowel atresias, such as duodenal or jejunal atresia, do not result in microcolon, since the remaining portion of small bowel produces sufficient succus entericus to maintain normal colon caliber.

Congenital causes of low bowel obstruction

Hirschsprung's disease

Hirschsprung's disease is characterized by absence of myenteric plexus ganglion cells, a condition that results in failure of normal colonic relaxation with subsequent obstruction. Most infants with Hirschsprung's disease are examined during the first 6 weeks of life for abdominal distension, vomiting, and, occasionally, enteritis. This disease is more common in boys than in girls, and it rarely occurs in preterm infants; approximately 5% of trisomy 21 patients have Hirschsprung's disease. Conventional radiographs show variable bowel distension, but often without rectal gas ³ (figure 1A). Thickening of the bowel wall and pneumatosis may be seen with entercolitis complicating Hirschsprung's disease; contrast enema is contraindicated in these situations. Cecal perforation is rare. Contrast enema should be begun in the lateral projection to demonstrate the transition zone between normal dilated colon and contracted, aganglionic distal colon and rectum (figures 1B and C). This transition zone is located most frequently in the rectosigmoid region. Although a transition zone is obvious in only a minority of neonates with Hirschsprung's disease, the aganglionic segment may be narrower than normal innervated bowel. ³ An irregular, serrated aganglionic segment is an additional sign (figure 1D). Approximately 80% of patients with Hirschsprung's disease have short-segment distal aganglionosis; 10% to 15% have long segment involvement, and 5% to 10% have total colonic involvement. ⁴ The ultrashort segment is extremely rare and involves only the internal sphincter. Total colonic Hirschsprung's disease is uncommon, but its incidence is equal between male and female infants and it may be inherited. Results of contrast enema evaluation may be normal, or the colon may be foreshortened with flexures pulled down (figure 2A). In addition to a short colon, total agan- glionosis may involve the small bowel, and a megaileum may be identified (figure 2B); a true microcolon appearance is rare.

Meconium plug syndrome

Meconium plug syndrome is also known as small left-colon syndrome, functional immaturity of the colon, or colon inertia of prematurity. Its exact etiology is unknown, but it is thought to be associated with immature myenteric plexus ganglia ^3,5 ; it tends to be self-limited. Meconium plug syndrome has been associated with maternal drug ingestion, diabetes, eclampsia, and prematurity; it is very rarely associated with cystic fibrosis. Conventional radiographs of the abdomen show distal bowel obstruction, although typically less severe than with other causes of distal neonatal bowel obstruction (figure 3A). ³ Air-fluid levels are typically absent in the first 48 hours and "soap-bubble" meconium may be seen in the collapsed left colon. ⁶ Contrast enema shows a relatively normal rectum with small-caliber left colon to the level of the splenic flexure with dilated proximal colon containing filling defects due to meconium plugs (figures 3B and 3C). Contrast enema is usually therapeutic; if symptoms persist, long-segment Hirschsprung's disease should be considered. ⁴

Meconium ileus

Meconium ileus is caused by abnormal meconium inspissation in the distal ileum. It is usually associated with cystic fibrosis; 10% to 20% of these patients initially present with this condition. ⁷ Meconium ileus develops in infants with cystic fibrosis who produce in utero meconium that occludes the small intestine, resulting in a high-grade distal small-bowel obstruction. ⁶ Conventional radiographs demonstrate multiple dilated bowel loops, indicative of low obstruction, and meconium with a soap-bubble appearance in the right side of the abdomen (figure 4A). Air-fluid levels are generally fewer than in patients with small-bowel atresias and may be skewed on erect or decubitus views. Meconium ileus is complicated in approximately 50% of cases by volvulus, perforation, atresia, or peritonitis; multiple air-fluid levels suggest the presence of a complicating feature. Dilated echogenic bowel may be identified on sonograms (figure 4B). Contrast enema is diagnostic, as it demonstrates a microcolon with a collapsed meconium-filled distal ileum (figure 4C). Water-soluble contrast is preferred, and attempted reflux of contrast into the ileum is recommended. The reported overall success rate for treatment of uncomplicated meconium ileus is 50% to 60%. The perforation rate is 2% to 3%. ⁸

Ileal atresia

Ileal atresia is caused by ischemic injury to the developing gut. ⁹ The ischemia may be primary, due to a vascular accident, or secondary, due to meconium ileus. Conventional radiographs of the abdomen demonstrate typical findings of low obstruction (figure 5A). Contrast enema usually shows a microcolon; the more distal the atresia, the smaller the colon (figure 5B). Atresias may be multiple.

Microcolon of prematurity

Microcolon of prematurity is due to a functional obstruction in a very premature infant and presumably is the result of immaturity of the neuronal apparatus. ¹⁰ Mothers of these very low birth-weight infants may have experienced toxemia or may have received magnesium sulfate. Conventional abdominal radiographs demonstrate marked distension with or without air-fluid levels (figure 6A). Results of contrast enema evaluation may be normal or may demonstrate a microcolon (figure 6B). Enemas are frequently therapeutic, producing a nondistended state in 24 to 36 hours.

High intestinal obstruction

High intestinal obstruction is defined as obstruction proximal to the distal ileum. Conventional abdominal radiographs typically show one, two, or a few dilated bowel loops. Congenital gastric obstruction is quite rare. In most newborns with gastric distension and apparent gastric outlet obstruction, obstruction is at the level of the duodenum. ⁶ Common congenital causes of upper gastrointestinal obstruction are duodenal atresia, stenosis or web, malrotation, and jejunal atresia. ⁶

Duodenal atresia

The most common cause of high obstruction in the newborn is intestinal atresia; the duodenum is the most common site. Duodenal atresia is more common than duodenal stenosis or duodenal web. It occurs in one of approximately 3400 live births and is associated with Down's syndrome (30%), congenital heart disease (20%), and intestinal malrotation. ⁴ Conventional radiographs show stomach distension with a rounded, markedly dilated duodenal bulb, called the "double bubble" (figure 7A). These classic findings are diagnostic, and an upper GI series is not indicated. Atresia or stenosis occurs, usually in the region of the ampulla of Vater, when the duodenum fails to recanalize partially or completely. Annular pancreas is present in approximately 20% of patients with duodenal atresia or stenosis. ¹¹ In duodenal stenosis, whether the result of segmental narrowing or of a web or diaphragm, the stomach and bulb are distended, but air is present in the distal bowel (figure 7B). If an upper GI series is performed, duodenal stenosis appears as a narrowed area in the second portion of the duodenum (figure 7C). Distinguishing duodenal web from duodenal stenosis may be difficult; the "wind-sock" appearance is not usually seen in newborns. ⁶

Malrotation

The most important surgical emergency in association with duodenal obstruction is malrotation. Emesis is nearly always present and is usually bilious. The classic temporal sequence is the acute onset of bilious vomiting at 2- to 3-days-old, with 39% of presentations occurring during the first week of life. ¹² The most common type of malrotation requiring surgical intervention is incomplete rotation, which results in a shortened mesenteric root that has a tendency to twist on its axis (volvulus). An infant with this condition may also develop dense peritoneal bands (Ladd's bands) extending from the cecum to the liver hilum across the duodenum. Conventional radiographs of patients with intestinal obstruction secondary to either volvulus or Ladd's bands usually show a distended stomach and duodenum. However, duodenal dilatation is usually less pronounced than in duodenal atresia, which is excluded by the presence of distal bowel gas (figure 8A). Plain abdominal radiographs of the infant with intestinal malrotation and volvulus may demonstrate partial or complete duodenal obstruction, ileus pattern, or the results may be normal. If bowel ischemia accompanies volvulus, bowel wall thickening, ascites, or pneumatosis may be evident.

Bilious vomiting in an infant is a surgical emergency, and in the absence of another defined cause, intestinal malrotation must be excluded. An upper GI series should be performed to document the ligament of Treitz and to evaluate for duodenal obstruction. With malrotation, the duodenal and jejunal junction (ligament of Treitz) is not to the left of the spine, not at the level of the duodenal bulb, or both. A spiral or corkscrew appearance of the duodenum with tapered obstruction of the third portion indicates midgut volvulus (figure 8B). Most neonates with obstruction due to malrotation have associated midgut volvulus. Obstruction in malrotation is due to either volvulus or Ladd's bands; distinguishing these two entities during an upper GI series is not usually possible unless the actual volvulus is seen. If the location of the duodenal jejunal junction is indeterminate, a small-bowel follow-through is recommended to identify cecal position.

Sonography has also been used in the diagnosis of malrotation. In malrotation, the superior mesenteric vein (SMV) lies to the left of the superior mesenteric artery (SMA). ¹³ However, some patients with abnormal SMA and SMV do not have malrotation, and as many as one-third of patients with surgically proven malrotation have a normal SMA-SMV relationship. ¹⁴

Jejunal atresia

Jejunal atresia is the result of an ischemic injury to the developing gut. ⁹ The ischemia may be caused by a vascular accident or may be secondary to a mechanical obstruction, such as an in utero volvulus. Jejunal atresia may be membranous or fibrous, or it may involve the entire jejunum. Conventional abdominal radiographs usually show a few dilated bowel loops relatively high in the abdomen; these suggest mid­small-bowel obstruction with air-fluid levels (figure 9A). Calcification may be seen within the bowel walls and is due to prior ischemia or infarction. Meconium peritonitis with peritoneal calcification may be evident if the ischemic event producing the atresia caused a bowel perforation. Occasionally a "triple-bubble" sign is seen (figure 9B). No further radiographic investigation is usually required; however, surgeons may request an enema to exclude second and third atresias lower in the bowel. Colon size is normal or near normal in isolated proximal jejunal atresia, but is much smaller in more distal atresias (figures 10A and B).

Two unusual forms of jejunal atresia are inherited. "Apple-peel" atresia consists of proximal jejunal atresia with absence of the distal superior mesenteric artery, shortening of the small bowel distal to the atresia, and absence of the dorsal mesentery. ¹⁵ The distal small bowel spirals around its mesentery and resembles an apple peel; the resultant very short intestine has a propensity toward necrotizing enterocolitis. ¹⁶ Apple-peel atresia is thought to be autosomal recessive and is associated with prematurity, biliary atresia, ureteropelvic junction obstruction, and imperforate anus. ⁴

Another familial form of jejunal atresia is that of multiple intestinal atresias with intraluminal calcification. ¹⁷ Also thought to be autosomal recessive, it is most frequently found in infants of French-Canadian ancestry. Multiple atresias occur from the stomach to the rectum and are associated with biliary dilatation and congenital heart disease. Conventional abdominal radiographs demonstrate extensive intraluminal calcification between the atretic areas, resembling a string of pearls. ¹⁷

Conclusion

Intestinal obstruction in the newborn may be classified as high or low. Conventional radiographs of the abdomen usually suffice to make this distinction. Newborns with complete upper intestinal obstructions usually have a few dilated loops and go to the operating room without further radiologic investigation. If obstruction is incomplete, an upper GI series may be useful. All neonates with gastric or duodenal obstruction should be considered to have malrotation with or without volvulus until proven otherwise. Newborns with low obstructions usually have many dilated loops; in these patients a contrast enema may be therapeutic as well as diagnostic. AR