Microgastria is a rare anomaly characterized by a small, underdeveloped stomach (3). Associated malformations are common, including polysplenia and asplenia, colonic aganglionosis, midgut malrotation, and skeletal abnormalities. Imaging findings are a small, tubular, midline stomach and esophageal dilatation (Fig. 7.6).

Duodenal atresia is the most common cause of proximal bowel obstruction in the neonate and is believed to be the result of failure of recanalization of the duodenal lumen during fetal development. The atresia obstructs near the ampulla of Vater and leads to vomiting of bilious material within the first 24 hours of life. There is usually little or no upper abdominal distention because of the high anatomic level of the obstruction and the vomiting. Other anomalies are present in about one half of patients, including Down syndrome (found in approximately 30%), malrotation, congenital heart disease, esophageal atresia, renal anomalies, and imperforate anus.

Abdominal radiography usually shows gas in a dilated stomach and duodenal bulb, producing a classic double bubble sign (Fig. 7.7). With this appearance, further imaging studies are unnecessary. However, if the patient has vomited or a nasogastric tube has been placed, the stomach and the duodenum may be collapsed. In these cases, a barium upper GI series may be needed for diagnosis (Fig. 7.8). Air may be seen in the distal small bowel in some patients with duodenal atresia if they also have a bifurcated common bile duct with one branch inserting above the site of atresia and one branch below beside the site of atresia. Patients with this form of the anomaly also may need an upper GI series for diagnosis.

Duodenal membrane or web is an intraluminal diaphragm that arises near the ampulla of Vater and may be complete or partial, depending on the presence or absence of an aperture. If the diaphragm is complete, the patient presents with bilious vomiting and a doublebubble pattern on plain radiographs, indistinguishable from that seen in duodenal atresia. If the web is incomplete, patients can present after the neonatal period with vomiting or abdominal pain. In these patients, a thin, convex curvilinear structure may be seen extending across the lumen of the second portion of the duodenum on the UGI study, producing a “windsock” deformity (Fig. 7.9).

Annular pancreas is an anomaly in which normal pancreatic tissue encircles the second portion of the duodenum. It is nearly always associated with duodenal atresia or stenosis, which is probably the cause of obstruction. If the obstruction is severe, patients present shortly after birth with bilious vomiting, mimicking duodenal atresia. With lesser degrees of obstruction, patients may be asymptomatic or present later in childhood with symptoms of obstruction. In these patients, barium examination shows duodenal narrowing. CT or magnetic resonance imaging (MRI) can confirm the diagnosis by showing the anomalous pancreatic tissue encasing the duodenum (Fig. 7.10). Treatment involves surgically bypassing the obstruction with a side-to-side anastomosis.

Jejunal atresia and stenosis are also causes of proximal neonatal intestinal obstruction. Atresia is more common than stenosis. Both anomalies are more common in the proximal jejunum and distal ileum than in other areas of the small bowel. Both are thought to result from an in utero ischemic injury. Affected neonates usually present in the first 24 hours of life with bilious vomiting, abdominal distension, and failure to pass meconium. Abdominal distension is more severe than with duodenal obstruction.

Plain radiographs show dilated loops of proximal small bowel. The remainder of the abdomen is gasless (Fig. 7.11). Air-fluid levels, peritoneal calcification due to in utero perforation, and intraluminal calcifications may also be noted. An upper GI series is not
needed for diagnosis, but some surgeons may request a contrast enema examination to exclude other sites of atresia in the distal small bowel. With an isolated jejunal atresia, the colon is of normal or near normal caliber, as adequate amounts of succus entericus reach it. If there are also sites of distal atresia, the colon will be small. Treatment is resection of the atretic sites and primary anastomosis.

The embryology and clinical findings of ileal atresia are similar to those of jejunal atresia. Plain radiographs show distal bowel obstruction (Fig. 7.12). Contrast enema demonstrates a microcolon secondary to the obstructive lesion in the distal small bowel (Fig. 7.13).

Meconium ileus is a congenital bowel obstruction that results from inspissation of abnormally sticky meconium in the distal small bowel. It is almost always a manifestation of cystic fibrosis, and approximately 10% to 20% of patients with cystic fibrosis present in the neonatal period with meconium ileus.

Plain radiographs show distal bowel obstruction indistinguishable from that seen in other distal small or large bowel obstruction. Water-soluble enema findings include a microcolon and impacted meconium, which appears as multiple ovoid or round filling defects in the terminal ileum (Fig. 7.14). Contrast usually can be refluxed into proximal distended loops of ileum. Therapeutic enemas are less likely to be successful in infants with meconium ileus complicated by other pathology (i.e., volvulus, perforation, atresia).

Meconium peritonitis is an aseptic peritonitis caused by antenatal perforation of bowel and intraperitoneal spill of meconium. It is usually associated with bowel atresias and meconium ileus. Calcifications can develop as early as 12 hours after in utero perforation (Fig. 7.15). Although peritoneal calcification is most common, calcifications also can be seen in the scrotum and bowel wall.

Meconium pseudocyst is a cyst-like mass that results when the extruded meconium becomes walled off by fibrous adhesions. The cysts can contain only meconium or a combination of meconium and trapped bowel loops. Plain radiography shows a soft-tissue mass, which may have an air-fluid level and a calcified wall (Fig. 7.16). Contrast enema can demonstrate communication with the bowel (Fig. 7.17). Sonographic findings include a fluid-filled mass, echogenic walls due to calcifications, and internal echoes due to the presence of air or debris (Fig. 7.18).

Hirschsprung disease is a functional colonic obstruction, which is characterized by the absence of ganglion cells in the distal colon. The aganglionic segment most often involves only the rectum and a part of the sigmoid colon (termed short segment disease). It also may extend into the descending colon (long segment disease) or it may involve the entire colon and distal small bowel (total colonic disease). Hirschsprung disease affects males more than females and is a disease of term infants. Patients present in the neonatal period or in childhood with constipation and abdominal distension. Enterocolitis is a complication of severe disease.

Plain radiographs demonstrate small bowel obstruction. A barium contrast enema provides a definite diagnosis. Barium is preferred over the water soluble contrast agents because it is better for demonstrating the poor colonic evacuation associated with Hirschsprung
disease. No bowel preparation should be given, and a catheter without a balloon should be placed low in the rectum to avoid obscuring the transition zone between the aganglionic and normal colon. The classic finding of Hirschsprung disease is a transition zone from small aganglionic bowel to more dilated normally innervated proximal bowel (Fig. 7.19). The transition zone may be difficult to detect in the newborns. In these patients, measurement of the rectosigmoid (R/S) index can help in the diagnosis. In neonates with Hirschsprung disease, the maximal diameter of the rectum is smaller than that of the maximal diameter of the sigmoid colon (R/S index <1) (Fig. 7.20). In normal newborns, the rectal diameter is larger than that of the sigmoid colon (R/S ratio >1). Other findings are irregular, sawtooth contractions of the aganglionic segment and delayed evacuation of barium on radiographs obtained 24 hours after the examination. In the presence of colitis, the enema may show mucosal thickening, due to edema and ulceration.

Findings of total colonic Hirschsprung disease include a normal caliber colon, a short foreshortened colon with absent flexures, and a microcolon (Fig. 7.21). Additional findings include dilated small bowel and intraluminal small bowel calcifications. Total colonic aganglionosis has an equal incidence in males and females and may be familial.

Functional immaturity of the neonatal colon, previously known as the small left colon or meconium plug syndrome, is a transient abnormality of intestinal motility presenting at
birth. The affected newborn fails to pass meconium and develops abdominal distention. This condition is found in premature infants, small-for-dates infants, and infants of diabetic mothers. It is usually self-limited, although cecal perforation has been described.

Plain radiographs show a distal bowel obstruction. Contrast enema reveals a narrow descending and rectosigmoid colon with a transition point to a normal caliber colon near the splenic flexure (Fig. 7.22). The rectum, transverse, and right colon are normally distensible. A variable amount of meconium is noted in the narrowed segment of colon. Water-soluble enemas are useful for diagnosis and also treatment, because they facilitate passage of meconium. Passage of normal stools is noted within several days. The normal size of the rectum, premature age of the infants, and quick improvement in symptoms helps differentiate functional colonic immaturity from Hirschsprung disease.

Anorectal malformation, also referred to as imperforate anus, is the result of abnormal separation of the hindgut from the genitourinary system. The absence of a normal anus is diagnostic on physical examination. Anorectal malformations are classified as high or low, depending on whether the rectum terminates above or below the levator ani. In boys with high lesions, the rectum usually ends with a fistula to the posterior urethra or rarely to the bladder. In girls with high lesions, the fistula is to the vagina or vestibule. In low lesions, the rectum has no communication with the genitourinary tract. Patients with low lesions may have a tiny perineal orifice that is anteriorly positioned relative to the expected normal position or the rectum may be completely covered. The differentiation between high and
low lesions can usually be made clinically, based on visualization of a perineal orifice or the passage of meconium through the urethra or vagina.

Plain radiographs reveal a distal bowel obstruction. Air may be noted in the bladder in boys with high lesions. Calcified meconium also may be noted in the bowel. Anorectal malformations are part of the VACTERL association (see prior discussion) (Fig. 7.23). Renal abnormalities include hydronephrosis, horseshoe kidney, renal agenesis and hypoplasia, and vesicoureteral reflux. Associated anomalies are more common in neonates with high lesions.

Surgery for the low-type imperforate anus is usually done soon after birth and involves closure of the fistula, creation of an anal opening, and repositioning the rectal pouch into the anal opening. Surgery for a high-type imperforate anus defect initially involves creation of a colostomy. The infant is allowed to grow for several months and then a complete repair is done. In this repair, the rectal pouch is pulled down, an anal opening is created, and the colostomy is closed. Incontinence or constipation are complications after repair. In these instances, MRI can be helpful in showing the position of the neorectum and whether it is adequately positioned within the levator sling (Fig. 7.24).

Colonic atresia is a relatively uncommon cause of neonatal bowel obstruction. The atresia occurs proximal to the rectum, usually in the descending or transverse colon. The two parts of the colon may be separated by a fibrous cord, complete gap, or membrane (web). Infants present immediately after birth with abdominal distention, bilious vomiting, and failure to pass meconium. Plain radiographs demonstrate a distal bowel obstruction and commonly disproportionately dilated bowel loops on the right side of the abdomen, representing dilated proximal colon (Fig. 7.25). Contrast enema shows a microcolon distal to the site of atresia and no flow proximally. In the membranous type of colonic atresia, the membrane prolapses in a retrograde fashion into the more proximal colon, creating a “windsock” deformity.

Megacystis-microcolon-intestinal hypoperistalsis syndrome is a rare cause of distal bowel obstruction. Contrast enema shows a microcolon, which is likely due to decreased small bowel motility. The small bowel also may be short and malrotated. Sonography reveals a large bladder, hydronephrosis, and ureteral dilatation. Long-term prognosis is poor.

Hypertrophic pyloric stenosis (HPS) is a disorder characterized by hypertrophy of the circular muscle of the pylorus, which results in gastric outlet obstruction (8). Boys are affected 4 to 5 times more than girls, and there is a familial predisposition. HSP develops in approximately 20% of sons and 7% of daughters of female index cases and 5% of sons and 2% of daughters of male index cases. Typically, patients present between 2 and 6 weeks of age with nonbilious vomiting, which often becomes projectile. The clinical diagnosis of HPS can be made when an olive-shaped mass is palpated in the epigastrium. The treatment for HPS is a Ramstedt pyloromyotomy, in which the hypertrophic muscle is split longitudinally without violating the mucosa.

Pylorospasm is a disorder characterized by transient spasm of the pylorus. Patients present with vomiting. Barium and sonographic findings include antral narrowing and delayed, but intermittent, gastric emptying. At sonography, pyloric muscle thickness and channel length are normal. The intermittent flow of gastric contents into the duodenum helps to differentiate pylorospasm from pyloric stenosis (Fig. 7.28).

Prostaglandin E is given to maintain patency of the ductus arteriosus in newborns with cyanotic heart disease and results in hyperplasia of the gastric mucosa, which may produce gastric outlet obstruction. The upper GI series shows an elongated pyloric channel. Sonography shows that the narrowing is secondary to mucosal hyperplasia and not to muscle hypertrophy. The hyperplasia disappears when therapy is discontinued.

Gastric volvulus is classified as organoaxial or mesenteroaxial. Patients present with vomiting. Organoaxial volvulus is rotation of the stomach around its long axis, which results in the greater curvature positioned superiorly and the inferior curvature positioned inferiorly (Fig. 7.29). It often is associated with diaphragmatic hernia or eventration. Mesenteroaxial volvulus is rotation of the stomach around an axis perpendicular to the stomach, which results in the gastric antrum lying superiorly and anteriorly and to the left of midline and near the gastroesophageal junction (Fig. 7.30). The obstruction may occur at the gastroesophageal junction or at the pylorus. In both types of volvulus, abdominal radiographs show gastric dilatation. Contrast studies reveal the abnormal rotation.

Malrotation refers to a spectrum of rotational abnormalities including failure of rotation (colon to the left, small bowel to the right), reverse nonrotation (small bowel to the left, colon to the right), reverse rotation (duodenum anterior and colon posterior to the superior mesenteric artery, small bowel to the right, cecum malpositioned), and incomplete rotation (a range of abnormalities between nonrotation and normal rotation). In malrotation, the
attachment of the bowel is abnormally short, resulting in a narrowed mesenteric pedicle and an abnormally positioned ligament of Treitz.

Symptoms in patients with malrotation are a result of a proximal bowel obstruction from extrinsic peritoneal bands (Ladd bands) crossing the third portion of the duodenum, midgut volvulus, or a combination of the abnormalities (11). Patients with malrotation usually present in the first month of life with vomiting, which is nearly always bilious. Older children can present with diarrhea and malabsorption caused by mesenteric lymph stasis or with intermittent abdominal pain and vomiting from episodic volvulus.