The Pediatric Gastrointestinal Tract: What Every Radiologist Needs to Know

Gastrointestinal pathology is relatively common in children. The clinical history as well as an understanding of disease categories affecting children of different age groups are of critical importance when formulating a differential diagnosis. In this chapter, we discuss obstructive, inflammatory, and traumatic causes of gastrointestinal pathology in children.


Ultrasound
Ultrasound (US) is one of the most valuable imaging tools for the initial evaluation of gastrointestinal disease, due to its high-resolution, real-time capability, wide availability and acceptance, and lack of ionizing radiation and because it rarely requires sedation. There are two key factors for success: (1) knowing how to deal with children of all ages and (2) using high-end US equipment/high-frequency probes.

Computed Tomography
In children, CT should be used with caution and is usually reserved for troubleshooting equivocal findings made in the initial diagnostic workup of acutely and critically sick children. CT is consequently often performed as first-line imaging for emergent indications, such as blunt and penetrating trauma. As a general rule, only one acquisition is needed, and multiphase contrast enhanced sequences should be avoided.

Magnetic Resonance Imaging
MRI is rarely used in the first-line evaluation of gastrointestinal pathology; however, it may serve as an adjunct in challenging/unequivocal cases. In children, MRI is typically performed in the assessment of intestinal pathology such as inflammatory bowel disease and appendicitis, focal and diffuse disease of the abdominal viscera, and diseases of the biliary tree.

Intestinal Obstruction
In neonates and young children, there is no radiographic "signature" for the large and small bowel. Plain film radiographs of the normal newborn demonstrate swallowed air

Learning Objectives
• To compare imaging modalities commonly used in the assessment of pediatric gastrointestinal disease. • To understand gastrointestinal pathology affecting children of different age groups. • To recognize classic imaging features of gastrointestinal diseases commonly encountered in infants and children.
reaching the rectum by 24 h of life. In a clinically obstructed newborn, the number of dilated loops ("dilated" meaning roughly wider than the vertebral interpedicular space) may be more helpful. Whether there is one gas bubble (gastric outlet obstruction), a double bubble (duodenal obstruction), 3-4 bubbles/loops (other high small bowel obstruction), or more (low obstruction), the bowel gas pattern can help decide which contrast study is appropriate. A bowel gas pattern suspicious for a high obstruction may be assessed further with an upper GI contrast study. A bowel gas pattern suggesting a low obstruction should be followed by a contrast enema.

Neonatal Obstruction
Neonatal intestinal obstruction can be classified by location, with "high" obstructions occurring through the level of the proximal jejunum and "low" obstructions occurring after the proximal jejunum. Etiologies of neonatal intestinal obstruction are commonly congenital. In most cases, a fluoroscopic contrast study is required.
The differential diagnosis for high intestinal obstruction includes atresia of the esophagus, stomach, duodenum and jejunum, duodenal stenosis from annular pancreas, duodenal web, and malrotation possibly complicated by midgut volvulus.
The differential diagnoses for low intestinal obstruction include ileal and colonic atresia, anorectal malformation, meconium ileus, colonic dysmotility syndromes, and Hirschsprung's disease. Congenital abdominopelvic cysts or masses can cause both high and low (usually partial) obstruction.
An understanding of clinical presentation as well as the expected postnatal bowel gas progression throughout the abdomen on radiographs can help to detect the level of obstruction [1].
Esophageal atresia (Fig. 15.1) is a clinical diagnosis. If the diagnosis is established, then any gas in the gastrointestinal system distal to the esophagus on plain radiographs means there is also a tracheoesophageal fistula. Radiology has a greater role in the follow-up of these children. Fistulae may recur, and the contrast esophagogram is the imaging test of choice for diagnosis of recurrence; this is performed by trained operators as there is a considerable aspiration risk. For suspected anastomotic stricture, contrast swallow is performed. There is almost always relative narrowing at the anastomosis, so it is a functional holdup which suggests a potential need for dilatation [2,3]. Esophageal atresia may be observed as part of the VACTERL association. Additional anomalies should be actively searched for. Duodenal obstruction ( Fig. 15.2) may have several causes. In duodenal atresia the classic radiographic findings are a double bubble and no distal intestinal gas. Duodenal atresia is thought to result from failure or incomplete recanalization of the intestine in utero. Annular pancreas is caused by failed migration of the anlagen, and it may be evident sonographically (or with CT/MRI). Duodenal stenosis and duodenal web cause partial obstruction and may be difficult to distinguish. The classic sign of a web on a contrast study is the "windsock sign" [4,5].
Malrotation with midgut volvulus (Fig. 15.3) is a surgical emergency in the neonate as well as in infants and children. The failed intestinal rotation manifests as a right-sided/midline low duodenojejunal flexure and a high cecum. This is associated with a short mesenteric root, which predisposes for volvulus. The normal position of the duodenojejunal flexure is at the height of the pylorus and to the left of the midline, at least as far as the left vertebral pedicles. Volvulus is suggested by bilious vomiting and is verified by the finding of a spiraling outline of a malrotated duodenum/proximal jejunum on the contrast study [6,7]. On US a whirlpool of vessels is typically seen.
Small bowel atresia and stenosis ( Fig. 15.4) are relatively common causes of complete and incomplete obstruction in the neonate. Multiple atresias may coexist; the etiology is thought to be in utero mesenteric vascular accidents. Accurate diagnosis is critical, since early surgical invention is usually performed. Bilious vomiting may be observed in those with obstruction beyond the level of the pancreatic ampulla. The typical findings of atresia on contrast enema are a small-caliber (unused) colon and distal small bowel and the inability to reflux contrast into dilated small bowel loops (distinguishing it from meconium ileus; see below) [4,5].
Meconium ileus ( Fig. 15.5) has a very high association (>80%) with cystic fibrosis. The distal ileum is impacted with sticky meconium. The findings on contrast enema are similar to those in small bowel atresia except that when contrast refluxes into the ileum, it outlines pellets of meconium, and it is usually possible to eventually fill dilated small bowel loops. In uncomplicated cases, (repeated) contrast enemas using water soluble contrast material are used therapeutically to soften and wash out meconium pellets. Both meconium ileus and intestinal atresias may be complicated by in utero bowel necrosis/perforation and meconium peritonitis [8,9]. Colonic atresia is exceedingly rare, and the diagnosis is often made clinically (vomiting and failure to pass meconium) on plain radiographs (no gas in the rectum). Contrast enema reveals failed reflux of contrast proximal to the level of the colonic atresia [10].
In Hirschsprung's disease ( Fig. 15.6), intermuscular and submucosal nerve plexuses are absent due to arrested migration of intestinal ganglion cells, producing a functional intestinal obstruction. Since this migration occurs from proximal to distal large bowel, the aganglionic segment extends proximally from the anus. The typical clinical presentation is delayed passage of meconium in the neonate beyond 24 h of age.
Most affected patients demonstrate short-segment involvement, in which the transition point between normal and aganglionic bowel is located in the rectosigmoid colon. Rarely, the entire colon may be involved. Meconium/fecal impaction, a zone of transition from dilated to non-dilated bowel, and abnormal peristalsis of the affected colon segment with "sawtooth" appearance of the wall are classical findings at contrast enema. Very short-segment aganglionosis lacks a clear transition zone and is therefore difficult to diagnose radiologically. Hirschsprung's disease can only be definitively diagnosed by rectal (suction) biopsy [4,11].
Functional immaturity of the colon, small left colon, and meconium plug syndrome (Fig. 15.7) are entities with considerable overlap. The risk is greater in babies of diabetic mothers. Radiologically there is a relatively smaller left colon and proximal meconium impaction. It is difficult to make a categorical diagnosis as similar findings may be seen in Hirschsprung's disease [12,13].
Anorectal malformation is a clinical diagnosis. These malformations are classified as "high" or "low" depending on the location of the distal-most bowel segment relative to the puborectalis sling, with high lesions terminating above and low lesions terminating below this level. The role of radiology is to assess the length of the atretic anorectal segment and to map the fistula(e) [14].

Intestinal Obstruction in the Older Neonate and Infant
In older infants and children, obstruction may be related to other etiologies, such as adhesions, hypertrophic pyloric stenosis, intussusception, incarcerated intestinal hernia, appendicitis, sigmoid volvulus, and Meckel's diverticulum. Plain film radiographs can occasionally suggest the level of obstruction in older infants and children. In many cases, however, ultrasound and, occasionally, CT or MRI may be necessary to reach the diagnosis. Hypertrophic pyloric stenosis (Fig. 15.8) classically presents with forceful "projectile" vomiting and failure to gain weight in an infant. The diagnosis is made sonographically a b Fig. 15.5 Meconium ileus represents obstruction of the distal ileum by viscous, impacted meconium pellets. There is a high association with cystic fibrosis (presenting symptom in 15% of neonates with CF). (a) Fluoroscopic contrast enema demonstrates contrast opacification of a "microcolon" (unused colon due to proximal obstruction). Contrast has refluxed into the distal ileum (arrowheads) and outlines numerous impacted meconium pellets. (b) Abdominal radiograph demonstrates punctate calcifications scattered throughout the abdomen, suggestive of meconium peritonitis which can occur with intestinal atresias as well as meconium ileus, and results from perforation of meconium-containing bowel where the typical findings are (1) thickened muscularis (>4 mm); (2) elongated pylorus (>15 mm), with redundant mucosa occasionally protruding into the antrum of the stomach; (3) gastric hyperperistalsis; and (4) persistent closed pyloric channel after a test feed. A contrast meal is rarely indicated except in a few equivocal cases [15]. Incarcerated intestinal hernia is a surgical emergency. The presentation and imaging findings are similar to those in adults.
Congenital abdominopelvic masses (Fig. 15.9) comprise a spectrum of abnormalities where gastrointestinal obstruction is relatively unusual. Sacrococcygeal masses (e.g., germ cell tumors, such as teratomas, or anterior meningoceles) may cause rectal obstruction. A cystic obstructive mass should raise suspicion for meconium pseudocyst or enteric duplication cyst. Duplication cysts may occur anywhere along the gastrointestinal tract and may have a relatively thick layered wall (akin to bowel) and a mucus-fluid level.

Intestinal Obstruction in the Infant and Older Child
Ileocolic intussusception (Fig. 15.10) is a surgical emergency, usually occurring in infants and toddlers with maximum incidence at just under 1 year of age. The typical presentation is with abdominal pain, bloody stools, and a palpable mass. A clear cause (i.e., a "lead point," such as a mesenteric lymph node, lymphoid hyperplasia, duplication cyst, mural inflammation caused by Henoch-Schonlein purpura, or Meckel's diverticulum) may or may not be established. The sonographic diagnosis is fairly straightforward with the finding of a concentric doughnut shape in the transverse plane and sandwiching of the involved bowel segments (the "pseudokidney" sign) in the longitudinal plane [17]. Long-standing intussusception, trapped fluid in the involved segments, and trapped lymph nodes are thought to be associated with lower success rates for reduction with pressurized gas or fluid.
Colonic volvulus results from twisting of the colon around its mesenteric root and is a rare cause of obstruction in children. In children, cecal volvulus is more common compared with volvuli occurring in the sigmoid and transverse colon. Those affected typically present with delayed passage of meconium. There is a reported increased incidence in babies with mothers who are diabetic or who have received magnesium sulfate during pregnancy for preeclampsia. This image from a contrast enema reveals a small caliber of the descending and sigmoid colon, which are filled with meconium, proximal dilatation, and a normal-caliber rectum Clinical presentation includes abdominal pain, vomiting, and distention. Depending on the type of twist, plain films may demonstrate marked distention of the colon and/or air-fluid levels resembling either a "bird's beak" or "coffee bean." CT demonstrates the "swirling" appearance of the sigmoid mesentery and its vasculature. While clinical and imaging findings are suggestive, sigmoid volvulus remains an often missed diagnosis which can be complicated by bowel ischemia, gangrene, perforation, shock, and even death [18][19][20].
Henoch-Schonlein purpura is an acute small-vessel vasculitis in young children typically involving the skin, kidneys, synovium, and bowel. Sonographically there is nonspecific mural inflammation [21].

Infectious, Ischemic, and Inflammatory Intestinal Pathology
Necrotizing enterocolitis (Fig. 15.11) is the most common gastrointestinal emergency in the newborn. The etiology of NEC is incompletely understood and is thought to involve multiple factors, including immaturity of the blood-gut barrier, early feeding, and bacterial colonization which ultimately lead to hemorrhagic and ischemic necrosis of the intestines. NEC typically occurs in preterm neonates especially with very low birthweight and less commonly term or late preterm neonates with congenital cardiac disease or Hirschsprung's disease. Although somewhat insensitive, plain films may demonstrate intestinal pneumatosis, portal venous air, and pneumoperitoneum. Intestinal ultrasound has proven useful in the diagnosis of NEC and may demonstrate the presence of free fluid and abnormal bowel peristalsis, increased wall thickness, echogenicity, and vascularity and may suggest impending complications such as perforation. Additionally, US has been shown to detect small amounts of intramural and portal venous "air bubbles" not demonstrated on plain film [22,23]. Acute appendicitis is the most common surgical indication in children. Appendicitis results from obstruction of the appendix which leads to distention, mucosal ischemia, bacterial overgrowth, inflammation, and potentially wall compromise and perforation. Early diagnosis is therefore essential in order to avoid such complications. US is the initial test of choice due to its lack of ionizing radiation and is optimal for imaging children. With an experienced sonographer and use of graded compression technique, US is highly sensitive and specific for the diagnosis of appendicitis and its complications and approaches the accuracy of CT. Use of MRI in the diagnosis of appendicitis is increasing, particularly for children with equivocal ultrasound [24,25]. Inflammatory bowel disease may be seen at any age, but early-onset disease is often more complex with associated immunological disorders. In older children and adolescents, the clinical picture and the imaging characteristics are more similar to those in adults. Conventional fluoroscopic contrast follow-through is rarely indicated. Both ultrasound and MR enterography are excellent for assessing the macroscopic disease: bowel wall thickening, mesenteric inflammation, mesenteric lymphadenopathy, hyperemia, and fistulation [26].
Meckel's diverticulitis is inflammation of a Meckel's diverticulum, an omphalomesenteric remnant containing gastric/pancreatic mucosa. Abdominal pain and melena are typical symptoms. The diagnosis is extremely difficult to make with radiographs and cross-sectional imaging alone, which are often insensitive. 99mTc-pertechnetate scintigraphy is typically diagnostic [27].

Gastrointestinal Trauma
Accidental and non-accidental traumas, particularly blunt trauma, are common causes of morbidity and mortality in children. Clinical and laboratory findings may suggest injury to the hollow and solid abdominal viscera, necessitating imaging follow-up. CT performed with intravenous contrast provides superb detail of vascular, solid organ, intestinal, and musculoskeletal anatomy, thus making it the imaging of choice in stable children presenting with trauma. US may be used in unstable patients and is especially helpful in the detection of hemoperitoneum. The liver, spleen, and kidneys are among the most common solid organ injuries observed in accidental trauma. In non-accidental trauma, the liver and pancreas are the most commonly injured solid organs. Intestinal injury is uncommon overall, yet occurs with more frequency in non-accidental versus accidental trauma.
Children with compensated shock following initial resuscitation efforts may demonstrate characteristic abdominal CT findings known as the hypoperfusion complex, which includes collapse of the aorta and inferior vena cava and hyperenhancement of the adrenals, kidneys, mesentery, and bowel walls. These findings signal impending hemodynamic collapse, and their immediate recognition is critical [28][29][30].

Take-Home Messages
• The differential diagnosis for neonatal high intestinal obstruction includes atresia of the esophagus, stomach, duodenum and jejunum, duodenal stenosis from annular pancreas, duodenal web, and malrotation with midgut volvulus.
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• The differential diagnoses for neonatal low intestinal obstruction include ileal and colonic atresia, anorectal malformation, meconium ileus, colonic dysmotility syndromes, and Hirschsprung's disease. • Congenital abdominopelvic cysts may lead to high or low intestinal obstruction. • In older infants and children, intestinal obstruction may be related to adhesions, pyloric stenosis, intussusception, incarcerated intestinal hernia, appendicitis, sigmoid volvulus, and Meckel's diverticulum. • Radiographic findings of NEC including free, intramural, and portal venous air are classic yet infrequently observed. Ultrasound improves sensitivity. • In the hands of an experienced sonographer and with use of graded compression technique, sensitivity and specificity of intestinal ultrasound for acute appendicitis approaches that of CT. • Contrast enhanced CT is the imaging standard in the evaluation of stable children with blunt or penetrating trauma.