Radiologists have a critical role in the diagnosis and management of abdominal distension in neonates, the most common abdominal emergency in this group. Proper selection of imaging techniques and recognition of certain patterns, correlated with patient age, can facilitate subsequent imaging and diagnosis.
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.
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
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
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.
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 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
(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.
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
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
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
; 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).
Air-fluid levels are typically absent in the first 48 hours and
"soap-bubble" meconium may be seen in the collapsed left colon.
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
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.
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.
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%.
Ileal atresia is caused by ischemic injury to the developing
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
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.
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
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.
Common congenital causes of upper gastrointestinal obstruction are
duodenal atresia, stenosis or web, malrotation, and jejunal
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
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.
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.
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.
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
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).
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.
Jejunal atresia is the result of an ischemic injury to the
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.
The distal small bowel spirals around its mesentery and resembles
an apple peel; the resultant very short intestine has a propensity
toward necrotizing enterocolitis.
Apple-peel atresia is thought to be autosomal recessive and is
associated with prematurity, biliary atresia, ureteropelvic
junction obstruction, and imperforate anus.
Another familial form of jejunal atresia is that of multiple
intestinal atresias with intraluminal calcification.
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.
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.