Intestinal Obstruction in the Newborn Workup

When considering intestinal obstruction in the newborn, useful information may be gleaned from plain x-rays and ultrasonography, which can be performed in the nursery. Water-soluble contrast enemas may be both diagnostic and therapeutic (meconium plug syndrome). Ensure the infant is well hydrated and kept warm during the contrast enema, and remind the radiologist to be gentle during instillation of the contrast material, because the colon may be tiny and unused.

Complete or incomplete intestinal obstruction

"Atresia" denotes a complete obstruction, whereas "stenosis" implies a partial obstruction, usually from a fenestrated web.

Malrotation/volvulus may simulate the double-bubble radiographic appearance of duodenal atresia, but the initial radiographs in malrotation/volvulus usually show air in the distal intestine, whereas duodenal atresia does not. These considerations may be helpful in determining the urgency of surgical intervention.

Paradoxically, infants with distal small bowel obstruction may pass meconium stools. Whereas stooling usually indicates a partial obstruction, the obstructing event (mesenteric vascular accident or volvulus) may have occurred after meconium reached the colon. Babies must pass transitional stools (digested milk and meconium) to demonstrate intestinal continuity (patency).

Rectal stimulation in babies with Hirschsprung disease usually elicits an evacuation, because aganglionosis causes spastic contraction of the rectum (± the distal sigmoid colon). It may occur in conjunction with the meconium plug syndrome, which is typically relieved following administration of a water-soluble contrast enema.

Imperforate anus

Imperforate anus occurs in conjunction with the VACTERL association (v ertebral, a nal, c ardiac, t racheal, e sophageal, r enal, and radius anomalies). The following investigations for concurrent anomalies are indicated:

• Ultrasonography of the kidneys and central nervous system (CNS)
• Echocardiography
• Plain radiographs of the vertebral column and forearm (radius)

Malrotation/volvulus

An upper gastrointestinal study usually suffices to diagnose malrotation/volvulus (see the image below), but computed tomography (CT) scanning may have been ordered prior to consultation.

 

Normally, the superior mesenteric artery lies to the left of the superior mesenteric vein. Reversal of this spatial relationship suggests malrotation with midgut volvulus (see the CT scan below).

Polyhydramnios occurs in 50% of pregnancies in which the fetus has duodenal obstruction. The dilated stomach and duodenum may be visualized with fetal ultrasonography, and the image is similar to the double-bubble appearance of the plain radiographs obtained after birth. Prenatal ultrasonography may also detect jejunoileal atresia. Polyhydramnios occurs with proximal intestinal atresia, but usually not when the obstruction is distal.

A complication of meconium ileus is in utero perforation, which ocurs proximal to an obstruction. Meconum escapes from the intestine and is then contained by adjacent loops of intestine, creating a pseudocyst. Intense inflammation follows, giving this condition its appellation “meconium peritonitis.”

Distinctive ultrasonographic features of meconium peritonitis include the following:

• Meconium pseudocyst (an echogenic mass)
• Abdominal calcifications
• Ascites
• Intestinal obstruction

If the perforation seals and only a small amount of meconium escapes, scattered calcifications will be seen on abdominal radiographs because extraluminal meconium calcifies. ^[21]

Duodenal obstruction

Plain x-rays of the abdomen show the classic “double-bubble sign” of duodenal atresia. Air is an excellent contrast medium and may be introduced into the baby’s stomach via a nasogastric tube if duodenal obstruction is suspected. Incomplete duodenal obstruction mandates contrast radiography to differentiate duodenal atresia from malrotation/volvulus. Significant dilatation of the duodenum indicates chronicity, whereas minimal dilatation suggest an acute event.

Malrotation/volvulus

If malrotation/volvulus is suspected, an upper gastrointestinal (GI) series should be obtained as soon as possible. ^{[22, 23]}

Corroborative radiographic findings include the following:

• Incomplete obstruction of the duodenum may occur from Ladd bands in the absence of volvulus (indicated by the spiral below the obstruction).
• The ligament of Treitz (duodenojejunal junction) is located to the right of the vertebral column (directly below the pylorus).

If a baby presents with abdominal distention and tenderness, with or without hematochezia, a barium enema is a more definitive study, because it helps to differentiate malrotation/volvulus from Hirschsprung enterocolitis.

Contrast enema signs of malrotation include the following:

• Absence of the splenic and hepatic flexures
• Incomplete descent of the cecum (it remains subhepatic)
• Impeded flow of contrast medium through the colon
• Volvulus: A "closed loop" obstruction; it involves the proximal and distal limbs of the intestinal loop

Jejunal atresia

In jejunal atresia, plain radiography of the abdomen reveals distention of the stomach and small bowel proximal to the obstruction. In 12% of newborns with jejunoileal atresia, intra-abdominal calcifications, which represent extraluminal meconium, are observed on plain radiography. This association occurs because interruption of blood flow to the atretic bowel is followed by sterile necrosis and perforation of the intestine with extrusion of meconium.

Occasionally, in proximal atresias with large mesenteric defects, the distal bowel obtains its blood supply by retrograde flow from the ileocecal vessels. The tiny, unused intestine spirals around these vessels, simulating an “apple peel” or “Christmas tree” appearance. The tip of the “Christmas tree” (the proximal end of the preserved distal intestine) may twist and obstruct its rather precarious blood supply, adding urgency to the evaluation and surgical correction of this condition (because the length of the intestine is already shortened by the atresia).

Distal small bowel obstruction

Typically, plain radiographs of distal small bowel obstruction reveal multiple dilated loops of intestine, air-fluid levels, and absence of air in the rectum. A diatrizoate (Gastrografin) enema is indicated and demonstrates “microcolon.” In patients with meconium plug, the diatrizoate enema is both diagnostic and therapeutic.

Meconium ileus

Bowel loops with varying diameters are seen on plain radiography, as well as a “soap bubble” or “ground glass” appearance from the sticky, inspissated meconium. Scattered calcifications suggest in utero perforation.

Meconium plug syndrome

A contrast enema should be performed in all infants with distal intestinal obstruction. It is diagnostic of meconium plug syndrome—a normal colon laden with meconium filling defects—as well as therapeutic, facilitating evacuation of a meconium plug. Hirschsprung disease may be associated with meconium plug syndrome in 4% of patients; therefore, some clinicians perform a rectal biopsy in patients presenting with meconium plug syndrome.

Hirschsprung disease

In babies with Hirschsprung disease, a contrast enema shows a transition zone, which is an abrupt change in the diameter of the narrow aganglionic rectum and the proximal obstructed colon. Failure to evacuate the contrast medium within 24 hours also suggests Hirschsprung disease.

Imperforate anus

A cross-table lateral x-ray, with the baby in prone “jack-knife” position, reveals the position of the rectum relative to the levator muscle complex and helps to differentiate high from low imperforate anus. This radiographic study is best performed 12-24 hours after birth to allow swallowed air to reach the rectum.

Newborns with a low imperforate anus have a perineal fistula; in babies with a high imperforate anus, the rectal fistula ends in the genitourinary system (the bladder, urethra, or vagina). Low imperforate anus may be repaired during the newborn period by perineal anoplasty. In newborns with high imperforate anus, a safer option is a temporary colostomy (3-6 months) followed by posterior sagital anorectoplasty.

Anal manometry

Anal manometry is utilized in patients with Hirschsprung disease to measure the pressure of the muscles of the anal sphincter. A balloon is inserted into the rectum and inflated. Normally, this elicits initial resistance, followed by relaxation. In Hirschsprung disease, the initial resistance (high pressure) never abates; the rectum never relaxes, following distention. Obviously, the study is not possible to perform in uncooperative children.

Rectal biopsy

If the contrast enema suggests Hirschsprung disease, a confirmatory suction rectal biopsy is performed.

• The instrument looks like a pencil with a side opening. It is inserted into the rectum (>2 cm) with the opening pressed against the posterior rectal wall.
• Suction is applied to pull a bit of rectal mucosa into the instrument.
• A trigger is pulled, amputating this bit of mucosa, which is submitted to pathology for examination.
• If ganglion cells are found, the baby is normal. If ganglion cells are not identified, a larger specimen is provided to the pathologist to ensure that the absence of ganglion cells was not because the specimen was inadequate. 

Pathologic examination of the rectal mucosa is facilitated by use of acetylcholinesterase, which stains the hypertrophic nerve fibers in Hirschsprung disease. ^[24]