Approximately 95% of intestinal obstructions diagnosed in the first 2 weeks of life are due to atresia and/or stenosis of small intestine.  Atresia is more common than stenosis and represents complete luminal obstruction of a hollow viscus, whereas stenosis involves partial occlusion. The incidence of atresia of the small bowel ranges from 1 case in 332 live births  to 1 case in 5000 live births.  No specific racial or sex predilection is known.
Most ileal atresias are diagnosed in the first 24 hours of life (range of age presentation, 1-30 d)  and represent 50% of small intestinal atresias (ileal atresia is more common than jejunal atresia. Although jejunal and ileal atresias uncommonly coexist and are multiple, the entire small bowel must be examined at surgery.).  Prematurity of the neonate is related to the severity of the subtype of atresia.  (See Anatomy, below.) At birth, complete obstruction is present with atresia, whereas various degrees of stenosis manifest with mild to severe sequelae. About 15% of ileal atresias occur proximally, and 30% occur distally. 
In proximal atresias, the colon is normal or nearly normal in size, because adequate succus entericus reaches the colon from small bowel distal to the obstruction. However, distal atresias are usually associated with an unused colon or microcolon less than 1 cm in diameter. When the ischemic event occurs later in gestation, the colon may be normal in caliber, even if the obstruction is distal.
Small bowel atresia, meconium ileus, volvulus, intussusception, and idiopathic causes can be complicated by perforation resulting in meconium peritonitis, which occurs in 5.2% of cases.  In utero perforation of bowel may allow spill of meconium into the peritoneal space. Fibrosis ensues, and peritoneal calcification may occur (see the image below). Calcifications may be irregular, linear, round, or generalized in appearance. Intraluminal calcifications may also occur in cases of severe obstruction.
Type I is characterized by a thin diaphragm that occludes the lumen. The survival rate is 85%. 
In type II, 2 blind ends are connected with a fibrous cord of atretic bowel. The survival rate is 66%. 
In type IIIA, 2 blind ends terminate with a V-shaped mesenteric defect; this is the most common type.
Type IIIB, apple-peel or Christmas-tree atresia, involves a large, V-shaped mesenteric defect in which the blind-ended bowel distal to the atresia is wrapped around its blood supply. That is, the proximal superior mesenteric artery is occluded, and the distal small intestine spirals around its vascular supply of collaterals from the ileocolic artery. This is associated with an absent distal superior mesenteric artery, shortening of the small intestine distal to the atresia, and absent dorsal mesentery. Prematurity, malrotation, and short gut occurs in most patients.  The survival rate is 57%
Type IV is defined as multiple atresias. The survival rate is 29%. There are 4 main types of ileal atresia with subtypes. Type I is characterized by a thin diaphragm that occludes the lumen. In type II, 2 blind ends are connected with a fibrous cord of atretic bowel. In type IIIa, 2 blind ends terminate with a V-shaped mesenteric defect; this is the most common type. Type IIIb, apple-peel or Christmas-tree atresia, involves a large, V-shaped mesenteric defect in which the blind-ended bowel distal to the atresia is wrapped around its collateral blood supply. Type IV is defined as multiple atresias.
In the newborn with suspected intestinal obstruction, the preferred initial examination is plain radiography of the abdomen. This examination allows for the differentiation of high (proximal) and low (distal) obstruction, and the results dictate the next appropriate step in management.
In infants, small bowel is impossible to distinguish from large bowel, but newborns are unique in that they are swallowing air for the first time. If bowel obstruction is complete, no air is present distal to the obstruction. Therefore, the abdominal radiograph is like an upper-gastrointestinal (UGI) series with air as the contrast medium. The number of loops of dilated, air-filled bowel differentiates high from low obstruction.
If only a few loops of dilated bowel are seen, a high obstruction is present. The plain radiograph is often diagnostic, as with the double-bubble sign of duodenal atresia.  All causes of high intestinal obstruction in the newborn are surgical; therefore, further imaging is not usually indicated. If surgery is to be delayed or if the plain radiograph is normal, a UGI series must be performed to exclude malrotation and/or midgut volvulus. This condition requires immediate surgery and is the diagnosis most likely to produce a normal radiograph in a neonate.
If the abdominal radiograph shows many dilated loops of bowel, a low obstruction is present. The differential diagnosis includes incarcerated inguinal or umbilical hernia, ileal atresia, meconium ileus, ileal duplication cyst, [12, 13] colonic atresia, functional immaturity of the colon, Hirschsprung disease, and imperforate anus. Hernia and imperforate anus are diagnosed on the basis of clinical examination. Otherwise, a contrast enema study is indicated to differentiate surgical from medical causes of distal bowel obstruction in the newborn.
Limitation of technique
Plain radiographs in the newborn are useful in determining the level of obstruction, but atresias are occasionally multiple, and the plain radiographs demonstrate only the most proximal obstruction. In addition, dilated loops of bowel filled with fluid may go undetected or may be mistaken for a mass (see the image below). Cross-table radiographs showing air-fluid levels are often helpful in such instances. Also, radiographs may be normal in cases of malrotation and intermittent midgut volvulus.
The UGI series may add no information to a diagnostic plain radiograph showing a pathognomonic finding, such as the double bubble of duodenal atresia.
If the referring physician requests both a UGI series to exclude midgut volvulus and a contrast enema study, one examination can interfere with the other. If there is an indication for both, the UGI series should be performed first. Contrast should be administered through an enteric tube; the enteric tube is then used to aspirate as much of the contrast agent as possible after the duodenal jejunal junction is evaluated.
Contrast enema is useful in demonstrating microcolon, but if contrast agent is inadequately refluxed into the distal small bowel, meconium ileus may be mistaken for ileal atresia; this mistaken finding may lead to unnecessary surgery.
Ultrasonography can demonstrate dilated fluid-filled loops of bowel, but this modality is of limited utility in the neonatal period, because it does not show the site of obstruction. Ultrasonography may be helpful for specific limited purposes, such as differentiating a dilated fluid-filled loop of bowel from a mass, diagnosing enteric duplication cyst, or helping to differentiate meconium ileus from ileal atresia (see Ultrasound).
Computed tomography (CT) scanning and magnetic resonance imaging (MRI) have no role in the diagnosis of ileal atresia.
Differential diagnosis and other problems to be considered
In addition to Hirschsprung disease, imperforate anus, incarcerated inguinal or umbilical hernia, ileal duplication cyst, and functional immaturity of the colon discussed in Preferred examination above, the differential diagnosis, includes meconium ileus, as well as meconium plug syndrome and colonic atresia. Furthermore, polyhydramnios occurs in one fourth of cases and is more common in proximal jejunal atresia than in distal ileal atresia.  However, extragastrointestinal anomalies are rare and occur in less than 7% of patients. 
Use of barium or of undiluted, hypertonic water-soluble contrast agents in the performance of a diagnostic enema study can cause untoward complications. Barium should be avoided in the newborn for a number of reasons. The main reason is the potential for spilling contrast material into the peritoneal cavity. Bowel adjacent to the atretic bowel is at risk of perforation and can be necrotic. On occasion, initial plain images do not demonstrate the perforation. Barium in the peritoneal cavity is not absorbed. Ensuing inflammatory response can lead to formation of granulomata and adhesions. In addition, barium can become inspissated in the colon, leading to obstruction. Patients at risk are those with Hirschsprung disease, cystic fibrosis, ileus, and blind loops of bowel. Use of undiluted, high-osmolality contrast agents in infants can also cause fluid shifts resulting in life-threatening serum electrolyte imbalances.
In addition, it is important to minimize radiation exposure in children. Important steps to include are effective immobilization, limiting the field size to the area of interest, and use of gonadal shielding whenever possible. During fluoroscopic procedures, examination time should be kept to a minimum; pulsed fluoroscopy further decreases radiation exposure to the patient.
Plain radiographic findings in ileal atresia include a dilated stomach (if no nasogastric [NG] suction was used), numerous loops of dilated bowel, multiple air-fluid levels proximal to the point of obstruction, and absent gas distal to the obstruction (see the following images). A dilated loop of bowel may be observed immediately proximal to the site of atresia; this is a common finding with atresias in general. This loop may be filled with fluid and resemble a mass. Ultrasonography may be helpful to show that this is fluid-filled bowel. 
Meconium peritonitis can occur when an atresia or other obstruction leads to an in utero bowel perforation. It is most commonly seen in the setting of jejunal or ileal atresia. Calcifications around bowel and in the peritoneum are evident in meconium peritonitis (see the image below). These calcifications can be focal, cystic or generalized. In the male patient, they may extend into the scrotum by means of a patent processus vaginalis.
Contrast enema study is indicated when plain radiographs demonstrate a distal obstruction (see the following images). In ileal atresia, the colon is diffusely small (< 1 cm; ie, microcolon) (see the images below). Filling of the distal small bowel with contrast agent is abruptly cut off, because material cannot reflux past the atresia into the dilated ileum proximal to the obstruction. It is this abrupt truncation of the contrast column that allows differentiation of ileal atresia, a surgical lesion, from meconium ileus, a medical lesion when uncomplicated.
Both meconium ileus and ileal atresia cause distal obstruction and microcolon. In contrast to ileal atresia, meconium ileus classically shows few, if any, air-fluid levels. Plain radiographs may show a characteristic bubbly appearance in the right lower quadrant, which represents inspissated meconium mixed with air (see the image below).
A definitive diagnosis of ileal atresia can usually be made by performing a contrast enema study. In meconium ileus, the contrast agent can be refluxed past the obstructing, inspissated meconium into the dilated proximal ileum. The inspissated meconium may appear as round or tubular filling defects in the contrast material–filled small bowel (see the following image). Furthermore, contact with the contrast material usually softens and loosens the inspissated meconium, allowing it to pass. This feature makes contrast enema study the preferred initial therapy for meconium ileus, whereas ileal atresia always requires surgical repair.
It is important to reflux contrast material as far into the small bowel as is necessary to allow differentiation between ileal atresia and meconium ileus and to allow the material to contact all of the inspissated meconium.
Choice of contrast agent
With regard to the type of contrast agent, dilute, water-soluble contrast material is preferred for the diagnostic enema study in the neonate. The rationale for using water-soluble, iodinated contrast instead of barium involves multiple factors. The first is the potential for spilling contrast material into the peritoneal cavity in the clinical setting of necrosis and perforation of the atretic bowel. On occasion, the perforation is not demonstrated on the initial plain image. Barium in the peritoneal cavity is not absorbed and may induce the development of adhesions. Even if no perforation is present, water-soluble contrast agent in the colon is preferable to barium if the patient needs immediate surgery after the examination.
Furthermore, 2 entities in the differential diagnosis of low intestinal obstruction, meconium ileus and functional immaturity of the colon, often clinically improve after an enema study performed with water-soluble contrast. A follow-up therapeutic contrast enema procedure increases the likelihood of successful treatment of meconium ileus. Therapeutic enema is more likely to be successful if the preceding diagnostic enema was performed with water-soluble contrast material rather than barium.
Another concern is that barium can become inspissated in the colon and may be difficult to evacuate. Finally, the improved depiction of mucosal detail with barium is not important in the newborn.
The iodinated contrast used for the enema study should be diluted to be nearly isosmolar to serum, yet it should be dense (opaque) enough to be adequately visualized. Use of undiluted, high-osmolality contrast agents in infants can cause fluid shifts from the intravascular space to the lumen of the colon and result in life-threatening serum electrolyte imbalances.
High- and low-osmolality agents provide adequate contrast if iodine concentrations greater than 180 mg iodine per milliliter are used.  Normal serum osmolality is 285 mOsm/kg of water. High-osmolality, water-soluble agents include sodium and meglumine salts of diatrizoate and iothalamate, and these range in osmolality from 400-2000 mOsm/kg of water. If diluted to be nearly isosmolar to serum, these agents may be used instead of relatively expensive, nonionic, low-osmolality contrast agents. The low-osmolality, water-soluble agents range in osmolality from 290 (isosmolar) to 844 mOsm/kg of water.
The osmolality of most commercially available contrast agents is specified on the product insert. If the package insert does not state the osmolarity of the product, a particular dilution is recommended (eg, 1:3-5 for agents with high osmolality). As an alternative, this information can be found in Appendix A: Contrast Media Specifications of the Manual on Contrast Media (version 7) from the American College of Radiology. 
Degree of confidence
The degree of confidence in plain radiographic findings is high for determining the presence of high versus low obstruction in ileal atresia. For low obstruction, further imaging is necessary to localize the site and nature of the obstruction.
Contrast enema study offers a high degree of confidence in the diagnosis if good reflux of contrast agent into the small bowel can be achieved.
Ultrasonography is valuable in the diagnosis of in utero bowel obstruction, as the dilated bowel is filled with fluid rather than air. The number of dilated loops indicates proximal versus distal obstruction, and findings may be specific in some causes of proximal obstruction, such as the double bubble of duodenal atresia. [9, 17, 18]
In the postnatal period, air is introduced into the gut, making ultrasonography less useful than before this period, particularly in distal obstruction, in which a great deal of air absorbs the ultrasound beam.
If the results of the contrast enema study are equivocal, ultrasonography may help distinguish ileal atresia from ultrameconium ileus in select cases. Meconium ileus is characterized by echogenic material within dilated loops of small bowel, whereas ileal atresia results in intestinal dilatation with anechoic fluid.  The following are radiographs and an ultrasonogram in the same patient with meconium ileus and cystic meconium peritonitis.
Ultrasonography may elucidate associated findings, such as meconium peritonitis. Fibrotic tissue with calcifications has an echogenic appearance with posterior acoustic shadowing. When peritonitis is generalized throughout the peritoneum, a snowstorm appearance may be demonstrated.
Although they typically do not cause obstruction in the newborn period, enteric duplication cysts are well evaluated with ultrasonography. The wall of the cyst is characterized by a layered appearance with an inner echogenic layer of mucosa, surrounded by a hypoechoic layer of muscularis propria, the so-called gut signature. Peristalsis may also be observed in the cyst.
In a systematic review of 16 studies including 640 fetuses, the accuracy of prenatal ultrasound in detecting nonduodenal small bowel atresia was very variable, ranging from 10-100%, with an overall prediction of 50.63%. The individual detection rates for jejunal and ileal atresia were 66.30% and 25.87%, respectively. Both dilated bowel and polyhydramnios had an overall low detection rate for these anomalies. 
Degree of confidence
In utero, the finding large number of dilated fluid-filled loops has a high positive predictive value, but this finding does not differentiate one cause from another.