Excerpt from Ileal Atresia

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Background

Approximately 95% of intestinal obstructions diagnosed in the first 2 weeks of life are due to atresia and/or stenosis of small intestine (Grosfeld, 1970). Atresia represents complete luminal obstruction of a hollow viscus, and stenosis involves partial occlusion. Atresia is more common than stenosis. At birth, complete obstruction is present with atresia, whereas various degrees of stenosis manifest with mild-to-severe sequelae. Ileal atresia represents 50% of small-intestinal atresias (Hernanz-Schulman, 1999). Ileal atresia is more common than jejunal atresia.

Pathophysiology

Ischemia is the cause for jejunal and ileal atresia and stenosis. The etiology of ischemia is most commonly a primary vascular accident and less commonly in utero obstruction, such as volvulus. Malrotation and internal hernia have also been noted. Because the bowel contents are sterile in the fetus, the necrotic tissue is resorbed or converted into fibrous bands.

The succus entericus, which forms the meconium, is composed of secretions from the stomach and small bowel, desquamated cells, and secreted fluid and bile. Exposure to succus entericus expands the size of the colon from its fetal size. The timing and location of atresia determines whether the succus entericus enters the colon during its development.

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. On the other hand, 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. In utero perforation of bowel may allow spill of meconium into the peritoneal space. Fibrosis ensues, and peritoneal calcification may occur. Calcifications may be irregular, linear, round, or generalized in appearance. Intraluminal calcifications may also occur in cases of severe obstruction.

Meconium peritonitis occurs in 5.2% of cases (de Lorimier, 1969).

Frequency

United States

The incidence of atresia of the small bowel ranges from 1 case in 332 live births (Walker, 2000) to 1 case in 5000 live births (Okada, 2002). About 15% of ileal atresias occur proximally, and 30% occur distally (Belknap, 1994).

Mortality/Morbidity

In general, early diagnosis of ileal atresia reduces the mortality rate.

• The overall survival rate for ileal atresia is 75.3%, whereas that for multiple atresias is 16% (de Lorimier, 1969).
• Martin and Zerella (1976) described the survival rate for each subtype as follows: type I, 85%; type II, 66%; type III (or type IV under the current classification), 29%; and type IV (or type IIIB under the current classification), 57%.

Race

• No specific racial predilection is known.
• Multiple atresias of the small bowel with luminal calcification occur in people of French Canadian descent, with an autosomal recessive inheritance pattern. The mortality rate for this condition is high, and it is associated with dilatation of the biliary duct.
• Nonhereditary forms of multiple intestinal atresias with intraluminal calcifications also exist.

Sex

There is no sex predilection for this disease.

Age

Most ileal atresias are diagnosed in the first 24 hours of life. The age at presentation is 1-30 days (de Lorimier, 1969). Prematurity of the neonate is related to the severity of the subtype of atresia (Martin, 1976). (See Anatomy below.)

Anatomy

Ileal atresia occurs in 4 main types with subtypes (see Image 1). 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 blood supply. Type IV is defined as multiple atresias.

In Christmas-tree or apple-peel atresia, the proximal superior mesenteric artery is occluded. 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 (Seashore, 1987). An autosomal recessive inheritance pattern is suggested.

Jejunal and ileal atresias uncommonly coexist and are multiple; therefore, the entire small bowel must be examined at surgery.

Clinical Details

Polyhydramnios occurs in one fourth of cases and is more common in proximal jejunal atresia than in distal ileal atresia (Stringer, 1989).

The clinical manifestations of ileal atresia appear in the first 24 hours of life. Symptoms of stenosis are less severe and may occur later. The most common presenting symptom is bilious vomiting, which may be associated with abdominal distension or an inability to pass meconium.

In one quarter of cases, a probable etiology is identified at the time of surgery. These causes include malrotation, volvulus, gastroschisis, or intussusception.

Extra-GI anomalies are rare and occur in less than 7% of patients (de Lorimier, 1969).

Preferred Examination

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-GI 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, an upper-GI 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, 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.

Limitations of Techniques

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. 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 upper-GI 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 an upper-GI 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 upper-GI 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 is of limited utility in the neonatal period, because it does not show the site of obstruction. It 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).

CT and MRI have no role in the diagnosis of ileal atresia.

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