Gastric outlet obstruction due to atresia or membranous antral web is rare, constituting one of the most unusual causes of gastrointestinal obstruction. On the other hand, gastric outlet obstruction due to hypertrophic pyloric stenosis is among the most common causes of gastrointestinal obstruction in the neonate. This condition only rarely presents antenatally (Zimmerman, 1978; Nebekura et al., 1983; Mitchell and Risch, 1993).

A classification system for pyloric stenosis has been proposed: type A, a pyloric membrane or web; type B, the pyloric channel is a solid cord; type C, there is a gap between the stomach and the duodenum. Prenatal diagnosis should preempt consideration of other intestinal atresias (Usta, 2000; Ilce et al., 2003).

The incidence of pyloric atresia has been reported at 1 in 1 million livebirths, representing less than 1% of all gastrointestinal atresias (Geber and Aberdeen, 1965; Thompson et al., 1968). Hypertrophic pyloric stenosis occurs in 1.5 in 1000 to 4 in 1000 livebirths among whites, but it is less prevalent in blacks and Asians (Mitchell and Risch, 1993; Grant and McAleer, 1996). The incidence of prenatally diagnosed cases of these conditions is unknown.

Prenatal diagnosis of pyloric atresia is associated with polyhydramniosin 61% of reported cases (Colin, 1989). The prenatal sonographic image usually shows polyhydramnios and associated gastric distention (Figure 71-1). If the fetus has recently vomited or if the obstruction is incomplete, the stomach may appear normal in size and polyhydramnios may be absent (Rizzo et al., 1995). This is especially true early in gestation, as polyhydramnios tends to develop during the third trimester. Sonographically, pyloric atresia and stenosis are associated with a “single bubble” as opposed to the “double bubble” observed in duodenal atresia. This appearance is due to either high-grade stenosis or complete atresia of the gastric outlet, resulting in marked dilation of the stomach. However, one should be cautious in the fetus with a “double bubble” sign, as this can be seen due to massive gastric distention and folding of the stomach on itself. The ultrasound beam can pass through the dilated fundus and then through the antrum, giving the impression of a double bubble. The sonographic features of hypertrophic pyloric stenosis are similar to pyloric atresia (Zimmerman, 1978; Nebekura et al., 1983). Isolated gastric distention is seen, but unlike in pyloric atresia, the hypertrophied pylorus is also seen. Because hypertrophic pyloric stenosis creates only a partial obstruction, more distal fluid-filled loops of bowel are seen in contrast to pyloric atresia, in which they are not observed. The hypertrophied pylorus seen in cross section has a characteristic “target” or “bull’seye” appearance. No prenatal parameters for mural thickness, diameter, or pyloric length exist. However, postnatal values for hypertrophic pyloric stenosis can be used. In the neonate, a pylorus that demonstrates a mural thickness > 4 mm, a diameter > 11 mm, or a pyloric length of > 16 mm is consistent with a diagnosis of hypertrophic pyloric stenosis. It should be pointed out, however, that measurements that do not meet these criteria do not rule out this diagnosis. Hypertrophic pyloric stenosis tends to be progressive, and increases in these parameters are to be expected. Because of the partial gastric outlet obstruction, polyhydramnios may be seen in pyloric stenosis, but not to the same extent as in pyloric atresia.

In addition to pyloric atresia, the differential diagnosis of a dilated fetal stomach includes duodenal atresia or stenosis (see Chapter 72), malrotation with midgut volvulus, duodenal duplication, and antral duplication. The presence of the double bubble sonographic sign is more consistent with the diagnosis of duodenal atresia, but in pyloric atresia, the stomach may become so grossly distended that in a single ultrasonographic image both the fundus and the antrum may give the appearance of a double bubble (Malone et al., 1997). Polyhydramnios is present in each of these conditions and is less useful in making a distinction between them. In pyloric or duodenal atresia, no distal bowel will be visualized sonographically, but in pyloric stenosis or duodenal stenosis more distal intestine will be seen.

Pyloric atresia associated with epidermolysis bullosa was first reported by Swinburne and Kohler (Swinburne and Kohler, 1968). Pyloric atresia, in addition to its association with epidermolysis bullosa, has also rarely been reported with esophageal atresia and tracheoesophageal fistula (Korber and Glasson, 1977; Peterson and Hertel, 1977; El Shafie et al., 1979) as well as with cardiovascular and genitourinary anomalies (Lepinard et al., 2000).