The Gastrointestinal Tract and Intraabdominal Organs
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The Gastrointestinal Tract and Intraabdominal Organs Normal Anatomy of the Meconium Peritonitis/ 243 Gastrointestinal Tract and the Hirschsprung's Disease/ 245 Anterior Abdominal Wall/ 233 Splenomegaly/ 246 Esophageal Atresia with and without Hepatomegaly/ 249 Tracheoesophageal Fistula/ 234 Choledochal Cyst/ 251 Duodenal Atresia/ 236 Mesenteric, Omental, and Bowel Obstruction/ 239 Retroperitoneal Cysts/ 253 Normal Anatomy of the Gastrointestinal Tract and the Anterior Abdominal Wall The stomach forms at about 4 weeks after conception are characterized by vigorous and fleeting movements by the development of a spindle-shaped dilatation of with a duration of less than 3 seconds. Later in the foregut caudal to the esophagus. The stomach gestation, peristaltic waves are more vigorous, descends into the abdomen at about 6 to 7 weeks after ubiquitous, and of longer duration. A small bowel conception, and by 11 weeks the muscles in its wall loop of more than 7 mm in internal diameter should are developed. Visualization with ultrasound is raise the index of suspicion for bowel obstruction.4 possible as early as the 9th week of menstrual age. Generally, segments of small bowel measure less than Different parts of the gastric anatomy (greater 15 mm in length. Meconium formation begins at 16 to curvature, fundus, body, and pylorus) can be seen by 20 weeks of menstrual age. Sonographically, the 14th week. Peristalsis is rarely visible before the meconium appears hypoechogenic in comparison to 16th week. Table 7-7 (see p. 254) displays data on the bowel wall. The large bowel appears as a large fetal stomach dimensions. Observations of this organ tubular structure in the periphery of the fetal abdomen. over a 3-hour period of time have demonstrated that Haustral clefts are sonographically apparent at 30 there are no dramatic changes in size and therefore weeks in 87 percent of the fetuses and in all cases at 31 these dimensions should be useful in evaluating weeks of menstrual age. Table 7-8 (see p. 254) shows stomach size.3 On occasion, an echogenic mass (fetal the correlation between transverse colonic diameter gastric pseudomass) is seen within the fetal stomach. and gestational age. The echogenicity of the colon is The origin of this monographic image is not clear. If it assessed and graded in comparison with bladder and disappears on subsequent examinations, it has no liver echogenicity. Grade 0 means the abdomen is proven pathologic significance.1 uniform in appearance and the colon is not identified. The small and large bowels can be distinguished Grade 1, the colonic appearance is hypoechogenic and from each other. The small bowel is centrally located, essentially identical to the stomach and bladder. and it changes its position and appearance with Haustra may be identified. Grade 2, the echogenicity peristalsis, which is visible as early as the 18th week of the colon is greater than the bladder, but less than of menstrual age. Early in gestation, peristaltic waves the liver. Grade 3, colonic 233 234 THE GASTROINTESTINAL TRACT AND INTRAABDOMINAL ORGANS ©1987-2002 Romero-Pilu-Jeanty-Ghidini-Hobbins content has echogenicity similar to the liver. Grade 2 ductus venosus is inconsistently visible in physiologic appears at 29 weeks and Grade 3 begins after 34 weeks circumstances. The ductus venosus drains into the left of menstrual age.2 hepatic vein or directly into the inferior vena cava. The pancreas can be seen in the third trimester in The portal system is best seen in transverse scans fetuses with the spine down. It can be found behind of the fetus, whereas the hepatic veins are better the stomach and in front of the splenic vein. visualized in longitudinal or oblique scans. The The gallbladder is a hypoechogenic structure in the hepatic veins can be seen when they enter the distal middle right of the abdomen. It is often confused with inferior vena cava near the right atrium. the umbilical vein. They can be differentiated because The aorta and its bifurcation into the common iliac (1) the gallbladder is right-sided and not median, (2) vessels are usually visible. The internal iliac vessels the umbilical vein can be traced outside the fetus into are more difficult to visualize, but their main branches, the umbilical cord and inside the liver to the portal the umbilical arteries, can commonly be seen along the system, and (3) the gallbladder neck is thinner than its fetal bladder. The external iliac arteries can be bottom, and, therefore, the gallbladder is conical, followed into the femoral artery. which contrasts with the cylindrical umbilical vein. The inferior vena cava is visible adjacent to the Absence of the gallbladder occurs in rare aorta. In the higher abdomen, the inferior vena cava chromosomal syndromes (G syndrome) and in about can be observed to be more anterior than the aorta. 20 percent of patients with biliary atresia. This is because the inferior vena cava bends anteriorly The size of the liver is affected by fetal nutrition. to enter the right atrium, whereas the aorta comes from There are four ductal systems in the liver: the portal the posterior aspect of the chest after ending the arch. circulation, the hepatic veins, the hepatic arteries, and The major collaterals of the inferior vena cava, visible the biliary ducts. The last two cannot be seen under with ultrasound, are the renal, hepatic, and iliac veins. normal circumstances. The umbilical vein courses cephalically after entering the abdomen, then follows REFERENCES the falciform ligament and enters the liver, where it receives blood from the venae advehentes. It 1. Fakhry J, Shapiro LR, Schechter A, et al.: Fetal gastric anastomoses with the left portal vein. The umbilical pseudomasses. J Ultrasound Med. 6:177, 1987. vein is, in fact, the left umbilical vein, since the right 2. Goldstein I, Lockwood C, Hobbins, IC: Ultrasound umbilical vein regresses around the 6th to 7th week. assessment of fetal intestinal development in the evaluation of gestational age. Obstet Gynecol (in press), From the junction with the left portal vein, the 1987. umbilical vein is called the "umbilical part of the left 3. Goldstein I, Reece EA, Yarkoni S, et al.: Growth of the portal vein." The oxygenated blood may reach the fetal stomach in normal pregnancies. Obstet Gynecol heart through the ductus venosus or the hepatic 70:(in press), 1987. sinusoids and hepatic veins. The sphincter at the 4. Nyberg DA, Mack LA, Patten RM, Cyr DR: Fetal bowel: origin of the ductus venosus is believed to protect the Normal sonographic findings. J Ultrasound Med 6:3, fetal heart from overload when uterine contractions 1987. increase the placental venous return. This also explains why the Esophageal Atresia with and without Tracheoesophageal Fistula Definition Epidemiology Esophageal atresia is the absence of a segment of the The incidence of esophageal atresia varies between esophagus. In most cases, this condition is associated 1:8001, and 1:5,000 live births.7,11 There is no with a fistula between the gastrointestinal and established sex preponderance. respiratory tracts. Embryology Etiology The esophagus and trachea develop from a common Unknown. There is no demonstrable genetic diverticulum of the primitive pharyngeal cavity. The predisposition, and the disease occurs sporadically. diverticulum is subsequently partitioned by the tra- ESOPHAGEAL ATRESIA WITH AND WITHOUT TRACHEOESOPHAGEAL FISTULA 235 ©1987-2002 Romero-Pilu-Jeanty-Ghidini-Hobbins Figure 7-1. Transverse scan of the fetal abdomen in a fetus with esophageal atresia. A normal stomach is not visualized. The arrows point to the collapsed walls of the stomach. Sp, Spine. Figure 7-2. Coronal section of the neck in a fetus with esophageal atresia. Arrows point to the esophagus, which ends blindly (*). cheoesophageal septum, forming the laryngotracheal tube and the esophagus. Development of the upper consequence of the esophageal obstruction. The prenatal respiratory and gastrointestinal tracts takes place diagnosis of isolated esophageal atresia has been reported 13 3,8,14 between the 21st day and the 5th week of gestation in a few cases. Failure to visualize the stomach in Given this common embryologic origin, it is not serial ultrasound examinations allowed the diagnosis surprising that anomalies of the trachea and (Figs. 7-1, 7-2). It should be stressed that this diagnosis esophagus are often associated. is only possible in 10 percent of patients. In the remaining patients, the gastric secretion or the occurrence of a tracheoesophageal fistula will allow visualization of Pathology 2 There are different types of tracheoesophageal some gastric distention. Any time the stomach is not abnormalities. The five major varieties include (1) visualized, the fetal face should be examined to exclude isolated esophageal atresia, (2) esophageal atresia otocephaly (see p. 110). with a fistula connecting the proximal portion of the esophagus with the trachea, (3) esophageal atresia with a fistula connecting the distal portion of the Prognosis esophagus with the trachea, (4) esophageal atresia The prognosis of esophageal atresia with or without with a double fistula connecting both segments of the tracheoesophageal fistula depends on three factors: interrupted esophagus with the trachea, and (5) associated congenital anomalies, respiratory tracheoesophageal fistula without esophageal atresia. complications, and gestational age and weight at The most common variety is the third one, which delivery. Infants born at term without anomalies or accounts