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 by the development of a spindle-shaped dilatation of the foregut caudal to the esophagus. The stomach descends into the abdomen at about 6 to 7 weeks after conception, and by 11 weeks the muscles in its wall are developed. Visualization with ultrasound is possible as early as the 9th week of menstrual age. Different parts of the gastric anatomy (greater curvature, fundus, body, and pylorus) can be seen by the 14th week. Peristalsis is rarely visible before the 16th week. Table 7-7 (see p. 254) displays data on fetal stomach dimensions. Observations of this organ over a 3-hour period of time have demonstrated that there are no dramatic changes in size and therefore these dimensions should be useful in evaluating stomach size.3 On occasion, an echogenic mass (fetal gastric pseudomass) is seen within the fetal stomach. The origin of this monographic image is not clear. If it disappears on subsequent examinations, it has no proven pathologic significance.1 The small and large bowels can be distinguished from each other. The small bowel is centrally located, and it changes its position and appearance with peristalsis, which is visible as early as the 18th week of menstrual age. Early in gestation, peristaltic waves
are characterized by vigorous and fleeting movements with a duration of less than 3 seconds. Later in gestation, peristaltic waves are more vigorous, ubiquitous, and of longer duration. A small bowel loop of more than 7 mm in internal diameter should raise the index of suspicion for bowel obstruction.4 Generally, segments of small bowel measure less than 15 mm in length. Meconium formation begins at 16 to 20 weeks of menstrual age. Sonographically, meconium appears hypoechogenic in comparison to the bowel wall. The large bowel appears as a large tubular structure in the periphery of the fetal abdomen. Haustral clefts are sonographically apparent at 30 weeks in 87 percent of the fetuses and in all cases at 31 weeks of menstrual age. Table 7-8 (see p. 254) shows the correlation between transverse colonic diameter and gestational age. The echogenicity of the colon is assessed and graded in comparison with bladder and liver echogenicity. Grade 0 means the abdomen is uniform in appearance and the colon is not identified. Grade 1, the colonic appearance is hypoechogenic and essentially identical to the stomach and bladder. Haustra may be identified. Grade 2, the echogenicity of the colon is greater than the bladder, but less than the liver. Grade 3, colonic
233 234 THE GASTROINTESTINAL TRACT AND INTRAABDOMINAL ORGANS
content has echogenicity similar to the liver. Grade 2 appears at 29 weeks and Grade 3 begins after 34 weeks of menstrual age.2
The pancreas can be seen in the third trimester in fetuses with the spine down. It can be found behind the stomach and in front of the splenic vein.
The gallbladder is a hypoechogenic structure in the middle right of the abdomen. It is often confused with the umbilical vein. They can be differentiated because (1) the gallbladder is right-sided and not median, (2) the umbilical vein can be traced outside the fetus into the umbilical cord and inside the liver to the portal system, and (3) the gallbladder neck is thinner than its bottom, and, therefore, the gallbladder is conical, which contrasts with the cylindrical umbilical vein. Absence of the gallbladder occurs in rare chromosomal syndromes (G syndrome) and in about 20 percent of patients with biliary atresia.
The size of the liver is affected by fetal nutrition. There are four ductal systems in the liver: the portal circulation, the hepatic veins, the hepatic arteries, and the biliary ducts. The last two cannot be seen under normal circumstances. The umbilical vein courses cephalically after entering the abdomen, then follows the falciform ligament and enters the liver, where it receives blood from the venae advehentes. It anastomoses with the left portal vein. The umbilical vein is, in fact, the left umbilical vein, since the right umbilical vein regresses around the 6th to 7th week. From the junction with the left portal vein, the umbilical vein is called the "umbilical part of the left portal vein." The oxygenated blood may reach the heart through the ductus venosus or the hepatic sinusoids and hepatic veins. The sphincter at the origin of the ductus venosus is believed to protect the fetal heart from overload when uterine contractions increase the placental venous return. This also explains why the
ductus venosus is inconsistently visible in physiologic circumstances. The ductus venosus drains into the left hepatic vein or directly into the inferior vena cava.
The portal system is best seen in transverse scans of the fetus, whereas the hepatic veins are better visualized in longitudinal or oblique scans. The hepatic veins can be seen when they enter the distal inferior vena cava near the right atrium.
The aorta and its bifurcation into the common iliac vessels are usually visible. The internal iliac vessels are more difficult to visualize, but their main branches, the umbilical arteries, can commonly be seen along the fetal bladder. The external iliac arteries can be followed into the femoral artery.
The inferior vena cava is visible adjacent to the aorta. In the higher abdomen, the inferior vena cava can be observed to be more anterior than the aorta. This is because the inferior vena cava bends anteriorly to enter the right atrium, whereas the aorta comes from the posterior aspect of the chest after ending the arch. The major collaterals of the inferior vena cava, visible with ultrasound, are the renal, hepatic, and iliac veins. REFERENCES
1. Fakhry J, Shapiro LR, Schechter A, et al.: Fetal gastric
pseudomasses. J Ultrasound Med. 6:177, 1987. 2. Goldstein I, Lockwood C, Hobbins, IC: Ultrasound
assessment of fetal intestinal development in the evaluation of gestational age. Obstet Gynecol (in press), 1987.
3. Goldstein I, Reece EA, Yarkoni S, et al.: Growth of the fetal stomach in normal pregnancies. Obstet Gynecol 70:(in press), 1987.
4. Nyberg DA, Mack LA, Patten RM, Cyr DR: Fetal bowel: Normal sonographic findings. J Ultrasound Med 6:3, 1987.
Esophageal Atresia with and without Tracheoesophageal Fistula
Definition Esophageal atresia is the absence of a segment of the esophagus. In most cases, this condition is associated with a fistula between the gastrointestinal and respiratory tracts. Etiology Unknown. There is no demonstrable genetic predisposition, and the disease occurs sporadically.
Epidemiology The incidence of esophageal atresia varies between
1:8001, and 1:5,000 live births.7,11 There is no established sex preponderance. Embryology The esophagus and trachea develop from a common diverticulum of the primitive pharyngeal cavity. The diverticulum is subsequently partitioned by the tra-
ESOPHAGEAL ATRESIA WITH AND WITHOUT TRACHEOESOPHAGEAL FISTULA 235
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. cheoesophageal septum, forming the laryngotracheal tube and the esophagus. Development of the upper respiratory and gastrointestinal tracts takes place between the 21st day and the 5th week of gestation13 Given this common embryologic origin, it is not surprising that anomalies of the trachea and esophagus are often associated. Pathology There are different types of tracheoesophageal abnormalities. The five major varieties include (1) isolated esophageal atresia, (2) esophageal atresia 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 esophagus with the trachea, (4) esophageal atresia with a double fistula connecting both segments of the interrupted esophagus with the trachea, and (5) tracheoesophageal fistula without esophageal atresia. The most common variety is the third one, which accounts for more than 90 percent of all cases.5 Associated Anomalies Cardiac, chromosomal (Down syndrome), gastrointestinal, and genitourinary anomalies are found in 58 percent of patients. The incidence of congenital heart disease varies between 15 and 39 percent.4,10 Atrial and ventricular septal defects are the most common cardiac abnormalities. Diagnosis An esophageal atresia should be suspected in the presence of polyhydramnios. The increased amount of amniotic fluid is related to decreased turnover as a
Figure 7-2. Coronal section of the neck in a fetus with esophageal atresia. Arrows point to the esophagus, which ends blindly (*).
consequence of the esophageal obstruction. The prenatal diagnosis of isolated esophageal atresia has been reported in a few cases.3,8,14 Failure to visualize the stomach in serial ultrasound examinations allowed the diagnosis (Figs. 7-1, 7-2). It sh