Embryology

Many of the gastrointestinal abnormalities seen in children and which are detectable with ultrasound arise from abnormal intrauterine development. Also, many of the anomalies are interrelated with other systems in the body. A basic understanding of the embryological development of the gastrointestinal tract will help the sonographer to perform a complete examination of all the relevant systems.

The primordial gut is divided into three parts: the foregut, midgut and hindgut.

The structures derived from the foregut are the pharynx, oral cavity, upper and lower respiratory system, the esophagus, stomach and duodenum, liver, bile ducts and pancreas. The celiac artery supplies the stomach, duodenum, liver, spleen and pancreas. During duodenal development the lumen becomes obliterated and then recanalizes. If this recanalization fails to occur then duodenal atresia or stenosis results. All atresias are distal to the second part of the duodenum (and thus entry of the common bile duct) so that infants present clinically with bile-stained vomiting. Atresias or stenoses in the jejenum and ileum are a later intrauterine event, and are thought to be related to a vascular compromise which results in a localized area of ischemia of the bowel.

The midgut forms the duodenum beyond the sphincter of Oddi (where the common bile duct enters the duodenum), the jejunum, the ileum, the cecum, the appendix and the ascending and right two thirds of the transverse colon. Due to the rapidly enlarging liver and two sets of kidneys within the abdomen there is a shortage of space within the abdominal cavity. The primitive midgut forms a loop, the midgut loop, the apex of which is continuous with the vitello-intestinal duct or yolk stalk. The midgut loop elongates rapidly on an elongated dorsal mesentery which is extruded into the extra-embryonic coelom at the umbilicus. This extrusion constitutes the physiological umbilical hernia. The major artery supplying the midgut is the superior mesenteric artery, which is central to this whole process of herniation and rotation (Fig. 6.1).

Figure 6.1 Embryology of the midgut. The diagram shows herniation of the bowel in early uterine life. The bowel is guided by the vitello-intestinal duct and rotates anticlockwise before re-entering the abdomen. The superior mesenteric artery is in the middle of the rotating bowel.

The return of the midgut to the abdomen occurs in the third month. The proximal limb (i.e. the small bowel) re-enters the abdominal cavity first. The cecum and appendix are the last structures to return to the abdomen. The cecum then descends to the right iliac fossa. The mesenteries shorten and disappear by a process of fusion, and the large bowel becomes fixed in a retroperitoneal position by a process of peritonealization. This is important because, if the bowel is malrotated, this process may result in abnormal bands from the undersurface of the liver, called Ladd bands, as the body attempts to fix the malrotated bowel in position.

Since the appendix develops during the descent of the colon, its final position frequently is posterior to the cecum or colon. These positions of the appendix are called retrocecal or retrocolic (Fig. 6.2).

Figure 6.2 Position of a retrocecal appendix. These can be very difficult to identify on ultrasound.

The hindgut forms the left third of the transverse colon, the descending colon, the sigmoid colon, the rectum and the upper part of the anal canal. The urogenital organs are separated from the primitive rectum by the urorectal septum (Fig. 6.3). The anorectal anomalies occur when there is abnormal separation of the rectum from the urogenital system by arrested growth or deviation of the septum. This results in atresia of the rectum and fistulas to the urethra, bladder or vagina. High anorectal atresias are all associated with a fistula. In addition the development of the rectum and urogenital system takes place at the same time as that of the spine, and hence there is a high association with spinal anomalies.

Figure 6.3 The normal separation of the rectum from the developing urethra and bladder by the urorectal septum. If the septum does not grow down, no separation occurs, resulting in a cloaca, i.e. only one orifice.

The development of the upper part of the anal canal differs from that of the lower part, and this is reflected in different epithelium, blood supply and lymphatic drainage. This is important in adults who have an anorectal malignancy, as the position of the tumor affects the lymphatic drainage and direction of metastatic spread.

The peritoneum is a membrane which surrounds the bowel and delineates the abdominal cavity. After the bowel rotates and becomes fixed, some organs come to lie behind the peritoneum, such as the kidneys and pancreas, in which case they are termed retroperitoneal. In addition, in some places the peritoneum fuses, resulting in sacs such as the lesser sac and ligaments.

Omphalocele

This involves herniation of abdominal viscera through an enlarged umbilical ring. The viscera which may be herniated include the liver, small and large intestine, stomach, spleen or gallbladder and are covered by amnion. The origin of the defect is a failure of the bowel to return to the body cavity from its physiological herniation. Omphalocele is associated with a high rate of mortality and is associated with other severe malformations.

Gastroschisis

This is a herniation of abdominal contents through the body wall directly into the amniotic cavity. It is the more common abdominal wall defect. It occurs lateral to the umbilicus, and the viscera are not covered by peritoneum. The umbilical cord is not involved. Unlike an omphalocele, gastroschisis is not associated with other abnormalities, so the survival rate is excellent. However, volvulus, that is twisting of the bowel, does occur.

Vitelline duct persistence

The vitelline duct or yolk stalk is the structure that connects the primitive gut and the yolk sac. If the vitelline duct persists it forms an outpouching of ileum which is called a Meckel’s diverticulum. This occurs in roughly 2% of the population and is located about 60 cm from the ileocecal valve. It is important because, if it contains gastric mucosa, it may ulcerate and bleed or perforate. Very rarely if the vitelline duct persists and remains patent, a direct communication will exist between the umbilicus and ileum. Infants present with a ‘weeping’ umbilicus, even gas expulsion, and sinography reveals a connection with the ileum.

Preparation

Ideally all children scheduled for abdominal examinations should be fasted so as to distend the gallbladder and decrease abdominal gas. In reality, this is not practical as it is far better to have a happy, drinking child with a full bladder so that the pelvis and lower abdomen can be fully examined. In addition, a hungry child is a fractious child, which makes ultrasound much more difficult. In our department we fast only those children for whom there are specific clinical queries about the gallbladder, biliary system or pancreas. No specific bowel preparation is necessary, although it is wise to ask parents to ensure the child avoids fizzy drinks.

Examination

Always start with a probe of the highest frequency and then decrease frequency if there is insufficient penetration. Generally 5–10 MHz is suitable for most children. Good near field visualization is needed. Start with a curvilinear probe and systematically examine the entire abdominal contents. Start with the intra-abdominal organs and ensure they appear normal. When looking specifically at the bowel, change and use a high-frequency linear probe. Starting in the right flank, gradually move around the abdomen in a clockwise manner. Gradually compressing the abdomen and gently pushing the bowel gas away is a useful trick allowing better visualization if the abdomen is very gassy.

Observations should include bowel wall thickness, bowel motility, bowel contents, free fluid or abscess collections in the abdomen, and any masses or cysts related to the bowel. Doppler should also be used and particularly in suspected appendicitis, inflammatory bowel disease and intussusception. Ultrasound plays a major role in some conditions, in particular the cystic masses, pyloric stenosis, intussusception and appendicitis.

In most instances, contrast examinations of the bowel are still the imaging modality of choice.

Technique of ultrasound examination

The baby may have a nasogastric tube draining the stomach, or have been vomiting severely, in which case the stomach will be empty. It is useful to be able to give the baby clear fluid, and this can be done via the nasogastric tube or with a bottle. The clear fluid will fill the gastric antrum so that it can be used as an acoustic window for the pylorus. Secondly, an assessment can be made of whether any fluid is passing through the pylorus into the duodenum. Normally fluid can be seen to pass through the pylorus and into the duodenum without delay (Fig. 6.5).

Figure 6.5 Normal pylorus. (A), (B) and (C) Transverse sonograms at the level of the pylorus. The patient has been given clear fluid in order to delineate the antrum and to observe fluid movement through the pylorus. The normal pylorus actively peristalses and fluid passes into the duodenum with ease. This is a good view to obtain when looking for pyloric stenosis.

Begin with the patient in the supine position, using a high-frequency 15L8 MHz linear transducer. If there is insufficient fluid in the stomach then fluid should be given. Be careful however not to overfill the stomach and provoke a further bout of vomiting. A very full stomach may also distort the antrum, making the pylorus difficult to see.

Start by scanning longitudinally in the right upper quadrant just medial to the gallbladder. Once the ‘doughnut’ of the transverse section through the hypertrophic pyloric stenosis is identified, pivot on the axis through 90° to get the longitudinal measurement. The trick in the transverse view is to identify the gastric antrum and, if the abdomen is too gassy, turn the infant right side down in order to displace gas and fill the antrum with fluid. The appearances to look for are the hypoechoic thickening of the pyloric muscle and elongation of the canal. Table 6.1 gives the published data for measurements of the canal in different series. Virtually no authors are in entire agreement, which makes remembering the measurements even more difficult! Broadly speaking, keep in mind 10 mm × 15 mm, i.e. take a pyloric length of over 15 mm and an overall width of 10 mm as abnormal in an average weight for term baby. These measurements are a slight overestimation but easily remembered. In the many series in the literature there is an overlap in measurements between the normal and abnormal pylorus. To the experienced eye, if the pylorus is abnormal it is easy to identify, often without any measurements at all (Fig. 6.6).

Table 6.1 Sonographic measurements of pyloric stenosis

Figure 6.6 Pyloric stenosis. (A) Transverse sonogram of the pylorus in an infant with projectile vomiting and suspected of having pyloric stenosis. There is thickening of the pyloric muscle around the echogenic mucosa centrally. The gallbladder is draped over the top. (B) Longitudinal sonogram of the pyloric canal. There is marked thickening of the pyloric muscle (between calipers 1) and lengthening of the canal (between calipers 2). (C) Barium has been introduced into the stomach via a nasogastric tube. There was very active antral peristalsis and an elongated and narrowed pyloric canal. There is the typical double channel of pyloric stenosis with delay in gastric emptying because of the gastric outflow obstruction.

Other important features to look for are the double mucosal channel of the pylorus, excessive antral peristalsis, delayed or absent passage of fluid into the duodenum and GER. It is important to note these additional features, as pylorospasm may mimic hypertrophic pyloric stenosis and the sonographer may make a false-positive diagnosis.

Infants who are premature, small and underweight may have pyloric stenosis in the presence of measurements that are below those quoted in the series. Pyloric volume measurements have been reported although they are not widely used or accepted.