Normal Anatomy and Abdominal Wall Defects

Normal Anatomy

• The diaphragm defines the superior aspect of the abdominal cavity
• The diaphragm is a hypoechoic curved line that separates the echogenic lungs from the liver and stomach
• The lies occupies most of the upper abdomen
• Its left lobe is larger than the right lobe
• Gallbladder is seen in the anterior right abdomen, inferior to the margin of the liver
• The spleen is located in the left upper abdomen, posterior to the stomach, echogenic, and homogeneous

Abdominal Wall

• The development of the anterior abdominal wall requires normal herniation of the viscera into the umbilical cord base during the first trimester
• Contents return to the abdominal cavity after undergoing mid-gut rotation
• This happens by the 12th week of gestation
• Anterior abdominal wall evaluation should be performed in prenatal scans

Abdominal Wall Defects - Gastroschisis

• Gastroschisis is a small defect involving all three layers of the abdominal wall
• This small defect allows intestines to protrude into the amniotic cavity
• It is lateral to the umbilical cord insertion, usually on the right
• Not covered by a membranous sac
• Gastroschisis results from a muscular defect, possibly related to vascular disruption
• It is sporadic, and has no associated anomalies and no increased risk for chromosomal abnormalities

Sonographic findings for Gastroschisis

• Variable amount of bowel floating in amniotic fluid
• Cord is located adjacent to the defect
• There is no membranous sac covering the herniation

Abdominal Wall Defects - Omphalocele

• Omphalocele results from the failure of the intestines to return to the abdomen during the second stage of intestinal rotation
• May contain a single loop of bowel or most of the abdominal contents
• The defect is covered by a membrane which is a layer of amnion and peritoneum
• It is typically seen at the level of the umbilical cord insertion
• Rupture of the sac during vaginal delivery could cause sepsis
• Patients receive genetic counseling because of the high association between omphalocele and other chromosomal abnormalities

Sonographic findings in cases of Omphalocele:

• Extra-abdominal mass consisting of bowel loops, liver, or other abdominal organs
• The mass is contiguous with the umbilical cord and covered by a membrane
• Omphaloceles that contain ONLY bowel are more likely to be associated with chromosomal abnormalities

Bladder Exstrophy

• Midline defect involving the lower abdominal wall and the anterior wall of the bladder
• Usually an isolated defect
• Exposure and protrusion of the urinary bladder
• Can be associated with genital anomalies such as cleft clitoris, epispadias, wide separation of pubic bones

Sonographic Findings in cases of Bladder Exstrophy:

• Bladder not identified after over 30 minutes of scan time, normal AFI
• Possible soft tissue protrusion from the lower abdominal wall
• Separation of pubic bones
• Microphallus in male fetus

The GI System – normal anatomy

• Esophagus is difficult to image unless fetus is swallowing or there is stenosis present

• Transverse views of the stomach show an ovoid fluid collection in the left upper abdomen

• Coronal imaging can demonstrate the stomach's fundus, body, and pylorus

• Muscle layer is very thin in a fetus

• There will occasionally be echoes (cellular debris) in the fluid being that is swallowed and in the stomach

• Specific segments of the intestines are difficult to see unless there is fluid to provide contrast

• Normally mixed echogenicity to cystic in appearance

• Peristalsis can be seen in the late second trimester of pregnancy

• Meconium (mixture of bile and swallowed vernix, cells, hair) fills the colon and may appear highly echogenic

• The colon is most obvious in the late third trimester

• Colon is peripheral and small bowel is centrally located fetal abdomen

Esophageal Atresia

• Occurs 1:2000 to 3000 live births
• It is a discontinuity of the esophagus
• 90% are accompanied by a distal tracheo-esophageal fistula (TE fistula)
• 30-70% is the association with other congenital malformations
  • Cardiovascular anomalies
  • GI defects
  • Genitourinary issues
  • MSK anomalies

Sonographic Findings in cases of Esophageal Atresia:

• Small or absent fetal stomach
• Failure to demonstrate stomach on serial sonograms
• Polyhydramnios
• *Not always able to detect sonographically, this depends on whether or not TE fistula is present.

Duodenal Atresia

• Duodenal obstruction is the MOST COMMON perinatal intestinal obstruction
• 65% have either a karyotypic abnormality or an associated abnormality at birth (most common are cardiac and vertebral)
• 30% of infants with duodenal atresia have trisomy 21
• More common in boys

Intestinal Atresia

• Obstruction of the intestine with subsequent distention of bowel loops
• May occur anywhere along the intestinal tract all the way to the anus

Sonographic findings of Intestinal Atresia:

• Multiple fluid filled distended bowel loops
• Small bowel internal diameter of more than 7 mm
• Increased peristalsis
• Perforation indicated by abdominal calcifications and ascites
• Polyhydramnios

Meconium Peritonitis

• Chemical peritonitis caused by a small bowel perforation in utero
• Perforation can occur after bowel obstruction caused by atresia or volvulus (A volvulus is the loop of intestine twists around itself and the mesentery that supports it, resulting in a bowel obstruction)
• Perforation can also occur by meconium ileus (obstruction due to thick meconium).
• Cystic fibrosis is the etiology in 35-40% of cases because the meconium is sticky and thick

Sonographic findings in cases of Meconium Peritonitis:

• Calcifications in the fetal abdomen
• Fetal ascites
• Polyhydramnios
• Meconium pseudocyst
• Meconium is the earliest stool of a mammalian infant, and it is composed of materials ingested during the time an infant spends in the uterus: intestinal epithelial cells, lanugo(hair), mucus, amniotic fluid, bile, and water

Midgut Volvulus

• An obstruction caused by the bowel twisting upon its blood supply
• If the small bowel fails to completely return to the abdominal cavity and rotate properly, the bowel may twist about the axis of the superior mesenteric artery, resulting in poor vascular flow distal to the point of obstruction
• May also occur if the long mesenteric attachments that fix the bowel to the posterior abdominal wall fail to develop, leading to infarction if this is not corrected surgically
• Midgut volvulus is usually diagnosed in the first days of life; the infant may present with distention or obstruction, but, most typically, with bilious vomiting

Sonographic Imaging in cases of Midgut Volvulus:

• A fluid-filled proximal duodenum with an arrowhead twist at the point of descending or transverse duodenal obstruction.
• This is often seen with Mild polyhydramnios
• Echogenic mass under the fetal liver
• Slightly dilated bowel loops are other findings
• Because of the twisting around the mesenteric vessels, the characteristic “whirlpool sign”
• The color Doppler exam may also demonstrate the twisted vessels

Hyperechoic Bowel

• Increased echogenicity of the fetal bowel identified on a second trimester US
• Hyperechogenicity can be diffuse or focal
• Similar to or greater echogenicity than bone – no shadowing
• Primarily in the lower fetal abdomen and pelvis
• Bowel tends to look more echogenic when using higher frequencies or harmonics
• A lower frequency and avoid harmonics should be used if its suspected

Common etiologies of Hyperechoic Bowel:

• Normal variant
• Trisomy 21
• CMV infection (cytomegalovirus)
• Cystic fibrosis
• Swallowed intra-amniotic blood – very rare

Large Bowel Obstruction

• Normal colon tends to image only in the third trimester and is frequently observed without any obvious bowel peristalsis
• The major causes of large bowel obstruction are imperforate anus, meconium ileus and Hirschsprung disease
• Hirschsprung disease is a functional disorder of the distal colon that results in perpetually contracted bowel
• The colon does not relax because of the congenital absence of the neuroenteric ganglion cells in the mucosal layer of the bowel
• Those cells control the relaxation phase of peristalsis and consequently affect the movement of meconium resulting in a functional obstruction
• More common in males and usually affects the distal portion of the bowel
• Other conditions that may affect the motility of the fetal bowel and formation of a meconium plug, including maternal preeclampsia, maternal diabetes mellitus, maternal administration of magnesium sulfate, prematurity, sepsis, and hypothyroidism

Ascites

• Most commonly associated with fetal hydrops.

Persistent Right Umbilical Vein

• The umbilical vein courses to the LEFT side of the fetal abdomen
• In Persistent right Umbilical vein, the Umbilical vein Is actually entering the right portal vein of the liver rather than the left.

Criteria for diagnosis of Persistent Right Umbilical Vein:

• The diagnosis of persistent right umbilical vein is made by the findings observed in the transverse section of the fetal abdomen
• Portal vein curved towards the liver
• Fetal gall bladder located medially to the umbilical vein between the umbilical vein & the stomach
• Umbilical vein is abnormally connected to the right portal vein instead of to the left portal vein
• Color doppler is necessary to identify the type of persistent right umbilical vein

Overview of the normal anatomy of the abdomen, including the diaphragm, liver, gallbladder, and spleen. Also describes abdominal wall development relating to Gastroschisis. Evaluation should be performed in prenatal scans.